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Hi, I noticed when playing my Dyna ST-35 amp in a dark room that the 2 front power tubes ,one on each channel were not lighting up as bright as the same power tube behind it, on each channel. Even with my hand on the cage as a heat test, the rear power tubes, one on each channel seemed hotter. The amp sounds great and I can't hear any problem. Has anyone ever noticed this and is it something to jump on quick before something fails? It's always been like this for quite a while and I'm finally asking this question because else wise it's been working and sounding perfect.......thanks for any advise......Mark Korda
The ST-35 is cathode self bias with a 'common' single resistor/capacitor attached to the cathode of all four EL-84 tubes.
To my mind that might suggest using matched tubes?
First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass
It may be nothing of any consequence - check my Tube FAQ here at the Asylum. Item #7 may be what is happening in your amp.
Hi, thanks you guys,all great answers and I'll start with the tube switch and also the resistor check. Jim McShane, I just skimmed thru quickly but will study all that info you sent. Thanks again guys...Mark Korda
Hi, I made a dumb mistake and found the problem.I apologize for taking up your time but thank you all for taking the effort to help. My mistake was so evident and stupid I don"t want to waste your time but do want to thank you guys again for jumping in to help, sincerely, Mark Korda....I
I have an ST35 (the basic shoebox) and thru years of use I've found that if it's the stock circuit, it's sensitive to the matching of all four 6BQ5s. This is because the stock circuit is cathode biased using only one resistor for all four finals. Although there are a few technical benefits of biasing this way, it pretty much demands a matched quad of 6BQ5s. The ST35 manual notes this but underplays the need. Proper tube matching for a design like this should include both bias match and somewhat less importantly, transconductance match. If the amp is otherwise in good condition, first thing I'd check is bias match for all four 6BQ5s. Another alternative fix is to operate the amp only when there's enough ambient light to mask all glow. ;-)
How old are the tubes?
I could only guess that perhaps the bias has changed. Checking it might give you some ideas. Otherwise, I wonder if you swapped the tubes and the back ones would run hot like the front ones.
Beyond that and my knowledge which is dangerous, I'd check voltages on those sockets. Bad resistor?
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