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Old 7th March 2013, 01:51 PM   #581
iko is offline iko  Canada
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Quote:
Originally Posted by regal View Post
I understand the 306A may have better linearity but less B+ capability? This would put the 306A at the same disadvatage as the 4P1L, 2A3, etc.
I have exactly one lonely 306A looking for a mate. Just can't find one anywhere for a couple of years...
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Old 7th March 2013, 02:04 PM   #582
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Originally Posted by Rod Coleman View Post
Personally, I would find some way of obtaining real measurements before spending money on transformers.

The fact that the data-sheet says explicitly that the class-A performance of the 307A was compromised in order to get good class-C operation is a warning. It gives us exactly zero confidence that the WE 307A can compete with the same company's 300B design, where class-A linearity was the actual design goal. But still, measured curves will answer this question, too.
Rod, I agree 100%! It'd be nice to see some real measured curves. The attached are from Pete Millett's site, the only sample I could find.
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File Type: gif 307a.gif (66.2 KB, 187 views)
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Old 7th March 2013, 02:10 PM   #583
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Originally Posted by Rod Coleman View Post
But relying on a driver circuit that can only support 307A and not reach the extra for a 300B, seems like asking for trouble, IMV.
+1
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Old 7th March 2013, 02:20 PM   #584
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Originally Posted by ErikdeBest View Post
Hi Regal,

thanks for the explanation, so it is indeed mostly about DCR on secondary.

I agree that one needs a large double C core for 8W of Single Ended power when one wants wide response and low DCR. I had a pair of audionote OPTs that confirm that. IIRC Pieter (from tribute audio) spoke that his SE transformers for 300B have about 0,2R on the 8 ohm secondary. If that secondary is made of 4 0,8R windings in parallel, rearranging for series parallel would result in 0,8R DCR output on a 32 ohm tap. But indeed, those are expensive transformerts (even though a pair is still probably much cheaper than the HE-6).

But a 35W OPT for a GM70 will also be expensive. Maybe a 35W (SE) EI GOSS core costs less than a smaller (8W SE) double C-core, but it will be a monster, and those dimensions will make that each secondary winding will be long and therefore contribute to high DCR as well, I think (sorry, can't do the transformer math).

And a high voltage amp brings all other sort of costs - and risks - with it.

Now this all sounds very rational (and maybe it is just wrong), and at least in the audio hobby I am not rational myself (just trying to not kill myself and others with high voltages, but for the rest all sort of extravagancies are allowed), therefore I repeat that I only asked because I didn't understand.

many thanks! Erik
I agree that GM-70 type tubes and headphones seem a risky proposition. I believe I checked with Tribute and they do not do custom windings, so asking for an 8+8+8+8 secondary would be impossible, correct me if I am wrong.


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Originally Posted by Rod Coleman View Post
It's possible, but this could go either way!

My point was that we need some kind of reliable characterisation of the 307A - in triode mode - before dropping cash on the iron. Nothing about the data sheet suggests more than middling performance, and 50-70 years lying around soaking up gas does not improve anything - gassy grid current is noisy, for starters.

(I speak for myself, but) I don't see the worth in designing with an unknown quantity. Real curve tracing, like the Audiomatica samples linked earlier, will allow:

- meaningful (rather than anecdotal) comparison with 300B;
- checking grid current in ancient samples;
- checking the operating point (especially voltage) that gives reasonable linearity - before the high-voltage degradation sets in.

OTOH, the latest 300B EH Gold is very well made (a major improvement on previous issues), despite moderate cost, and sounds excellent, provided only that the B+ and Filament supplies are worthy of it.

And this 300B is a 40W anode, rather than 15W+ some fraction of 6W for G2. If we are looking for headroom in our head-fi, the 300B gives much more.

I suppose one could build with a 360V B+ and a 5000-ohm OT, and try them both.

But relying on a driver circuit that can only support 307A and not reach the extra for a 300B, seems like asking for trouble, IMV.
PMillet and Eddie Current both designed headphone amps with the 307A that were well received. The Eddie current model even had the ability to switch to 300B's and several reported liking the 307A's better. I know that's just anectodal evidence, but a thread here reported that the screen is fine operating at 20W's (I think Tubelab reported 25W.) I have nothing against Sovetek 300B's or any other 300B but I haven't seen any independant tube tracing of these either, so I'm going with the $12 tubes. I agree though that the design should accomodate both just in case.
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Old 7th March 2013, 02:32 PM   #585
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Originally Posted by ikoflexer View Post
Rod, I agree 100%! It'd be nice to see some real measured curves. The attached are from Pete Millett's site, the only sample I could find.
Yes, Let's hope for a 307-A and a Tracer meeting up soon.

The other route, if a bit more headroom is the goal, is via the EML super 300Bs: 300B-XLS, EML 520 and 1605.

Emission Labs - Introduction

you can pick your operating point from 360V/60mA to 600V or more, and all points between.

This way, we can avoid the high [800 ... 1000V] voltage of the GM-70, and its high-Z transformer preference. The big EMLs can work with 4500 ohm or less.

I know they are costly, but given the cost of the headphones, and the transformers, going after bargain tubes seems out-of-line with the rest of the project, IMHO.
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Old 7th March 2013, 02:55 PM   #586
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For example, one of the moderate operating points for the EML 300BXLS is:

475V - 90mA - 4200 ohm anode load........... for 14W ouput.

We would be unlucky not to be able to deliver 5W cleanly from this, even with terribly inefficient transformer solutions (though I would solve this problem by using the Sowter custom Transformer - the offer mentioned at the start of the thread, or the Tribute Custom OT).

This platform can be used easily with any old 300B to get started. The 4200ohm load is near enough to optimum, and the voltage is perfect when used with cathode bias (etc) giving about 395V A-K...perfect! My Filament heating kits can be supplied to switch between 2A and normal 300B [1.2 .. 1.3A] filaments, easily enough.

This way, the whole design can be built with a ready upgrade route, for those who prefer incremental builds, or just a bit more choice.
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Old 7th March 2013, 03:03 PM   #587
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I agree that GM-70 type tubes and headphones seem a risky proposition. I believe I checked with Tribute and they do not do custom windings, so asking for an 8+8+8+8 secondary would be impossible, correct me if I am wrong.
I only used Tribute's example because I remember Pieter citing a very low DCR value for the 8R secondary of a 300B transformer. As I am sure he interleaves his trafo, I just assumed 4 windings, which in parallel result in 0,2R, and that in series parallel would be 0,8R DCR. But that is just some assumptions.
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Old 7th March 2013, 05:37 PM   #588
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I believe I checked with Tribute and they do not do custom windings
According to the website he does pretty much anything within reason for the customer.
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Old 7th March 2013, 05:48 PM   #589
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Originally Posted by ikoflexer View Post
Rod, I agree 100%! It'd be nice to see some real measured curves. The attached are from Pete Millett's site, the only sample I could find.
I'd be more than happy to trace them but unfortunately I don't have any
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Old 7th March 2013, 06:35 PM   #590
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Quote:
I only used Tribute's example because I remember Pieter citing a very low DCR value for the 8R secondary of a 300B transformer. As I am sure he interleaves his trafo, I just assumed 4 windings, which in parallel result in 0,2R, and that in series parallel would be 0,8R DCR. But that is just some assumptions.
From Pieter Treurniet:

@Erik de Best:
"I am finishing a 3k5/8R SE output transformer right now; it has 40 ohm primary DCR, and 0.13 ohm secondary DCR. Primary inductance is 30 Henry (static measurement)."

@Regal:
"Virtually all my windings are custom."
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