Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on June 18, 2022, 09:13:47 pm
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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
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That amp is a beast! Sounds like your friend rides it hard too.
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My friend is not as deaf as me though...
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All 4 x Rg2 blown? Check the bias supply is working at the grid pins
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Yep the 4 blown. Thanks for the hint
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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
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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.)
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Memory fades. Ampeg stuffed 7027.
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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.
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They are JJ 's 7027 . I ll replace the g2 resistors and put it on test for a while.
Thanx guys
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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 ?
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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 ?
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Disregard , last issue fixed
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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.