Ampeg V-4 arcing?

[themusic]

I recently came upon an Ampeg V-4 . Caps had leaked badly. Clean chassis and installed a new cap kit
from Fliptops and also new tubes. Brought up everything nice a slow with my variac.
Ran well for about 15 min. then noise started to build till all power tubes started to arc. (I don't know if that's the right term)
Shut it down... waited 10min ...Turn on and this time started within a minute same thing.
I look over solder joints and chopsticked everything before I had to shut it down. Should I
pull all the all the 7027A power tubes and start with the meter ? Thanks

[nashvillebill]

Did you confirm you have the -62 volts on the bias circuit?

edit: this is for my original version of the V4, other later models varied considerably...

[g1]

I think you may mean red-plating, where the plates of the tubes turn red? This is due to loss of bias. Yes you should remove the power tubes and start with the meter. You want to make sure you are getting the proper negative voltage on Pin 5 of the power tubes.
You may want to use the variac set a little lower than 120V. With the power tubes removed the B+ voltage will increase and can stress your new caps, depending on their voltage rating. Running with the variac a bit low will help avoid this problem.

[Jecarroll7]

I doubt this is the problem, but It wouldn't hurt to confirm all the new caps are installed correctly. I reversed the leads on a filter cap in a blues junior once, seemed to me it took about 10 min before the amp started acting up.

If one is reversed, use a new one to replace it.

[themusic]

thanks...sorry about getting back late to everyone...damn work...
reading -51v on pin 5. also checked cap wiring seems ok. This appears to be original first version V4, no master volume.

[pdf64]

Does the -51V stay solid after 15 minutes?
The new tubes may need more than -51V bias, in which case you may need to amend resistor values in the bias supply.
Are 1 ohm resistors fitted in the power tube cathode returns, or did you use a bias adaptor (or any other method), to check the power tube plate dissipation?
I just noticed the 10 ohm resistors in series with the power tube plates., on this version -
http://schems.com/manu/ampeg/v4poweramp.pdf
The voltage drop across those could be used to check plate current / and then its dissipation.
Pete

[nashvillebill]

Mine is the very first version of the V4, no master volume, with the toggle switches for power and standby (not rocker). My schematic, which was glued to the lid so I'm reasonably sure it's applicable, does not show what the voltage is on Pin 5 but it does show -62V at the junction of C23 and R60, so that would be a good place to start if it were mine. As pdf64 asks, does that stay solid?

[themusic]

about 2 min in my -51volts is all over the place...thanks

[pdf64]

Maybe a bias supply cap has been fitted in the wrong polarity, or is just plain bad?
Pete

[nashvillebill]

That schematic that pdf64 posted was the Piazzo redrawn version, which does not show voltages and which may contain the occasional error, and it may a later revision...

This schematic I've attached is the Ampeg schematic from the lid of my V4. It shows the C23/R60 junction I referred to earlier, with the -62 volts shown there, and that's the reference voltage to start with IMHO. It must be stable as well as correct. If not stable then as pdf64 said, check the bias cap polarity, solder joints, and the related caps and resistors in the bias voltage divider network.

[themusic]

I took some pics and I'm in the process of checking everything once again...thanks

[audiopete]

Bias cap is on the circuit board IIRC. Why is that cap hanging off there if you installed a cap kit from fliptops? That is a prime opportunity for it to vibrate loose and blow the amp up one day....

[themusic]

Finally back at this ... had a bad accident but getting back at it.
I just hadn't glued the cap back on the board yet. The original cap was a sprague 100mf x 350v.
The replacement cap in the photo is a 47uf x 500v. I checked every new cap connection "I took photos before I replaced the new ones"
and everything seems ok I just can't get that bias to settle ? Thanks
P.S. The accident wasn't because of the amp !!!

[g1]

What kind of variation are you seeing at the point you are measuring? The amp will need to sit idling for a few minutes to stabilize.
If you are getting significant changes, check if it is the same on either side of R60, or if it is stable on one side.

[themusic]

Took some time to double check the new caps and all of the board components, cold joints. Replaced D5 , rechecked tubes.
With tubes out after about 3min reading -55.7v both sides of r60 . Replaced power tubes
and within a minute large voltage swings , and also in the tubes till I shut it down. Thanks