Strange soft-clipping artifact

dtbradio

New Member
Jun 11, 2023
16
5
3
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I found a working 5U4 in my random tube box and connected it up. I'm getting 390 volts with just bleeder loading, and with the two 6BQ5's in and biased at about 7 watts each I'm seeing about 369 volts. Bias current is sitting at about 19mA per tube, using separate bias controls for each tube to compensate for some tube mismatch. With a 1KHz sine wave coming into the PI, I can get about 15 watts RMS into a 50-ohm load with some crossover and saturation distortion. If I back down the input to fairly clean sine wave, its about 12 watts RMS. I did change the .1 power tube coupling caps to .047uf, but left the PI input caps at .1uf. I ran a heat and current test by injecting a 1KHz sine wave at a level which gave the highest plate current draw and running the amp at that level for 2 minutes. The tube temperature never went above 370 degrees F, and the current draw held steady with no surging. After removing the input and allowing the amp to come back down to idle current/temperature, the envelopes were holding steady at about 369 degrees F. See thermal images below. The "Max:" reading near the top left of the image is the envelope temperature.


Max load temperature:

max-load-6BQ5.jpg


Idle load temperature:

idle-load-6BQ5.jpg

The amp has been powered up with different input levels for roughly an hour and no misbehavior whatsoever has been observed. I think I can now move on to adding some pre-amp stages.
 

sparkie

Member
Dec 7, 2023
68
16
10
Rapid City, South Dakota
I found a working 5U4 in my random tube box and connected it up. I'm getting 390 volts with just bleeder loading, and with the two 6BQ5's in and biased at about 7 watts each I'm seeing about 369 volts. Bias current is sitting at about 19mA per tube, using separate bias controls for each tube to compensate for some tube mismatch. With a 1KHz sine wave coming into the PI, I can get about 15 watts RMS into a 50-ohm load with some crossover and saturation distortion. If I back down the input to fairly clean sine wave, its about 12 watts RMS. I did change the .1 power tube coupling caps to .047uf, but left the PI input caps at .1uf. I ran a heat and current test by injecting a 1KHz sine wave at a level which gave the highest plate current draw and running the amp at that level for 2 minutes. The tube temperature never went above 370 degrees F, and the current draw held steady with no surging. After removing the input and allowing the amp to come back down to idle current/temperature, the envelopes were holding steady at about 369 degrees F. See thermal images below. The "Max:" reading near the top left of the image is the envelope temperature.
close, you need to drop another 60 volts so your voltage is 330V with bleeder.
 

dtbradio

New Member
Jun 11, 2023
16
5
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close, you need to drop another 60 volts so your voltage is 330V with bleeder.

Not trying to argue, but the Baldwin 46C organ that this amp unit came out of was running full voltage from its 5U4-based supply (which I am now using exactly as it was wired in its original configuration) up to the eight original 6BQ5's. The unit was built in the 1950's and the power amp section was still working when I stripped it down earlier this year. I do understand that the BQ specs call for lower max voltage, but if the original organ amp ran fine in its original configuration (with the higher voltages) for roughly 70 years, why wouldn't it still do so today? As always, I'm trying to learn things.
 

sparkie

Member
Dec 7, 2023
68
16
10
Rapid City, South Dakota
Not trying to argue, but the Baldwin 46C organ that this amp unit came out of was running full voltage from its 5U4-based supply (which I am now using exactly as it was wired in its original configuration) up to the eight original 6BQ5's.
And how much resistance was in the cathode circuit originally? Because one tube didn't drop all that voltage either.
Also, the impedance of the transformer primary would be smaller with 8 tubes that it would be with two tubes. So to drive the transformer properly with two tubes, you would drop B+ for linear audio operation.
 

dtbradio

New Member
Jun 11, 2023
16
5
3
53
And how much resistance was in the cathode circuit originally? Because one tube didn't drop all that voltage either.
Also, the impedance of the transformer primary would be smaller with 8 tubes that it would be with two tubes. So to drive the transformer properly with two tubes, you would drop B+ for linear audio operation.

Ok, thank you! That makes 100 sense. Always learning!
 
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