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Old 8th September 2004, 10:34 AM   #1
GAK is offline GAK
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Default EL 34 30W ultra linear

Any comments for this schem?

I need a simple design at 30 watts.
I think it's a classic topology,easy to build, and it won't cost me too much.
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Old 8th September 2004, 10:52 AM   #2
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Just some offhand comments (I haven't had time to really look at this in depth):

Stabilizing this might be a bit of a challenge. One disadvantage to using a pair of cascaded triodes for the input stage is the number of poles this introduces. ("Stan, this is Wanda. Wanda, this is Stan.")

I couldn't make out the plate-to-plate Z. If it's 6K or so, you might consider going up on the B+ and down on the idle current.
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Old 8th September 2004, 10:58 AM   #3
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the schem
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Old 8th September 2004, 11:27 AM   #4
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Thanks for the larger version!

One more offhand comment: split-load inverters really don't need an adjustment, and without matched o'scope probes and a good sense of what you're doing, it's likely to end up misadjusted anyway. Just use matched plate and cathode resistors and it will work fine. Fewer parts, better reliability.
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Old 8th September 2004, 12:16 PM   #5
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Quote:
Any comments for this schem?
Soundwise, it's not likely to beat a basic Gainclone.
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Old 8th September 2004, 06:40 PM   #6
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Default RC coupling vs DC coupling

@GAK

I would get rid of the RC coupling between the ECC81 and the ECC82 and use direct coupling into an ECC82 long-tailed pair. The circuit in your schem would be very diffcult to stablilize. Jax has a nice driver-amp/LTP posted somewhere on this forum. Or you could use the same circiut, but leave the 1st stage out of the feedback loop, using a 5687 as the 1st stage.
A 12AZ7 or a 12AV7/5965A would be much better than a 12AT7/ECC81 IMHO.
Another way you could go would be to use a Dynaco type driver stage such as a 6U8/ECF82 triode pentode, the pentode section direct coupled into a triode concerta phase splitter.
If you like I could post a couple of schems or I could email you a dozen or so for you to look over (as long as you post to let us all know how everything turns out!). The June/04 issue of Audio Express has a very good and fairly simple article on a 35W Triode/60W Ultralinear Control Amp which looks very similar as to what you're trying to do, except it leaves the 1st stage out of the feedback loop. It uses 6550's but can adopted to use EL34's

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Wayne
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Old 8th September 2004, 06:57 PM   #7
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Default equal plate/cathode loads

@SY

Quote:
One more offhand comment: split-load inverters really don't need an adjustment, and without matched o'scope probes and a good sense of what you're doing, it's likely to end up misadjusted anyway. Just use matched plate and cathode resistors and it will work fine. Fewer parts, better reliability.
Well yes you are correct but it depends on how the OPT primary is wound. Sometimes the lower output tube needs more drive as in the amp-kit I have, a mod'ed VAL VAA-70MkII. The concerta phase/splitter has a 36k anode load, while the cathode load resistor is 43k. I'm planning on changing that to a 6SL7 feeding a 6SN7 LTP soon.

Cheers,
Wayne
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Old 8th September 2004, 07:06 PM   #8
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If you've got an output transformer that's unbalanced in the bass and midrange, you've got problems. I suppose you could band-aid it a bit by deliberately unbalancing the phase splitter, but why select such a transformer in the first place?

I much prefer your suggested topology. In fact, I'm building one much like it right now, a 6SL7 cascode direct-coupled to a 6SN7 LTP.
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Old 8th September 2004, 07:44 PM   #9
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@SY

It really has to do with the DC resistance of the two sides of the OPT primary. I didn't choose the OPT's, that's what the kit came with. Quite a few OPT's where made that way, it has to do with the way the primary was wound. Someone else posted about about this very same thing a while ago, but I can't find it. I'm also planning on chaging the OPT's also, some Hammond 1650N's at this point.
Using the trannies it came with, I get good bass, it's flat to below 20Hz, using a resistive load of course. I had to back off the feedback a little bit from the original circuit to get it stable at low frequences using my Polks as the load! Got supersonic bursts of oscillation riding on the waveform. Around 360kHz IIRC. There was also quite of bit of ringing with 10kHz square waves. Decreasing the feedback did the trick.

Here's a schem of how it stands at this point.

Wayne
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Old 8th September 2004, 08:35 PM   #10
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Ah, a variation of the Dyna circuit. Primary resistances on trannies like that rarely vary more than 20 or so ohms from side to side; the Eico HF87 transformers in front of me now run 130 and 150 ohms on each half. That's pretty insignificant imbalance compared to the 6600 ohm p-p impedance. At idle, you're drawing more-or-less 50 ma per tube, meaning that the plate voltage mismatch is roughly 1 volt. That's out of 380-400V. So I don't understand the rationale for unbalancing the split load, especially to the degree that you have done. The more significant error terms will be things like transconductance mismatch between the output tubes, which can be partly compensated for by unbalancing the idle currents just a bit to minimize 2nd HD.

One thing you might try for HF stabilizing is putting a high voltage cap across the lower resistance half of the opt (i.e., from the end to the CT). Something on the order of 270-470 pF would be appropriate.
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