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24.27.41.140
In Reply to: RE: Swapped Output Tubes posted by JKT on June 17, 2024 at 13:46:16
"Now the left channel is running higher at 46mA and the right lower at 42mA. Not the 55mA 35mA difference there was before. Weird."
I would swap them back to see if it repeats the original difference. Could be an intermittent socket or a small oscillation. Do you have grid and screen stoppers installed at the outputs? Separate cathode resistors for measuring individual cathode currents?
Follow Ups:
I thought there might have been intermittent oscillation in that channel initially. Ran some tests with the tubes swapped and measured 30W at clipping in triode mode with 0.8V RMS input. The Eico reached full output at 0.5V RMS input. Makes sense since the Eico input stage had a gain of ~70 and my input stage has a gain of ~50. It's configured as a half mu stage (see my post below in the SRPP question). May consider a tube with a lower mu than an 12AX7. Maybe a 5751. Waiting for the tubes to cool down to swap them back.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Swapped them back again. Same as before: 55mA and 43mA.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
"Swapped them back again. Same as before: 55mA and 43mA."
You didn't say anything about stoppers. A 1K carbon comp next to each grid and 150Ω to 220Ω at each screen will help quench HF oscillation. A small 1Ω or 10Ω resistor at each cathode will provide a monitoring point for bias and also for viewing with a scope. Any oscillation should be visible there.
I replaced the output tubes, which were well used, with a fresh quad and they are behaving well. There are 1R resistors at the cathodes for monitoring the bias, 1K grid stoppers and 100R screen resistors. Haven't seen any HF oscillation on scope.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
So... maybe the tubes were gassy (or something else internally wrong), and that was causing a difference simply due to slightly different layout between the two channels. Glad the new tubes fixed it!
I think if you lookout the picture you will see why the used set of tubes were more sensitive to small changes. ;~D
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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