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Old 24th April 2013, 06:34 PM   #701
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Originally Posted by iko View Post
The good side effect of this is that the price of 307a has dropped significantly.
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Old 24th April 2013, 06:39 PM   #702
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Originally Posted by regal View Post
The Tribute I ordered one is of nano-tech non-amorphous core (finemet), its fairly experimental for this application although has been done with the Japanese Feastex Takatsuki 300B commercial amp.

I would recommend contacting Tribute thru email for pricing, but he has a lot of business, so since this is a developmental OPT you may want to wait till it is a standard product.

I will be building my DHT SET headphone amp with 2A3 output tubes after Ale's not so good measurements of the 307A. I think the new JJ-2A3-40 and Chinese 2A3-C's which are basically 300B's with 2.5V filaments are ideal (the sims show much better linearity with 300B than an other DHT short of the HV transmiters.) Would be interesting to see Ale's measurements of these modern 2A3's-300Bs.

I plan on IDHT drivers in order to keep it 2 stage, just not enough gain from small signal DHT's for 2-stage.
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Old 25th April 2013, 08:59 AM   #703
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Originally Posted by Rod Coleman View Post
The JJ 2A3-40 does not inspire confidence, for working above the usual 2A3 voltages.

compare the published curves:

http://www.jj-electronic.com/pdf/2A3.pdf


And, to keep the comparison fair - intramural, even - please see their 300B:

http://www.jj-electronic.com/pdf/300%20B.pdf

And see how the 2A3's behaviour goes bad, much above 400V.

The 40W thermal rating is deceptive. It's the internal geometry that makes the 300B the beauty it is (when well fed, and watered).

.
That or the curves are marketing mumbo jumbo.
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Old 5th May 2013, 09:59 AM   #704
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Can I use a standard OPT 5K / 8 ohms be suitable for the 300 ohms headphones?
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Old 5th May 2013, 10:10 AM   #705
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It'll work just fine.

Since for headphones, output power is not paramount, I'd sugguest going parafeed. Put a gyrator on top of the output tube, and use a 10k primary OT. Much, much better linearity - with headphones you can really hear small differences in sound quality.

I would also sugguest going fixed bias, since you have a coupling cap anyway.
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Old 5th May 2013, 10:13 AM   #706
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Thanks, in place of a gyrator I have in mind to use Salas shunt reg SSHV2, sorry my ignorance how can go fixed bias?
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Old 5th May 2013, 10:29 AM   #707
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Originally Posted by MrCurwen View Post
It'll work just fine.

Since for headphones, output power is not paramount, I'd sugguest going parafeed. Put a gyrator on top of the output tube, and use a 10k primary OT. Much, much better linearity - with headphones you can really hear small differences in sound quality.

I would also sugguest going fixed bias, since you have a coupling cap anyway.
This is absolutely right for most headphones. It is the right approach unless you have headphones (like the new planars) that require 1W+. Sowter makes a perfect parafeed OPT for this.
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Old 5th May 2013, 10:35 AM   #708
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Quote:
Originally Posted by MrCurwen View Post
It'll work just fine.

Since for headphones, output power is not paramount, I'd sugguest going parafeed. Put a gyrator on top of the output tube, and use a 10k primary OT. Much, much better linearity - with headphones you can really hear small differences in sound quality.

I would also sugguest going fixed bias, since you have a coupling cap anyway.
Fixed bias for the output tube?
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Old 5th May 2013, 10:50 AM   #709
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Originally Posted by merlin el mago View Post
Fixed bias for the output tube?
For low power parafeed my experience is to just build a spud amp with a high gm high mu IDHT. You only need to move to two stage with planars.
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Old 5th May 2013, 11:09 AM   #710
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merlin el mago, this schematic demonstrates what I meant.

L1 is OT primary.

Fixed bias instead of RC, gyrator plate loads on all stages, parafeed output.

Quality of C5 is very important, so go Russian NOS. Best quality for little bucks. Go as small as you can get away with, 2F is ok for 10k primary.

B+ should be regulated as you were planning to do.
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