D3a/EL34 power amplifier

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sbench
Posts: 296
Joined: Fri Jan 28, 2005 3:45 pm

D3a/EL34 power amplifier

Post by sbench »

This is one of those "no R, no C" designs. I've attached the schematic gif file as well as the LTSpice ASC source file. Note that you need the include files modeling the tubes too, see SPICE thread for those files.
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El34_d3a.asc
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El34_d3a.gif
El34_d3a.gif (9.65 KiB) Viewed 13319 times
dave slagle
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Post by dave slagle »

steve wrote:I mentioned I'm going to do a fully integrated amp. I'd kinda like to make as much as possible using glass and iron.

Anyhooooo,, for the EL34+D3a PA, figuring on about 160v/1.4V bias on the D3a which is about 25mA (triode connection). Im thinking cathode winding is about 50mH (I'll refine this later), plate winding is about 60Hy. I'd actually like to use the same scheme on the "line amp" stage with a third (output) winding of about 6.4Hy (3:1 turns ratio). Thus one transformer (with 3 windings) would fit in 3 different spots...

1. Input "bottom half of totem pole" for power amp
2. Line amp stage
3. (bonus application) Final D3a phono stage transformer.

Here's the power amp. Note that I've got some pretty good values for the transformer. Simulation is good enough to proceed. This circuit produces slightly over 10 watts and distortion at 1 watt level looks like it ought to be about 0.1%, mostly 2nd.

The driver transformer is shown. The resistances of the windings are fairly important, as this is a no R no C design (so the R's hide inside the transformer). Hopefully, they're reasonable. (if not lemme know).
I worked through some numbers for the initial design and everything looked workable except for the 110 ohm value in the cathode the entire winding turned out to be 140 turns of #44 wire which resulted in this response.
Yep, those parameters look pretty good. Whenever we get set up, I can send the LT Spice .ASC file too. That lets us fool with various parms. I started out with running the D3a (which, by the way is really a darned nice tube, very linear, quiet, hi mu low rp, not microphonic) at 25 mA. Now that allows that tertiary winding to have 58 ohms resistance (instead of 110), but also requires the sum of the resistance in the other windings to be 400 ohms instead of 600 ohms (to bias the EL34 the same). That still remains an alternative, except I suspect that is less realistic for the transformer.
I really think the 58 ohms is a more desirable number, it will allow us to at least parallel two 140 turn #44 ga windings. Lowering the DCR to 400 ohms also isn't a problem, I actually had inflated the DCR a bit by going to thinner wire than needed.

dave
Bas Horneman
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Post by Bas Horneman »

This looks like an interesting design Steve! Have you built one yet?
To infinity and beyond!!!
sbench
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Joined: Fri Jan 28, 2005 3:45 pm

Post by sbench »

In Queue.
:lol:
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