Problem with a Fisher 400

[fourkids]

All,

I have searched and searched, and can't seem to find what I need.

I have a Fisher 400 tube amp...family heirloom as it were...and I
recently did a rebuild on it based on Charlie Kittleson (?) kit. I am
an EE...so good and nerdy, fairly competent with a soldering iron, etc
etc. I also have the original schematic my dad got when he bought the
thing.

So it works, but I have a couple of significant problems:

1) The tubes still run VERY hot...they ran hot before, and frankly, I
think they are actually running hotter. I was a bit surprised. The
rectifier has been replaced, which if I read things correctly, should
almost make it run TOO cool. The mesh cover over the tubes is almost
too hot to touch.

I have read about checking the bias voltage, and I think that I know
where I need to check it, but I am leary of having this thing open on
the bench and on knowing the voltage levels in there. Besides, I am
not sure how to get it turned over and on without damaging the
tubes....

2) There seems to be a bit a signal leak at least at the headphone
jack...with the volume all the way down, I get music through the right
ear...

The second one is pretty minor. I guess what I am looking for is some
guidance on checking voltages, etc, to make sure I havn't screwed
something up. The schematics show a number of voltage check points,
which I can do. I have a goof Fluke MM, but no osciliscope, so I can't
check signal anywhere.

Anyone out there done a "rebuild" and have any pointer for me???

Thanks in advance.

Dave Early
Colorado

[Jon Yaeger]

Post by fourkids
All,
I have searched and searched, and can't seem to find what I need.
I have a Fisher 400 tube amp...family heirloom as it were...and I
recently did a rebuild on it based on Charlie Kittleson (?) kit. I am
an EE...so good and nerdy, fairly competent with a soldering iron, etc
etc. I also have the original schematic my dad got when he bought the
thing.
1) The tubes still run VERY hot...they ran hot before, and frankly, I
think they are actually running hotter. I was a bit surprised. The
rectifier has been replaced, which if I read things correctly, should
almost make it run TOO cool. The mesh cover over the tubes is almost
too hot to touch.
I have read about checking the bias voltage, and I think that I know
where I need to check it, but I am leary of having this thing open on
the bench and on knowing the voltage levels in there. Besides, I am
not sure how to get it turned over and on without damaging the
tubes....
2) There seems to be a bit a signal leak at least at the headphone
jack...with the volume all the way down, I get music through the right
ear...
The second one is pretty minor. I guess what I am looking for is some
guidance on checking voltages, etc, to make sure I havn't screwed
something up. The schematics show a number of voltage check points,
which I can do. I have a goof Fluke MM, but no osciliscope, so I can't
check signal anywhere.
Anyone out there done a "rebuild" and have any pointer for me???
Thanks in advance.
Dave Early
Colorado

Dave,

Fisher pushed the output tubes back "in the day" when the mains voltages
were about 117. Now that average voltages are higher, it pushes the tubes
even more.

You should raise the negative bias voltage on the 7868 tubes.

1. Install 10 ohm resistors between each 7868 cathode and ground. These
resistors also act as fuses and can protect the outputs in the event of a
short.

2. R34 is a 2.2K shunt resistor that follows R35, a 1K resistor. Raising
the value of the shunt resistor will raise (i.e. make more "negative") the
bias voltage.

3. I'd replace the 2.2K shunt resistor with a 1K resistor wired in series
with a suitable 5K pot. This will allow you to vary the bias.

4. Measure voltage across the 10 ohm cathode resistors. 0.25 volts
corresponds to 25 ma current. Set the bias between 15-30 ma (I usually pick
about 23 ma) per tube.

5. Since there are no individual bias pots you really need a matched quad
of 7868s. Failing that, you can install multiple pots to adjust pairs or
individual tubes.

6. You might have a leaky speaker switch or perhaps an imbalance between
the two outputs causing sound in the headphone jack. Note that the
impedance selector wires connect to each side of the earphone jack through
330 ohm resistors. Also check the value of these resistors.

- Jon

[p***@aol.com]

Or, put a dropping resistor into the line-cord. Figure you want to get
the voltage that the unit "sees" around 110V. Figure to start with a
50-ohm @10-watt power-resistor in series with the line cord.

