Discussion:
Tube mono-block amp with SIX 6L6 outut tubes
(too old to reply)
t***@myshop.com
2018-12-28 03:11:35 UTC
Permalink
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.

I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.

They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.

Obviously the purpose is to increase wattage output.

I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.

Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.

Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
Tim Williams
2018-12-28 08:40:04 UTC
Permalink
Well, yeah, it just scales. More in parallel, more (total) bias and load
current, lower load impedance.

By the time you get to 8 or 10 or more, you should probably be asking
yourself if a one of those fancy DSP pedals plus a class D amp is cheaper
(not to mention more efficient, as we start running into the limitations of
a 120V 15A circuit around this level..), or at least another tube type with
better economy of scale (i.e. a transmitter tube of some sort), give or take
whether we're talking production here (6L6s are still around, if shitty --
the cheap ones that is) or one-offs from NOS.

Heh, funny, come to think of it, that matched tubes are very common, and
independent grid bias is reasonably common, but independent grid drive level
is not at all common. Really all that matters is balance at the OPT, for
magnetic reasons. The more tubes you wire in parallel, the less critical
their matching is (assuming independent variables, and assuming any
individual does not exceed its plate dissipation rating).

Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
u***@downunder.com
2018-12-28 08:45:13 UTC
Permalink
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
Tauno Voipio
2018-12-28 12:19:50 UTC
Permalink
Post by u***@downunder.com
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
In the 1960's, I made a guitar amplifier with six EL500's. The tubes
are not characterized for linear use, and they were not matched in
any way. Each tube had an own cathode resistor, and there was no
evidence of any overload from unbalanced operation.

The tubes had a tendency of parasitic oscillation, so I added a
ferrite bead on the control grids and an inductor-resistor parallel
combination on each plate (for constructions, see nearest ham handbook).
--
-TV
u***@downunder.com
2018-12-28 13:11:45 UTC
Permalink
On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
Post by Tauno Voipio
Post by u***@downunder.com
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
In the 1960's, I made a guitar amplifier with six EL500's. The tubes
are not characterized for linear use, and they were not matched in
any way. Each tube had an own cathode resistor, and there was no
evidence of any overload from unbalanced operation.
Using individual cathode resistors to make the whole grid bias helps a
lot to equalize the current in each tube.

In general one tries to avoid cathode resistor bias in power stages,
since quite a lot of the output voltage swing is lost in the cathode
resistor(s) and use fixed low power negative grid bias supply instead,
but this may require separate bias adjustment for each tube.

I am not familiar with 6L6, but the similar EL34 is used in pairs for
30 W for HiFi, using 4 or 6 for "100 W" guitar amplifier with 450 V
anode voltage. One bass qui tar amplifier is rated at 100 W using only
two EL34 tubes, but runs at 800 V, but only Telefunken "Special
quality" EL34s seemed to survive more than a few gigs.
Post by Tauno Voipio
The tubes had a tendency of parasitic oscillation, so I added a
ferrite bead on the control grids and an inductor-resistor parallel
combination on each plate (for constructions, see nearest ham handbook).
t***@myshop.com
2019-01-03 22:54:33 UTC
Permalink
On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
Post by Tauno Voipio
Post by u***@downunder.com
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
In the 1960's, I made a guitar amplifier with six EL500's. The tubes
are not characterized for linear use, and they were not matched in
any way. Each tube had an own cathode resistor, and there was no
evidence of any overload from unbalanced operation.
The tubes had a tendency of parasitic oscillation, so I added a
ferrite bead on the control grids and an inductor-resistor parallel
combination on each plate (for constructions, see nearest ham handbook).
Back in the late 60s early 70s, I had several (self refurbished)
mono-block power amps that used four 6L6 output tubes. I did not even
know about matching those tubes. I just put in any tube marked as a 6L6.
I recall having both the glass (GC) types mixed with the black metal
ones. I always had good sound and lots of power. I do recall that
replacing the metal cased ones with 6L6GC did increase my power though.
But until I could afford new tubes, I used what I had. And even with the
new tubes, I never matched them.

