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PostPosted: Thu, Aug 09, 2018 2:42pm 
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Hack

Joined: Mon, Oct 23, 2017 1:50pm
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Hey all,

Just hoping one of you amp dudes out there can help me out.

I'm working on a Peavey VB-3 all tube bass amp that I picked up from another tech off of reverb for $300.
This thing had the boner of all tech boners pulled on it, and I've almost got her sorted, but I'm reaching out for help with the final issue.

Here's a little context.

Sooo, I got this thing, and the dude said it was blowing electrolytics all over the place. So I opened her up, and sure as s**t, there was a couple blown on the power amp PCB (C7-C8), and one on the preamp PCB (C85). That's when I noticed that the cable assembly that has all of the main power rails (PPRE1) was plugged in backwards. DOH! It was sending B+(250 ish), and B++(220 ish), down the +-19 volt rails, and reversed the polarity on the +-30 volt rails. Soooooo, I proceeded to replace every IC on those rails. I was hoping for the best ya know. Put some tubes in there, and fired her up, and Yaaayyy!!! She lives!!! Until you flip the standby switch. :doh:

It does pass signal. It's just really low and distorted. I fed the preamp through my orange, and it's good. Which is a huge success as far as I'm concerned. I tested the OT as well as you can with a DMM. The continuity readings made sense, and there were no shorts between the primary and secondary. I think it's good. I don't have a matched octet of el34s, and I would rather not blow 180 bucks on em if I'm not sure that they'll get this beast goin. I used a couple of used quads, and mixed em so that there were 2 from each quad on each side. It seems through my scope that the signal is hugely amplified on the plates of the el84s, but the output is choked.

Any ideas?? I did a bunch of other stuff too, but this post is getting long. If you think of something, let me know, and I'll let you know if I did it.


Thanks yall. :D


Attachments:
File comment: VB-3 power amp page
VB-3_Power_Amp_Schematic.pdf [156.79 KiB]
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File comment: VB-3 power supply
VB-3_Power_Supply_Schematic.pdf [346.72 KiB]
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File comment: VB-3 Preamp page
VB-3_Pre_Amp_Bd.pdf [627.59 KiB]
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File comment: VB-3 Patch PCB
VB-3_Patch_Schematic.pdf [133.86 KiB]
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PostPosted: Thu, Aug 09, 2018 10:58pm 
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Hack

Joined: Mon, Oct 23, 2017 1:50pm
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Ok. Update. The plate signal is not hugely amplified. I injected a 1Khz 1VAC signal directly into the power amp input, and traced the signal all the way through to the grids of the power tubes. The screen resistors are good, and the grid resistors are good. The signal coming from the plates is tiny and squashed. Anything besides bad tubes that would cause that?


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PostPosted: Thu, Aug 09, 2018 11:25pm 
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Plank Cranker
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Well, a little beyond my abilities here, but it seems to me that if the tubes are being presented with the correct voltages (plate, screen, cathode, grid/bias), then the only things left would be the phase inverter, the power tubes, the output transformer, or the speakers. I'm guessing you ruled out the speakers long ago. Could definitely be the power tubes, though. I know you've tested the OT, but I've seen OTs that tested good with a multimeter but were still bad. I'm guessing also that the tube sockets are good and are making good contact with the tube pins. Other than that... IDK. Good luck, and definitely post your progress. :)

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PostPosted: Mon, Aug 13, 2018 1:09am 
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Hack

Joined: Mon, Oct 23, 2017 1:50pm
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Yeah, according to my bias probe, plate voltage is 510vdc, Although, I've just figured out that when I poke the power supply PCB, the plate voltage drops dramatically. to around 180vdc. So that makes me think that the power amp PCB is not the issue. Now I gotta find the fault in the SMPS. UGH!


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