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Old 17th November 2010, 07:03 AM   #1
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Default DHT Driving Pentode Output Stage Question

Had an idea to try something like a 26 or 01a in front of a splitter transformer or autoformer feeding into an easy to drive pentode/ultralinear/triode strapped output stage using either 6BQ5, 6V6, or 7591 in push pull.

The idea is to get most of the tone from the DHT front end with the power/current drive of the PP output. 7591 may be preferable for this given its higher ratings.

Does anyone know if this has been tried? Is this feasible, or just crazy?
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Old 17th November 2010, 10:17 AM   #2
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Sure is!
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Old 17th November 2010, 10:21 AM   #3
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Yes, it is!
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Old 17th November 2010, 01:25 PM   #4
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Originally Posted by Wavebourn View Post
Yes, it is!
Would that be "yes, tried", or "yes, feasible, or "yes, crazy" (seriously)

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Old 17th November 2010, 03:30 PM   #5
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It should work just fine with a tube like the 6BQ5 and perhaps the 7591 as well given its high transconductance.

It's an interesting idea...

Note that with an interstage you probably don't want to use global feedback, but if you do, and some Lundahls have good enough phase margins that this might be possible then you would want the IT to provide some limited gain. Since a lot of loop gain is not available regardless of the design choices made I would stay away from pentode operation and use UL instead. Given the moderate rp of the 26 I'd probably make provision for triode mode as well.

Another tube to consider would be the 12/112.. The 01 with 3mA of plate current might be just a little anemic compared to a 26 running at 6mA, but the 12 and 01 would interchange easily. The 12 incidentally has an rp of under 5K which might give you a wider choice of transformers.

Another really good candidate which is not a dht would be the 5842/417A which would provide you with enough gain for useful feedback if you wanted it. An IT with 2:1 stepdown ratio could be used here to reduce the open loop gain if desired..

I've just added a bit to the confusion..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 17th November 2010, 09:31 PM   #6
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Default Global Feedback & Other Thoughts

The feedback issue did cross my mind. Not much gain with the DHT tubes mentioned. Could split the signal before the driver tubes and use 4 DHTs cap coupled to the output stage to eliminate the trans/autoformer in the global loop. Might be more complex than necessary though.

Thinking about feedback:

The 26 has a gain of 8 if I'm remembering correctly, not much headroom for feedback but I think 3-6dB feedback could be squeezed out of them and still drive the 6BQ5 with 2 volts in from a digital source... Not sure if that's enough feedback to use pentode or ultralinear mode, but then we wouldn't want very much fb to keep from homogenizing the DHT sound. It would be easy to get more volts out of my digital source if that would help too. Then comes the question about how to apply feedback to a DHT.. The heater is the cathode, does that change anything about bringing the feedback loop back to the driver tubes cathode?

Thinking about output stages:

The Ars Sonum Filarmonia uses pentode mode with regulated G2 voltage and 6 dB of feedback. I wonder if something like that could be done to the 7591?

Or just triode strap the output stage and don't use global feedback allowing the use of the trans/autoformer splitter and only 2 26 tubes... Less output power, but simpler to build. Have to admit I like the sound of push pull pentodes though.

@kevinkr: Thanks for the detailed input. The 417 would definitely work, but the oddball notion is to use a DHT for its tone I have 26 and 01a on hand, will likely try 26 thanks to your reminder about its higher current capability and the vast 26 preamp thread to draw ideas from. Have to look at the 112, but I don't have any of them here.

@ Semper Fi and Wavebourn: 2 and 3 word answers that don't indicate what they mean in the context of the question posed... You can write in more detail if you'd like . I'll read, and even appreciate any insight you can provide
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Old 17th November 2010, 09:36 PM   #7
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Default More thoughts

Also found this thread about a single tube EL-84 amp designed by Gordon Rankin.

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Change the EXO 173 for a splitter transformer, put a 26 driver in front of it and scale the output to push pull... Seems feasible

Could also be tried single ended first to see if the idea has merit.

Have to read up on the 7591 because my speakers like extra power. El-84s sound ok on them, but lose scale and body.

Last edited by TV Man; 17th November 2010 at 09:37 PM. Reason: Typos
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Old 17th November 2010, 10:22 PM   #8
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Sorry, I was in that mood. Sure is a good idea and sure it's been done before. I am not convinced u'll get any more DHT sound than PP sound tho. And if u add gNFB then the sound will be a mix of all worlds, won't it? Using quality transformers, low distortion DHT driver, class - A output stage... I really can't see the need to mess it up with gnfb.
Thorardson and WE had something similar some 70-80 years ago.
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