Phil Hobbs
2018-12-28 15:43:18 UTC
Permalink
Post by u***@downunder.com
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
Unless the tubes are arranged truly symmetrically, e.g. in a circle with
vertical air flow, they won't all run at the same envelope temperature,
which means they won't have the same cathode temperature either.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
bitrex
2018-12-28 17:16:42 UTC
Permalink
Post by u***@downunder.com
Post by t***@myshop.com
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)
All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.

Not putting that into an amp with the kind of non-inconsiderable expense
one will sink into building it on the grounds of "purism" or whatever is
mad, the kind of irresponsible behavior that the less well-heeled of the
world get called out on. Work hard. Save money. Don't burn up 2 grand of
parts/risk your life cuz one thinks silicon is too new-fangled.
Big Bad Bob
2018-12-31 19:25:08 UTC
Permalink
Post by bitrex
All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.
I think it works ok without "all that". fuses where needed of course,
to avoid things catching on fire. But tubes are amazingly strong at
absorbing conditions that transistors would melt under. Sure the plates
turn pink [like when half of a push-pull transformer melts away] but if
you don't push them to the edge of physics they generally take it ok and
survive getting a new output transformer. Or if a single tube fails in
multi-pair configuration, the others will take up most of the slack
without too much bad behavior. You'll probably hear the bad quality
sound at high volumes, but without extended operation "that way" the
other tubes should survive.

the more you have paralleled, the less impact a single tube failure will
have on the other tubes.

Even briefly overvolt or overcurrent on a typical transistor, and you'll
be replacing it VERY soon. I made the mistake of designing a circuit
that operated close to the maximum Vceo (these 60V transistors should be
able to handle 45-50V right?), and the transistors never lasted long
under load (replaced 3 times, and 3 blown fuses that were supposed to
protect them). Replaced with transistors that had twice the Vceo and no
problem.
--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me

'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"
Tim Williams
2019-01-01 04:32:00 UTC
Permalink
I think it works ok without "all that". fuses where needed of course, to
avoid things catching on fire. But tubes are amazingly strong at
absorbing conditions that transistors would melt under. Sure the plates
turn pink [like when half of a push-pull transformer melts away] but if
you don't push them to the edge of physics they generally take it ok and
survive getting a new output transformer.
A lot of sweep tube datasheets contain the rating, "short term overload
duration: 220W for 60s" or something like that. Transistors can only dream
of such abuse, gone in milliseconds!

But the purpose of that rating must be understood. These were TV tubes, and
the sweep tube in particular often took a beating as other tubes heated up
and other signals stabilized: horizontal oscillator and sync, and the damper
diode especially -- its high cathode voltage isolation takes a long time to
warm up.

There was definitely no money in adding a protection circuit! Burn a tube,
pop it out, take it down to the corner drugstore and buy a new one for a
buck or a few.

Nowadays, with both tubes and transformers being rather pricey, let alone
the repairman -- the balance changes, and especially with how little
hardware is involved in adding a protection circuit (if one does not mind
that it contains silicon), it's well worth it.

Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/
bitrex
2019-01-01 06:32:52 UTC
Permalink
Post by Tim Williams
I think it works ok without "all that".  fuses where needed of course,
to avoid things catching on fire.  But tubes are amazingly strong at
absorbing conditions that transistors would melt under.  Sure the plates
turn pink [like when half of a push-pull transformer melts away] but
if you don't push them to the edge of physics they generally take it
ok and
survive getting a new output transformer.
A lot of sweep tube datasheets contain the rating, "short term overload
duration: 220W for 60s" or something like that.  Transistors can only
dream of such abuse, gone in milliseconds!
But the purpose of that rating must be understood.  These were TV tubes,
and the sweep tube in particular often took a beating as other tubes
heated up and other signals stabilized: horizontal oscillator and sync,
and the damper diode especially -- its high cathode voltage isolation
takes a long time to warm up.
There was definitely no money in adding a protection circuit!  Burn a
tube, pop it out, take it down to the corner drugstore and buy a new one
for a buck or a few.
Nowadays, with both tubes and transformers being rather pricey, let
alone the repairman -- the balance changes, and especially with how
little hardware is involved in adding a protection circuit (if one does
not mind that it contains silicon), it's well worth it.
Tim
Yep, the OP is talking about monoblock amp with 6, 8, ten power tubes.
The power supply iron and output transformer will be large, possibly
custom, and not cheap.

