[JackD201]

I have a collection of 6922/6dj8s and 12BH7s. I'd like to start a project using the the 6922s as input tubes and the 12BH7s as drivers for a 2a3 Parallel Single Ended amplifier. I can't find any designs though. Can anyone point me in the right direction? Thanks in advance!!!!

[Niteshade]

For a  low impedance drive, parallel the sections of a 12BH7 and then cathode-drive the 2A3's. Use a 6922 to drive the  12BH7. Use a 1K resistor for the 12BH7 unitized anode and maybe a 30K for the unitized cathode. I don't like tying the plate directly to B+ in this application. Your B+ for this stage should be around 250v.

[JackD201]

Thanks Niteshade I'll check it out

Any schematics of an amp with this tube combination anybody out there has seen or better yet built?

[Niteshade]

Thanks Niteshade I'll check it out

Any schematics of an amp with this tube combination anybody out there has seen or better yet built?

I think 2A3's in push-pull would be sweet! It's not done too often....but it IS, without a doubt a good idea.

[ESaudio]

Hi Jack!

Am not sure if you came across with this 2A3 SET-Parallel monoblock amp designed by MR three (3) years ago. The power tube is a Shuguang 2A3C almost the same size of a 300B . Power supply is fully tube regulated and tube rectified. The driver tube is 6S4A and the input tube is either 6DR7/6EW7/6DE7.

This has a twin brother in the States also made by MR. Mine is the first.

MR can also design and use the 6922 as input tube and the 12BH7 as driver tube. But the 12BH7 is a double low mu triode and not sure how it would perform as a driver tube.

Hope this will help you point to the right direction.

Tnx!

[JackD201]

Hi Pat,

I'm doing this project with another local builder to help him out. As you'll notice I'm shooting for a stereo amp with speed;) I have OTHER plans with MR. I'm thinking compact GM70 monoblocks pure SET shooting for weight and focus. Don't tell him yet though as that project might be some time coming as the house is nowhere near built yet. Both amps will be rolled with the slim 95dB ribbon speakers you heard at my place last time.

Jack

[ESaudio]

Great!

Despite our long session last Tuesday, we surely missed the chance to talk about the GM70.  Ding had brought out and proposed this tube to MR a year ago and we had a lengthy discussion about this tube with MR and Ding in his recent visit. If by chance we had another meeting with MR, you should discuss this with him also coz he is very familiar with the charateristics of this tube.

Patrick

[JoshK]

I'm thinking compact GM70 monoblocks pure SET shooting for weight and focus.

I got to wonder why.     Compact means that you can't use any of the components that really make a SET a SET, like big choke input chokes, motor run caps, VT rectifiers.  Plus the GM70 has 60 watt heaters.  If you want to build a long-lasting and reliable amp you must ensure space around the output tube especially so that heat doesn't cook your components.

Most amps commercially build are a compromise, we diy'ers don't have to make such compromises.  why would you compromise intentionally? 

If you intent to build compact gm70's, you will be using ss diodes, no big chokes, no seperated filament windings, electrolytic caps throughout.  If that is the case I have to ask why bother?

[turkey]

I'm thinking compact GM70 monoblocks pure SET shooting for weight and focus.

I got to wonder why.     Compact means that you can't use any of the components that really make a SET a SET, like big choke input chokes, motor run caps, VT rectifiers.  Plus the GM70 has 60 watt heaters.  If you want to build a long-lasting and reliable amp you must ensure space around the output tube especially so that heat doesn't cook your components.

Most amps commercially build are a compromise, we diy'ers don't have to make such compromises.  why would you compromise intentionally? 

If you intent to build compact gm70's, you will be using ss diodes, no big chokes, no seperated filament windings, electrolytic caps throughout.  If that is the case I have to ask why bother?

I have to wonder why not.

What is so magical about the things you mention that "make a SET a SET?"

