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Transformer coupled cathode follower power?
Transformer coupled cathode follower power?
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Old 23rd December 2016, 04:56 PM   #1
Sodacose is offline Sodacose  United States
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Default Transformer coupled cathode follower power?

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Although I understand the drawback of displacing distortion from the output to the input stage of an amp, I have this urge to try using a cathode follower in a low power amplifier output. The two stages of 6SN7 also make me feel all warm and fuzzy because of how often it appears in classic DHT designs.

There's a little serendipity with the above schematic as it looks to me like I can swing 400V ptp and the Vhk of the 6L6GC is +/-200V. Probably good for a bit under 4W into an 8 ohm speaker (also want to try headphones with and without a resistor to load the primary).

I've got all the parts and tubes aside from the Hammond chokes (outputs are Edcor XSE15). Anyone care to talk me out of this or suggest tweaks?
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Old 23rd December 2016, 04:58 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Yesssss‼ A good decoupling cap in parallel to the 400ΩR, otherway you will lost almost all signal across it.
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Old 23rd December 2016, 05:17 PM   #3
Sodacose is offline Sodacose  United States
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Hi Osvaldo. Thank you! I didn't consider that. The transformer will reflect a 5k impedance in series with the cathode resistor, so I don't think I'd lose that much AC signal (or am I missing something?). But I can't think of a reason not to bypass it.
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Old 23rd December 2016, 05:30 PM   #4
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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400Ω / (5000Ω + 400Ω) = 400/5400 = .074, then about 7.4% of the signal will be lost in it. But still worse is that it increases the output impedance looked by the reflected load through the transformer, loosing the main advantage of the cathode follower.
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Old 23rd December 2016, 05:31 PM   #5
Sodacose is offline Sodacose  United States
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Thank you for doing the math for me I will bypass the resistor!
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Old 23rd December 2016, 05:33 PM   #6
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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OK, I learned this thanks for your "ancestors" and teachers; Seely, Wallman, Valley, Terman, and many other electronic pioneers.

Perhaps if you like falsely increased bass response, leaving the resistor unbypassed, will reinforce speaker's resonance in the bass range. Undesirable for me, but I see that someone like this behavior.

PD: Why not to try a pentode cathode follower, with higher gain and lower output impedance?
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Last edited by Osvaldo de Banfield; 23rd December 2016 at 05:36 PM.
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Old 23rd December 2016, 05:45 PM   #7
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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A circuit like this, but with the output transformer in place of the "resolver stator" (G. Valley, H. Wallman - Vacuum tube amplifiers - 1948, page 350).
I would use the choke for the cathode, and from it, bootstrap the load anode resistor in the 6SN7 stage to improve linearity and gain.

My only idea.
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Old 23rd December 2016, 05:59 PM   #8
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Why not ground the transformer and use fixed bias; it shouldn't be much of a risk with this low gm valve.

Actually, that idea of Osvaldo looks very interesting.

Last edited by piano3; 23rd December 2016 at 06:02 PM.
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Old 23rd December 2016, 06:01 PM   #9
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Depending on the trafo's DCR, perhaps no external bias nor resistor needed, but the guy didn't say anything about it.
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Old 23rd December 2016, 06:11 PM   #10
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
PD: Why not to try a pentode cathode follower, with higher gain and lower output impedance?
And much more power out.
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