FirstTimer here so please be gentle on Tech Terms.
Just finished wiring a FenderBassman style 6L6 PP with UL OPT and 12ax7a Long-tail PI. Problem is pin3 of bluelead primary with tubes out, measures 420v (same as other three) when I put the tubes in to setup Bias, pin3 of bluelead is unstable reading as high as 900v and 113ma. Swapped Tubes but still same problem. Is my OPT shot or is it something else. Wired circuit same as F.Bassman AA864
Any Ideas
Just finished wiring a FenderBassman style 6L6 PP with UL OPT and 12ax7a Long-tail PI. Problem is pin3 of bluelead primary with tubes out, measures 420v (same as other three) when I put the tubes in to setup Bias, pin3 of bluelead is unstable reading as high as 900v and 113ma. Swapped Tubes but still same problem. Is my OPT shot or is it something else. Wired circuit same as F.Bassman AA864
Any Ideas
Are you running it without a load? If you are, then suspect oscillation. OPT voltages can soar like that if there's a signal, but no load on the secondary. You do not want to run hollow state amps with OPTs without a load. The overvoltage can poof finals as well as OPTs.
OPT primary leads swapped resulting in very low frequency oscillation?
Never run without a load on the secondary of an OPT as this is a good way to have an insulation busting internal arc-over due to flyback effect that will destroy the transformer.
Never run without a load on the secondary of an OPT as this is a good way to have an insulation busting internal arc-over due to flyback effect that will destroy the transformer.
Spidy -- With the PI tube removed, see if the same problem still exists. If it does, you likely have your UL wires crossed (or the primary wires of the OPT in some way mis-wired), so that the output stage is becoming an oscillator when both output tubes are installed. That will cause the wild fluctuations you are getting, and also prove that the problem originates in the output stage. If it happens only with the PI tube installed, then it is likely some sort of wiring error or lead dress issue causing the problem in and around the PI stage.
If it only happens with the PI tube installed, and ANY combination of output tubes installed (one, the other, or both), then the phase of the NFB has likely become reversed due to a wiring error, or from using an OPT transformer with a different color code scheme to indicate phase. If that is the case, simply reversing the OPT primary connections from one output tube to the other will resolve the issue. Good luck with it!
Dave
Dave
If it only happens with the PI tube installed, and ANY combination of output tubes installed (one, the other, or both), then the phase of the NFB has likely become reversed due to a wiring error, or from using an OPT transformer with a different color code scheme to indicate phase. If that is the case, simply reversing the OPT primary connections from one output tube to the other will resolve the issue. Good luck with it!
Dave
Dave
Thanks Dave
Reversed OPT leads and it seemed to solve the voltage on Pin3 420v. But now the current bias on the new BlueLead Tube jumps as high as 110ma. So I disconnected NFB but output is low on full volume. when i jumper the NFB from either secondary's back to 820R the current fluctuates up over 100ma.
This was built out of junk and Found parts so lead colours and ratings are questionable. My concern is that OPT secondarys have three colours Black/ Green/ Yellow and resistance from Blk-Gre is 1 ohm and from Blk/ yellow is 0.8 ohm; does this mean it requires a 1 ohm load where would i connect NFB for 8 ohm.
I calculated the Impedance Of OPT at 4.7K on a 4ohm load on Blk/Green secondary.
Probably sounds like i shouldn't be touching amps but I'm a really fast learner. (decided to build this two months ago)
I'll go back in and check my leads and placements.
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