$5-10 worth of microcontroller or ICs and relays to at the least monitor
tube cathode currents and grid voltages and cut the HT if things start
going tits-up compared to a melted half of an output transformer that
might cost $3-500. Big BUMMER!
Kevin Aylward
2019-01-01 15:27:45 UTC
Permalink
All amps of that scale should have active protection circuitry/monitoring
of currents, biases, and temperatures via microprocessor to avoid
potentially catastrophic faults.
Even briefly overvolt or overcurrent on a typical transistor, and you'll be
replacing it VERY soon. I made the mistake of designing a circuit that
operated close to the maximum Vceo (these 60V transistors should be able to
handle 45-50V right?), and the transistors never lasted long under load
(replaced 3 times, and 3 blown fuses that were supposed to protect them).
Replaced with transistors that had twice the Vceo and no problem.
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating at
high voltage is much lower than their power ratings would imply.

I designed mosfet amp in the early 80s. I tested it by putting in a full
level signal with an output s/c. The only protection was a zener across the
gates to limit the current to the device ratings. I left it cycling with its
90 Deg heatsink thermal cutout for 3 days. No problems.

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html
bitrex
2019-01-01 23:02:06 UTC
Permalink
Post by Kevin Aylward
Post by bitrex
All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.
Even briefly overvolt or overcurrent on a typical transistor, and
you'll be replacing it VERY soon.  I made the mistake of designing a
circuit that operated close to the maximum Vceo (these 60V transistors
should be able to handle 45-50V right?), and the transistors never
lasted long under load (replaced 3 times, and 3 blown fuses that were
supposed to protect them). Replaced with transistors that had twice
the Vceo and no problem.
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their
rating at high voltage is much lower than their power ratings would imply.
Ya, BJT amps tend to have oversized output devices as compared to what
their rated maximum "RMS power" output would imply. 75 watt-rated
devices in 25 watt amps. The problem is risk of second breakdown when
working into reaactive loads, not de-rating BJTs appropriately when
they're handing significant powers into reactive loads common newbie
mistake
Post by Kevin Aylward
I designed  mosfet amp in the early 80s. I tested it by putting in a
full level signal with an output s/c. The only protection was a zener
across the gates to limit the current to the device ratings. I left it
cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems.
-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html
Tim Williams
2019-01-02 06:16:56 UTC
Permalink
Post by Kevin Aylward
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating
at high voltage is much lower than their power ratings would imply.
Used to be true -- modern MOSFETs are more than current-dense enough to
exhibit 2nd breakdown. I shouldn't actually say modern, because apparently
SuperJunction process has... PTC source connections or something? I haven't
seen one without a square SOA yet I don't think. So by now, it's actually
previous generation that you have to watch out for. I forget if lower
voltage (SJ goes away under ~400V I think it was?) processes are still
prone.

Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/
Kevin Aylward
2019-01-03 21:25:34 UTC
Permalink
Post by Tim Williams
Post by Kevin Aylward
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating
at high voltage is much lower than their power ratings would imply.
Used to be true -- modern MOSFETs are more than current-dense enough to
exhibit 2nd breakdown. I shouldn't actually say modern, because apparently
SuperJunction process has... PTC source connections or something? I
haven't seen one without a square SOA yet I don't think. So by now, it's
actually previous generation that you have to watch out for. I forget if
lower voltage (SJ goes away under ~400V I think it was?) processes are
still prone.
Exicon are the audio mosfets of choice today, apparently. They are laterals.

http://www.exicon.info/

They have the usual power limited SOA.

