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I have a rebuilt Dynaco ST 70 using a VTA mod board and a couple of issues have developed and any help you can give would be greatly appreciated.1) One channel has begun to have a higher level of hiss or distortion than the other.
2) The same channel as described above has an unstable bias after the amp warms up. The bias will not remain stable - it fluctuates from .31 - .55 and never stays stable (i.e. the meter reading never sits at the same number for more than a second) The other channel bias reading sits solid as a rock.
The same channel as describe above had a tube that glowed bright orange and I replaced it with a new one. I thought that the output transformer was going so I got a new one and it has the same issue. These happened previously to me realizing that the bias is fluctuating upon warm up.
Thank you for your time and any advice you can give..
Follow Ups:
Take out your output tubes and measure the negative bias voltage on pin 5 of the tube socket. You should read a negative DC voltage like -25 or something. Compare this voltage in both amp channels. I would suspect that you may have a bad bias adjustment potentiometer, a bad solder joint, a bad connection from the tube socket to the tube pins. A leaky cap will often just run up a positive DC voltage and smoke your tubes. Pins 2 and 7 are the heater pins of the output tubes. Measure these in AC. Your DC voltage measurements on these pins is meaningless.
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Long Live Dr.Gizmo
Thank you very much sir for the good explanation. I did as you said and each channel is reading at -35 in DC voltage for pin 5. ANy other thoughts?
One question:Are we talking about tubes in the old sockets, or in the new board?
I worked on Fender Bassman amp once, four output tubes. One pot set bias for all four. No matter which tube was in which socket, the one closest to the middle of the amp glowed fairly bright orange after an hour or so of playing, and the next one outboard glowed a little. The other two were always fine. Drove me nuts! Checked every passive component twice (all were within spec), put in a set of matched tubes, rewired the B+ lines to the output tubes, even CLEANED the tube sockets. It got better, but not completely cured. The final fix? New tube sockets, and all was just fine.
Yes, changing tube sockets can be a PITA, but, at least it's not a tube tester! (YIKES).
Good luck,
Additional info:As well as the Bias instability here is another voltage reading that seems odd.
Good channel tube socket pins 2 & 7 = .26 - .27 volts.
Bad channel tube socket pins 2& 7 = .10 - .13 voltsDoes that indicate anything?
B+ = 442 volts
a
Thank you for your response! Could you please let me know what the best way to check for a leaky coupling cap is?
So leaky in this context refers to leakage or passage of Direct Current thru the capacitor. Simply measure for any DC voltage @ the output side of the capacitor to circut Common.
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