EML 20B V4 bias

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EML 20B V4 bias

Postby Crazy Bill » Tue May 03, 2016 5:39 pm

Hello all,
Building an all EML amp, with most of the stuff on the way. EML 20B V4 IT coupled to 2A3 mesh V4.
Since the 20B biases with a higher voltage than the 2A3, I was having some questions as to how to configure the power supply. I was pretty much resigned to having two completely separate supplies; one for the 20B and one for the 2A3.
A gentleman here in a forum on this side of the pond suggested Hitting the 20B w/ 250 VDC, 16ma with -4V on the grid. This would allow a normally configured PS where both stages would be supplied from a single point of the B+ rail, simplifying everything.
But of course the 20B is being hit at a lower voltage and current than recommended on the data sheet.
So, does hitting the 20B as specified above have any sonic disadvantages that anyone may be aware of ? The curves still look good there, with plenty of swing to drive the 2A3, but this amp will be all about how it sounds, and I don't want to compromise the end result if possible, though it would be great if that operating point on the 20B is feasible.

As always, thanks in advance for and advice and help. Much appreciated.

Cheers, Crazy Bill
Crazy Bill
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Re: EML 20B V4 bias

Postby Crazy Bill » Sun May 08, 2016 2:44 am

Hello all,
a little further study on this site has answered my questions.

Thanks, Crazy Bill
Crazy Bill
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Re: EML 20B V4 bias

Postby Admin19 » Tue May 10, 2016 2:06 pm

Hello Bill,

when you use the 20B with a plate choke, this is going to help VERY much in getting nice signal out of it, at low distortion, using low voltage power supply. Moreover 2A3 needs 250V from plate to heater, but has 45V bias. And some volts DC loss over the SE transformer. So with auto bias you have more than 300Volts available. When using a plate choke, that should work well with a 20B. Only thing I do not know ( you need to draw curves, or just try it out....) if 20B needs more anode voltage to be able to use enough signal voltage on the 20B grid. My guess is, it will work at 300V.

Take care to get no oscillations, with two tubes off the same power line. So use a small resistor + capacitor of it's won, for the 20B power supply.

So when driving the 2A3 then with a 20B, the input signal on the 20B is appr 2 Volts. To increase the input sensitivity you can use a 1:2 or 1:2.5 step up transformer like LL1528 . These work virtually free of noise and hum, and almost zero distortion at that kind of signal level. You can't beat that with a tube. Pricewise it's the same. Also the amplifier input can be made floating when you use an isolated ground RCA plug. That obsoletes a ground loop if there is any.

Good luck with it!
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Re: EML 20B V4 bias

Postby Crazy Bill » Mon May 23, 2016 12:58 am

Hello Jac,
Thanks so much for the reply. Sorry it took so long to get back, but crazy busy doesn't even begin to describe what's going on ( but in a good way ).

When I first posted this question, I was working on martini # 2.5, so the thought process was a bit skewed.

I had totally forgot to figure in the voltage drop across the cathode resistor of the 2A3 and the OPT, so, of course, you are right in that I will have 300+ VDC to play with for the 20B. No problem, as using an IT will still give me plenty of B+ available after splitting dual mono with an RC section for the 20BV4.

BTW, all iron ( except PS ) will be nickel core from Dave Slagle at Intact Audio. So very high quality stuff.

Will be using DC on the filaments, presently leaning towards the Colman regs. I initially leaned towards the Tentlabs because of their ease of implementation, but they oscillated into the tertiary winding of Dave's nickel IT's, and the Colman regs are stable. I've read the thread on current sourced DHT filament heating, so I will tread here very carefully. I read your suggestion for battery supplies in the EML footnotes, but Dave advises that will not work with the tertiary winding on the IT. Bummer. Batteries would have been easy and oh so quiet.

On the input, Dave has new silver winding autoformers for volume control w/ 5 db of gain available. They're still in prototype, but the first pair is mine when available.

They'll be feeding the 20B's.

I'm sure I'll have more questions as this project moves forward.

Appreciate your support and encouragement !!

Thanks, Crazy Bill
Crazy Bill
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Re: EML 20B V4 bias

Postby pktube » Fri Jan 20, 2017 5:16 pm

Crazy Bill,

I am considering the 20B as a driver for my 2A3 amp. Please share you thoughts on this combination and are you getting sufficient output? If possible, would you be willing to share your schematic?

Thanks for any help,

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Re: EML 20B V4 bias

Postby J. Leven » Fri Jan 20, 2017 6:58 pm

A sketch, schematic, would simplify the discussion
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