When they get that leaky, the leakage currents are never going to match. You will get one that is leakier, and will hog all the bias current. You would know it, by the one tube glowing orange and the smell.
Has ANYbody suggested that an intermittent issue with operation might be caused by a loose tube socket connector. perhaps a bit of corrosion down inside it?
Cleaning, and pinching for a tighter grab on tube pins is something us professional techs have done for ages.
Cleaning, and pinching for a tighter grab on tube pins is something us professional techs have done for ages.
And has Lucky simply tried replacing the k-follower with a new tube (even if it appears to “still be good”)?
Yes I've tried 3 pairs of ECC82/12AU7 in V2/V3. One pair brand new, the other 2 NOS, one of those mullards. No effect.
No amount of tapping or banging the pcbs or chassis will make the fault happen, nor make it go away once its happened. It stays like that until standby is cycled, the amp is turned off, or a probe or meter is connected to the grid circuit of V2/V3. Then theres a loud pop and normal operation is restored for a few minutes.
I've remade most, if not all, solder joints anyways.
No amount of tapping or banging the pcbs or chassis will make the fault happen, nor make it go away once its happened. It stays like that until standby is cycled, the amp is turned off, or a probe or meter is connected to the grid circuit of V2/V3. Then theres a loud pop and normal operation is restored for a few minutes.
I've remade most, if not all, solder joints anyways.
This amp monitors individual 6550 cathode currents. If any one bias goes out of range leds change colour or go out - which doesnt happen. The bias appears to remain in range for all valves. IF 6550s really can leak 600uA then they all do it and stay properly biased into the bargain. All 6550s appear calm and normal.
This thread got moved to Amps and Instruments from Tubes and Valves. Really I'm most interested in apparently odd theoretical behaviour of valves in this circuit.
I'll post an update for the record when and if I (eventually) resolve and understand it.
Thanks for all pointers and help.
I'll post an update for the record when and if I (eventually) resolve and understand it.
Thanks for all pointers and help.
It would just be highly unlikely that 6550’s that leak on the order 600 uA out of the grids would all leak the same amount. If one was at 300 and another at 600 - which is far more likely- they simply wouldn’t bias the same.
At last me the fast scope and the amp in same place. No oscillation at CF even when fault. Scope/probe good for 150Mhz. Cathode of V2/V3 - 48V normal even with fault my bad must have measured on CF grids. Tried grid stoppers on CF down to 47k from 15Ok no effect. Run out of ideas...... hate to change everything in sight cos might introduce new probs.......last resort. Weird yoda thinks.
Thanks for all pointers and help. If and when I find the fault I'll post. I don't get it. Signing off and thanks again.