Can you monitor both the plate and bias voltages as you go through this warm switching?
New power tubes are in, same issue but it seems the him is less loud and quicker until it gets to the pop.
Martin, will do. Do you mean to monitor at second start up? Or first?
I just did the second start up, all the way from on to the pop, the plates were between 400 and 425V. Mine is normally low at 425V. Didn't see much fluctuations
The bias was all over the place, went from 0 to 50mV then back to a low number (10mV ISH) after the pop and then slowly increased to 37mA which is where it's set. I did a quick video of the bias at second start up and you can notice now how much quicker the pop happens as well *edit* video down
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Last edited by Bombacaototal on Thu Feb 03, 2022 5:36 pm, edited 1 time in total.
I replaced the CF tube and do not have the pop anymore. The bias also doesn't do that very high spike at the start. It goes nicely and gradually, from 0mA to 3mA to 6mA onwards. Maybe the CF tube was the cause of all this or maybe just another independet issue? Overall to summarise the issues: bad bias cap, low current fuse at PT HT, standby switches going bust and the CF tube..
I've had such a bad spell with NOS tubes lately. Either creating hum in the circuit, or pops and crackles, two broke and got air in and now this one...
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Bombacaototal wrote: ↑Thu Feb 03, 2022 3:02 pm
I replaced the CF tube and do not have the pop anymore. The bias also doesn't do that very high spike at the start. It goes nicely and gradually, from 0mA to 3mA to 6mA onwards. Maybe the CF tube was the cause of all this...
I think yes, probably h-k insulation failure. Some data sheets specify +/- 100V, some are as much as +/-200V Vh-k max. Remember you have cathode voltage at nearly half of HT until the CF tube heats up and begins to conduct. This is one reason I'm not a fan of the HT-derived bias for this tube. I like the bipolar supply.
Bombacaototal wrote: ↑Thu Feb 03, 2022 3:02 pm
I replaced the CF tube and do not have the pop anymore. The bias also doesn't do that very high spike at the start. It goes nicely and gradually, from 0mA to 3mA to 6mA onwards. Maybe the CF tube was the cause of all this...
I think yes, probably h-k insulation failure. Some data sheets specify +/- 100V, some are as much as +/-200V Vh-k max. Remember you have cathode voltage at nearly half of HT until the CF tube heats up and begins to conduct. This is one reason I'm not a fan of the HT-derived bias for this tube. I like the bipolar supply.
Many thanks Martin, Tim and Richard for all the help in solving this, I really appreciate its. What a fight! Let's hope it is the end of troubles.
Congratulations! That must have been frustrating, but following the diagnostic really does make you appreciate what a fabulous and helpful forum this is - I learn so much
timrobertson100 wrote: ↑Thu Feb 03, 2022 8:44 pm
Congratulations! That must have been frustrating, but following the diagnostic really does make you appreciate what a fabulous and helpful forum this is - I learn so much
Yeah absolutely, very kind, generous, helpful folks
I typically use 100VDC h-k as the limit for new production tubes and 200VDC for old stock (but try not to run them that hot). Any amp I build with a cathode follower gets DC elevation whether it needs it or not.
One thing I wanted to ask, I wonder if my power tubes were fine, given after the change the PT hum went on for a much shorter period and less loud. Makes me wonder if there was some sort of issue there too, as this was prior to the CF replacement. Obviously, after the new power tubes and CF is when things seem to have fallen into the right place. The ultimate question would be, should I dispose of my old power tubes..
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