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Old 13th April 2006, 04:27 PM   #1
cwerner is offline cwerner  United States
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Default hh scott bias issues

Hey all,
I've been restoring an HH Scott 233 integrated tube amp -- replaced all the "ceracaps" and a few resistors. It sounds great but the problem I'm having is, if I use the cathode current test points to check the bias on the output tubes (7591a's) I can only raise the bias to about 35mA or so per channel, whereas the recommended is about 65-70mA. Any ideas what could be causing this or alternate test methods? The tubes are quite old, but the sound is still good. For reference here's a schematic: http://www.hhscott.com/pdf/fs/233_299C.JPG Thanks.
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Old 13th April 2006, 06:40 PM   #2
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Could be worn output tubes - low emisssion, could be drifted resistors. Voltages check out?
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Old 13th April 2006, 09:20 PM   #3
kevinkr is offline kevinkr  United States
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Did you or someone else replace the bias rectifier with a modern silicon bridge or diodes? If so your bias voltage is now about 10 - 15 % higher than with the old siemens selenium bridge that was probably originally installed. A small amount of additional resistance right at the bridge will fix the problem - you will have to determine it empirically.

At least you have a good schematic to aid in trouble shooting.

Note that the bias supply also powers the filaments in the pre-amplifier section of most of these amplifiers and ran their 12AX7A at about 11V each connected in series, this should give you a clue as to how far off you are.

Another possibility is that R213 has drifted upwards in value from the original 33K which would create just the problem you are having.

Are these original 7591 or the current EH type which might bias slightly differently? In this case R213 is the key.

Note that the higher line voltage may play a role in this as well, however B+ and bias track reasonably well in my experience in these designs.
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Old 14th April 2006, 06:41 AM   #4
cwerner is offline cwerner  United States
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Thanks for the tips, I did replace the old diodes, so I'll have to check that -- as well as the resistor value. I'll report back my findings. The tubes are original (I believe, most of the writing has worn off, but they do say HH Scott on the base) and they seem to show signs of being a bit gassy.
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