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Old 14th October 2010, 08:35 PM   #1
rongon is offline rongon  United States
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Default Looking for simple Push-Pull tricks (from Baby Huey) #1

I saw something in the Baby Huey thread (great threads, and Kofi's is priceless!) and I'm hoping to get more info on it...

In the original version with cathode-biased EL84 outputs, there's a bit about using using two separate and bypassed cathode resistors (one Rk and Ck for each output tube) with the bottoms tied to a single 39 ohm resistor. This is supposed to reduce IM products and helps the clarity of the output.

I've attached a quick drawing of the circuit as I understand it from the Baby Huey thread (see below).

I understand Allen Wright's PP-1C EL34-triode amp uses this same trick (and used it a long time ago). That schematic is here:
Click the image to open in full size.

I'm hoping this is applicable to all tube push-pull output stages that might wander into class AB. Questions:

1) How would one go about deciding on the proper value for the shared cathode resistor (Rk3 in my drawing)?

2) I noticed the Baby Huey uses 39R for Rk3 while the PP-1C has 68R. Does that mean the value is relatively uncritical?

3) Can something like this be accomplished with a single, common cathode resistor such as would be necessary with a pair of DHT's using a single filament transformer? Maybe a small value unbypassed resistor (say 47R) in series with the bypass capacitor, between the cap and ground? Or would that simply introduce a shelf in its low frequency response due to partial degeneration?


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File Type: png PP_AB_cathode_bias_BH.png (22.3 KB, 496 views)

Last edited by rongon; 14th October 2010 at 08:39 PM.
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Old 14th October 2010, 09:25 PM   #2
rongon is offline rongon  United States
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...And there's another, related, push-pull trick I'd like to ask about.

Lynn Olson, in a presentation made to the 2004 European Triode Festival, mentioned a Western Electric PP triode output circuit that's pretty standard except for a 10 to 40 uF capacitor from the B+ tap to the OPT center tap to the junction of the two cathodes in the output tubes pair.

Click the image to open in full size.

Lynn was talking about the signal path in amplifiers and how they follow the current through the circuit (if I understand correctly, and it's likely that I don't!). I think the 10 to 40uF capacitor from B+ to the output tubes' cathodes bypasses the unbalanced current going through the tubes to ground to above ground, from cathode to plate to OPT winding to OPT CT back to cathodes, and so on.

Later on, Lynn mentions use of a CCS to force balance.

Click the image to open in full size.

Hmmm... This is getting interesting.

Since I haven't seen these tricks put to use in any other amplifiers, I was wondering if there was any particular reason why -- especially the really easy trick of bypassing the OPT CT (B+ feed) to the joined cathodes of the output tubes. The reason I bring that one up is that I tried it with a 20uF 630V Solen cap and it seems to work really well. Nothing to measure with, unfortunately. I'm hoping to bring the amp to a friend's shop and see if any differences are observable on his 'scope.

Anyway, if anybody has experience with any of these strategies, I'd like to hear what you think of them. Always a learning experience...

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Old 14th October 2010, 09:45 PM   #3
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Default Modified cathode circuit

The way the cathodes are connected somewhat resembles a differential amplifier that is commonly used in modern transistor amplifier input stages (transimpedance amplifier). That is it looks like the emmiter coupled circuit, only with cathodes. It should also provide a small amount of coupling out of phase/negative feedback, and balance, between the tubes. It is also in a way a pair of emitter resistors.

I am referring to the small schematic in the first post.

Last edited by epitaxial; 14th October 2010 at 09:48 PM. Reason: Add more
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Old 14th October 2010, 10:19 PM   #4
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Rk3 also means that Ck1 and Ck2 share the same current.
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