RH2A3

If they glow, this is the place to be
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Alex Kitic
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#1 RH2A3

Post by Alex Kitic »

I have published a new design on my blog, the RH2A3 amplifier.

Image

An additional feature of this amplifier is 1619 tube compatibility... or it can be build with 6B4G and 6L6 tubes in mind.

All interested in RH amplifiers might check it out...
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Paul Barker
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#2

Post by Paul Barker »

Great,

and by the way I shall be sending you some 1619's, I am just very far behind on everything one side of the work//life balance.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Paul Barker
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#3

Post by Paul Barker »

Alex in simple terms what is the loss of gain with your feedback. Is it in this case 50% of the forward voltage gain without it?

Thank you?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Ali Tait
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#4

Post by Ali Tait »

Paul,

I put your quote into Google translate and this is what came out -

It is a constant race between doing things zaps, and the company fill big boobies.



:D
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pre65
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#5

Post by pre65 »

Babylon translate says

It is a constant race between doing things zaps them, and the society to fill with major idiots.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
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Paul Barker
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#6

Post by Paul Barker »

Alex does your CCS in the cathode behave in the following way with the cathode by-pass cap?

Take the 2a3

Rl 3k ohm
Ri 800 ohm
mu 4.2

A normal cathode resistor would be 800 ohm.

From above I get your 22uF bypass cap is -3db at 20hz.

But your ccs would make the cathode resistor 1 meg ohm which makes your -3db 11 hz.

Am I right? Does feedback alter the equation?

Another question. On your ECC81 you use half degenerative feedback. No arguments there. I am guessing the Ri and the transconductance. I have put in values of 13k ohm and 4 S for those. With your 150 ohm cathode resistor I see a gain of 28, if you had 300 (no partial bypass) gain would be 21 and if you bypassed it all 45. Now of your 28 (this comes back to an earlier question) how much is lost in the 100k feedback? 50%?

Thank you?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Alex Kitic
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#7

Post by Alex Kitic »

Paul,

If you take a closer look at the simulation shown on my blog, just below the initial schematics, you might notice that there is an AC analysis that gives you all the answers.

c-d=12.60dB which is the difference in gain between the input signal to the 2A3 and the output signal, equating to a voltage gain of 4.26

Unfortunately, there is not enough space as to clearly show the gain of the first stage, which is 28.61dB equating to a voltage gain of 26.95

I guess from there you can calculate the amount of feedback (you should calculate it on the driver tube, comparing with open loop gain on the driver, actually). An additional element for your calculation should be the input impedance of the cathode follower tube which with 6SN7 is about 2.35meg ohm.

From the same graph you can get the answer to your bypass cap question: indeed there are two factor to take into account, besides the CCS, the second factor is feedback... thus things are not as simple as in a "no feedback" SE amp.

Regarding the cathode resistor of the driver tube, the "cathode degeneration" is necessary for the circuit to function as supposed - without it the feedback is much lower, and the effects negligible (thus you cannot use a LED or similar to bias the driver tube in RH amps).

The reason for splitting the resistor is the need to have a given current draw (thus a voltage drop across the anode resistor-s) biasing correctly the cathode follower tube. The most important problem if it was left unbypassed would be the input sensitivity (besides other implications...).

The input sensitivity of the RH2A3 for "full power" (as shown) is 1.148V RMS, which is quite reasonable, I believe. When 1619 tubes are used, the input sensitivity is higher, and the circuit actually functions differently in more ways than one.

I hope the explanation helps :)
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#8

Post by Cressy Snr »

Alex,
Will you reduce the size of that picture please, it is playing havoc with my web browser safari on my ipad.
The print is so small I can't read it. :)
Last edited by Cressy Snr on Sat Feb 08, 2014 9:51 am, edited 1 time in total.
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Paul Barker
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#9

Post by Paul Barker »

Thank you Alex.

Steve sell the ipad.
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#10

Post by Cressy Snr »

Amp looks great Alex by the way. :D
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Alex Kitic
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#11

Post by Alex Kitic »

Steve,

I can't reduce it, it is a link to the picture on my blog.
I have no problems viewing in Opera on my Galaxy Pad... try Opera instead?

And, thanks for the compliments, it is definitely a keeper, I'll try to pioneer an aluminum box this time.
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Paul Barker
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#12

Post by Paul Barker »

Certainly got my attention.

Have you thought about stepping up your technique to GK71 pentode or GM70 Triode output valves? Looks like your method would be ideal. You wouldn't manage grid current but you could bias them away from it.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Alex Kitic
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#13

Post by Alex Kitic »

Indeed I have: an RH813 is already on paper, for an estimated 30 to 40W at the usual 1% distortion. Key features are unusual operating point and "safe" voltage.

I haven't got the tubes, and would need to wind a 50W capable output transformer for that one, but it is scheduled already. Ongaku eclipse, that is the code-name.

Imagine 40W of SE RH power... a dream come true for "normal speaker owners".
Last edited by Alex Kitic on Sun Feb 09, 2014 6:45 am, edited 1 time in total.
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Paul Barker
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#14

Post by Paul Barker »

I enjoyed 813 triode mode. But as a triode I quickly moved over to 211. But I imagine that as a Pentode in your style of design the 813 would really come into it's own.
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Alex Kitic
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#15

Post by Alex Kitic »

I believe that the best way to use beam tetrodes and pentodes is as they were meant to be used, not triode strapped. When I look at the fabled DHT tubes, my first taught is "it's a shame it has no g2 that I could use as a lever".

The RH2A3/1619 amp is a good illustration of that taught. Good and beautiful as the 2A3 is, if you forget about looks, the 1619 steals the day. If it was to be boxed in a closed AN kit style box, it would mostly work with 1619s.

The other interesting comparison could be 307A vs 1619, pentode vs beam tetrode.
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