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Old 4th September 2012, 06:40 AM   #1981
dhtrob is offline dhtrob  Netherlands
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Hi Rod,
Thanks.. Some time ago I stumbled on the same challenge: Only on filament winding on power transformer but wanting to use directly heated Triodes. Following "old school" diagrams I came upon this. Since the filament is grounded, there is no (audible) noise penalty when using LM317 as current source..
For completeness of the post I attached the diagram..

Rob
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Old 4th September 2012, 07:08 AM   #1982
dhtrob is offline dhtrob  Netherlands
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Since I am posting, also an idea for the bias resistor..
Use graphite!
De-wood a HB pencil and use the graphite rod as bias resistor. Sounds better to my ears than wire-wound resistors. Wattage about 10 Watts (although they can be heated until they glow bright red )

See for more how-to details on my site (Dutch), but pictures are more or less self-explaining..
Also on Troels site in English (I used his as reference..)

Rob
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Old 4th September 2012, 07:09 AM   #1983
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Rob, Works OK when the filament Regulator has dynamic output impedance much higher than the filaments (1.5 ohm). Crosstalk between L & R channels is degraded by leakage capacitance (or stability capacitors of voltage regulators). If you have a Turntable with a moving-coil cartridge, this high impedance must reach to 50kHz, to keep out crosstalk of scratches/defects!

What's the Tribute Transformer? Is it high performance?
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Old 4th September 2012, 07:18 AM   #1984
dhtrob is offline dhtrob  Netherlands
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Rod, thanks for elaboration! Clear as always..

Tribute high performance? Mwah... (sorry Pieter, could not resist )
Actually it was designed as a 2A3 1:2 step-up interstage. I use it in reverse.. It has excellent bandwith (up to 80khz, no saturation at low frequencies), has an amorphous core. Sound is very much okay..

Rob
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Old 4th September 2012, 07:51 AM   #1985
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Hi Rob,

Quote:
Originally Posted by dhtrob View Post
Actually it was designed as a 2A3 1:2 step-up interstage. I use it in reverse..
Are you really terminating this with 750 Ohms on the output as indicated in the schematic you posted? That would load the 26 with 3 kOhms...

Thomas
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Old 4th September 2012, 08:17 AM   #1986
dhtrob is offline dhtrob  Netherlands
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Hi Thomas,
Yes, when measuring the whole set-up, that gave the best performance.. Btw the secondary measures almost 180Henries static inductance (that is used as primary now)..
Rob
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Old 4th September 2012, 08:25 AM   #1987
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Hi!

I'm surprised, 3k seems like a very low load to me. I can see how that probably swamps out many properties of the transformer since the 3k dominates the load, so the inductance does not play that big of a role any more. Gain is also drastically reduced due to the loss across this load.

I can see how that equalises frequency response over a wide bandwidh, but I'd expect that it increases distortion considerably. Probably not as much at line levels but should be noticable.

I never liked the sound of stages with such a low impedance relative to the output impedance, that's why I'm surprised

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Old 4th September 2012, 08:28 AM   #1988
dhtrob is offline dhtrob  Netherlands
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Yes, you are quite right to be surprised...
My rant.. Just checked the value in the preamp and it is 2K2... Drawing error on my behalf..
Thanks for pointing out!

Rob
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Old 4th September 2012, 10:51 AM   #1989
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by dhtrob View Post
There is one possibility: At ground potential you can get away with one filament supply (transformer and Coleman). Put both filaments in series, adjust for 3 Volts. Ground the filament between the two filaments, so you have one 26 at 1.5Volts and one 26 at -1.5Volts. Bias the 26 with a battery (9Volt).
I have tried this succesfully, however not exactly cathode-bias, sound is excellent..

Rob
your schema is so nice . I will try on your way. Hope everything good

May I can adjust bias by 2 separate negative voltage so that 2 tubes are biased equally.
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Old 4th September 2012, 11:03 AM   #1990
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Hi!

2,2 reflects to 8,8k load for the 26. That's still very low in my book. Sure by burning up 50% of the gain, you equalize the frequency response and extend it. I would rather step down further and get real low output impedance. A 4:1 step down would yield a similar overall gain but less than 500 Ohms Zout

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