I don't know who actually makes them, but my guess is someone like XFAB.

Standard fab vendors will make any asic for any fabless company, even if the
asic is just the one big transistor!

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html
Peter Wieck
2018-12-28 13:49:03 UTC
Permalink
RCA Theater amps back in the day used between two and six 6L6 tubes depending on the required function. They were all monoblocks, sometimes from a common power-supply. Towards the end of their heyday, when Stereo sound-tracks were becoming more common, the center channel (the speaker directly behind the screen) would run from the optical sound-track, and the left and right from the magnetic sound track.

Peter Wieck
Melrose Park, PA
Chris
2018-12-28 23:32:00 UTC
Permalink
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
bitrex
2018-12-29 01:18:07 UTC
Permalink
Post by Chris
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
the two big issues with OTL tube amps is the intrinsic impedance
mismatch of a tube cathode vs a tube plate in a totem pole arrangement
looking into the load, and related lack of any intrinsic power supply
noise rejection in the output stage the way a push-pull
transformer-coupled output stage has when both halves have similar
output impedances working into the same reflected load.

Global negative feedback can't do anything about the second and there
isn't usually enough open-loop gain available to do a good job of
correcting for distortion caused by the first across the audio band.
often leading to a kinda poor-performing amp.
bitrex
2018-12-29 01:20:10 UTC
Permalink
Post by bitrex
Post by Chris
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
the two big issues with OTL tube amps is the intrinsic impedance
mismatch of a tube cathode vs a tube plate in a totem pole arrangement
looking into the load, and related lack of any intrinsic power supply
noise rejection in the output stage the way a push-pull
transformer-coupled output stage has when both halves have similar
output impedances working into the same reflected load.
Global negative feedback can't do anything about the second and there
isn't usually enough open-loop gain available to do a good job of
correcting for distortion caused by the first across the audio band.
often leading to a kinda poor-performing amp.
These issues can be addressed but it requires more work than I'm
guessing they put in in the mid 60s (can't immediately find the article
in question online)
Tauno Voipio
2018-12-29 18:27:21 UTC
Permalink
Post by Chris
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
I found it in my copy, 16th edition, 1963.

The amplifier is not direct coupled, there are several stages
with coupling capacitors. The final is series-connected with
3 series pairs of 6082 triodes in parallel and direct feed to
a 16 ohm speaker. The power supplies are + and - 140 V.
--
-TV
Big Bad Bob
2018-12-31 19:14:33 UTC
Permalink
Post by Tauno Voipio
Post by Chris
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
I found it in my copy, 16th edition, 1963.
The amplifier is not direct coupled, there are several stages
with coupling capacitors. The final is series-connected with
3 series pairs of 6082 triodes in parallel and direct feed to
a 16 ohm speaker. The power supplies are + and - 140 V.
sounds like a gimmick. Tube plate/cathode currents are just way too
small to properly drive a speaker without an impedence matching transformer.

Had they thought of it "back then" they could've used toroidal
transformers with PWM push-pull amplifiers modulated up to 70 or 80
percent as a class 'G' amplifier. By the time that was invented, it was
all transistors, though. [and no benfit to using tubes].

I suspect that the push-pull configuration's advantages [and
disadvantages] disappear when you use a transformerless configuration,
and new ones take their place.

