6AS7 Triode PP Amps-experience?

[c***@hotmail.com]

I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

[Yves Monmagnon]

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

There was so many designs with that tubes.
Here is mine:

http://www.dissident-audio.com/PP_6AS7/Page.html

I obtained 7 watts of really good quality

Yves.

[c***@hotmail.com]

Yeah, I'm going to build a 6AS7 amp on a testbed (not pretty) basis
using an existing power supply I built for another amp, and if I like
it it may become part of a concept amp I've thought about for awhile.
I'm thinking of putting a 6AS7 triode PP channel and a pair of big beam
power tubes on one chassis with the electronic x/o. That would be
great for a two-way (Altec 604s) but I'm using three way speakers-I
think the bass will get its own channel and the mid and tweeter will
share the small one with an optimized passive x/o.

Just a thought.

[george craig]

calcerise wrote,
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.
John Stuart designed a bootstrapped driver for the 6AS7 output contact
Mr Stuart...................GC

[Yves Monmagnon]

[ . . . ]

John Stuart designed a bootstrapped driver for the 6AS7 output contact
Mr Stuart...................GC

Indeed, he is the King ;>))

Cheers, Yves.

[John Stewart]

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

If you have old copies of Glass Audio magazine see both Issue 2 & Issue 3
from 1999. Issue 2 describes a very straight forward PP 6AS7/6080 using a
pair of those tubes. It manages 30 watts at clipping. The drive is by a
boot strapped technique, thus avoiding the IT transformer. The results are
very good & supported by some test data.

More information is in Issue 3 of the 1999 series of Glass Audio magazine.

If you like contact me direct.

Cheers, John Stewart

[c***@hotmail.com]

The Glass Audio CD has been on my list of things to order for some
time now, so I will, and not bother you until I have it in front of me
if at all.

[Fabio Berutti]

I made a simple 6AS7 PP amp based on a modified Williamson schematic like
this:
- a gain stage (1st half of a 2C51/Russian 6N3P, unbypassed cathode
resistor)
- a DC coupled split-load phase splitter (2nd half of the 2C51)
- a choke-loaded driver stage (Russian 6N1P loaded by 80H, 20 mA chokes,
unbypassed common cathode bias resistance like in a differential amplifier)
- 6AS7 (Svetlana 6S13P) operating in fixed bias at about 230V a-k and 52 mA
into a 2K5 a-a load; bias voltage needs to be about -130V with WIDE
regulation capability, I found discrepancies like -80 to -150V among the two
triodes in the same glass bulb.
The driver stage, being choke-loaded, is fed at about 200V / 6 mA per triode
and delivers a voltage swing of about 300V peak-to-peak.
The only DC feed is provided by a SS bridge thru a CLC filter
(100uF/4H/100uF) and then divided between the two channels by a RC filter
(47R/220uF per channel).
No feedback is used, but some can be introduced (right now sensitivity is
about 0.8V p-p for full power).
The amp delivers some 14-15W with a very "dynamic" sound and good bass
extension due to the very low internal resistance of the 6AS7 tube.
Nevertheless it is not an audio tube and its linearity is not as good as
(say) a 6B4G, therefore now I'd prefer build a similar unit using a PP pair
of these DHTs instead, having lower bias and driving requirements.

Ciao

Fabio

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

[c***@hotmail.com]

The 6AS7 is specifically not rated for fixed bias use, and generally
most people follow the admonition in all tube manuals against this
practice. Simplicity and availability are the motivations behind
building 6AS7 amps. The 6B4G is an expensive tube since they are in
demand among audio people. (Although many have no knowledge of how to
use them, or any other tube for that matter.)

[Patrick Turner]

Post by c***@hotmail.com
The 6AS7 is specifically not rated for fixed bias use, and generally
most people follow the admonition in all tube manuals against this
practice. Simplicity and availability are the motivations behind
building 6AS7 amps. The 6B4G is an expensive tube since they are in
demand among audio people. (Although many have no knowledge of how to
use them, or any other tube for that matter.)

