weird amp problem

[ALBATROS1234]

i got a kodak av154 projector. I extracted the p.p. 6L6 amp promptly . this has been over 2 years ago. i modded the input because it was goofy and designed for a projector . it sounded great and has for 2 years. All of a sudden today i was playing through it, as i was yesterday and the day before. It all of a sudden started sounding like I had a tremolo engaged which is impossible since I use no pedals. A pretty rapid oscillation has suddenly appeared. I turned it off and waited but still when I fired it up it goes into automatic tremolo mode. When I initially changed the input which was not conducive to guitar amps I also changed coupling caps between stages. The power supply caps seem fine and never have produced hum albeit I brought it up slowly on a variac to reform and they surely did. There is still no hum just it sounds like someone stepped on a tremolo footswitch. Where do I begin?

[ALBATROS1234]

basically I put a standard input to the 1st stage, 1 Meg to ground and a 47k grid stopper because the input level was super low so I put a standard guitar input on it as well as changing the coupling caps. this is a simple amp . I don't get what could make it sound like a tremolo?

[sluckey]

Filter caps are my number 1 suspect.

[ALBATROS1234]

I didn't realize filter caps could cause a tremolo like oscillation. I figured it would be a cap but maybe something on a cathode.

[sluckey]

Get a 20, or 30, 0r 40µF axial cap. At least 450V, better if 500v. Connect a gator clip lead to the negative cap lead. Put some insulation on the positive cap lead but leave about a 1/4" of lead exposed at the end. Now connect the negative cap lead to chassis. Hold the body of the cap in your hand and use it as a test probe. Touch and hold the positive tip to each of the filter caps, one at a time. Be sure to discharge the test cap by touching the positive tip to chassis before touching the next cap. Be prepared for a spark. This is a good tool for finding many filter cap problems such as hum or decoupling issues. Back in the '60s most radio/tv shops had a test cap hanging over the workbench.

[Willabe]

Hold the body of the cap in your hand and use it as a test probe. Touch and hold the positive tip to each of the filter caps, one at a time. Be sure to discharge the test cap by touching the positive tip to chassis before touching the next cap. Be prepared for a spark.

If your going to do this, I wouldn't, at least wear a rubber glove on the hand holding the cap, better yet, on both hands.

Discharging an E-cap, or any cap like that is very hard on the cap. I wouldn't do this either. Discharge the cap threw a R, like we always do.

(Sorry Sluckey, I have great respect for you, but surprised by this.)   

[sluckey]

Everybody held the cap in their bare hand. The body of the cap is insulated and even if not, the body is connected to the negative terminal so no danger. Everybody discharged the cap as I described. No reason to fiddle with a resistor. It's a test cap. A disposable tool. I never saw one fail because of discharging by shorting the leads. It's good for the nerves. Sometimes you can get quite a spark!

[shooter]

We regularly  used a shorting bar to discharge caps that sat at 56kv, loved the zzzzzzzzzzZAP blue flame 

[Willabe]

It's good for the nerves. Sometimes you can get quite a spark!

 

[Willabe]

We regularly  used a shorting bar to discharge caps that sat at 56kv, loved the zzzzzzzzzzZAP blue flame 

    

[astronomicum]

Get a 20, or 30, 0r 40µF axial cap. At least 450V, better if 500v. Connect a gator clip lead to the negative cap lead. Put some insulation on the positive cap lead but leave about a 1/4" of lead exposed at the end. Now connect the negative cap lead to chassis.

Thanks for this trouble-shooting tip. Going to make me some of these. I know that this procedure is for the amp in question but just a word of caution for general use that not all filter caps are grounded like in cases where caps are in series for higher voltages and caps in voltage doublers (may be others). The negative test cap lead (gator clip) would need to be moved to the negative side of the cap being tested otherwise, the test cap could be subjected to voltages higher than its rating.

[sluckey]

This is definitely a shop tool to be used by someone that knows what they are doing.

[PRR]

> I didn't realize filter caps could cause a tremolo like oscillation

Big output signals sneak-back to sensitive input stages through the DC power line. Capacitors reduce mids-highs leakage well, not so much in bass. If you get rising leakage below the audio band, you can get a few-Hz oscillation throughout the amplifier. "Motorboating". Maybe not with C1 C2 C3 all good, but not when C2 quits so you have C1 XX C3, only two filter steps.

[Keppy]

Big output signals sneak-back to sensitive input stages through the DC power line. Capacitors reduce mids-highs leakage well, not so much in bass. If you get rising leakage below the audio band, you can get a few-Hz oscillation throughout the amplifier. "Motorboating". Maybe not with C1 C2 C3 all good, but not when C2 quits so you have C1 XX C3, only two filter steps.

I'm getting the unauthorized tremolo problem on an amp and thought of the filter caps, but they measure at the correct value. Is this likely to happen with caps that still measure in spec? Is there another factor besides capacitance? Not to hijack, but the info seems relevant to discussion here.

[shooter]

I also changed coupling caps between stages.

did that include C5 & C10?
if not, as a WAG, maybe try those?

[ALBATROS1234]

Thanks all, I will definitely be trying this as soon as I can. I like the test rig idea, i appreciate the help and will report my findings

[ALBATROS1234]

That was it, filter cap. I clipped a 22uf cap to ground with an alligator lead and used another alligator lead to clip to the base of the can cap which has 4 stages. When I clipped to 1 the oscillation disappeared so I learned something new today. Thanks sluckey and all.

[sluckey]

I'm getting the unauthorized tremolo problem on an amp and thought of the filter caps, but they measure at the correct value. Is this likely to happen with caps that still measure in spec? Is there another factor besides capacitance? Not to hijack, but the info seems relevant to discussion here.

Just try it. Only takes one minute to know if your problem is a filter cap.