Obtain a '1900' box, single-receptacle cover, hospital-grade
receptacle, 5' of 16-2/w/ground SJO cord and a three-prong plug. Obtain
a 1/4" Glass fuse holder, and a few 10-watt resistors.... say 40, 50, &
100 ohms. Install the fuse in line with the "HOT" lead. Then the
resistor, starting with the 50-ohm unit. Install a 3A DUAL-ELEMENT
fuse.

With this arrangement, you can precisely fuse the plugged in unit, vary
the resistance until you get to a cool operating condition, and have
the whole shebang serve multiple units. Jon is dead-on that the
line-voltage at your receptacle may be (within tariff) up to 127V. You
want net-110V to your receiver.

Also, check _all_ the low-value caps on the boards. Leaky caps are the
single greatest threat to vintage tube equipment.

Peter Wieck
Wyncote, PA

[fourkids]

This is GREAT stuff! Just what I needed to know and frankly this
should be written up somewhere...

I am just new to the whole tube thing...not entirely sure where I am
at.

I measured the volts at my outlet...125.

One more question (for the moment):

I can guarantee you that I don't have a matched set of tubes...so I am
interested in adding the trim resistors. However, I not sure what,
exactly I am balancing...do I need to make the current the same at each
bias resistor?

Sorry for the newbie questions...but many thanks for the great
responses.

Dave Early
Colorado

[Jon Yaeger]

Post by fourkids
This is GREAT stuff! Just what I needed to know and frankly this
should be written up somewhere...
I am just new to the whole tube thing...not entirely sure where I am
at.
I measured the volts at my outlet...125.
I can guarantee you that I don't have a matched set of tubes...so I am
interested in adding the trim resistors. However, I not sure what,
exactly I am balancing...do I need to make the current the same at each
bias resistor?
Sorry for the newbie questions...but many thanks for the great
responses.
Dave Early
Colorado

Dave,

Let's say you install just the cathode resistors first and don't modify the
bias circuit just yet.

Ideally, you want the current through each tube to be the same, measured as
voltage across the 10 ohm cathode resistor.

1. If you can get hold of a good tube checker, test each 7868 to be sure
there are no gross defects. Note also the transconductance for each tube
and mark them on the tubes. If the values are close to one another, the
tubes may be roughly matched in circuit, but not necessarily!

2. If they are dissimilar, separate the tubes into pairs that are most
alike, if you can. (This may not be feasible, depending upon the values).

3. Plug in the most alike tubes in the same channel.

4. Do a quick check of the current through each tube when the receiver
warms up. I'll bet you find the idle current in each tube to be 50 ma range
(just a guess). See if the values have any correspondence to what you saw
on the tube checker. Gassiness, age, and other problems can cause higher
current draws than expected. You are looking for no more than 4-5 ma
difference between pairs for a "match."

5. Let's say you have two "matched" pairs of tubes. Then you would need
two pots, one for each channel. If the tubes aren't even close, then you'd
need separate pots for each tube.

6. Once you bring the current down to acceptable levels, check the cathode
voltages (i.e. currents) again. You'll probably be surprised what you
measure in terms of "balance" at the lower currents.

7. Rematch each pair of tubes under the lower current, if necessary. Warm
up and recheck / adjust bias as needed. Tubes that have thermal runaway
need to be discarded.

8. And by all means do use fresh output coupling caps!

- Jon

[Jon Yaeger]

Post by Jon Yaeger

Post by fourkids
This is GREAT stuff! Just what I needed to know and frankly this
should be written up somewhere...
I am just new to the whole tube thing...not entirely sure where I am
at.
I measured the volts at my outlet...125.
I can guarantee you that I don't have a matched set of tubes...so I am
interested in adding the trim resistors. However, I not sure what,
exactly I am balancing...do I need to make the current the same at each
bias resistor?
Sorry for the newbie questions...but many thanks for the great
responses.
Dave Early
Colorado

