Originally Posted by X–Ray
Originally Posted by bitsleftover
(in a color a bit easier to read than black ;~)
I need help.
Ive done all the mods apart from the Cathode Bias Resistor (R27) and the output board wire.
Also I have fitted the Hall Amplification VVR board and applied it to the Power section only. The B+ voltages seem to work and adjust fine.
I fitted a master volume (as Halls recommend) and followed Lyles advice a few posts back by removing R24 and R26 and installing a 1meg pot as a voltage divider in their place.
All other mods are as per the schematic.
The problem I have now is this.
When I hit a chord with the amp turned up loud, ie, Vol 10 Master 10 and VVR 10 it sounds great for a second or so, but as the chord begins to decay slightly I get the horrible loud pulsing Cha-cha-cha-cha-cha-cha which overpowers the guitar sound. It sounds kind of like a very distorted pulsed crackle at about one per second or sometimes very fast ie 10 per second.
The longer the amp has been powered up the better it gets and after an hour or so it sometimes stops altogether.
It seems to start at anywhere above 8 on the vvr control and anywhere above 7-8 on the master volume. By which I mean if either one is on full the other can only go up to about 7or8 and seems to be very interactive between the 2.
I could understand it if it was the other way around ie having to turn the Master Vol down a bit when the VVR was turned down but with the vvr maxed it should be as if it isn’t even fitted.
What am I doing wrong?
Am I over-overdriving my power amp? This is not a musical type of distortion in any way, more a kind of “something is F*@#&d” distortion!
With R24 and R26 replaced by my 1meg pot, when I have it up to 10 am I over-overdriving my power amp? would I be better re installing R24 and R26 and fitting the pot between these resistors so that the Gain level of that stage will never exceed the value you have set with your mods?
Or do I have another problem?
Wierdly enough, I also did the headphone trick, (the stereo jack plug on the speaker cable plugged into the headphone socket) and in headphone mode the problem disappears so its good for quiet practice.
What can I do? Please help someone. As it is Im getting the piss taken out of me by the guys for messing with forces I dont fully understand and destroying my new amp!
Have you put a scope on the B+ to see what's going on there? If you haven't access to a scope, you might be able to see the pulsing with a fast reading DVM—an analogue meter would be better, in this case. Seeing what the B+ is doing would offer a clue.
Does the oscillation rate change with control settings? Does it speed up or slow down as you reduce the MV setting? It could be common motorboating, because of an error in decoupling the B+ for the preamp valves.
You should be running the regulated B+ to the PT, and a separate, non-regulated HT line to R4. Is there a diode on the unregulated B+ out? (I don't have a schematic for the Hall board, so I'm shooting in the dark.)
If I recall correctly, Lyle's mods for R24/R26 set them at 470k/1M, which reduces the signal at V3's grid by about 0.7. If you're going to use a 1M MV pot, you should leave R24 in the circuit, as running the full tilt signal out of V2 into the EL84 is overkill. You can put R24 back in the circuit easily enough by wiring it in series with the top lug of the pot. That would be the first thing I'd try, just to get another data point. It will likely change the symptoms, but probably won't cure it. I think power supply decoupling is the more likely suspect.