2A3 Monoblocks

Cressy Snr · Post by **Cressy Snr** » Sun Sep 25, 2011 11:16 pm

After much experimenting over the past few weeks I've sorted out a pair of 2A3 monoblock amps, that run into 5K output transformers

Here is the audio circuit schematic:

Input/driver is my favourite pentode at the moment, the 6AU6.

The 2A3 runs at 287V and 40mA with 60V cathode bias.

Cressy Snr · Post by **Cressy Snr** » Sun Sep 25, 2011 11:23 pm

And here is the power supply.

Oh my!

This is responsible for the solid sound of these amps.

There is one of these per channel. Overkill? On the sonic evidence. provided by these amps I don't think so.

Dave the bass · Post by **Dave the bass** » Mon Sep 26, 2011 7:57 am

Ooooo-errrrrrrrrrrrrr!

Pentode driving 2A3 + a stiff supply. Hope for it to be ready for Owster's Steve? I'd like to hear it, I've been living in 2A3 land for a long time but mostly triode driven.

DTB

Cressy Snr · Post by **Cressy Snr** » Mon Sep 26, 2011 8:48 am

Aye it'll be ready for Owston.

New amp and new speakers will be on show.

I think with SETs and SEPs for that matter, inevitably one has to come to the
realisation that the power supply is everything and overrides all other considerations.

Ok so I'm an excitable person, much given to hysterics over amps but taking things calmly and rationally, this is the best 2A3 amp I've built. You can build a nice 2A3 amp, much the same way as you can make a nice EL84 amp but
make a 400W power supply and ask it to produce 4W and the results are startling.

Some people think he is a lunatic, but though highly opinionated, and controversial, Jeff Medwin on the SET asylum seems to talk sense. He is a big advocate of massive power supplies feeding weedy amps, hates 300Bs with a passion
and rewires power transformer leads with Kimber Cable

.

I doubt that my power supply with its 3 section RC filtration after the choke/ capacitor filter would meet with his complete approval, cos he is not a fan of regulators, but it does seem to allow the 2A3 to put up a great performance unhindered by sag at high volumes.

There may be more I can squeeze out of it with shunt regulation of the HT rather than the series reg I have at the moment, but shunt regs are another ball game entirely. Paul Nick and Andrew will have to teach me about them at Owsters.

I'm using some chinese 2A3B single plate valves at the moment and would love to do a bit of tube rolling.

I'd like to try the JJ2A3 40, Shuguang 2A3C, TJ 2A3 mesh plate but most of all the Emission Labs 2A3 mesh window plate. It's bloody expensive but the quality of manufacture looks staggeringly good, the valves themselves look fantastic too.

One can dream I suppose.

Steve

Dave the bass · Post by **Dave the bass** » Mon Sep 26, 2011 10:30 am

If you fancy I can bring TJ Mesh 2A3 and 2A3C if you want to try Tube rolling at owsters Steve.

DTB

Cressy Snr · Post by **Cressy Snr** » Mon Sep 26, 2011 10:31 am

Dave the bass wrote:If you fancy I can bring TJ Mesh 2A3 and 2A3C if you want to try Tube rolling at owsters Steve.

DTB

That'll do nicely sir!

The 2A3B Shuggies are very nice but a bit microphonic.

Dave the bass · Post by **Dave the bass** » Mon Sep 26, 2011 10:33 am

No prob's, remind me before hand as I'm likely to forget otherwise.

DTB

Paul Barker · Post by **Paul Barker** » Mon Sep 26, 2011 12:50 pm

Remind me and I'll bring RCA dual plate and Sovtek monoplate.

Cressy Snr · Post by **Cressy Snr** » Mon Sep 26, 2011 12:58 pm

Paul Barker wrote:Remind me and I'll bring RCA dual plate and Sovtek monoplate.

Cheers Paul

Much appreciated.

Cressy Snr · Post by **Cressy Snr** » Tue Sep 27, 2011 7:21 pm

I've just changed out the driver valve on one of the monos for a 12HG7 hi-gm pentode.

I've had these for ages and it seems a pity to let them languish in a box.
The idea is that if I ever want to experiment with those 300B belonging to our Ant, then this driver ought to
cope with them nicely, whereas the 6AU6s would probably have struggled running at 8mA quiescent current.