Dave,
Let's say you install just the cathode resistors first and don't modify the
bias circuit just yet.
Ideally, you want the current through each tube to be the same, measured as
voltage across the 10 ohm cathode resistor.
1. If you can get hold of a good tube checker, test each 7868 to be sure
there are no gross defects. Note also the transconductance for each tube
and mark them on the tubes. If the values are close to one another, the
tubes may be roughly matched in circuit, but not necessarily!
2. If they are dissimilar, separate the tubes into pairs that are most
alike, if you can. (This may not be feasible, depending upon the values).
3. Plug in the most alike tubes in the same channel.
4. Do a quick check of the current through each tube when the receiver
warms up. I'll bet you find the idle current in each tube to be 50 ma range
(just a guess). See if the values have any correspondence to what you saw
on the tube checker. Gassiness, age, and other problems can cause higher
current draws than expected. You are looking for no more than 4-5 ma
difference between pairs for a "match."
5. Let's say you have two "matched" pairs of tubes. Then you would need
two pots, one for each channel. If the tubes aren't even close, then you'd
need separate pots for each tube.
6. Once you bring the current down to acceptable levels, check the cathode
voltages (i.e. currents) again. You'll probably be surprised what you
measure in terms of "balance" at the lower currents.
7. Rematch each pair of tubes under the lower current, if necessary. Warm
up and recheck / adjust bias as needed. Tubes that have thermal runaway
need to be discarded.
8. And by all means do use fresh output coupling caps!
- Jon

PS: I assume you've already replaced the selenium bridge with a silicon
one. This will raise the bias voltage significantly. Add a dropping
resistor or change the pot values if you can't get the bias within range.

[p***@aol.com]

Post by fourkids
I can guarantee you that I don't have a matched set of tubes...so I am
interested in adding the trim resistors. However, I not sure what,
exactly I am balancing...do I need to make the current the same at each
bias resistor?

Mpfffff.......

Matched tubes is about 30% real fact, 70% myth and 80%
not-quite-necessary in a well-designed system. If you question as to
that adding up to 180%, you are right. It is about that way.

Some systems greatly benefit from matched pairs/quads due to sometimes
marginal design or lack of adjustable bias, or a marginal bias
adjustment process, lack of general maintenance, etc. Carefully matched
tubes greatly reduce the stress on the entire system as long as the
match holds. The problem is that these days, the match typically is
badly done initially and rarely holds all that long either. (Often
tubes are sold as 'matched' with identical lot and date codes, no
more). Assume that no tubes are other than reasonably matched (20%),
and work from there with the rest of the circuit.

Clarification:

Match = w/in 5% on the following values, at least:

Plate current
Grid Current
Bias Voltage (usually externally fixed)
GMhos
Filament Current (at a fixed voltage)

At a range of operating voltages. It can be done, but it is a snap-shot
condition, not permanent after significant use.

Added Trim Resistors are a last resort. Your amp has survived lo, these
last 40+ years as-designed. With proper maintenance, it can last
another 40.

It is 'written up' in several venues. rec.antiques.radio+phono will
direct you to many good sites on recapping and drifting resistors, tube
bias, and so-forth.

At some point the Chorus will start ranting here. Not such that you
should ignore them, not hardly. There is considerable wisdom herein.
But beware the overly complicated. You could shotgun the small-value
caps, check all the critical resistors, and test all your tubes on a
good GM-type tester, replacing individual and *pairs* as needed and be
90% there. Were your unit mine, I would go further and carefully test
all the electrolytics, clean the controls, replace any selenium diodes,
and (again) install a dropping resistor to get the voltage that the
unit 'sees' to about 110V. And, where it mine, when I found that magic
resistor value, I would obtain a 30-watt Dale finned aluminum resistor
for permanence. I would also use my current meter to install the
correct DUAL-ELEMENT fuse such that I could actually protect the
equipment, not just the real-estate. Dual-element fuses are NOT
"slow-blow" fuses, BTW. They are fuses that at turn-on can absorb a
massive overcurrent (cold filaments conduct well) but once the surge
has passed function as quick-blow fuses thereafter.

On a detail note: Jon suggests pots in series with fixed resistors so
bias becomes adjustable. This can be (is) an excellent expedient,
however and CAUTION: Use the best-damned-quality pots you can find at
any cost. Cheap carbon-element pots can drift with heat, deteriorate,
go intermittent, all sorts of problems such that you can introduce a
condition that could be fatal... such as 0-bias voltage... can you say
Runaway? . Make sure that the pots are rated at the current they will
be carrying, with a margin of at least 40%.

Music/sound/noise at the headphone jack could be so many things *right
now* that I would not be concerned until I had dealt with all the
small-value caps. Just one leaky cap could explain both the heat,
noise... and any other wildness so that ab-initio, I would ignore that
issue until far down the restoration process. Cap leakage _can_ be
temperature-dependent as well.

