Gibson GA-100 amp is a troublesome critter...

[dchang0]

nickb could be right about the ticking noise coming from outside the amp.

While working on an amp for a buddy, we were tearing our hair out trying to figure out why the thing hummed so much. Turned out to be the mains power in his apartment. We figured it out when I took the amp to my home and plugged it in. His other amps seemed to be more tolerant of the hum, so we thought it was just that one amp.

It's worth taking the amp somewhere else where you know there is clean power. Could save you a lot of trouble.

[Britt Rossman]

Just wanted to give an update here. Apparently, my client got fed up with the wait and wanted the amp back regardless of condition. So, I put the amp back in the metal chassis to prepare for travel and booted her up once more to try the parallel cap suggested by nickb, and the amp stopped being rude! I disconnected the cap, and nothing changed! Apparently, I WAS picking up interference, but it was either from the input hovering over the driver circuitry, or the tone stack hanging over the power amp circuit...

I'm not kidding, I pulled my hair out for a month. I will henceforth always consider the possibility of just putting things back together and seeing what happens. Mind your lead dress, folks! Uuugh... I hope this helps SOMEone!

[dchang0]

Glad to hear it! All's well that ends well, except that you and I have less hair than we should have with our troubleshooting episodes...

[Enzo]

Cell phnes are notorious for introducing clicks and other noises into amps, and a lot of guys set their cell phone on top of their amps.

I have a wireless land line phone at the shop, and it radiates a digital sort of noise into amps within a couple feet.

[Britt Rossman]

Okay, another update...

Upon getting EVERYTHING back in the amp and buttoning up, I noticed that under even moderate power conditions (volume around 6/10), there is a pretty hard sag, wherein I see the 0C2 turn off (stop glowing) synchronized with the sound loss (and it's near complete loss). Is there some sort of extreme load that is just smashing the PS when the input is hit?

[ThermionicScott]

It's not plugged into a light bulb limiter or anything like that, is it? What does your B+ sag to?

[Britt Rossman]

Nope, just the wall, though through a strip. I can't turn it on to test again tonight, so can't measure the B+ sag, but I'm not sure I could capture it since its only ducking for a split second... I'll try again soon.

[ThermionicScott]

It's a lot easier to check with an analog voltmeter, since the display isn't jumping around so much. Easier still if you have a signal generator to feed the thing. But failing those things, you can route the amp into a big dummy load, and try to strum the guitar in a more or less consistent way with the amp cranked to gauge the sag.

How are you verifying that the tubes are good? There are a lot of unknowns still.

[Britt Rossman]

Well, you got me I guess... I bought the 0A2 NOS. Checked all tubes on my EICO 666. Everything seems to check out fine there. I don't have an analog meter or a dummy load. Signal generator I have, but havent checked functionality... ever?

As I may have said before, I'm just an idiot with a multimeter!

[ThermionicScott]

Well, you got me I guess... I bought the 0A2 NOS. Checked all tubes on my EICO 666. Everything seems to check out fine there. I don't have an analog meter or a dummy load. Signal generator I have, but havent checked functionality... ever?

As I may have said before, I'm just an idiot with a multimeter!

Okay, a working tube tester is good! I was just checking so we could rule out shorted or gassy or otherwise-bad tubes. If all the tubes are good, the components functional and wired in correctly, I'm wondering if something may be wrong with the PT that the B+ sags so much. It ought to be a pretty tight amp with that GZ34 rectifier and regulated screen supply.

Don't be too hard on yourself -- we all started somewhere, and like you, I still have much to learn.

[Britt Rossman]

I should note that for the purposes of debugging safely with the HV issues I was having earlier, I wired some 30ohm 10W resistors and 1n5408 diodes in series with the GZ34... Should I remove them?

[g1]

yep.

[Britt Rossman]

For sake of knowing, how would I use Ohm's Law to calculate the additional drop (or load?) occurring across these components? I've done my best to figure it out, but I don't think ~3v quite cuts it... I tried V=(0.05A*30R)+1.5 <-- the second term would be forward voltage drop for both diodes.

By putting these in, I figured I could lower the voltage coming in (for components' safety), but I may have limited my current thus causing the extreme sag (cutoff)?

[nickb]

For sake of knowing, how would I use Ohm's Law to calculate the additional drop (or load?) occurring across these components? I've done my best to figure it out, but I don't think ~3v quite cuts it... I tried V=(0.05A*30R)+1.5 <-- the second term would be forward voltage drop for both diodes.

By putting these in, I figured I could lower the voltage coming in (for components' safety), but I may have limited my current thus causing the extreme sag (cutoff)?

The formula is correct but the current is not. At full whack the 6L6's will draw around 275mA on the peaks. That's about 175mA average. Your resistors are on the rectifier side where there are bug gulps of current that charge the smoothing caps. For that reason the peak current through the resistors will be more like 1A so the drop will be ~30V. Google Duncan's PSU Designer - it's a good tool for this sort of thing.

[Britt Rossman]

Thanks Nickb, That's what I was thinking... I knew idle wasn't affecting anything, but had no idea about the peak current. It figures under idle conditions there a little bit of sound, and when I whack it and pull a gulp of current... Hmm, go figure, we're not idling anymore!! I think after the rectifier, that could be enough PS starvation to drop the OA2 out of its conductive range... Thanks!