Ampeg V4B 1971

[Colas LeGrippa]

Hola amigos,


I can't stay calm here and only shut up and play my guitar. A friend of mine came in this morning with an Ampeg V4B amplifier from 1971.
I just opened it up and checking the voltages I saw the 4 screen grid voltages were 0 V due to the 4 screen resistors opened. The schematics show 470R @ 2W but in there I discovered 4 X 1k @ 5W. It seems to me that there is a problem that makes bust.open the screen resistors, because the original ones  has been already replaced by stronger ones. I  want to solve that problem permanently . I get  good screen voltage just before the resistors and 0 V after.
My ohmmeter shows the 4 resistors open.


Thanks for advice

[sluckey]

That amp is a beast! Sounds like your friend rides it hard too.

[Colas LeGrippa]

My friend is not as deaf as me though...

[tubeswell]

All 4 x Rg2 blown? Check the bias supply is working at the grid pins

[Colas LeGrippa]

Yep the 4 blown. Thanks for the hint

[Colas LeGrippa]

Minus 64 V on the 4 control grids


Never seen 4 screen grid resistors blown at the same time. There ought to be something else I can't figure out

[PRR]

EL34/6CA7, or 6550/KT88?

These amps were designed for 6550. These are expensive. Someone put EL34 in and it worked for weeks. That's what the factory mostly put in. 'My' VT40 had a note to use 6550 "for professional use".

When 'my' VT40 blew-up, it smoked the power transformer, and I think it started with melting screen grid.

I would argue for the best 6550-types you can get today, and go back to the stock G2 resistors value and power. However it won't be me paying for this. (What I did on 'mine' was a low-voltage PT for a reliable 30W with any large octals instead of a TOO DANG LOUD and unreliable 60W.)

[PRR]

Memory fades. Ampeg stuffed 7027.

[owen]

yeah what power tubes are in there? I put JJ “7027” in one of my ampeg v4 and it ate two of those resistors even after a tech confirmed proper bias.  he stated something was drawing excess current but it’s been a while.

bought a quad of NOS 7027A and didn’t have an issue.

I have seen a lot of threads following ampeg repairs that replace those resistors with cement 5w 1k.

[Colas LeGrippa]

They are JJ  's 7027 .  I ll  replace the g2 resistors and put it on test for a while.
Thanx guys

[Colas LeGrippa]

The only thing I can see, after a night with Linda,  is the tube sockets.  A power tube trying to draw all the needed current through a bad connection would definitely put a stress on the grid resistors. What do you think ?

[Colas LeGrippa]

Replaced all 4 sockets of the power tubes. Replaced the 4 x 1k g2 resistors. Issue solved
Channel 1 : very loud


Other issue: channel 2 very weak even with vol at max


Any idea ?

[Colas LeGrippa]

Disregard ,  last issue fixed

[owen]

Disregard ,  last issue fixed

Heck yeah! good job! My v4 was the first amp I ever worked on, and I knew NOTHING then (and next to nothing now). so I just took my time to troubleshoot. I never replaced sockets but that sounds a lot like what was plaguing mine.

I just adore the eq in this amp. nothing tops it for my needs.