Do you have a means to measure operating current draw in real-time?

I have rebuilt any number of tube amps, tuners and pre-amps. I have
rebuilt dozens of vintage radios and more than a few solid-state amps
and tuners. It ain't nohow rocket science if common sense and common
cautions are followed. But, and for the record, I would not take on
your unit without some pretty basic equipment including an Isolation
Transformer, a metered Variac capable of very fine and accurate
measurement of voltage and current, and a good VOM. A scope is rarely
necessary unless/until you need an alignment, and then much more than a
scope is needed. I have all of the above and much more, but really,
most of it is unnecessary at this point.

Keep in mind that you are NOT the manufacturer, you do not need to
purchse your restoration parts from the lowest bidder and at minimum
specifications. The incremental cost between say... a 0.022uF cap at
100V, and that same cap at 600V is negligible. So, when replacing
non-electrolytic caps, you want to match the CAPACITANCE value, but you
are permitted (and should by one increment at least) to increase the
operating VOLTAGE. Modern caps are so much smaller (and generally
better) than the caps on your unit that going up in voltage even
several increments will not cause size issues. Electrolytics are
another dissertation. Replacement options there depend on what they are
doing and where they are in the circuit.... and their value.

Peter Wieck
Wyncote, PA

[fourkids]

Jon,

Your R numbers don't match up to my schematic...so I have a couple of
questions:

1) There is a straight wire ground connection from pin 3 of the
7868...I assume that this is the line that needs to be replaced with a
10 ohm resistor.

2) The bias voltage (marked on my schematic as -15V) is connected to
pins 2 and 6. There is a 1k resistor (R106/107/108/109) that connects
to a pair of 330k resistors that connect to the negative side of the
diode bridge (via a 5.6k resistor R102). The coupling capacitors (.047
C62/63/64/65) have been replaced:

Tube 1----pins 2/6 -------(1k ohm)-------||(coupling capacitor)------
|
|
(330k)
|
|
*-------(5.6k)-----(negative side
of bridge)
|
|
(330k)
|
|
Tube 2----pins 2/6 -------(1k ohm)-------||(coupling capacitor)------


hope that come through OK (looks ok in monospaced font)

The point is, the -15V bias voltage appears to be affected by the 1k
ohm resistors and the balance of the 3330k's...if the 330k's are out,
there will be different bias voltages on each tube.

OK, so here is what I am assuming I should do:

1) Check the 1k's and 330k's to make sure that they are significantly
out of balance.

2) Add the 10 ohm resistors between pin 3 and ground on each tube.

3) check the voltage across each 10 ohm resistor

That last step will give me an idea of how close my tubes are "matched"
(I have read Peter's post)

4) Assuming that the currents are fairly close, I can adjust the bias
voltage overall by placing a larger power resistor in line as described
by Peter...this will effectively lower the voltage seen by the system,
and thus lower the bias voltage for all tubes

5) Now things seem to get complicated. If the current draw as
indicated by measuring the voltage drop over the 10 ohm resistor is
significantly different between the pairs of tubes, it would appear
that I need to make some kind of adjustment. This is where I am
getting a litle fuzzy.

Based on the information thus far presented, it would seem that the
system can tolerate a certain amount of differences between the tubes.
The question I have is how bad does it have to be before I should make
the adjustment.

I don't have all the equipment listed in Peter's post. I do have a
good Fluke DMM...a good place to start anyway. And while there are
more complex issues with "matcing" the tubes, it would appear that if I
can at least get the current through the 10 ohm resistor to be about
the same for each tube pair, that is at least a gross adjustment to the
system.

I assume that each resistor should have about the same current. Also,
I assume fromeverything that has been posted thus far that I adjust
that current by fiddling with the bias voltage.

Have I missed anything? It is late, and I will read through this again
in the AM

Dave Early
Colorado

[Jon Yaeger]

Post by fourkids
Jon,
Your R numbers don't match up to my schematic...so I have a couple of

*** I'm using the Service Manual for the 400 for reference, beginning with
chassis S/N 48001. There were a couple of versions of the 400; that may or
may not explain the bias circuit differences. Look near the bridge
rectifier to find the 2.2K shunt resistor. Physically, it is mounted on a
terminal strip near the large two section cap that's mounted under the
chassis (and is probably leaking).

