A2 Amps

[BTH K10A]

I've been collecting parts for some amps using 45 triodes but yesterday heard an A2 amp using high mu transmitting triodes.

I was blown away by the capability of what was a circa 10W single ended amp. As well as having a great midrange and top, the bass extension and clarity was exceptional.

I am now tempted to try this approach but I fear it will be outside my comfort zone.

Has anyone here heard or have experience of A2 operation for amps?

[Ali Tait]

Yes, Nick (Lurcher) has lots of experience with A2 amps. He uses a 211 A2 he built himself. There are several folks with experience of this over on Audio-Talk, might pay to join and ask for advice there. They will be only too happy to help.

[lurcher]

I must rebuild that 100th SET again some time :-)

Just to avoid confusion having read that, not the one hundredth single ended amp I have made, but the breadboard I did some time ago using EIMAC 100TH high µ triode.

[Ali Tait]

Aye that was a good'un Nick.

[BTH K10A]

Yes, Nick (Lurcher) has lots of experience with A2 amps. He uses a 211 A2 he built himself. There are several folks with experience of this over on Audio-Talk, might pay to join and ask for advice there. They will be only too happy to help.

I've registered with Audio-Talk previously but never received a confirmation email, so have never been able to log in. I tried a little earlier but it states my email is in use by another account?

[BTH K10A]

I must rebuild that 100th SET again some time :-)

Just to avoid confusion having read that, not the one hundredth single ended amp I have made, but the breadboard I did some time ago using EIMAC 100TH high µ triode.

I was hoping to use Eimac 35T or 35TG triodes, possibly driven by a 45 if feasable for A2 configuration. I assume it would be possible to configure the grid wiring to allow either to be used.

The (stereo) amp I heard used 811 output valves and seemed to give the best attributes from valve and SS in one package.

[Ali Tait]

I've registered with Audio-Talk previously but never received a confirmation email, so have never been able to log in. I tried a little earlier but it states my email is in use by another account?

Just get in touch with Nick. It's his forum.

[lurcher]

I was hoping to use Eimac 35T or 35TG triodes, possibly driven by a 45 if feasable for A2 configuration. I assume it would be possible to configure the grid wiring to allow either to be used.

The (stereo) amp I heard used 811 output valves and seemed to give the best attributes from valve and SS in one package.

Well, looking at the 35T curves, 50ma @ 800v with the grid at 0v would work. But the only problem I can see is the plate resistance looks like 25k or more, that would need a output TX that would be close on unwindeable with any hope of maintaining HF response.

And not wanting to be entirely negative, but a 45 would work through a interstage, but it would not be my choice, I have found that interstage coupled A2 drive never works as well as some would suggest, you may get away with a valve like the 35T that needs so little grid swing, but I find a cathode follower with a high gm triode or pentode and a active or choke load works much much better.

Let me know your user name on AT and I will take a look at whats gone wrong.

[lurcher]

Found a picture of the 100th amp

[anthonyTD]

Found a picture of the 100th amp

And not something you want to be hovering over with a cup of tea!
Great pick Nick.
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