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    Building, troubleshooting and testing of these amplifiers should only be
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Best center tapped filament transformer AC heating 300B

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T

TBM

Member
Joined 2005
#1
Hi!

Regarding AC heating of SET 300B, how much does a center tapped filament transformer influence the sound after the hum pot has been adjusted?

Any experiences and recommended brands for the very best filament transformer?
Cost no object, sound is...


Best, Tom 🙂
 
#2
Years ago, when building a PP 2A3 amp i spent a day listening through a dozen different transformers. Shocking, how big a difference they made. Most were quite intolerable, per example the great otherwise EI transformer i was using for B+. The best, by a large margin were a pair of 280VA Chinese R-cores which also had windings for B+ but those i never liked. So, after an extended comparison settled for the rather ugly option to have a pair of transformers for each channel's outputs: an EI for B+ and an R-core for the filaments. Input valves obviously had their own 🙂
 
#3
Very, very interesting!! 😀 :up:

In a way, I am not surprised, since it is indirectly in the signal path 😉

I must try to google and see if I find a R-core center tapped filament transformer, if you or anyone has links to such transformers, that´s very welcome...!

Best, Tom 🙂
 
#5
Something of appropriate size made by your local winder on a split bobbin. Better to have the windings side by side rather than on top of each other to help reduce any high frequency coupling.

If not DC, then then perhaps a very low frequency AC signal generator. Not seen this done. Maybe a subwoofer amp with a low frequency signal genrator would be something to investigate.

Edit: on second thoughts a sub amp probably might not like the low impedance.
 
Last edited:
#6
TBM said:
Regarding AC heating of SET 300B, how much does a center tapped filament transformer influence the sound after the hum pot has been adjusted?
Click to expand...


The most interesting subject I've seen for a while! Filtering the filament side of the transformer isn't possible because 50/60 Hz is in the audio range. Assume that hum pot center to signal ground is imperfectly zero. How could an audio band impedance, significantly larger than the imperfectly zero desired impedance, be made? ***



Or, float the filament transformer, fed by its own low distortion oscillator/amplifier?, and isolate it by heroic measures. Maybe just a really good isolation transformer.


*** This is what the Rod Coleman type DC filament regulators aim to do. They have great isolation, but for a very small overhead. You still have to keep their overhead burden way under a volt or couple.


YOS,
Chris
 
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