Post by fourkids
1) There is a straight wire ground connection from pin 3 of the
7868...I assume that this is the line that needs to be replaced with a
10 ohm resistor.

** Yes.

Post by fourkids
2) The bias voltage (marked on my schematic as -15V)

*** Definitely too low. Should be about -21 volts, IMHO.

is connected to

Post by fourkids
pins 2 and 6. There is a 1k resistor (R106/107/108/109) that connects
to a pair of 330k resistors that connect to the negative side of the
diode bridge (via a 5.6k resistor R102). The coupling capacitors (.047
Tube 1----pins 2/6 -------(1k ohm)-------||(coupling capacitor)------
|
|
(330k)
|
|
*-------(5.6k)-----(negative side
of bridge)
|
|
(330k)
|
|
Tube 2----pins 2/6 -------(1k ohm)-------||(coupling capacitor)------
hope that come through OK (looks ok in monospaced font)

*** On my schematic there is no 5.6K part and the bias is connected to the
junction of the 330K resistors. Follow that 5.6K resistor back to the
negative pole of a 1000 MFD cap. The resistor that is across this cap to
ground is the shunt resistor that can be adjusted.

Post by fourkids
The point is, the -15V bias voltage appears to be affected by the 1k
ohm resistors and the balance of the 3330k's...if the 330k's are out,
there will be different bias voltages on each tube.

*** Correct. I often replace these resistors with precision parts.
Originally, they were closely matched.

Post by fourkids
1) Check the 1k's and 330k's to make sure that they are significantly
out of balance.

*** I think you intended them to be NOT out of balance . . .

Post by fourkids
2) Add the 10 ohm resistors between pin 3 and ground on each tube.
3) check the voltage across each 10 ohm resistor
That last step will give me an idea of how close my tubes are "matched"
(I have read Peter's post)
4) Assuming that the currents are fairly close, I can adjust the bias
voltage overall by placing a larger power resistor in line as described
by Peter...this will effectively lower the voltage seen by the system,
and thus lower the bias voltage for all tubes

*** That's one option. One benefit is that it'll keep the filament
voltages closer to spec.

Post by fourkids
5) Now things seem to get complicated. If the current draw as
indicated by measuring the voltage drop over the 10 ohm resistor is
significantly different between the pairs of tubes, it would appear
that I need to make some kind of adjustment. This is where I am
getting a litle fuzzy.

*** Just to confuse you further, you could "simplify" the process by
obtaining a new quad of matched 7868s. Note that if you use current
production 7868s then you MUST get the bias right. Then you'd only have to
change one resistor (the shunt resistor) and the bias should be about the
same across the quad (unless you have other issues).

*** Also note that the pins of current production tubes are larger than
the NOS tubes and they will likely render your sockets useless for NOS tubes
later on . . . .

Post by fourkids
Based on the information thus far presented, it would seem that the
system can tolerate a certain amount of differences between the tubes.
The question I have is how bad does it have to be before I should make
the adjustment.

*** I'll leave this question to the experts. As I mentioned, I'd seek no
more than 4-5 ma difference in each pair. A distortion analyzer will help
you to quantify the matter . . . .

Post by fourkids
I don't have all the equipment listed in Peter's post.

*** My recommendations assume that you have a good DMM. Any other gear is
optional.

I do have a

Post by fourkids
good Fluke DMM...a good place to start anyway. And while there are
more complex issues with "matcing" the tubes, it would appear that if I
can at least get the current through the 10 ohm resistor to be about
the same for each tube pair, that is at least a gross adjustment to the
system.

*** Yes. You want to optimize what you've already got, at least. If you
find that a tube won't adjust in range (too much or too little current
compared to the others) then a tube purchase is in your future . . .

Post by fourkids
I assume that each resistor should have about the same current. Also,
I assume fromeverything that has been posted thus far that I adjust
that current by fiddling with the bias voltage.

*** Yes. You'll note that a bias voltage of about 21 volts is more
suitable for the 7868. This should produce current range as op. cit.

Post by fourkids
Have I missed anything? It is late, and I will read through this again
in the AM

*** Only the cabernet knows for sure . . . .

Post by fourkids
Dave Early
Colorado

[fourkids]

Well, I hope you guys are still out there....