I'm sure it is possible to build an excellent SET amp without them. In fact, the end result could sound better. (Or not. It's hard to tell with this kind of thing.)

[JoshK]

Do you use electrolytics in the signal path for coupling caps?  The output of the power supply (last cap) of most conventional SET amps is in the signal path (think AC current loop).  Motor run caps have an order of magnitude lower reactance (ESR, ESL, etc). There are a whole lot of other hotly debated reasons why not to use electrolytics, but the above is the main reason I see.

Plus, designing the power supply of a class A tube amp (high voltage, low current) is a bit different animal than designing a class A/B SS amp.  If you brick wall filter the power supply, ss diodes and a sh**load of capacitance then what happens to the transient response?  With HV and low current it is (arguably) better to design for optimal damped supplies and that usually means that motor runs and large chokes win out over brute force capacitance. 

The question of whether to use SS diodes versus VT diodes is one of preference and I won't argue that SS diodes can't be appropriate but I wouldn't then use coupled filament windings with the HT supply (thus needing more space).  The switching noise will couple to your B+ supply and visa versa through the parasitic capacitance.

[JackD201]

Hi Josh,

By compact I'm referring to a slim profile 10" for the front max but it can be as deep as needed which will probably be 2' or a bit more. It's a practical rather than a sonic consideration given the placement. They'll be situated beside slim ribbon/hybrids. Between the speakers is a sliding door to a balcony, don't worry there will be some heavy drapes. ICs will be run under the floorboards. I don't have the space for typical 17" or 19" amps as I won't be able to walk around them. The GM70 is a big tube and a hot one at. Anyhow, that's what I meant by compact. The fun with having amps built (I'd build it myself except I'd probably burn the house down with my lack of dexterity) is you can get it how you want it. I'll leave it to MR to design around what I need. He knows what I like hearing and he has an uncanny way of getting there. I suppose 30 years with valves counts. As long as he does, he can put whatever he wants into that amp.

As noted below I am a Lamm dealer unfortunately the ML3 is not "compact" but more importantly, I can't afford it. I can't afford an ML2.1 for myself either

Back to the 2a3 project, the other guy is a builder and not a designer. Great casework, fantastically neat point to point wiring runs but like I said he isn't a designer so he needs a schematic to work with.

Jack

[JoshK]

Well you are going to have a hard time finding something exactly like you are looking for without designing it yourself.  But you can find schematics that are close enough that you can modify them slightly, like this one.



I don't know what type a 6072 is off hand, but you have enough volts to modify the op point for a 6922.  I'd personally stick with 5687 as the driver, but 12BH7 will do just fine as well, since they are similar enough.  Again, I would parrallel the sections of the driver to give more drive for the parrallel output tubes.   I'd also not be tempted to use the other section of the input tube for the other channel to avoid crosstalk.

I have no idea if the above schematic is a good amp, but it looks reasonable and close enough to what you ask.  It was the second thread with a google search on parallel 2a3 schematic.  The thread from which it came is here

http://www.users.globalnet.co.uk/~valveamp/SEPowerAmp.htm

Edit: 6072 is a 12ay7 with a mu of 40, so pretty close to 6922 in gain.

There is of course a no holds barred attempt at PSE 2a3 with Andrea's design:
http://www.audiodesignguide.com/my/pse3.html#MAIN

If you want to go PP 2a3 then you have the SETH
http://www.magnequest.com/diy_lessard_2a3pp.htm

Some more schematics....but not exactly what you are looking for.  What I would do is borrow ideas and them piece them together.

http://digilander.libero.it/paeng/a_2a3_PSE_AMP.htm
http://www.magnequest.com/diy_lessard.htm
http://www.electra-print.com/pushpull_a2.php
http://www.audiodesignguide.com/New2A3/new2A3.html

and a whole slew of ideas can be wrought from here
http://www2u.biglobe.ne.jp/~tossie/contents-E.010106.html