The 6AS7 has an Ra of only 280 ohms per 1/2 section.
With two tubes, each with Ra of 140 ohms each total, you get
Ra = 280 ohms a-a, so an RLa-a = 1k is about right for good AB1.
With 6AS7, the supply voltage is less than half for the larger octal
power tubes.
AB2 will give a little more power.
So if you get 15 watts into 1k, then that's 122Va-a, and since µ = 2,
gain is only about 1.6, so about 76Vrms grid to grid AB1 is needed for
drive,
which is very similar to when using 6550 in triode.

I'd prefer 6550, KT88, or KT90 in triode any day.

The 6B4 is like a 2A3 with 6v heaters, is it not?

Patrick Turner.

[d***@rushmore.com]

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

drtechno at rushmoredotcom
rca had a good one in my old manual... ignore the first stage and
build from j2 and over. the first stage is an phono pre

[c***@hotmail.com]

RC-something-which one?

[John Stewart]

Post by c***@hotmail.com
RC-something-which one?

AFAIK the referenced circuit appears only in RC-16. The tube driving
the 6AS7 is a 6SN7 running 450 volts on its plates.

Somewhere in my pile I have copy of an article describing the
circuit. The description is not in
RC-16. JLS

[Yves Monmagnon]

Post by John Stewart

Post by c***@hotmail.com
RC-something-which one?

AFAIK the referenced circuit appears only in RC-16. The tube driving
the 6AS7 is a 6SN7 running 450 volts on its plates.
Somewhere in my pile I have copy of an article describing the
circuit. The description is not in
RC-16. JLS

Perhaps this one (just one page, there is 16-12 printed at top left corner)

www.dissident-audio.com/Yves/276.gif

Yves.

[John Stewart]

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

I used a 400 volt plate supply & cathode bias. Each cathode really does
need to have its own resistor to prevent thermal runaway. I tried a common
cathode resistor but managed to quickly burn a hole thru one of the grids.
Ik for each triode is 52 ma.

You could also use bias servos if ambitious!

The plate-to-plate load is 2150R, a special made for me by Hammond based on
their 60 watt 1650N. There is no need for an IT at all. Nor is the driver
circuit complicated.

At 10 watts & zero NFB the 2H is down about 45 db while the 3H is down
about 42 db.
At 25 watts & zero NFB the 2H is down about 38 db while the 3H is down
about 28 db.
In the final version I used 13 db NFB so that the resulting THD & IMD
results were quite good.
Clipping occurs at about 30 watts in this arrangement.

The 2nd article shows how the circuit can be modified so that the
bootstrapping is done without the benefit of UL connexions on the OPT.

I've never seen either of these driver circuits ever used to drive low mu
triodes by others previously. I believe it to be unique & original. Its
simplicity will surprise you!

For more information on biasing the 6AS7/6080 family see

http://frank.pocnet.net/sheets/030/6/6080.pdf

Page 4, Note 6.

Cheers, John

[george craig]

Calcerise wrote,
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.
Here is another reg. tube amp www.wdehaan.demon.nl/mono/6336.html
...................GC

[Sander deWaal]

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

Here is another reg. tube amp www.wdehaan.demon.nl/mono/6336.html
...................GC

That's a "404" here.............

[John Stewart]

Post by Sander deWaal

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

Here is another reg. tube amp www.wdehaan.demon.nl/mono/6336.html
...................GC

That's a "404" here.............

I got that too. But got in via http://www.wdehaan.demon.nl/

Then look down the page to the 6336 link. It works OK.

Cheers, John Stewart

[Fred Thompson]

Post by Sander deWaal

Post by c***@hotmail.com
I was doing some housecleaning and came across the old Audio Anthology
with C.G. McProud. He really likes 6AS7's, but usually with
transformer-coupled drivers. The driver xfmr would cost a lot plus,
phase shift makes applying feedback difficult. Has anyone built a nice
6AS7 resistance-coupled amp recently with good results? I'm thinking
they are good for ten watts with a single tube in push-pull.

Here is another reg. tube amp www.wdehaan.demon.nl/mono/6336.html
...................GC

That's a "404" here.............

Try http://www.wdehaan.demon.nl/mono/mono/6336.html

Post by Sander deWaal
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

[Sander deWaal]

Post by Fred Thompson
Try http://www.wdehaan.demon.nl/mono/mono/6336.html

Thanks Fred (and John, too) .
I was too lazy do do that myself :-)