Hi all,
Just a quick question regarding cathode bias voltage and, in particular, stability.
After catastrophic failure caused by an overused output tube (runaway cathode voltage). I rebuilt the amp and installed meters to indicate what the cathode bias voltage was on the KT66's (as pictured).
Generally, they do vary slightly (albeit always 1v apart) but occasionally, the left channel (blue) will skip very quickly between 38v - 39.5v. It sort of cycles from 38v then jumps quickly to 39.5v then back down, then back up. This happens over a few minutes and all the time the right channel (red readout) is stable. After a few minutes, it becomes stable too.
Happened today shortly after switch on, yesterday after about 2hrs use.
Schematic attached.
Just a quick question regarding cathode bias voltage and, in particular, stability.
After catastrophic failure caused by an overused output tube (runaway cathode voltage). I rebuilt the amp and installed meters to indicate what the cathode bias voltage was on the KT66's (as pictured).
Generally, they do vary slightly (albeit always 1v apart) but occasionally, the left channel (blue) will skip very quickly between 38v - 39.5v. It sort of cycles from 38v then jumps quickly to 39.5v then back down, then back up. This happens over a few minutes and all the time the right channel (red readout) is stable. After a few minutes, it becomes stable too.
Happened today shortly after switch on, yesterday after about 2hrs use.
Schematic attached.
Attachments
No actual question was stated, but if these are new tubes, just burn them in at least 100 hours before more bias checking.
If they are well used tubes, and vary somewhat similarly, it could be your AC line.
If you really want to isolate the problem, ground the output tube grids and observe the bias current.
If problem now is gone, there is LF crud from the first stage causing the erratic behavior, which
indicates better supply filtering is needed, or possibly the feedback resistor is the culprit.
If they are well used tubes, and vary somewhat similarly, it could be your AC line.
If you really want to isolate the problem, ground the output tube grids and observe the bias current.
If problem now is gone, there is LF crud from the first stage causing the erratic behavior, which
indicates better supply filtering is needed, or possibly the feedback resistor is the culprit.
OK, well the question was whether the varying cathode voltage was normal, especially as it only affects the one channel.
Tubes, at a considered guess, would have over 100hrs but not by a great margin,
AC line could be ruled out as on 1 channel is affected.
I did however, yesterday, install another 6n1p. After a couple of hours, one channel skipped voltage as described in 1st post. Then, before switch on today, I changed the 6n1p back to what I was using originally. The voltage skip occured again at switch on, but went after a few minutes and has not returned.
Therefore, as you say, LF crud from 1st stage (or specifically an old 6n1p)
Thanks.
Tubes, at a considered guess, would have over 100hrs but not by a great margin,
AC line could be ruled out as on 1 channel is affected.
I did however, yesterday, install another 6n1p. After a couple of hours, one channel skipped voltage as described in 1st post. Then, before switch on today, I changed the 6n1p back to what I was using originally. The voltage skip occured again at switch on, but went after a few minutes and has not returned.
Therefore, as you say, LF crud from 1st stage (or specifically an old 6n1p)
Thanks.
OK, so another night of stability with the previous 6n1p which, as rayma suggests, indicates a bad tube that caused the fluctuations.
I'm glad I put these volt meters on the output tube cathodes. Visual indicators that something is wrong, not only with the output tubes but also the driver.
Thanks for the advice
I'm glad I put these volt meters on the output tube cathodes. Visual indicators that something is wrong, not only with the output tubes but also the driver.
Thanks for the advice