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Old 3rd January 2003, 12:47 PM   #1
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Default Push pull dht triode with diode bias

I have been playing with 2A3 pp designs to compare to a 2A3 PSE amplifier that my son is building. I have come up with this design and I would like your views on how it might be improved, whether its rubbish or not, suggested component choices, etc. etc.

Basically any help or critisim you care to give...

Thanks

James D.
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Old 3rd January 2003, 12:56 PM   #2
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James,

Was there supposed to be a schematic attached?
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Old 3rd January 2003, 01:03 PM   #3
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Hi Brett,

There is supposed to be a schematic attached and an image embedded.

I'm having problems with both... Sorry.

I'll keep trying..

James
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Old 3rd January 2003, 01:09 PM   #4
dhaen is offline dhaen  Europe
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James, if you preview, the attachment gets lost unless you add it second time.
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Old 3rd January 2003, 01:23 PM   #5
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John,

Thanks for the rminder. I did forget that the first time! But know it says my picture size is too big and must be under 800x1200. But my image size is under that....

I'll try again and see what happens. It might be finger trouble as I'm mildly dyslexic and that can affect my typing...

lets see if it attaches here ...

James
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Old 3rd January 2003, 01:24 PM   #6
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Some success - sorry about the poor quality...

I'll keep trying to improve it
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Old 3rd January 2003, 01:30 PM   #7
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Take 2...

(Sorry about this...)
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Old 3rd January 2003, 01:31 PM   #8
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Default I hope this is better...

Sorry about this trouble
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Old 3rd January 2003, 03:15 PM   #9
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Default [insert expletive] Microsoft!

James,
I had a long reply in the process when IE crashed and took out the 'puter. So here's a synopsis;

Diode bias; never tried it, most people I have corresponded with about it didn't like it for a variety of reasons. However, it's only a resistor and a cap to change. There's info on this at Steve Bench's pages.

Fixed bias 6C45: My (limited) experience with biassing direct on line level tubes has been a bit uninspiring. However, other's like Joel like it. Frank has suggested bypassing the battery with some good polypro's. Again a couple of resistors makes it cathode bias. I'd suggest trying both for a while once it's built. Ditto for the diode biassed OP stage. You <i>might</i> need to put input caps, as some of the TVCs are very sensitive to any DC bias or offset. (I use S&B TX102/2)

The op point on the 6C45's at Va=175V and Vg=-2.5V shows about 15mA Ia. The anode voltage is a bit above the 150V max rating on the sheet, but I onow of people who've run them at 5W+ at 250V for long periods. Tough little beast.

Tubes: Matched sets of 2A3 (easy & cheap with Sovteks), and especially 6C45's as they vary a LOT sample to sample IME. I like them though, and they're nice and grunty, driving 300Bs well so a 2A3 should be a piece of ****. Sound good too.

Driver gain will be around 50, with an input sensitivity of 800mV or so.

Grid stoppers <b>required</b> on the 6C45s.

Signal Iron: There isn't any need to spec them for the current, except perhaps for the IT, where an imbalance of 5mA or so should be allowed for even with well matched toobs. My suggestion would be a Lundahl LL1635/5mA or similar. Any quality PP OPT will be fine. No significant airgap is neccessary, construction butt gaps are plenty. It'll lower the inductance lots for no benefit.

PSU: I like, no significant quibbles. The chokes are rated higher for current than they need to be. No problem with this, but it might save some $ and weight with smaller ones.
Suggested addition would be a glow diode shunt reg for the driver stage.

My fave writer and experimenters with PP DHT are <a href="http://www.nutshellhifi.com/triode1.html">Lynn Olson</a> and Gary Dahl. Lots of good stuff in the <a href="http://www.nutshellhifi.com/library/index.html">Library</a>

HTH
Cheers
Brett
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Old 3rd January 2003, 03:51 PM   #10
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Default SYMBOLS

Hi,

Quote:
Suggested addition would be a glow diode shunt reg for the driver stage.
I think there is one already...James used a neon symbol in the diagram though.

Correct me if I'm wrong.

Cheers,
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Frank
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