Amp crackle with note - I'm stumped!

[Rick1114]

My GA-5 fried the power tranny a few weeks ago in a sort of dramtic event (the amp went BZZTTT and smoke started coming out from the PT). I replaced it with MM clone and replaced all the electrolytic capacitors as well. When I turned the amp on again it now worked but there is a "crackle/buzz" that appears with the note. It is more prominent with the lower strings, but it can be heard in the upper registers as well if you listen closely (particulary if you bend or sustain the note). It sounds very much like a blown speaker.

So far I've tried:
*checked the speaker cone - no damage
* different set of tubes
*replaced a blown plate resistor and it still crackles (most of the other resisitors are within 10-15% of spec).
*replaced the coupling cap that is attached to the volume pot - no dice.
*resoldered the filter cap and PT connections and anything that looked funny
*tried wiggling the tubes to see if I could loose socket

The only possible clue - I did the chopstick test and none of the connections made noise except around the input jacks - the two resistors connected to the input jack made noise (but they tested fine at 45k), and the volume pot connection to the preamp tube made noise when struck with the chopstick. The volume knob itself is noisy even though I cleaned it with contact cleaner. Not sure if any of this is related to the crackle though.

Any ideas would be appreciated - I'm stumped!!!

Rick

[6267]

Have you tried isolating the speaker from the combo? It could be mechanical vibration related. It could also be that you are getting more power out than before, and the speaker can't handle it...I'd try a higher wattage speaker too. If it is mechanical, it's likely a cold solder or loose connection, so I'd reflow all suspect solders.

[Rick1114]

Does the symptom I'm describing (buzzing with notes) sound like a blown Output Transformer? I called the guy at Mercury and he said it could be (but he's trying to sell transformers, of course)

Rick

[gmoon]

and the volume pot connection to the preamp tube made noise when struck with the chopstick. The volume knob itself is noisy even though I cleaned it with contact cleaner. Not sure if any of this is related to the crackle though.

I had a similar situation which was directly linked to a bad (probably oversoldered) pot. Amp sounded fine until it was cranked; then it went all fuzzy.

Bridge the pot with a jumper to check...

[Rick1114]

Bridge the pot with a jumper to check...

How do you bridge the pot with a jumper cable - just clip one lead on each outer lug and it takes it out of the circut?

[Chuck H]

The volume knob itself is noisy even though I cleaned it with contact cleaner. Not sure if any of this is related to the crackle though.

Have you checked the volume pot for DC voltage? If you have a leaky decoupling cap or any something else letting DC into the circuit where it should not be that may cause your problem. Ive seen bad caps, bad preamp tubes, tiny chips of carbonized flux or lead insulation and old black circuit board that have gone conductive all cause DC bias shifts in a preamp tube that behaved like you describe.

Chuck

[Rick1114]

Have you checked the volume pot for DC voltage? If you have a leaky decoupling cap or any something else letting DC into the circuit where it should not be that may cause your problem. Ive seen bad caps, bad preamp tubes, tiny chips of carbonized flux or lead insulation and old black circuit board that have gone conductive all cause DC bias shifts in a preamp tube that behaved like you describe.

Chuck

I have a feeling there is some DC getting to the volume pot & that's why I tried changing the coupling cap attached to it to no avail. There is only one other coupling cap in the amp, so I can try that next, but I'm getting pretty down about the whole thing. I'm worried it's going to be the OT, and then this amp will start to be getting $$, especially if I bring a tech into the picture.

PS - how do I check the pot for DC with my multimeter?

Thanks,

Rick

[gmoon]

How do you bridge the pot with a jumper cable - just clip one lead on each outer lug and it takes it out of the circut?

Generally, a volume control is a simple voltage divider, so one end is the incoming signal and the other end is GND; the center is the divided signal (output.) Jumper the incoming side to the center (and turn the pot all the way up.)

No idea if it's the cause, but at least it's an easy thing to check...

[Rick1114]

I solved it!!! I was a dirty/loose socket on the 12AX7 preamp tube. I had retensioned the other tube sockets with a jewelers screwdriver, but the preamp tube had a cover to keep it in secure and the sockets were so tiny, so I just left that one alone. Because i was getting some noise inside the amp (during the chopstick test) on one of the preamp pins, I decided to take the tube cover off and see if adjusting by hand had any effect. Lo and behold the buzz vanished!

Now I just have to install a 3 prong cord and tweak the amp to taste

Thanks everyone here for their suggestions.

Rick