If you want high power, you should consider using KT88's and a matching
transformer from Hammond. A single pair should get you at least 100W
RMS. I saw a stereo amplifier built using these at a state fair once,
in the 70's, basically a clone of the GE tube manual's reference design.
It was on an oversized chassis, though. I would've preferred it as 2
"monoblock" amplifiers that could fit together on a shelf...

even the high heater current requirements of a KT88 would be smaller
than 6 or 8 6L6's [and is physically smaller]. But I suppose it looks
'cooler' to have all of those bottles grouped together.
--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me

'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"
Tim Williams
2019-01-01 04:39:02 UTC
Permalink
sounds like a gimmick. Tube plate/cathode currents are just way too small
to properly drive a speaker without an impedence matching transformer.
So what, nothing comes out?...

They were definitely below the maximum power point (at clipping), if that's
more accurately what you meant.

6AS7/6080, 6S33S and the other regulator tubes have peak cathode current in
the ballpark of an ampere, so a modest number of tubes offers a modest power
output, say 20 or 40W, preferably into a higher load like 16 ohms. The
efficiency is poor, with more heat dissipated in the heaters alone, than
delivered to the output.

If you put dozens in parallel, the efficiency keeps going up as you get
closer to matching, but now your whole system consumes multiple kilowatts...

There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms
voice coil at good efficiency, but those speakers are so rare that this is
practically a unique case.

Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/
u***@downunder.com
2019-01-01 12:34:54 UTC
Permalink
On Mon, 31 Dec 2018 22:39:02 -0600, "Tim Williams"
Post by Tim Williams
There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms
voice coil at good efficiency, but those speakers are so rare that this is
practically a unique case.
Usually closer to 1000 ohms.

These speakers were sold for a few years, but later on, they went on
sale. Adding an 8:1000 ohm transformer and you could cheaply build a
good quality speaker box for an ordinary 8 ohm tube or transistor
amplifier :-)
Tauno Voipio
2019-01-01 19:00:36 UTC
Permalink
Post by Tim Williams
sounds like a gimmick.  Tube plate/cathode currents are just way too
small to properly drive a speaker without an impedence matching transformer.
So what, nothing comes out?...
They were definitely below the maximum power point (at clipping), if
that's more accurately what you meant.
6AS7/6080, 6S33S and the other regulator tubes have peak cathode current
in the ballpark of an ampere, so a modest number of tubes offers a
modest power output, say 20 or 40W, preferably into a higher load like
16 ohms.  The efficiency is poor, with more heat dissipated in the
heaters alone, than delivered to the output.
If you put dozens in parallel, the efficiency keeps going up as you get
closer to matching, but now your whole system consumes multiple kilowatts...
There was also the Philips "SEPP" with a pair of EL86? driving a
100s-ohms voice coil at good efficiency, but those speakers are so rare
that this is practically a unique case.
Tim
The final triodes (6080) are special high-current tubes. The amplifier
designer rated the thing at 25 W into a 16 ohm voice-coil speaker.
--
-TV
u***@downunder.com
2019-01-01 12:21:45 UTC
Permalink
On Mon, 31 Dec 2018 11:14:33 -0800, Big Bad Bob
<clip>
Post by Big Bad Bob
Post by Tauno Voipio
Post by Chris
If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...
I found it in my copy, 16th edition, 1963.
The amplifier is not direct coupled, there are several stages
with coupling capacitors. The final is series-connected with
3 series pairs of 6082 triodes in parallel and direct feed to
a 16 ohm speaker. The power supplies are + and - 140 V.
sounds like a gimmick. Tube plate/cathode currents are just way too
small to properly drive a speaker without an impedence matching transformer.
Search for 6082 "50 ohm" and you find quite a lot hits with single
pair driving 50 ohms.

With tree pairs, the load could be 16 ohms.
Les Cargill
2018-12-30 17:51:22 UTC
Permalink
Post by t***@myshop.com
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.
I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.
They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.
Obviously the purpose is to increase wattage output.
I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.
Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.
Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.
The largest production tube MI amp I am aware of is the
Mesa Boogie 400+. It uses ( I believe ) 12 power tubes,
essentially 6 push-pull pairs ).

SFAIK, it exists only to compete with the Ampeg SVT.


I wouldn't want to lift it.
--
Les Cargill
Loading...