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Old 19th December 2009, 11:46 PM   #1
sphinx is offline sphinx  Canada
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Question bizarre bias, ST-70 experts step inside

Just bought a bone stock crusty ST-70. One side of the amp wasn't working because of a colder solder joint. I fix that and tried to bias the amp. Right side, perfect, no problems. Left side, bias flies up to 2.76V across the 15.6Ω resistor and will increase if I turn the pot CW.

Bad pot? no...at the min bias pot position, the voltage is -39V, same as the other side at min bias.

If I remove the 7199, I can set the bias normally

Bad 7199? no, swapped from other side , no change.

Bad EL34s? no, swapped from other side , no change.

Bad 0.1uF caps leaking DC? no, swapped from other side, no change.

There is some ripple on the cathode and anode of the 7199 triode but the magnitude is about the same as the other side.

How can the 7199 be influencing the output stage bias if it's only AC coupled to the EL-34s?
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Old 20th December 2009, 12:19 AM   #2
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by sphinx View Post
Just bought a bone stock crusty ST-70. One side of the amp wasn't working because of a colder solder joint. I fix that and tried to bias the amp. Right side, perfect, no problems. Left side, bias flies up to 2.76V across the 15.6Ω resistor and will increase if I turn the pot CW.

Bad pot? no...at the min bias pot position, the voltage is -39V, same as the other side at min bias.

If I remove the 7199, I can set the bias normally

Bad 7199? no, swapped from other side , no change.

Bad EL34s? no, swapped from other side , no change.

Bad 0.1uF caps leaking DC? no, swapped from other side, no change.

There is some ripple on the cathode and anode of the 7199 triode but the magnitude is about the same as the other side.

How can the 7199 be influencing the output stage bias if it's only AC coupled to the EL-34s?
How about a bad/open grid bias resistor?

Also note that you really should have put the amp on a variac and brought it up slowly, better still having just ascertained the PA and opt were ok - rebuild the amplifier without having applied power at all.

You've been fairly lucky so far - I wouldn't push your luck too much further.. You need to replace the quad cap, the bias rectifier and filter caps, the coupling caps and check all resistors. It's a wonder your amp didn't immolate itself upon application of power..
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Old 20th December 2009, 02:06 AM   #3
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Originally Posted by sphinx View Post
How can the 7199 be influencing the output stage bias if it's only AC coupled to the EL-34s?
Maybe that side is going into oscillation, and finals are drawing lots of current as they are driven to "eleven"?
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Old 20th December 2009, 08:04 AM   #4
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Even if swapping the coupling caps did not indicate a problem it is still worth replacing them. Quite likely they are leaky or about to become.
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Old 20th December 2009, 12:55 PM   #5
john65b is offline john65b  United States
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I had a modded (VT70) ST-70 that had one tube red plating. It would not hold bias...

Turned out to be a bad EL34. But was scratching my head for a while until I followed the directions - first was to check the input section without output tubes and with Rectifier.

If all good, add then one side output pair at a time with input tubes, rectifier and speakers attached and bias pot turned down all the way . You should now be able to bias up to bias point. Then do other pair. You will have to go back and forth on both sides a couple of times since they will drift from other side being dialed in.

I did not have the speakers attached when trying to bias, so my bias was drifting all over the place....maybe 4 or 8 ohm load resistors (high wattage) could be used?

Given this is an old amp, bring up on a variac would be best...
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Last edited by john65b; 20th December 2009 at 12:58 PM.
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Old 20th December 2009, 05:37 PM   #6
sphinx is offline sphinx  Canada
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Decided to probe the voltage across the bias resistor with my oscilloscope. Found that it was oscillating. The waveform wasn't symmetric so my DMM read it as 2.76Vdc. I removed the feedback cap and resistor, oscillation gone - but those parts were both ok. I checked the grounds to the 7199 and found that the trace from the 18K grid resistor was cracked and open. I shorted it with a wire and the amp now works perfectly!
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