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PostPosted: 13 Jul 2013, 17:29 
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Location: Arizona, USA
Hi, Unfortunately the PCB is not for sale. Actually the one I used is a discontinued one and I believe the last of its kind. The going from over 200 V (actually more like 270) down to the 150 range does not have any particular advantage. It is just what was readily available. Tube power trannies tend to be costly so when you find one that is under $20 and works in a lot of projects it is a good thing. To get sufficient filtering of the B+ any initial value over about 200 volts (vs 270) should be fine. There is no reason a regulator could not be used either. If you used one for each channel it would within the ratings for a LR8 series one. There are other high voltage ones as well.

Good listening
Bruce

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PostPosted: 17 Jul 2013, 12:49 
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Bruce,
Thanks for the info and the tips, very helpful. You are certainly right about the cost of tube power supply xformers, the Edcor is pretty darned cheap. And the regulator idea is worth a breadboarding.

Best regards,
Blake


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PostPosted: 20 Jul 2013, 14:30 
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Hi Bruce,

I want to get some experience designing a single-ended headphone amp before diving into your push-pull design. The Edcor output transformers you use in your headamp design seem applicable to a single ended amplifier design, just ignore the center tap on the primary and use the entire primary as the plate load impedance. What do you thought about that?

Also, I have some 6BF6 tubes which I used to build a one-tube regen radio receiver, and I set up a breadboard to measure the tube characteristics and design a simple resistive coupled amplifier that seems to be working well and has pretty good bandwidth. Just haven't put sound through it. Are the 6BF6s any good for audio in your experience?
d
Really appreciate your opinion on this, Bruce, thanks.

Best regards,

Blake


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PostPosted: 20 Jul 2013, 15:39 
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Hi Blake, They might work OK. I have never used them though. The trannies could be used SE, but the ease of building the PP version makes it an ideal first project. An SE phone amp would likely be more complicated.

Good listening
Bruce

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PostPosted: 01 Aug 2013, 02:12 
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Location: Thailand, Bangkok
Hi , I love 6DJ8 tube circuits , Can i play this Amp with my Headphone which have impedance of 32 ohms - they are Grado 325i.

Looking for best sound with these headphones?
Thanks


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PostPosted: 01 Aug 2013, 09:10 
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Hi, The amp will work fine with them, but I believe there is another of the same type transformer in the Edcor line that will match better. I thought I saw a one with 70/35 ohm secondaries. It would seem that having the low impedance load and its effect on lowering the anode to anode load on the tubes would cause audible problems. It does not seem to do that. My thoughts are that first there is excess power available and the mismatch really doesn't matter. Efficiency was not a goal in this design. Second as there are no coupling capacitors there can be no change in the F3 Low to mess up either as a result of the reflected change in impedance.

Good listening
Bruce

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PostPosted: 20 Sep 2013, 09:17 
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Location: Minneapolis, Minnesota, USA
Hi Bruce,
I am considering this for a first (in several decades) project. The cost looks right, and since I don't have any high-end gear, if I blow up a pair of headphones or something, it will fall into the learning category.

I have a pair of cheap Philips Noise Cancelling headphones, that I use all the time. I measured them with my trusty meter, and they seem to be 32 ohm. (Sure, I know I need to do this measurement way differently, because the resistance varies by frequency, but, my point is, they aren't 300 or 600 ohm or anything like that.)

In the past, I've read a lot about having the output impedence for solid state headphones amps be much lower than the actual impedence of the phones. (Not sure exactly what the theory is here, but, my cheap FiiO E5 does sound nice) I'm thinking, the theory might be the same for tube amps.

Sooooo:
32 ohms isn't far from 8 ohms, and < 1/2 watt of output power isn't going to be a lot, but, it would be fun to play with a pair of little speakers. If I got the Edcor GXPP10-8-10K (10k : 8ohm), would this setup 1) work, and 2) give me a starting place for experimenting with either my headphones and/or little speakers? My hope would be the output transformers could be reused in future projects, but still "work" for this first project.

Thanks in advance!
Dave


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PostPosted: 21 Sep 2013, 09:49 
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Hi, It would work. But bear in mind that the really clean level of output power is only in the 100 milli watt range. Not much for use with speakers. There was really no attempt to get more power without making the circuit more complicated. In theory you could get about 1/2 watt from the tubes, but by audio standards they are rather diminutive in capability for class A operation. A possible variation would be to use something on the order of 6CG7 tubes The circuit changes would be few and they could deliver about 1 watt rather easily. All these changes though are not what the design was all about....it is a simple, cheap headphone amp. If you want a bit more power I would probably go for a single ended design using a tube like the 6GV8 (or any of the related ones..). Something like in the schematic I attached. A generic output transformer would be fine. The sound is not as clean as the headphone amp, but still rather nice and a better amp for use with speakers. The "upgrade" in the version in the schematic is increased B+ filtering and higher quality components. For casual use the 400uf filter capacitors can be replaced with 100uf ones.
Attachment:
Alpha March 18, 2012.jpg

Good listening
Bruce


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PostPosted: 21 Sep 2013, 13:26 
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Oh wow, thanks! I wasn't expecting so much help and another circuit. Very nice.

This morning, I was trying to understand how the circuit of the headphone amp works. At first it looks like it is push pull, at least the way the cathodes are wired to the transformer, but, the input signal is only on the screen of the top triad, and the bottom triad's screen seems to be a constant through the 47k to ground. So, for the bottom one to conduct, either the cathode has to increase, or the anode decrease. Some "magic" through the transformer? Or is all the magic in the LM317L?

Thanks again for your help. I have lots of reading to do, and I think I need to get some sort of emulator working on my computer so I can study some more.
Dave


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PostPosted: 21 Sep 2013, 13:37 
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Hi, Just magic. :)

Yes it is a push-pull design. Back to tubes 101... the signal driver for a tube is normally between the grid and cathode. It doesn't care which one you change. The LM317 establishes a fixed current flow. The voltage across it however will fluctuate in step with the signal. So as the driven tube conducts more the second one will conduct less. It is not intuitive how this behaves. You actually have to study each part. If you look at the write ups for some of my bigger amps I explain it in greater detail. I like this sort of circuit as it has a high level of power supply noise rejection, only needs single ended drive and inherently cancels even harmonic distortion. Better yet is its simplicity. Few components generally results in less harm done to the sound.

Good listening
Bruce

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