Excess Clipping/"Farting" in old tube amp

[EETStudent]

I recently took on the project of restoring an old tube amp of mine. The most simplistic circuit you could imagine for a tube amp. 12AU6, 50C5, 12AV6 tubes into a filter and a micro transformer. I am an electronics student and I am pretty new to this field so bear with me. I have heard and read countless times that replacing the caps is almost mandatory in serving old, unattended guitar amps. There were two caps rated .03-400V and I replaced those with .04-400v. There were 4 other caps rated .05-400V and I replaced those with .06-630V. I increased all of the sizes because I read that doing this will increase the gain in their respective stages. I believe I did this successfully, but although the difference was small, there still was some improvement. I then to replace the resitors...all 6 of them. Pretty much the same deal here. Everything seemed to be fine, and then i woke up one morning and plugged and as the title of the thread would imply, i started getting the "farting" type response. The volume seemed somewhat lower and the performance was clearly lower. I dont really know where to start in checking this problem out. any assistance would be awesome

[CarlZ]

My first question to you would be to ask if the amp is farting as in low frequency motorboating or is the low end caving in and distorting momentarily? Two totally different problems with different cures.

But to help you along somewhat, you took a good first step by replacing all the E-lytics in the amp. Now what you've got to look for is any dc on the grids of the tubes. If there's ANY present, then you most likely have a leaky coupling cap. Another thing is to try replacing the tubes with known good ones. This may not be the solution, it sure is a quick and easy thing to check.

Finally, and this is what MAY have bitten you in the backside, is that when you replaced those coupling caps you went too big with them. The original values were probably much more than adequate in the original design. And while going from .03 to .04 and .05 to .06 on one or two of them won't necessarily make or break anything performance wise, if you went large on all of them it may have pushed the amp over the line and may be the cause of your problem.

-Carl

[EETStudent]

Hey Carl, thanks a lot. I did have some spare tubes that I checked but that didnt seem to be the problem. however i dont have a tube tester so maybe those couldve been equally damaged as far as i know, but i will surely look into that. you also mentioned checking for dc voltage on the grids of the tubes, and im not 100% sure how to do that. For one, i couldnt identify what the grid is. After locating that, would you simply check for dc with a multimeter?

As for the farting, I wouldnt say that its just low frequency distortion, but probably your second suggestion would be more accurate. To be as practical as possible, it just sounds like too much is being applied to the signal and the circuit cannot handle it. The overall attack and sustain are significantly lower. I do know that the original 6" speaker is blown, and it has been for a while, but never did it produce these results. I checked again with a 12" speaker that could easily handle any clipping from a low wattage amp, and although it wasnt as prominent, the farting was still there, which leads me to believe somewhere the signal is overly affected. The funny thing about the new capacitors is that when I initially installed them I had good results(well, at least not bad results) and as i mentioned i just woke up one morning and this was happening. That baffles me, but could that be more common than i would know?

[Steve Conner]

Did you actually replace the electrolytic caps? The small values you mentioned, like .03, are not electrolytics. They have values like 8uF, 25uF, or whatever, and look like metal canisters.

Also, if it has a 50C5 tube in it, it may well be one of the old AC/DC amps that ran directly off line voltage. The chassis and connectors may be live with the line voltage, so watch out!

[EETStudent]

I did replace a cap that measured .1MF and looked like a metal canister, and also one that measures .047-400DC made by SPRAGUE and it looks like a canister too. I suppose these might be electrolytics? I did replaced that though, and these are the biggest capacitor back there. lol I have definitely tried to be cautious around this little guy, however i have been zapped maybe four or five times and none of them were any more severe than that of a shock pen, and i was being careless. So all in all, I did replace all BUT one capacitor, which is ceramic disk cap reading Z5U on the front and I dont know how to read that, and I have also replaced most of the resistors even though the old ones checked out pretty well.

An interesting thing did happen though. I was checking the voltage at different joints and came across a joint that I had desoldered and i hadnt soldered it back so i believe it was loose, and when i put my probe down sparks starting coming up from the Tone pot, which is also the On/Off switch. Maybe I shorted something out there?

[ValveBytes]

sounds like you should be careful with that little amp!

what kind of filter caps does it have after the rectifier? did you replace them? could be leaky may have drifted.

if resistors measure ok then generally they should be ok although they can have crackles and pops under operating conditions.