Feedback in reverb Bandamaster Reverb AA1069

[jc_giggs]

I am having trouble locating the source of feedback coming from my reverb circuit. With or without the reverb tank plugged in or the footswitch plugged in there is a very loud squelch when the reverb pot is turned up to around 4. The volume can be turned to 0 and it will still do it. It is not gradual or increase in volume as the reverb pot is turned up. I have inspected the joints and they all test good. I have swapped tubes with no success. I am discounting the tank since it doesn't have to be plugged in to produce the sound. I appreciate your help.

[jc_giggs]

I have not yet found the source of the feedback in the reverb circuit. Ill restate that the oscillation occurs with or without the pan and or pedal plugged in. With the pot and pan unplugged, I have swapped tubes with no effect. I have changed reverb pots with no effect. When the pot is turned up from zero(with volume at 0) there is increase in midrange buzz. Feedback occurs when verb pot reaches 4. The feedback begins to oscillate if verb pot is turned up past 8. I can set the pot to 3 and sometimes make it feedback by grounding the outside of the pan input jack. Please help

[jpfamps]

I would try changing the cathode bypass cap on the reverb return.

The cathode bypass cap is shared between the reverb return and the reverb mixing amp. This is bad design, because if the cap fails then there will positive feedback around this stage.

If you have the schematic this is the caps connected to E.

[pdf64]

See
http://www.webphix.com/schematic%20h...1069_schem.pdf
for the schematic.
If one cathode bypass cap has failed, you may consider replacing them all (7 x 22uF@25V I think). The others will just lose gain and dull the tone, rather than causing oscillation.
Obviously also the other electrolytic caps in the B+ and C- supplies, if they haven't already been replaced. Pete.

[jc_giggs]

thank you for the lead. Although I have a scematic and have been studying it for several weeks, I dont follow your reference to "E". I see two places on the schematic that show E, both having to with the 1/2 7025 valve, but I dont know which bypass cap you are saying E is connected to. The only cap I see near E is the +25/25- and Im not sure how to test a cap that is polarized. All of the caps are new, replaced about two days before the problem arose, however the reverb circuit did not work at all prior to the cap job. As a matter of fact, the majority of the caps were missing. Let me know what you think

[überfuzz]

I guess you've triple checked the wiring, or you probably never changed it. Further you say that the problem arose a couple of days down the road for the new outfit. (I take it you don't have a oscilloscope.) Do you have any extra or spares of the components you replaced. It's a bit tedious but might be the only way of tracking down the gnarly. Try replacing one of the new component at the time and see if you nail it. Be gentle with the new parts because most of them are probably doing just dandy and can be put back after the test.

Every now and then you get new 'tip top' components that act out after some hours, like you said a couple of days.

[überfuzz]

Double post

[jc_giggs]

I have fixed the problem by swapping out some caps, specifically the +25/25- bypass cap on V6. Now however my voltages on the "vibrato" side of the amp are way off. The "normal" side and Valve are spot on excect the heaters, which are all at +3.8v.

[überfuzz]

The vibrato-side is somewhat wide... Could you narrow it down?

Don't forget, some voltages/measurements can appear to be fishy as they are oscillating. Further, some others are most likely very high when the tremolo is OFF, since there's no current, you know Ohms law.