Unexpected increased distortion (=earlier breakup) in vntage Silvertone 1482 tube amp

[slidincharlie (Carlo P)]

Hi all.
I've been missing from this forum for a few years. In fact I haven't done any amp job for a long while.
My 1967 Silvertone 1482 amp has started to sound more distorted (I would call it an earlier breakup) since the last few live gigs.
Same guitars, more or less same volume (pot around 4/10) and tone (8-10/10) settings, but definitely more distorted.
I have swapped speaker (vintage Jensen C12Q and P12Q) and tubes but the result is essentially the same.
Power tubes were well-used Philips JAN 6V6GT, RCA 6V6GT, and NOS Marconi/Westinghouse 6V6G. Preamp tubes were used GE or Sylvania 12AX7.
I did not try any recent production tube. I have put some JJ's in the shopping cart of TAD (www.tubeampdoctor.com) to buy them ASAP.
From a visual inspection there seems to be no burned resistors. A few years ago I replaced the stock cap can with separate Sprague Atom filter caps.
Is there anything else that I should check/try to find the cause of this increased distortion?
I attach the amp schematic.
Thanks in advance.
--Carlo

[Helmholtz]

Verify all DC voltages shown in the schematic.

Do you have a scope?

[pdf64]

The SOP for valve amp users with concerns about their amp’s performance / behaviour is to eliminate there being a failing / failed valve.

A ‘known good’ spare or 2 of each valve type in their amp is required. And preferably a whole spare set of output valves, that preferably all pass similar idle anode or cathode current, and which are reasonably close to the valves the amp is set up (biased) for, if the amp is fixed bias.

When problems emerge, swap a good spare valve into each socket in turn. If the spare doesn’t help with the issue, refit the previous valve and move along to the next socket.

When initially checking unknown or suspect output or rectifier valves in an amp, it’s beneficial to power the amp via a light bulb limiter. The wattage of the bulb used should be about twice that of the amp’s rated power output (eg so around a 100W bulb for a 50W ish amp). As otherwise, if the new / unknown valves short, they may stress and damage the amp.

[Chuck H]

Rather than trying to walk you through a repair procedure I'll let Helmholtz and pdf64 handle that. And you should follow their advice.

Here's what "I" think it could be and would check first. You mention the output tubes are old. Check those first. Check bias on the output tubes. Check the capacitor for the cathode bias circuit. The schematic says it's only rated at 25V which seems low considering the circuit. If that cap is shorted, even partially, the bias will be much too hot. That could cause red plating and the symptoms you describe. And then there's the possibility of a leaky coupling capacitor from the phase inverter. It's an older amp so I wouldn't be surprised if some older caps started leaking. This could also cause a hot bias and your symptoms.

[Helmholtz]

What is a "SOP"?

[slidincharlie (Carlo P)]

Thank you guys for your feedback.
pdf64: new tubes are in order. I had already tried other tubes (used preamp tubes and never used NOS power tubes; no rectifier tried yet) as you said but I could not hear any appreciable sonic difference. Also, I have not taken my measurements with those tubes yet.
Helmholtz: comparing my readings to the schematic that I shared (two other schematics show slightly different voltages) this is what I found:
- V1 is on specs;
- V2 is a little below specs (168 V instead of 188 V on pin 1, 128 V instead of 138 V on pin 2);
- plate voltage on the power tubes is lower than specs (304 V instead of 346 V);
- readings at the filter caps are lower than specs (5 uF: 231 V intead of 260 V; 10 uF: 294 V instead of 330 V; 20 UF: 308 V instead of 350 V).;
- the VAC reading out of the power transformer with the wires connected to the circuit are lower than specs (301 Vac instead of 335 Vac).
What is your take?
Now I'm going to take new readings with different tubes, starting from the rectifier.

[Helmholtz]

What is heater voltage?

What is the cathode voltage of the power tubes?
Does the 270R cathode resistor measure in spec?
And check the cathode bypass cap for leakage. You can just lift one end and see if this changes the cathode voltage.

[slidincharlie (Carlo P)]

(...) You mention the output tubes are old. Check those first. Check bias on the output tubes. Check the capacitor for the cathode bias circuit. The schematic says it's only rated at 25V which seems low considering the circuit. If that cap is shorted, even partially, the bias will be much too hot. That could cause red plating and the symptoms you describe. And then there's the possibility of a leaky coupling capacitor from the phase inverter. It's an older amp so I wouldn't be surprised if some older caps started leaking. This could also cause a hot bias and your symptoms.

Hey Chuck. I forgot to say that I had fully overhauled the amp a few years ago, replacimg all caps (tone, bypass, filter) and checking/replacing all resistors. I will check the bias now, although there is no red plating. I'll keep you posted.

[Chuck H]

What is a "SOP"?

Standard operating procedure.

[slidincharlie (Carlo P)]

What is heater voltage?
What is the cathode voltage of the power tubes?
Does the 270R cathode resistor measure in spec?
And check the cathode bypass cap for leakage. You can just lift one end and see if this changes the cathode voltage.

Heater voltage is 2.9 VAC on each filament.
Cathode voltage on the power tubes is 310 V (lower than specs).
The 270R cathode resistor is exactly in spec.
Re: checking the cathode bypass cap for leakage: I've read that this is hard to do in the circuit and without proper equipment. May I simply and safely lift the + end and see what happens to the cathode voltage of each 6V6?

[Helmholtz]

Heater voltage is 2.9 VAC on each filament.

So you should have 5.8V between the heater pins, right? This is low by 10% similar to your other voltages.

Cathode voltage on the power tubes is 310 V (lower than specs).

That's not cathode voltage.

Re: checking the cathode bypass cap for leakage: I've read that this is hard to do in the circuit and without proper equipment. May I simply and safely lift the + end and see what happens to the cathode voltage of each 6V6?

Lifting one end of a cap is the best and safest way to check for leakage effects.

[slidincharlie (Carlo P)]

So you should have 5.8V between the heater pins, right? This is low by 10% similar to your other voltages..

Exactly.

.

That's not cathode voltage..

Oops... sorry. Cathode voltage is 18 V and it stays there also with the cathode bypass cap lifted.

[Helmholtz]

Assuming that all DC voltages (including preamp cathode voltages) are within +/- 20% there's no indication of a problem so far.
I assume voltages have always been low.
I might make sense to try a new 6X4 rectifier, though.
Also clean the output jack and speaker plug with Deoxit D5.

Again: Do you have a scope?

[Helmholtz]

... 128 V instead of 138 V on pin 2);

Do you mean pin 6?

[slidincharlie (Carlo P)]

I might make sense to try a new 6X4 rectifier, though.

I tried a different 6X4 that I have at home: all voltages increased a little bit, but the heater voltage remained the same. It must be the power transformer I guess.

Again: Do you have a scope?

Sorry, I thought that you asked if I had a mutimeter - I have a Fluke digital multimeter, which I have used for my readings.
Or did you mean something else? I do not have any other testing equipment.

Do you mean pin 6?

Yes, sorry. I meant pin 6. Too many errors today...