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RE: Filament bias capacitor question

23.125.184.79

In Reply to: RE: Filament bias capacitor question posted by 1973shovel on January 24, 2024 at 12:19:24

The cap you note is in the design to allow the filament circuit to develop a positive DC bias while simultaneously "draining" off noise by possessing a low high frequency AC impedance. Since the cap carries no signal, the primary cap characteristic of interest is performance at high frequencies with very low DC leakage. Most ceramic disc caps are very good at this bypassing function so nothing fancy like COG/NP0 is needed. Most film caps will work well here too but aren't necessary to perform the function Dyna designed for. OTOH, the Dyna method of filament biasing is notable primarily for its cleverness and cheapness. If circuit authenticity isn't a priority, the voltage divider method described by others is the way to go for both reliability and predictability of the desired effect. As always, YMMV.

Follow Ups:

RE: "notable primarily for its cleverness and cheapness" - 1973shovel 05:35:07 01/26/24 (2)
That always seemed to be Dynaco's claim to fame, perhaps only surpassed by the quality of their output transformers.

My plan is to rebuild these to stock, if you don't count things like the the SDS B+ board, the 'protection diodes' added to the 5AR4 sockets, etc. Once they're done, I'll consider things like a new input stage, floating the heaters, etc if I think they're worth keeping. I don't need their power, my main amp, at 10 WPC drives my speakers well enough. I'm not expecting a great deal from the MK-IIIs, but who knows, maybe they'll surprise me.

Thanks for letting me know that a 'nothing fancy' ceramic will do the job, Steve O.
- just another testament towards the genius of Ed Laurent-nT - Cleantimestream 04:02:22 01/30/24 (1)
  !
  The Mind has No Firewall~ U.S. Army War College.
  - Yes sir! (nt) - 1973shovel 05:13:47 02/02/24 (0)

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