New Project: A SET Transformer-coupled 2A3 Headphone Amplifier
Aug 29, 2009 at 12:37 AM Post #16 of 20

UglyJoe

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I'm having difficulty finding information on how to design a choke-loaded driver stage. I assume that at the operating point the DC conditions will be set by the resistance in the choke and the current pulled by the stage. Is there any way to do this with LED biasing of the input valve? Also, for signal, the reactance of the choke would give a loadline that changes with frequency. So the value of the choke should be determined by the lowest frequency needed (say 20Hz) and the resulting reactance that would give a large load... say 10k. I assume that this would mean distortion of the stage would be worse for low frequency signals?

Assuming I have this right, what's the best way to go about determining the operating point and the correct choke, as well as optimal biasing scheme? Is there anyway to apply local feedback to the stage to increase the input sensitivity and decrease the output impedance if it is needed for the stage?
 
Aug 29, 2009 at 1:42 AM Post #17 of 20

the_equalizer

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
It seems Budda doesn't know what he's talking about here and he's factually incorrect. The transformer will not attempt to develop more voltage on the secondary.

The reason you don't want to run a tube amp without a load is because without a load, the primary is little more than a large inductor. If you run DC current through a large inductor, and then suddenly turn off that current, the inductor responds with a very large flyback voltage. It's this flyback voltage that can fry the amplifier. Has nothing to do with the transformer's secondary.

k



Interesting! So, running the output transformer will no load might kill it, but not for the reasons I thought. I'll look for info about this on line.

Cheers!
 
Aug 29, 2009 at 11:24 PM Post #18 of 20

FrankCooter

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1)A general “rule of thumb” for determining the value of a plate choke is about 15H per 1K of Rp. A 6SN7-type voltage amplifier has about 7K Rp and a plate current of about 7 MA. Therefore, a 100H @ 10 MA plate choke, which is a standard Electra-Print or Magna Quest value, is suitable for this purpose. The resistance of these chokes is about 1.5K ohm. The plate voltage on the tube would be the B + minus the resistive loss of the choke (1500 x .07 = 10.5V, in this case. For practical purposes, your calculations are no different than using a plate resistor.
2)Bias schemes and calculations for an inductive load are no different than a resistive load. An LED would work just fine.
3)I don’t understand your feedback comments. Any negative feedback, be it local or global, will reduce rather than enhance gain. Usually, DHT amps are designed to run Class A with no global feedback.
4)Again, I wouldn’t worry too much about Miller Capacitance or excessively low driver output impedance in an amp like this. Large DHTs, such as the 211 and 845, have high Miller Capacitance and need a very low impedance driver stage. Small ones, such as the 71A, 45, and 46 were designed to be cap coupled to 6SN7-types like the 26, 27, 56, 7N7, 6SN7, etc.
5)In designing an amp like this, computer modeling is of limited utility. After settling on basic topology, tube types and “ball park” operating points, there is no substitute for trial-and-error bread boarding. In the end, a well-stocked junk box becomes much more useful than a computer.
 
Nov 8, 2010 at 1:55 AM Post #19 of 20

lolitsluis

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Hi all, I was wondering if there had been any progress on this amp over the past year?
 
O am interested in DIY amps and I was wondering what were some recommended source of information. I really hope this amp come to fruition!
 
Nov 8, 2010 at 10:41 AM Post #20 of 20

nikongod

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Quote:
I am interested in DIY amps and I was wondering what were some recommended source of information. I really hope this amp come to fruition!


Sometimes if you want something to get built you have to do pretty much everything* yourself.
smily_headphones1.gif

 
If your interested in learning how to design and build this yourself, I would start off by reading Valve Amplifiers by Morgan Jones. The MJ book is an awesome starter course, then you can go off and learn about other stuff in greater depth.
 
If you do take off on this trip take pride in knowing that at the end of the journey the amp you build will be 100% yours. Building an amp from a blank piece of paper and an idea is a very rewarding thing on many levels.
 
*excludes making your own transformers, resistors, caps, or tubes - but you could if you really wanted to.
 

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