70s Ampeg V4B Blows Fuses

[ca7922303]

I was unable to see the tubes light up until I hit 100v on variac is reason I went to 100v.

[Enzo]

What I am seeing is that it burns something up and even blows fuses. SO whether or not it makes sound is irrelevant, at least at this point. We need to get the amp stable at DC voltages. SO that is why I suggested removing the tubes, and turning the standby switch ON, meaning the position like you would be playing. That means all the circuits are fully on. In that condition, does it still burn up or blow fuses?

If the amp sits there burning something and blowing fuses with NO tubes in it, what is the point of installing them?

I earlier suggested you add a light bulb limiter to your setup, that way even if you still over crank the variac, the bulb will prevent th further damage that occurs every time you smoke something.

And instead of edging that variac up and watching it burn, PLEASE add a AC current meter in the mains circuit. If you start turning up the variac and see the amp already drawing 2 or 3 amps when the voltage is still only 20v or something, then you know to stop and back off.

[ca7922303]

Ok, with 2-5881s out, there is no smoke or smell of smoke and no hum with amp fully on. Also, I notice now both power lights are on.

[ca7922303]

Amp has not blown 6 amp fuse for me. Guy I got it from claimed it was blowing fuses.

[ca7922303]

Looks like all 4-pin#8s(according to the bottom of a 5881) so continuity to ground on power tube sockets. I guess 7027s or V4 schematic is different.

[ca7922303]

Shows continuity*

[g1]

If those are the Russian style '5881/6L6' tubes, they will work fine here. Not sure what you mean about the pin 8's, but you should not have to worry about it. Any regular 6L6 variant will work ok with the amp wired up stock. Real NOS 5881 or the authentic re-issues will not be able to handle the high voltages.

It is powering up without the power tubes? Then something power tube related was probably causing the fuse to blow. Check for proper DC voltages at power tube sockets before you put them back in. Especially pin5, which needs to be somewhere in the --60VDC range.
Whatever you smelt around the 6K11 probably got burnt and will have to be dealt with as well.

[ca7922303]

All four(4) pin#3&4 read 480v on 500v range. All four(4) pin#5s read -364mv on 2000mv range.

[vintagekiki]

All four(4) pin#5s read -364mv on 2000mv range.

What did you measure on pin5 ???
Pin 5 is g1

See post
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/928158-70s-ampeg-v4b-blows-fuses?p=928217

[ca7922303]

-364mv on 2000mv range

[Helmholtz]

Not much sense measuring grid voltage with a cheap DMM, as the reading will depend on meter resistance.
Bias should be measured at the negative terminal of C23 instead.

[vintagekiki]

-364mv on 2000mv range

Pin 5 is g1
You no negative bias voltage.

[ca7922303]

C23 reads -454mv on 2000mv scale.

[Helmholtz]

C23 reads -454mv on 2000mv scale.

If that's DCV, you have a bias supply problem. Acc .to schematic you should have -64VDC here. Low bias voltage causes excessive power tube currents.
Check all bias supply components.

[g1]

Like the others said, you are missing the bias voltage. There should be AC voltage on both sides of C16, which sometimes explodes in these amplifiers.