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#1 2013-03-01 17:01:36

MikeFef
New member
Registered: 2013-03-01
Posts: 2

I am about to give up on this VT40. :-)

First of all, this site has helped me dozens of times in the past - THANKS!  But this time I have an Ampeg that I'm stumped on and I felt it was worth registering and posting about.
It's a VT-40. 
Here's the deal... It's sound like the speaker is blown and crumbly - but with a solid amp, the speakers are fine and sound perfectly solid.  If I plug the VT40 amp into a "known good" cabinet, it sounds like hell.  Obviously it's the amp.  I ruled out the pre-amp section right away.  I'm plugging a clean source into the Power Amp In jack.  Without getting into too much detail, I've re-capped the whole darn thing, it's got all new tubes now, and I've replaced most but not all of the diodes in the power supply section.   But the voltages have no AC shake to them, so I'm pretty certain the remaining diodes are ok anyway. Scopes don't lie.
Here's the issue:  I've got -63V bias and 412V at the screens.  All other voltages match the schematic (e.g. plate).   I haven't measured bias current yet because the voltage is so out of whack it seems pointless.  But let's pretend the bias is normal... with a scope on the input to the tubes, the signal is clean coming in.  Output on both tubes to the transformer is choppy and nasty looking (like the load might be squashing it - I also have a lot of flat transitions at the crossover but that's probably because the bias is so out of whack).  Power supply voltages remain stable. Ohms look good on the output transformer - both across winding and to the chassis.  It doesn't appear shorted or open, but then again under load, who knows what's happening in there.   
I'm so frustrated - this isn't a complicated design! :-)  Could it be the output transformer?  I'd really hate to drop the money on that and fine out I missed something completely goofy.  Any suggestions??

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#2 2013-03-01 18:41:43

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: I am about to give up on this VT40. :-)

Is this a distortion model?

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#3 2013-03-03 05:41:17

MikeFef
New member
Registered: 2013-03-01
Posts: 2

Re: I am about to give up on this VT40. :-)

Yes it is. (sorry for the slow reply - was traveling)

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