I have added the 10 ohm resistors...happened to have 4 precision 10
ohms laying around.

And it turned out that the 5.6k ohm resistor I referenced earlier had
already been doubled, so I modified it again for a theoretical value of
1.3k ohm. This brought my bias voltage down to -20.5 volts, measured
at each tube.

The voltage over the 10 ohms is .25 to .26, except for one tube: which
measures .1 volts...I appear to have a bad tube?

OK so my problem: the tubes are still running WAY hot...they are very
hot in just a couple of minutes. Now maybe I don't understand how hot
they are supposed to be, but i have nothing to compare it to.

I have already ordered a new quad of 7868EH tubes...should get them in
a couple of days. The tubes that are in the set now are all
"Realistic" tubes...not the greatest brand, and probably all bad having
been run hot for some time.

Anyway, any further wisdom would be greately appreciated.

Dave Early
Colorado

[Jon Yaeger]

Post by fourkids
Well, I hope you guys are still out there....
I have added the 10 ohm resistors...happened to have 4 precision 10
ohms laying around.
And it turned out that the 5.6k ohm resistor I referenced earlier had
already been doubled, so I modified it again for a theoretical value of
1.3k ohm. This brought my bias voltage down to -20.5 volts, measured
at each tube.
The voltage over the 10 ohms is .25 to .26, except for one tube: which
measures .1 volts...I appear to have a bad tube?
OK so my problem: the tubes are still running WAY hot...they are very
hot in just a couple of minutes. Now maybe I don't understand how hot
they are supposed to be, but i have nothing to compare it to.
I have already ordered a new quad of 7868EH tubes...should get them in
a couple of days. The tubes that are in the set now are all
"Realistic" tubes...not the greatest brand, and probably all bad having
been run hot for some time.
Anyway, any further wisdom would be greately appreciated.
Dave Early
Colorado

Sounds like you've got a bad tube. Swap it in another position and see if
is still too low.

Tubes are supposed to run very hot. But not so hot that the plates glow
dull orange. Look at them in a darkened room.

Note that most Realistic tubes - which were made by other companies and
banded Realistic - have a lifetime warranty, which they still honor!

[Jon Yaeger]

Post by Jon Yaeger

Post by fourkids
Well, I hope you guys are still out there....
I have added the 10 ohm resistors...happened to have 4 precision 10
ohms laying around.
And it turned out that the 5.6k ohm resistor I referenced earlier had
already been doubled, so I modified it again for a theoretical value of
1.3k ohm. This brought my bias voltage down to -20.5 volts, measured
at each tube.
The voltage over the 10 ohms is .25 to .26, except for one tube: which
measures .1 volts...I appear to have a bad tube?
OK so my problem: the tubes are still running WAY hot...they are very
hot in just a couple of minutes. Now maybe I don't understand how hot
they are supposed to be, but i have nothing to compare it to.
I have already ordered a new quad of 7868EH tubes...should get them in
a couple of days. The tubes that are in the set now are all
"Realistic" tubes...not the greatest brand, and probably all bad having
been run hot for some time.
Anyway, any further wisdom would be greately appreciated.
Dave Early
Colorado

Sounds like you've got a bad tube. Swap it in another position and see if
is still too low.
Tubes are supposed to run very hot. But not so hot that the plates glow
dull orange. Look at them in a darkened room.
Note that most Realistic tubes - which were made by other companies and
banded Realistic - have a lifetime warranty, which they still honor!

PS - I forgot to ask . . . . How dies it sound??

[fourkids]

It sounds...mushy I guess...kind of lacks preceision in the high end,
like it is slightly distorted and just now it crackled slightly for a
couple of seconds and then smoothed out. Not sure what that was.

I did switch the tubes around...some story paired with any of the other
tubes - I have one that only puts out .1V to the 10 ohm...do you know
that the "means"?

And I am going to try and get the realistic tube replaced, but
everything I have read indicatest that it will take a long time, and I
don't want to wait...if I can get it in, I will keep them as spares or
sell them off...not sure which yet.

Thanks for all the help...the amp is humming along next to me and is
running MUCH cooler...warm, but not scary hot.

Dave Early
Colorado

[p***@aol.com]

Have you done anything with the general low-value caps? Mushy... betcha
several leaking caps in the tone control section. Lack of precision...
several leaky caps in the driver section.

