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#1 2012-03-28 13:18:11

Tim
New member
Registered: 2012-03-28
Posts: 3

VT-22 Low Bias

I've got a 70 VT-22 I've been working on, and it's eating my lunch. Here are the issues.  I have replaced all the tubes and electrolytics, and most of the Bias circuit.  A, B, C, D, and E all read about 8-10 volts high with no power tubes inserted, which I think is probably normal. With the power tubes in, the bias is only about -30 volts, and the tubes will redplate if I leave it on. I also get a buzz when I power on (before I take off standby). Originally C15 had fried.  I have replaced C11, C12, C13, C15, and C16, as well as R49 and R50. I subbed another diode in D6, but I can't find what the correct diode should be anywhere.  Does anyone know the numbers for the 6 diodes in an early VT-22?  That would be D1-D4 (rectifier), D5, and D6. My schematic just shows 5 F-10s and an F-8. I've pulled and checked all the resisters in the power amp circuit. Any ideas?

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#2 2012-03-28 17:56:45

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: VT-22 Low Bias

Have you checked the tube sockets for signs of arcing? 

Or try lifting each R35 and r36 (of course keep the power tubes out)
You may find that lifting one causes the bias to jump down.

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#3 2012-03-28 18:48:06

Tim
New member
Registered: 2012-03-28
Posts: 3

Re: VT-22 Low Bias

Thanx Hangman!  That didn't fix it, but I was at my wits end and needed some moral support, I guess.  I swapped the D6 diode with another random diode and it seems to be working. I hate to leave an unknown component in there, do you happen to know what the two types of diodes in this amp are?

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#4 2012-03-29 02:09:27

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: VT-22 Low Bias

You can use 1n4007.   I have used them in the past with no problems.

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#5 2012-03-29 02:24:31

Tim
New member
Registered: 2012-03-28
Posts: 3

Re: VT-22 Low Bias

You da man!

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