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Old 30th January 2007, 07:35 AM   #1
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Default All pentode PP amp?

Has anyone tried building an amp like this? The Cit II had a reputation for accuracy and clarity, without either 'tubey' or SS sound, i.e. neutral.

Out of curiosity, I tried modeling a Mullard-type topology (using LTSpice), with 6AU6s in pentode mode for the voltage amp stage and LTP splitter, and EL34 OP tubes in pentode mode. The FFT analysis gave very low distortion, less than any other design I've tried, certainly less than using triodes in the voltage gain and splitter stages. The AC analysis was excellent after a bit of tweaking.
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Old 30th January 2007, 07:47 AM   #2
Yvesm is offline Yvesm  France
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Hi ray_moth,

Not really an "all penthode" project since 6L6 are rather said "tetrodes" (but who cares ! ) and that it uses triodes for grid current buffering.
This helps a lot in driving lo Z loudspeakers !

http://www.dissident-audio.com/PP_6L6/Page.html

BTW, you mention using LTSPICE.
Being very dumb about simulators could you tell me how you obtained tubes models ?
And, better, could you publish your .asc file ?

Thanks in advance.

Yves.
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Old 30th January 2007, 08:41 PM   #3
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I got the 6AU6 model from someone in this forum, I'm sure the thread still exists but I can't remember its name. The EL34 model is called 6CA7 and was from Duncan amps, I think. (Sorry to be vague, I've had these models a long time).

.asc is not one of the accepted file formats for uploading but I'll try to do it as a jpeg later - don't have time now.
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Old 30th January 2007, 08:50 PM   #4
Yvesm is offline Yvesm  France
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Thanks to you,

My query about .asc was to help me by having a starting base I could learn about and modify rather than having to create one from the scratch.

Not for publication here of course- sorry for my poor English- but says, for demonstration.

Yves.
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Old 30th January 2007, 09:10 PM   #5
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Hi Yves,

This was the best pure pentode ...errr.... beam power finals amp I ever auditioned so far. Very detailed while unobtrusive, also good bass control and decent power. A combination rarely to find among amps using straight pentode/beam-power finals, not to talk about the ingenious usage of ECF80 compound tubes as pentode LTP splitter plus CF drivers.

Putting aside all the technical stuff, it was just fun to listen to in any regard.

Tom

P.S./EDIT: Oh, never mind my signature is not applicable in this case
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Old 30th January 2007, 09:22 PM   #6
nhuwar is offline nhuwar  United States
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Yves

try this page for spice tube models http://www.duncanamps.com/spicevalves.html

Nick
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Old 30th January 2007, 10:50 PM   #7
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Ray,

As a Cit. 2 owner, I can tell you that all "pentode" is superb, if close attention is paid to details. You must have the very best O/P trafos money can buy. The entire circuit relies on bandwidth to function well. Plitron toroids might do the job. The huge bandwidth keeps the amp from running out of steam driving the NFB circuitry.

BTW, don't forget the high current, low impedance, B+ PSU. The kick butt bass performance is not an accident.
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Old 31st January 2007, 12:52 AM   #8
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Thanks for the responses. Now I can try posting the two tube models and the .asc file in a .zip file (I've just noticed .zip in the list of acceptable upload formats).

Eli, regarding the low impedance PS, I take your point. For a pentode mode PP OP stage, I intended using a simple series CMOS regulator for the screens and preceding stages (with decoupling between stages, of course). If it was a UL design, then the whole of B+ could be regulated (easy these days with MOSFETS), probably with an error amp, or I could look for suitable low DCR power tranny + choke).
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File Type: zip el34 pentode pp ab1 mul 6au6 6au6 gfb.zip (4.8 KB, 60 views)
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