My very first breadboard

[Cressy Snr]

Hi Guys

This will be my very first breadboard amp. I want to try out a few different output valves with it.
It is based on the 2a3 push pull amp on Tossie.jp's Valves world site in that it uses the same valves. However this one is single-ended and uses choke loading of the input and driver stages.

The input stage is a choke loaded 76, direct coupled to a 6A6 configured as a driver.

According to the VT52 spreadsheet, the 6mA standing bias current on the parallel 6A6 driver stage puts the -3db point at a smidge under 50khz driving a 2A3 to full power. In operation it'll swing more than that so the driver should be free of slew limiting. 6A6s are cool-looking and cheap but use a UX7 base.

The 6N7 is the same valve but has a more common octal base. I have the 6A6.

Unlike Tossie's output stage that dissipates around 20W per triode, ruling out NOS valves, the 2A3 on my amp is run at the conventional op point, so NOS RCA twin plates might be a possibility.

There will be three power supplies, one per stage.
I'm hoping to bring it to Owston

Schematic below



Any comments?

Steve

[Andrew]

Hmmm, 76 and 2A3 I know....6A6 is a new one on me...

On the schem, the B2+ on the 6A6 looks too high to me for the quoted anode voltage, is that a typo? Or is it too late at night and my maths is foobar.

A.

[Cressy Snr]

Hmmm, 76 and 2A3 I know....6A6 is a new one on me...

On the schem, the B2+ on the 6A6 looks too high to me for the quoted anode voltage, is that a typo? Or is it too late at night and my maths is foobar.

A.

No Andrew your maths is OK It's a typo. The B+ voltage should be 375V or thereabouts. How 429 got in I don't know, Must be losing my marbles or maybe it could be the fact that I was on my second bottle of Newcastle Brown Ale as I was finishing the drawing.

Steve

[Cressy Snr]

6A6 is a new one on me...

The 6A6 is a high mu twin triode designed in the 1940s for use as a class B push-pull output valve for radios. It is also designed to be used as a class A1 audio driver valve if both sides are connected in parallel.

The cathodes are commoned so it can also be used as a differential phase splitter stage with a CCS in the tail.

Quite a versatile valve but not used much in anything really these days. However such valves are nice things to play about with when you can't afford the "in-demand" ones.

One of these days I'll build a PX4 or 300B amp or such like but until I have sufficient spare cash to fork out 250 quid a pair for the good ones then the parts box stuff will have to do.

Steve

[Cressy Snr]

BTW

If anyone going to Owston wants a pair of 4019A, 4V, direct-heated, tilted-plate triodes from the 1930s in the proper boxes, drop me a PM and I'll bring them with me.

I really can't see me ever getting around to using them, so they'd be far better off going to someone who can make use of them, maybe Will or Steve S.

Steve.

[simon]

Not a million miles away from what I'm (going to be) dabbling with Steve. I'd like to have a build up and running for Owston but that's unlikely - you teachers with your spare time

[Cressy Snr]

Not a million miles away from what I'm (going to be) dabbling with Steve. I'd like to have a build up and running for Owston but that's unlikely - you teachers with your spare time

Sounds interesting Simon. We'll have to compare and contrast when the builds are finished.

Steve

[simon]

Hoping to do some drilling of ally this weekend, then I can make a start on the 76-45-2A3 in different configurations. But I have further plans beyond this...

Would be great to have it ready for Owston to compare but I think it'll be after that unfortunately.

[Cressy Snr]

Hmm DHT driver eh!

Now you've got me thinking that maybe biased at 9mA those 4019As might come in handy in this little project! with a gain of 7 after the 76 (gain 14) amplifies the voltage, those two stages will turn 2V from a CD player into 196V p-p at the grid of the 2A3.

That's a total gain of 98 before the signal hits the output valve, which is near enough the gain of the single parallel 12AX7 that Philip@Bluebell used with the Loftin White 2A3. Those of us who were weaned on that amp can testify as to how much ass it kicked!

Now that'll be a very interesting comparison indeed. Much more interesting than boring old IDHT drivers. I'll see if I can work up another schematic tomorrow

Steve.

[ed]

Hi Steve

It sounds very beefy. I'm a bit curious though, what bias point are you going to use on the 2a3?

If its the 45v shown on the above diag above aren't you going a long way into a2...its just that I've never seen a 2A3 cct do this before.

Ed

[simon]

There's so many valves to go at Steve . Also on my list to try are 26s, 10Ys and 300Bs. But I've said too much already . If I ever get this ally plate punched the full extent of my madness will be revealed .

[Cressy Snr]

Hi Steve

It sounds very beefy. I'm a bit curious though, what bias point are you going to use on the 2a3?

If its the 45v shown on the above diag above aren't you going a long way into a2...its just that I've never seen a 2A3 cct do this before.

Ed

Hi Ed

The 2A3 is biased at the conventional operating point RCA quoted for this valve ie 45V at 60mA with 250V anode to cathode, which is why you see 295V sitting at the bottom of the output transformer.

This, if you look at the curves for the 2A3 with a 2k5 load, corresponds to just about perfect centre biased class A operation, giving about 3 and a half watts per channel at full power with 5% THD.

Steve

[Cressy Snr]

There's so many valves to go at Steve . Also on my list to try are 26s, 10Ys and 300Bs. But I've said too much already . If I ever get this ally plate punched the full extent of my madness will be revealed .

I too have a pair of 26s off Will so these could be pressed into service if the 4019As are not a success. Having heard the 4019A under less than ideal conditions, they did seem a bit bright and microphonic but that was as a preamp valve. We'll have to see how they fare as a driver, probably a much better application for them as they were originally used to power the line driver transformers in telephone repeater station amplifiers.

Looking at the data for the 26, it should, like the 4019A, be quite easy to use as a driver valve, much easier than trying to make a first stage with it, what with all the hum and microphonics I had with that too in the first position.

Steve

[Cressy Snr]

Here's the circuit diagram reworked for a 4019A driver valve.
The amp seems to have worked out very well in terms of the gain structure. Using one of STC's recommended operating points for the driver allows it to drive the output stage to full power with a bit left over.

I could have upped the driver current by biasing at 9.8mA and -12 grid volts with more HT but then I would have been running the chokes beyond their 8mA current ratings, so I made the compromise and stayed the right side of the anode choke limit.



There are no electrolytic caps in the signal path either, so it will be interesting to see how it sounds when built.

Steve

[ed]

Hi Ed

The 2A3 is biased at the conventional operating point RCA quoted for this valve ie 45V at 60mA with 250V anode to cathode, which is why you see 295V sitting at the bottom of the output transformer.

This, if you look at the curves for the 2A3 with a 2k5 load, corresponds to just about perfect centre biased class A operation, giving about 3 and a half watts per channel at full power with 5% THD.

Steve

That wasn't what I was curious about Steve.

What I was referring to was the 190v pk-pk which I thought was driving the grid 50 odd volts positive..is this normal for driving a 2A3?

Ed