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by **Rand Reich** » Thu Sep 30, 2010 12:29 pm

I'm prototyping a GSG variant with a SRPP driver. Since heating the driver tube with the same winding as one of the OT's heater (as suggested in the GSG schematic) would exceed the driver tube's heater-cathode max potential (for the top triode in the SRPP), I need to use a separate heater winding for the driver tubes.

I only have 2 6.3v windings on my power xformer. But they are beefy (3A), so I'm considering powering the heater for both OT's (6B4G) from one winding. My concern (and the point of my question), is whether I'd incur any ill effects from doing that (crosstalk, etc.)

I believe I could just parallel feed the inputs to both rectifier bridges from the same winding, or maybe even have one rectifier/smoothing/biasing circuit for both tubes (with a doubled wattage bias resistor of course).

Any suggestions would be greatly appreciated.

by **Geek** » Thu Sep 30, 2010 1:11 pm

Hi,

I'm actually doing similar for a client at the moment with a 2A3 build and mu-follower driver. Just run the bottom tubes from the main heater supply and the top tubes from something like a Hammond 166G6, 6.3V @ 0.6A. Also don't forget you HV bleeder, making each cathode about 50VDC over the heater (ignore if you're using DHT in the driver heater line... the DHT's will do it all by themselves).

As for the second part of your question, meh, I just bought a seperate heater tranny for each output tube on the 6B4 sockets (166K6, 6.3V @ 1.2A) :))

Cheers!

by **Ty_Bower** » Thu Sep 30, 2010 2:34 pm

Rand Reich wrote:I'm considering powering the heater for both (6B4G) from one winding.

I was under the impression that directly heated tubes required separate heater supplies for each tube. At the moment, I'm at a loss to explain exactly why. Hopefully someone will chime in and set us straight.

I guess the issue is exactly what you guessed - signal from one channel will bleed into the other.

by **EWBrown** » Thu Sep 30, 2010 6:53 pm

The main reason that separate heater supplies would be needed, is if the DHTs are "cathode" biased.

Both ccould be powered from a single winding, and the "cathode" resistor would then be half tha resistance, and twisc the wattage, but that would require that both tubes are very closely matched.

This is better applied to DHT PP amps, than single ended, as any hum issues are effectively cancelled out by the action of the PP OPT.

With fixed biasing, then the filaments can be powered from a single winding or supply (for DC powered filaments), as they are normally connected (more or less) directly to the zero volts return..

I'm considering modifying one of my GSGs to use 6AV5GAs, and with their indirectly-heated cathodes, the rectifier circuitry can be eliminated and the cathodes connected to the top of the (approx) 900 ohm cathode resistors. I'll try using some Dale "heatsink" type 880 ohm 25W resistors, mounted on the aluminum chassis plate. .

/ed B