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In Reply to: RE: So what have I got wrong this time? posted by vinnie2 on May 24, 2016 at 08:48:37
I finally found the problems, and there were two of them. While trying to trace a signal through the amp with my scope, I did not see any amplification in the signal coming out or the second 26. Upon looking around that circuit for mistakes in wiring, etc, I noticed that the filaments for that tube were not glowing. Checking the connections to the filament transformer I found that one of the clip leads had worked lose and was not making contact. No wonder I could not hear any difference between two 26's and only one. After I go that fixed I cranked it back up and noticed the B+2 for the second 26 was quite a bit lower than the first 26. Tracing through that circuit I found that I had taken the b+2 voltage off the wrong side of the resistor reducing the b+2 on the first 26 and so was getting a much reduced b+2 that I was further reducing with the resistor for the second 26. Got that all straightened out and now everythsing seems to be working as it should. The gain is much improved and it sounds pretty good. Now I will have to go back and plus everything up to the operating points we came up with the first time around, and then maybe I can try incorporating these changes into the real amp. Thanks for all the help folks, it got me looking in the right direction.
Edits: 05/25/16 05/25/16 05/25/16Follow Ups:
IMO/IME, You would be better off going for a DC 45 final circuit and using something like a Triad C56-U as an input choke to get the B+ a bit higher.For the cap coupled final, you will need to drop more voltage off the rectifier somehow for lower B+ for the 45 plate, then decouple for the 26 driving stages.
I have done that before with a big power resistor and cap off the rectifier. Or with a power resistor in series with the primary HOT lead on the PT.
The major issue is the HV secondary of the PT in your circuit.
It is better suited for a DC 45 final.
Do you have another PT with a lower HV secondary voltage?
Edits: 05/25/16
I have a feeling by the time I get both channels of this thing operational the B+ won't be too high anymore.
Right .
A second channel should drop the B+ a good bit.
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