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Old 6th October 2008, 11:54 AM   #1
vladn is offline vladn  United States
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Default budget xformer for 829B pentode mode PP (class AB) ?

What are the low cost PP xformer options for 829B amp in pentode connection (separate g2 VCC) and NFB ? I want to bias it for class AB operation with class A extending to ~5W (normal listening). However I need as much headroom in class B as the tube allows since I'd like to use it with my digital piano (large dynamic range). I think I need one in the 30-40W range but I would like some comments how much can I expect from 829B in pentode mode class AB.

I've looked at edcor xformers - there is a price/size gap between 25W and 50/60W models. Also triode electronics has TF110 for cheap but I think it is way too small. Any other half-decent options below ~$50 apiece ? Obviously with 829B tube I can not use UL taps.
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Old 6th October 2008, 11:35 PM   #2
Jeb-D. is offline Jeb-D.  United States
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I want to do a 829B PP Pentode as well some day. When designing from scratch,it's good see what power transformers are also available. Then you either have to simulate it or plot composite load lines different combinations to see what works best.

Since there aren't many PP Pentode 829B designs floating around out there, it may be good to reference some 807 amplifier designs and application data. The 829B is pretty much equivalent to two 807's in 1 envelope.

Hammond or Edcor is about as budget as your iron is going to get in the 30+ W range. Running in AB with some NFB, you may be able to get by with slightly undersized iron, since the nominal power consumption for audio will be quite a bit lower than running it with sine or square waves. However it's always better to go on the larger side.
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Old 8th October 2008, 02:10 PM   #3
vladn is offline vladn  United States
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This is my first tube amp (but I have EE background). I am pretty much set on the topology - pentode-cathodyne front end, pentode mode output, class AB1 with fixed bias, global NFB. I am inclined to go with edcor 25W CXPP xformer. Do you think it is enough or should I shell out for 50W/60W CXPP ? 829B is close to a pair of 6l6 AFAIK.
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Old 8th October 2008, 03:59 PM   #4
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Pentodes and OPTs and non-DC couplings will make errors.
Less phase shift between errors and corrections the better.
In the real world may mean you need more than just one
all-encompassing feedback loop to get the job done right.

I would recommend local feedback around the OPT, and rely
less on global. I know GNF is what you are most comfortable
and familiar with, but its hard to fix an error after the fact.
Especially when dealing with less than perfect components.

Don't overlook that Triodes have intrinsic internal negative
feedback that keeps them linear. The reduced "performance"
relative to Pentodes may be well worth the reduced local
feedback complexity to include them.

I recommend you dig up a PDF copy of OHShade's infernal
diatribe on the 6L6, its in one of RCA's books dating 1938.
Many ancient forbidden secrets revealed...
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Old 8th October 2008, 06:41 PM   #5
Jeb-D. is offline Jeb-D.  United States
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Triode or UL wired 829B won't work if doing a single envelope push pull, since the screens are tied together internally. Paralleling the devices would kind of defeat the purpose of choosing the dual device envelope, IMO. Also, some pentodes do really well triode strapped, but doing so to an 829B, 807, 6l6 neuters the tube.

The point about keeping phase shifting to a minimal is a critical one when using GNFB. Try to keep it a 3 stage design as you are planning. You should be able to direct couple the input stage to the phase splitter if you play with it a little. That will remove 1 phase shifting object from the chain. As stated, finding clever ways to add local feedback, to reduce global is desirable, but sometimes it is not possible. Your probably already aware that compensation circuitry will require some tweaking.

You may want to look into using a high mu triode on the input rather than a pentode. If you can find one with enough gain to allow for the amount of feedback you plan to use. Either case, you will probably have to leave the input tubes cathode resistor partially bypassed. That way open loop gain will still be on the higher side, but you can still return feedback to that point.

I think the 25W CXPP xformer should do the job. I'd be surprised if the amp puts out much more than 30Wrms when it's done(unless your going AB2, but I don't think you are). Full power performance with sine waves may not be good, but nominal power for audio is much lower than sine waves in class AB.
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Old 8th October 2008, 06:59 PM   #6
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Exactly why I pointed him to O.H.Shade... Common screens tied
B+ not a problem with his local feedback scheme. Also does not
cripple the Pentode's capabilities as true triode strapping would...
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Old 8th October 2008, 07:03 PM   #7
vladn is offline vladn  United States
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Thanks for the input. It is AB1 so there will be no additional drivers after cathodyne, it is essentially a 2 envelope per channel design with something like 6gh8 in the front end.

Pentode-cathodyne will be directly coupled. I am also playing with the idea of bootstrapping from cathodyne output to the input pentode plate circuit. Doing some spice simulation.

I understand the problems with global NFB. But I can not use UL with 829B and I want to use single tube per channel. PP xformer will be smaller/cheaper than SE one. Also the amount of NFB will be fairly limited as I want fairly large voltage gain, sort of an integrated amp.

The feedback will go to the pentode cathode circuit. For now I am thinking of taking NFB from OPT speaker output plus a small cap from one of the final plates.
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Old 8th October 2008, 07:06 PM   #8
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You'll find that paper here: http://www.one-electron.com/Misc_Docs.html
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Old 8th October 2008, 07:55 PM   #9
Jeb-D. is offline Jeb-D.  United States
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Interesting article. I'll have to keep a copy of that one.
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Old 8th October 2008, 09:48 PM   #10
vladn is offline vladn  United States
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I briefly looked through the paper - lot's of useful info. However the proposed LNF has it's drawbacks. The cathodyne driver/splitter has asymmetric output impedance and does not fit well with feedback into output stage grids. Also it would be nice to use transformer output for feedback source but symmetric output winding is needed.
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