Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on April 03, 2018, 05:34:38 pm
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Have a Ampeg V4 on the bench and it needed a lot of love.
I have recapped the filters and associated resistors.
I have changed the flyback diodes.
I rewired the PPIMV that amp already had.
I rebuilt the power tube sections. Plate resistors (10ohm), Screen Resistors (1K), and Grid resistors.
Swapped the parts in the Reverb section.
And changed another million parts that were either wrong or toast.
Voltages look good!
Here's the rub.
The amp with master volume down is nice and quiet with no hum or audible signal noise.
There is a little bit of a static and crackle sound in the background intermittently, especially when just turned on to play mode.
I am wondering if anyone has any tips on what to look for to find the source of the "noise"?
I even rewired the primary side of the PT.
All jacks and mechanicals have been cleaned. I changed 1 Power tube socket that had a little carbon from a previous arc.
I can't seem to instigate the noise with probing or resettling the power tubes.
Anybody got any ideas?
Thanks
EDIT... Removed tiny font formatting
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bad tube, plate resistor breaking down, flaky socket, or leaking coupling cap(s).
i bought my V4 as box of parts with a bad OT and i did nearly the same thing to mine - after all that + new OT, i had a crackle, hiss and popping on Ch2: turns out i had a bad plate resistor, a flaky preamp tube socket, and bad Z selector switch that was probably what blew the OT on the previous owner. the bad Z switch was not contributing to the noise problem, just adding to the frustration, since sometimes the amp would have no output at all. i found the bad socket by wiggling one of the preamp tubes. the resistor i found by shotgun replacement of every plate R in the Ch2 path.
on a footnote: if your amp has color-coded chicklet caps, you may want to replace all of them. nearly every one of them on my amp was leaky and/or had hairline cracks.
--pete
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Yeah, that's what I was thinking. It has been an arduous repair.
Turns out that once I boxed it up, there was no noticeable crackling. I gonna have it the rest of the week, so I will be able to see if it still has issues.
Everything in the power section, post secondary, has been replaced at this point, aside from 3 tube sockets.
Might have to shoot those before it's said and done.
Thanks DummyLoad for the ubiquitous help!
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Highjack alert ...
Back in the day, a bandmate was playing through one of the original VT22's. That left a lasting impression.
One of the comments that one sees online about the VT22 and the V4 is that they get really loud, really quickly.
I spent some time earlier today studying the schematics for these, as well as the V4B. One thing that jumped out at me was that the Volume pot's were 1M-Linear Taper. So, I'm thinking, of course they get loud quickly.
As these amp's evolved, did Ampeg/Magnavox/et al. continue this feature?
Ed
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As these amp's evolved, did Ampeg/Magnavox/et al. continue this feature?
Maybe you could spend a little more time studying the later schematics and give us another report. :wink:
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Steve:
The answer to my question looks like a definite "it depends". For instance, there's a schematic out there, dated 1976, showing Channel 1 with "Distortion" and Channel 2 without. Channel 1 Volume shows 1M-?, while Channel 2 shows 500K-Log. On this site "RevG" (date?) shows 1M-Linear on both channels. I saw another schematic dated later than '76 earlier, but can't find it now. Suffice to say these amps, and Ampeg, went through some changes over the years.
My interest is piqued somewhat by there being an early V4 head for sale locally.
Perhaps I'm making some overly broad assumptions here - thinking that Audio/Log taper is the more typical practice for Volume pot's. I notice that some other Ampeg models show Linear.
Granted, Ampeg's original design philosophy was, uh, different.
Ed
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The SVT/VT era amps are quite different from earlier Ampegs.
There were several known Ampeg designers; I don't know their names/credits.
When a design was produced and distributed, push-back from dealers and customers sometimes suggested changes. If the VOL action was too quick, an Audio pot was one possible fix. It means pulling the LIN pots from the assembly room and ordering-in a crate of Audio pots, but that may be easier than circuit changes.
And the schematics have typos. The one inside my VT40 has a missing connection and it could not work as-drawn. I never troubled to check all lin/log notes and these may be in error also.