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RE: 300B - Is this thermal runaway ?

Posted by Burney (A) on October 29, 2021 at 13:26:38

In Reply to: RE: 300B - Is this thermal runaway ? posted by Paul Joppa on October 28, 2021 at 13:10:41:

Thanks Mr Joppa for the pointers.

I should have added in my OP that I have rechecked all solder joints, caps etc several times. Coupling caps tried included Mundorf Al oil 450v, Soviet K75 0.1uf 500v and some Arcotronics FF.

The 300B tubes work fine in an ebay PCB module that uses 6SN7's in cascade. In that board Vk is about 70vdc with an 1k Rk.

For 300B filaments I am using an ebay module that uses 1 qty of LM317. This ramps up the 5vdc slowly. Could this module be causing this grief? Current starvation? Filament voltage is stead at 5vdc as set.

Would it be helpful if I remove this regulator module and go AC or a simple bridge+caps arrangement for the filaments?

Thanks once again.

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