The 12HG7 is idling at 36mA plus 6mA of screen current. That ought to be
right for a 300B.

I'll do the other one tomorrow unless some work comes in. It's getting
round to that time now.

IslandPink · Post by **IslandPink** » Wed Sep 28, 2011 5:58 pm

Good stuff Steve

Your power supply is especially cool .
I would guess the 12HG7 is overkill for 2A3 but certainly man enough for 300B .
The one problem which I guess you may have found is that because of the high transconductance it will be hard to get a solution to the 12HG7 operating point that doesn't develop insane amounts of gain .... if fed from the same B+ as the output valve.

One other point, that I haven''t calc-ed yet, is that on the 6AU6 option, the decoupling cap for the screen, at 1uF , looks a bit small . This might affect your bass. I used to calculate with some simple rule based on 2piRC where R is the resistor to ground. James D pointed out that you need to include the impedance of the screen ( itself ) in the calc. If you have done this, then great . if not, then just try eg. 5uF in there if you can and see if it makes any difference .

ILPS

Cressy Snr · Post by **Cressy Snr** » Wed Sep 28, 2011 6:14 pm

IslandPink wrote:Good stuff Steve
Your power supply is especially cool .
I would guess the 12HG7 is overkill for 2A3 but certainly man enough for 300B .
The one problem which I guess you may have found is that because of the high transconductance it will be hard to get a solution to the 12HG7 operating point that doesn't develop insane amounts of gain .... if fed from the same B+ as the output valve.

One other point, that I haven''t calc-ed yet, is that on the 6AU6 option, the decoupling cap for the screen, at 1uF , looks a bit small . This might affect your bass. I used to calculate with some simple rule based on 2piRC where R is the resistor to ground. James D pointed out that you need to include the impedance of the screen ( itself ) in the calc. If you have done this, then great . if not, then just try eg. 5uF in there if you can and see if it makes any difference .

ILPS

Cheers Mark

Thanks for pointing the 6AU6 screen cap problem. On the diagram the actual value for the screen decoupling cap should read 10uF not 1uF.
Additionally one of the resistors on the plate of the 6AU6 is wrong too, should be 22K and 10K, not 2x22K

I'll amend the diagram and repost it below

EDIT
corrected schematic below:

I'll post the diagram with the 12HG7 as driver later on too. I did manage to solve the high gain problem.

Steve

Andrew · Post by **Andrew** » Wed Sep 28, 2011 7:26 pm

IslandPink wrote: One other point, that I haven''t calc-ed yet, is that on the 6AU6 option, the decoupling cap for the screen, at 1uF , looks a bit small . This might affect your bass. I used to calculate with some simple rule based on 2piRC where R is the resistor to ground. James D pointed out that you need to include the impedance of the screen ( itself ) in the calc. If you have done this, then great . if not, then just try eg. 5uF in there if you can and see if it makes any difference .

ILPS

Well spotted Mark, 10uF might be even better.

Cressy Snr · Post by **Cressy Snr** » Wed Sep 28, 2011 7:54 pm

And.....here is the amp with 12HG7 driver

The HT was tweaked up to 400V with a 1uF cap placed, before the choke.
The driver is based on the old Pete Millett circuit he used on his 2002 813 SE triode strapped amplifier, just a slight change of plate and cathode resistor values.

The excessive gain problem was solved by Pete's method, which was by the omission of the cathode bypass cap. The higher driver stage output Z is not a problem with the 2A3.

If a 300B is ever used I can boost the front end gain and lower output Z by using a bypass cap. This will up the gain to 100.

Cressy Snr · Post by **Cressy Snr** » Thu Sep 29, 2011 11:30 am

Thought I'd be silly and try Ant's Shuguang Mesh plate 300Bs in there at 2.5V.

They played and produced a decent sound, which was surprising. Lovely sparkly treble cos of the "proper" driver.

2A3s back in though now of course.
Still, it proves the concept and confirms that there is plenty of room for development of this amp, either down the exotic 2A3 route or with 300Bs.

IOW it's not a dead end amp like some I have built.

audio-talk

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