Ampeg V4 eating tube s

[melvin]

I think a good mod would be to put in an auxiliary transformer just for a decent bias supply. Also, putting a HT fuse in might be good.

[Chuck H]

You could make changes within the limitations of the PCB that would change the bias supply to a more typical type and move the standby switch to the power tube HV rail. I didn't trouble to work out the voltage divider for the bias supply.

[melvin]

I like the idea, but I wonder if you'll end up with too much voltage to drop to make it a good source for current.

[Diablo]

Can someone explain why they think there is a problem with the design of the bias supply in the V4? Do you measure a bias voltage with the amp in standby?

[Chuck H]

Well I think that comes down to the voltage divider values. Keep 'em low enough and the overall resistance/impedance of the circuit should be fine. The stock circuit has a load of 75k but the impedance is quite low because the decoupling cap hangs right off the DC source. I doubt you'll draw much DC current through that cap . AFAIK the actual voltage isn't the source from which the current is required, It's the resistance of the circuit affecting the time constant for DC. Sometimes it seems like resistance and impedance are interchanged inapropriately in these discussions. But I'm no tech and my intrinsic understanding could be wrong.

[km6xz]

I ran into the bias stability problem years ago when we were doing a lot of Ampeg's and offered a mod that used couple caps for coupling into a 1/2 wave rectifier, and filter followed by a LM7924 with the ground reference raised to 33 volts giving a stable -57 from which bias was derived. A higher voltage can be set if a bias adjustable divider is deired. 6L6's of any stripe are not as tough in the grid so any grid current pulled can damage them, when the original 7027's handled it with ease. The Svetlana(real one's not the Sovtek knockoffs) with the Wing-C logo should handle a little grid current also, and 585V anode potential is not a problem at all.
A really proper way to tame this bias design problem is to mount a suitable small transformer. Depending on a cap reactance to stay consistent for bias is only suitable for small consumer hi-fi's and AB TV sets from the 50's and 60's. No quality units used it. A difference in mains voltage, or more, line frequency such as when using a UK/European version in the US means all wrong biasing. Heat, aging etc all changed the bias level. Even in the cheapie line operated hi-fi's, some type of regulation was used.

[Diablo]

Perhaps this has nothing to do with the intermittent bias, but the way I understand the circuit is that when the amp is in standby, the return path for the bias current has to go through the two 100K-2W bleed / balance resistors on the main filter cap. That's the only ground connection I see when the amp is in standby. Perhaps someone removed those resistors or changed the value much higher so there is no bias in standby. When you flip the standby off, you have a moment with no bias on the power tubes.

[melvin]

This thread is the most interesting discussion I've ever come across on the V4.

[Chuck H]

I've only ever worked on one. That was enoug But it's kinda funny that most guy's have worked on at least one. I've read here that there are shops that won't work on them for some reason. Actually it was not that hard to work on, but what a brute of an amp.

[Ronsonic]

I've only ever worked on one. That was enoug But it's kinda funny that most guy's have worked on at least one. I've read here that there are shops that won't work on them for some reason. Actually it was not that hard to work on, but what a brute of an amp.

That bias problem can be extraordinarily frustrating. The cost of parts and labor for a recap is on a whole 'nother level from say a Fender or Marshall and involves some parts shopping, wrestling the beast around a bench is something a lot of guys don't want to do. Add all that up and it means more of them for me to own and repair.

The modern "oh no, I can't use an amp that loud and heavy" mindset means these things are a bit out of fashion anyway. What's the point if you can't let it roar. The V4 makes a brilliant bass amp and that's how most of them seem to be used these days.

[Gtr_tech]

I've worked on a few of them and did end up changing the bias supply a bit, but kept the cap "tap" in place. The biggest problem these had were the cheap sockets in the front end and cracked solder everywhere.....

[Chuck H]

Since I have guys here that either own several or have worked on a few, I'd like to ask a Q. Not hijacking though. I did a recap on one (yes the parts had to be found but it wasn't THAT bad. Expensive though). It also had a faulty switch. The EQ switch that affects specific frequencies. I cleaned it as best I could and it was working when I returned the amp but in case it comes up later... Does anyone know where to find a suitable replacement for that switch?

[pontiacpete]

is this it?

Fliptops

[Chuck H]

The white rocker for 25 bones looks right. IIRC the switch on this amp was black and selected from a couple of different cap and inductor circuits. Not certain if the switch had four contacts or three. Fliptops is a good source for this stuff, but real pricey. I bought my caps there for the recap I did. US $129.00... FOR CAPS!?! But the the price was rolled into the service so it's a blessing that Fliptops has this stuff. I looked there for the switch before but may have missed it. Thanks for the link.

[Prairie Dawg]

I'm finishing up one, basically going through it and renovating potential problem areas. Going to power it up tomorrow and start fine tuning it, and a customer has a newly acquired one he wants me to look over and bless with the holy water...