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Old 2nd April 2009, 05:04 AM   #1
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Default Cathode Windings

Hi ,

I salvaged a pair Fisher output transformers with tertiary windings to be used as cathode windings .The outputs are 4300k CT primary winding , 280 ohm CT tertiary primary winding and 16 , 8 ohm secondary at about 30 watts and is slightly larger than Dynaco ST 70 transfomers .I am wanting to use these with PP 5881 or 6CA7 in triode mode using the cathode windings .Will i have to stick with cathode bias or is a seperate bias supply practicle . Would a driver PCB such as a Posiden or Triode electronics MK4 have enough gain to drive the output using the cathode feedback windings .I'm open to driver curciut sugestions the only circuit i have found that use cathode windings like this is the Quad II and the Fisher model 80AZ .
http://www.ampslab.com/vintage_fisher80az.htm

Mark
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Old 2nd April 2009, 05:30 PM   #2
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Default Re: Cathode Windings

Quote:
Originally posted by battradio
Will i have to stick with cathode bias or is a seperate bias supply practicle?
Fixed bias with that sort of cathode feedback is quite practical.
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Old 3rd April 2009, 12:20 PM   #3
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Default Re: Re: Cathode Windings

Quote:
Originally posted by Miles Prower


Fixed bias with that sort of cathode feedback is quite practical.
Yes it is. The DCR of each half provides a current sense resistance to determine idle current measurement. I have used just such an arrangement with Chicago output iron as well as the Dynaco A441 output TX's.
cheers,
Douglas
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Old 3rd April 2009, 04:08 PM   #4
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With both plate to screen (Triode Strap) and cathode Schading
in effect simultaneously, your output stage voltage gain may be
very very low. You are going to need a really big drive signal.

Drive of that magnitude might venture into grid conduction.
I would suggest you simulate in LTSpice before committing.
Will providing that current cause the very distortion you wish
all this local feedback to prevent?

Might want to keep your options open for a Pentode or UL
screen strapping. If you are going with strong local feedback
into the cathode...

Remember the work of OHSchade(RCA 1939). This sort of
feedback makes almost the same curves as Triode anyway.
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Old 3rd April 2009, 05:06 PM   #5
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Quote:
Originally posted by kenpeter
With both plate to screen (Triode Strap) and cathode Schading
in effect simultaneously, your output stage voltage gain may be
very very low. You are going to need a really big drive signal.

Drive of that magnitude might venture into grid conduction.
I would suggest you simulate in LTSpice before committing.
Will providing that current cause the very distortion you wish
all this local feedback to prevent?

Might want to keep your options open for a Pentode or UL
screen strapping. If you are going with strong local feedback
into the cathode...

Remember the work of OHSchade(RCA 1939). This sort of
feedback makes almost the same curves as Triode anyway.

hey-Hey!!!,
The grid-cathode drive requirements are the same in any of these topologies, including the unity-coupled McIntosh. The large voltage is seen grid-ground, but that becomes a driver capability issue, not a power final grid current problem.
cheers,
Douglas
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Old 3rd April 2009, 10:00 PM   #6
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Doh!, I forgot the cathode swings with the input, not against...
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Old 4th April 2009, 11:55 AM   #7
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Quote:
Originally posted by kenpeter
Doh!, I forgot the cathode swings with the input, not against...

heh-heh-heh...against would be positive FB. You'd most likely encounter this if the winding polarity was not established before hooking it up...
cheers,
Douglas
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