Crackling... Mpffff. If on FM, sounds like silver-mica disease. Let me
know, please, the precise etiology of the cracking, from onset to
smoothing, and if it cycles (repeats) or not. Is it FM or all sources?
This _is_ important.

Are the "bad" artifacts confined to one channel or not? Do they switch
with the tubes when you switch them? What is your source (do you use a
source other than the Tuner section)?

Peter Wieck
Wyncote, PA

[fourkids]

OK, I have written 2 responses to this alread...not sure where they
have gone. I guess the ethernet is at least part ether.

This was not an FM problem...I was using the Aux-Tape input. Not sure
if it happens anywhere else as I have not used the system for anything
but the ipod.

The crackle seemed to be from both speakers. It was transient enough
and my speakers are poorly placed right now, so I couldn't discern it
was from one or both speakers. The crackle find of faded it and faded
out....quickly, but there was no "pop" at either side of the crackle.

I have now replaced the 7868's with the new EH tubes. I would note
that first off, the amp puts out more volume for same knob position.
And I would say that it sounds somewhat more precise in the mids....not
sure if this is a placebo effect or not. Also, at low volumes the bass
is stronger now...before it was really weak at lower volumes.

One more note: there was a noticable hum when there was no source
attached...now there isn't, even if I turn the volume all the way up.

I took at look at the caps in the tone control section...they look
simple enough to replace...should I just go ahead and replace them all?
No skin off my nose, but if I don't have to, I can spend the money
elsewhere

I am considering experimenting with some other new tubes in the the
12AX7 positions...maybe a couple of 5751's in the first stage just to
see.

Tahnks for all the help...

DAve Early

[p***@aol.com]

Dave:

Note the interpolations.

Post by fourkids
OK, I have written 2 responses to this alread...not sure where they
have gone. I guess the ethernet is at least part ether.
This was not an FM problem...I was using the Aux-Tape input. Not sure
if it happens anywhere else as I have not used the system for anything
but the ipod.

Good. That eliminates Silver Mica disease, something that is a major
PITA to fix, and not typical of FM tuners. Count your blessings.

Post by fourkids
?The crackle seemed to be from both speakers. It was transient enough
and my speakers are poorly placed right now, so I couldn't discern it
was from one or both speakers. The crackle find of faded it and faded
out....quickly, but there was no "pop" at either side of the crackle.

This sounds like any of a couple of things, but if you have not paid
attention to the small-value caps and checked the main filter caps,
don't leap to conclusions - yet. It could be marginal electrolytics
leaking at turn-on and forming (somewhat) after some time. As it is
both channels, start with the main filter caps, and any electrolytics
common to both channels. It could be marginal tubes, also possibly
affecting both channels as the tubes in there were all likely about the
same vintage. If it has 100% gone away now, that is quite likely.
Small-value caps first, however.

Post by fourkids
I have now replaced the 7868's with the new EH tubes. I would note
that first off, the amp puts out more volume for same knob position.
And I would say that it sounds somewhat more precise in the mids....not
sure if this is a placebo effect or not. Also, at low volumes the bass
is stronger now...before it was really weak at lower volumes.

Not hardly Placebo. If the original tubes were that marginal, all of
the above is the least you might expect from new tubes. BUT---- has ALL
the cracking gone away?

Post by fourkids
One more note: there was a noticable hum when there was no source
attached...now there isn't, even if I turn the volume all the way up.

A good thing.

Post by fourkids
I took at look at the caps in the tone control section...they look
simple enough to replace...should I just go ahead and replace them all?
No skin off my nose, but if I don't have to, I can spend the money
elsewhere

I would shotgun them on general principle. First, they are 40+ years
old, made by the cheapest bidder, and worked pretty hard in a tube amp.
Especially if they are black plastic caps with "black cat" or all sorts
of colored strips on them. Stick with similar value in uF, with the
same operating voltage-or-higher. If by any chance you find
electrolytics of 1uF or smaller, replace those with poly or
epoxy-dipped caps. That will tighten things up considerably. Ceramic
disc and mica (domino-looking) caps are generally pretty rugged, so for
now leave those alone.

Post by fourkids
I am considering experimenting with some other new tubes in the the
12AX7 positions...maybe a couple of 5751's in the first stage just to
see.

