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

[nickb]

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!

There is a big difference before and after the rectifier - by that I really mean after the smoothing capacitor. After the smoothing cap it's 175mA but before it's ~1A. The reason is that most of the time the current is being supplied by the smoothing cap. That same cap gets that same amount of energy that was used during the very short time when the incoming AC signal out of the power transformer is greater than the voltage on that capacitor. Therefore a big 'gulp' of current flows.

[Britt Rossman]

Okay folks, I pulled those components off the recto and have the same issue. I'm gonna try re-tensioning the tube sockets and see if that helps... it seems the 0A2 doesnt have very good hold in there... Also, I'll check some voltages if I can...

[Britt Rossman]

Okay guys, idk if the last post went cuz I'm not seeing it. Here is what I got:

I pulled 30R and 1n5408's off the recto, and no dice. Changed lead dress, as AC mains wiring to switch was crossing over the filter caps - nothing. Read voltages... B+ at OPT center tap = 465vDC - WOW that's hot! (It's starting to get late here, so I can't keep testing) When I whack it, it goes down by 15-20vDC.

OKAY, checked on Variac to get B+ at 440V, and checked bias:

Va1=440vDC, Ia1=28.6mA, Pa=12.6W
Va2=443vDC, Ia1=21.1mA, Pa=9.4W

So these tubes are 25% mismatched... While 22W is OKAY, I dont think that's how I wanna be getting it. Besides this, I think I'm gonna have to get higher voltage-rated caps if it is to be plugged straight in... Ugh.

Either way, the "ducking" issue is still here. Could it be the drastic mismatch? Never heard of that...

EDIT:: dropped in my matched tubes from my other 6L6 amp early in debugging this guy and somehow killed one tube guess it was time for that one... I do beat them up hard. Should I be replacing with new?

[nickb]

Okay guys, idk if the last post went cuz I'm not seeing it. Here is what I got:

I pulled 30R and 1n5408's off the recto, and no dice. Changed lead dress, as AC mains wiring to switch was crossing over the filter caps - nothing. Read voltages... B+ at OPT center tap = 465vDC - WOW that's hot! (It's starting to get late here, so I can't keep testing) When I whack it, it goes down by 15-20vDC.

15-20V is not bad at all. That will not cause near total volume loss. You could use a 5W 15V zener before the first 20uF smoothing cap to drop the voltage instead of changing the caps. 465V compared to expected 430V is really not that unusual, less that 10%. I guess your caps are 450V.

OKAY, checked on Variac to get B+ at 440V, and checked bias:

Va1=440vDC, Ia1=28.6mA, Pa=12.6W
Va2=443vDC, Ia1=21.1mA, Pa=9.4W

So these tubes are 25% mismatched... While 22W is OKAY, I dont think that's how I wanna be getting it. Besides this, I think I'm gonna have to get higher voltage-rated caps if it is to be plugged straight in... Ugh.

Either way, the "ducking" issue is still here. Could it be the drastic mismatch? Never heard of that...

That is not a bad mismatch and it would not cause the problem.

Is this problem simply caused by the compressor? Does changing the position of the compressor switch have any effect? Try pulling V4 and V5 to ensure the compressor is disabled.

EDIT:: dropped in my matched tubes from my other 6L6 amp early in debugging this guy and somehow killed one tube guess it was time for that one... I do beat them up hard. Should I be replacing with new?

Butter fingers! New, old. who cares so long as they work.

[Britt Rossman]

Yea, didnt even think to isolate that. Come on, dude... Anywany, that did it... I must have shorted the compressor out in there somewhere. Frankly, not worth it to have that in there.

I put a 5V4 recto in there and I'm now gettin 455vDC B+ from mains of 121vAC I still think this is hot, but I really don't wanna have to buy more caps... I'm cheap!

If that's the safest route, however, I'm totally down.

[nickb]

Yea, didnt even think to isolate that. Come on, dude... Anywany, that did it... I must have shorted the compressor out in there somewhere. Frankly, not worth it to have that in there.

I put a 5V4 recto in there and I'm now gettin 455vDC B+ from mains of 121vAC I still think this is hot, but I really don't wanna have to buy more caps... I'm cheap!

If that's the safest route, however, I'm totally down.

Ah ha! Peace at last!

The zener at $0.52 is the cheapest option 1N5352B-TP Micro Commercial Co | Discrete Semiconductor Products | DigiKey if you're buying some other stuff.

[Britt Rossman]

Well, I have some F&T 22uF caps on hand, and they're rated 500v... I think I'll just swap those out since inrush on startup is peaking at ~490V... that's a little much for those poor 450v-rated lytics... Even with 15v zener, I'd be seeing voltages ~25v higher than the rating.

The REAL problem is that the caps are rated for a standby arrangement that doesnt apply B+ to anything until it's thrown, YET this amp just ties the 6L6 grids together. FYI, that still produces sound when the volume is crank and the input is given some juice... It's crackly and fizzy, so I'm sure it's GREAT for those power tubes. Maybe I should rewire the standby to go from the first filter node-->switch-->choke instead of the arrangement now. That's the method recommended by Merlin Blancowe...

[nickb]

Well, I have some F&T 22uF caps on hand, and they're rated 500v... I think I'll just swap those out since inrush on startup is peaking at ~490V... that's a little much for those poor 450v-rated lytics... Even with 15v zener, I'd be seeing voltages ~25v higher than the rating.

The REAL problem is that the caps are rated for a standby arrangement that doesnt apply B+ to anything until it's thrown, YET this amp just ties the 6L6 grids together. FYI, that still produces sound when the volume is crank and the input is given some juice... It's crackly and fizzy, so I'm sure it's GREAT for those power tubes. Maybe I should rewire the standby to go from the first filter node-->switch-->choke instead of the arrangement now. That's the method recommended by Merlin Blancowe...

The standby won't do any harm to the power tubes. I'm not a fan of standby switches in the power due to the buildup of cathode interface resistance shortening the tube life. You'd probably need a different rated switch anyway. If you really can't stand the current arrangement, move the switch so that isolates the two power tube cathode form ground.

[Britt Rossman]

I rearranged the standby, but the results were fizzy and distorted ... weird. So I changed it back. Installed those higher rated caps at the first two nodes that are receiving high inrush voltages. Buttoned her up, new speaker installed and original (still functional) in a box.

I'm now satisfied beyond any doubt that I can return this and not see it for a long while. Cannot wait to get this thing off my bench and back to an appreciative customer! I'll post some pics soon as I can figure out how...

[Tom Phillips]

...I'll post some pics soon as I can figure out how...

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