Ampeg V4 rebuild
I've got a VT-22 / V4 amp running 7027's on the bench right now.
1st channel has Vol/Distortion, 2nd channel has Vol/Bass/Mid/Treb/Reverb. Two white rockers controlling power/standby. There is no polarity switch on this amp. The schematic lists this version as 1976, this version is probably late '76 at the earliest.
Testing the amp/known issues:
The amp is a bit finicky, hiss and hum typical of an amp with original power section. The only real malfunctioning component is the distortion control. When you turn it up you can hear that signal is indeed breaking up, but the volume gets quieter as you approach the end of the pot's swing. Any ideas?
Grid bias -32vdc (-615vdc on standby)
I've got the electrolytics on order from Fliptops, I'm going to rebuild the power section, bias supply, all diodes and plate resistors.
As for the other power supply resistors - R218 (10W), 219 (20W) & 220(10W) all seem to be OK - they are of the hollow-tube style and they all measure within spec and don't look cooked. Is it okay to leave them be?
To make matters more frustrating, the aforementioned resistors are mounted on the amp chassis by the PT and filter caps...but I see those same resistor numbers printed on the power/bias supply circuit board. So i'm peeking around and see that some of the resistor numbers match up to the schematic, but others don't. I suppose I'll just have to replace the parts as they appear on the circuit board.
I think i'll be okay for most of this rebuild, I'd love to hear some ideas about the distortion issue, as well as adding an adjustable bias voltage. Would it be best to add two trim pots (one for each pair of output tubes), and if so, where in the schematic to insert them? I was thinking after point F - replace R211 and R212 (100K) with a lower value (47K?) and add a 100k trim pot in parallel with each resistor.
Also, which coupling capacitors are important to replace? Anything I've missed?
Thanks for reading! I'll keep this posted as the work commences.
The distortion is engaging the optos P101 and P102.
Here's a good read on the circuit: http://music-electronics-forum.com/t3030/
Thank you! I got kinda lost in the technical side of things, but am I coming to the correct conclusion that one option may very well be to replace V1 with 12AX7 and remove D1/D2? Is it that simple? The thread doesn't mention a jumper, should there be one to replace the diodes?
Also, the schematic in the above link seems to list the correct resistor numbers relative to the amp in front of me.
No jumper, just lift (desolder) one leg of each diode. (A jumper would shunt the control! ) And sub the 12AX7 in V1.
Oh, and you don't need to replace the wirewound resistors (the "hollow-tube" ones). I also see no need to replace the power supply diodes if they are good, they are silicon diodes so they are OK.
But I agree totally with replacing the plate resistors. I would maybe think about replacing the coupling caps.
Why replace the 12dw7 with a 12ax7? Nothing wrong with NOS 12dw7's, although JJ 12dw7's totally suck.
Finally, I've come full circle on the bias pot thing--I now just prefer to leave the bias set at the voltage called for by the factory.
Well fair enough.
Thanks for the advice. I'm still confused about removing the diodes - this renders the 2nd op-amp and 1/2 of the ganged distortion control removed from the circuit (or at least not influencing the circuit). Is that right?
What op-amps? There are no op-amps in this schematic.
I see that the optos are doing two functions: 1) P101 gives a little bit of feedback to the cathode bias on the first half of V1, and 2) P102 is controlling the amount of signal that is fed back from the clipping diodes D1 and D2.
Remove P101 and P102 and that first stage will be much more like a traditional gain stage. Still possible to overdrive the preamp, expecially by getting that extra bit of gain from the 12AX7 sub, but the "fuzz" clipping diodes will be gone.
Sorry, been a long day - I meant to type optos.
Thank you very much for tips.
A while back this amp came to me for an assessment. The only significant 'not working' component was the reverb. The PCB's were disgusting, and I thought the dirt may be shorting some traces together. Using a contact cleaner soaked toothbrush I tried to wash the boards. I did manage to restore the reverb but was never able to lift off more than 10% of the old grime. Any tips?
Here they are after the most recent attempt...I should have took 'before' pictures, you couldn't even read the component labels;
70 to 90% rubbing alcohol, may still need the brush.
Start cleaning with a shaving brush. They are soft enough not to damage anything and do a good job at getting into corners. These are also great for getting the dust out around the bridge on instruments or for cleaning rack gear.
I use a liquid product from MG Chemicals called Safety Wash. It was designed to clean circuit boards and is a blend of ethyl alcohol, isopropanol and ethyl acetate. It cleans but doesn't remove lettering or color bars on components. I use it with Q-Tips and someone else's soft toothbrush. This is a little stronger than just isopropyl alcohol (called IPA). IPA is cheaper and you can get it at a drug store.
For some reason I tried contact cleaner, and that would leave a white residue (though the bottle claims 'leaves no residue'...), the 99% IPA did the job well.
The rebuild is complete. I'm seeing double right now, so I'll be testing the voltages and sound this weekend. I'll post pics, I've got lots.
I was just curious if I should remove the 0.47 capacitor on the PT primary (between 1 leg of AC and ground. I normally only see these caps in amps that were originally 2-prong, and adding a chassis grounded plug renders the cap useless. In my experience with Ampeg, they're really meticulous about their circuit grounding so I just wanted to ask if this cap is necessary and/or posing a potential threat and should be removed.
Also, fyi, I noted the electrolytic cap drift values:
30uf - 37
40uf - 50
40/100 - 53/138
40/100 - 50/130
40/40/40 - all to 56uf
Plate supply 541vdc (old 550V)
Screen 353vdc (old 357)
Grid -32 (old -32)
It sounds way better, I think. The massive static when switching on/off standby is now gone, overall idle hum/hiss is lower (yet still present like all amps). The pots cleaned up nice and the reverb sounds very rich, also not as noisy as before when cranked all the way up. Haven't really been able to crank it up, but the distortion control is about 10000% better. Even if it isn't the best effect, its still nice to see more volume as opposed to less when you turn it up! Most people would think it was broken.
Some old components looking sad:
Beginning the swap. I replaced both rectifier's diodes with parallel 1N4007s (1000v/1a), the bias supply got a RL205 (600v/2a)
Changed out all the colour-coded capacitors and plate resistors:
Well, its been fun! Thanks again for the help!
This thread will very likely be of assistance to me when I pick up that VT-22 on Monday. :)
If it tests good you can keep it but most people remove the cap on the primary of the PS because it could fail, short, and put the line voltage onto your chassis.
If you want to replace it, look for a Type Y2 safety cap. They are designed for this application. If they self destruct, they do so without shorting. This will allow you to keep your ground switch functional.
This amp doesn't have a polarity or ground switch.
I popped it open again just before it was to be picked up and snipped out the cap. Better safe than sorry.
This amp sounds so good now, almost like it has confidence behind its tone? not sure how to explain it. It reminded me of my SVT (re-capped 73 blackline) - for guitar testing purposes, the V4 resembled it in that solid tone and not a hint of wavering. Maybe my ears are tricking me.
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