It is my considered opinion that the *SYLVANIA* mil.spec. 5751 will
blow the socks off even the legendary smooth-plate Telefunkens. See if
you can find those. Kelly Green label except for the tube number, and a
bunch of extra numbers. Note that the "first stage" is the
phono-pre-amp. So ignore that for now. And, the Japanese commissioned
the 7025 as a low-noise 12AX7. You might try some of those if you can
find them for cheap.


Tahnks for all the help...

Have fun. That's the point.

Peter Wieck
Wyncote, PA

[Jon Yaeger]

Post by fourkids
OK, I have written 2 responses to this alread...not sure where they
have gone. I guess the ethernet is at least part ether.
This was not an FM problem...I was using the Aux-Tape input. Not sure
if it happens anywhere else as I have not used the system for anything
but the ipod.
The crackle seemed to be from both speakers. It was transient enough
and my speakers are poorly placed right now, so I couldn't discern it
was from one or both speakers. The crackle find of faded it and faded
out....quickly, but there was no "pop" at either side of the crackle.

*** Have you lubricated the potentiometers? Corrosion or bad connections
on the tube sockets can cause intermittent noise.

Post by fourkids
I have now replaced the 7868's with the new EH tubes. I would note
that first off, the amp puts out more volume for same knob position.
And I would say that it sounds somewhat more precise in the mids....not
sure if this is a placebo effect or not. Also, at low volumes the bass
is stronger now...before it was really weak at lower volumes.
One more note: there was a noticable hum when there was no source
attached...now there isn't, even if I turn the volume all the way up.
I took at look at the caps in the tone control section...they look
simple enough to replace...should I just go ahead and replace them all?
No skin off my nose, but if I don't have to, I can spend the money
elsewhere

*** Assuming you've already replaced the output coupling caps, replacing the
coupling caps should set you back about $10-15? A decent bottle of wine?

Post by fourkids
I am considering experimenting with some other new tubes in the the
12AX7 positions...maybe a couple of 5751's in the first stage just to
see.

*** Play all you like but the 5751s have a lower mu and a different
operating point than the 12AX7. They are different tubes.

Post by fourkids
Tahnks for all the help...
DAve Early

[p***@aol.com]

Post by Jon Yaeger
*** Play all you like but the 5751s have a lower mu and a different
operating point than the 12AX7. They are different tubes.

That, they are. But some of them with specific reference to the
Sylvania tubes can sound remarkable. Whether one likes it or not is a
different question. It is fun to experiment.

Peter Wieck
Wyncote, PA

[Jon Yaeger]

Post by fourkids
OK, I have written 2 responses to this alread...not sure where they
have gone. I guess the ethernet is at least part ether.
This was not an FM problem...I was using the Aux-Tape input. Not sure
if it happens anywhere else as I have not used the system for anything
but the ipod.
The crackle seemed to be from both speakers. It was transient enough
and my speakers are poorly placed right now, so I couldn't discern it
was from one or both speakers. The crackle find of faded it and faded
out....quickly, but there was no "pop" at either side of the crackle.
I have now replaced the 7868's with the new EH tubes. I would note
that first off, the amp puts out more volume for same knob position.
And I would say that it sounds somewhat more precise in the mids....not
sure if this is a placebo effect or not. Also, at low volumes the bass
is stronger now...before it was really weak at lower volumes.
One more note: there was a noticable hum when there was no source
attached...now there isn't, even if I turn the volume all the way up.
I took at look at the caps in the tone control section...they look
simple enough to replace...should I just go ahead and replace them all?
No skin off my nose, but if I don't have to, I can spend the money
elsewhere
I am considering experimenting with some other new tubes in the the
12AX7 positions...maybe a couple of 5751's in the first stage just to
see.
Tahnks for all the help...
DAve Early

I forgot to mention that there are 1 uf caps in the output stage of the FM
section that are usually bad and replacement may offer a very audible
improvement. These aren't too easy to get at.

Jon

[p***@aol.com]

Post by Jon Yaeger
I forgot to mention that there are 1 uf caps in the output stage of the FM
section that are usually bad and replacement may offer a very audible
improvement. These aren't too easy to get at.

..... and don't use electrolytics if they were so originally.

Peter Wieck
Wyncote, PA