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

http://www.lurcher.org/nick/Audio/100th/a2-set-1.jpg

[anthonyTD]

Found a picture of the 100th amp

http://www.lurcher.org/nick/Audio/100th/a2-set-1.jpg

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

[BTH K10A]

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.



It may be that Silvercore in Germany could wind one. When searching the web Sunday evening I was quite taken by this 35TG amp. Maybe a version with less iron and a few more bottles to keep the weight ond costs down though.

http://blog.silvercore.de/?p=236

[lurcher]

Winding one isn't a problem, the problem is winding one with good performance, you are fighting against the laws of physics more that the ability of a particular winder. Dave over on AT produced a very nice sounding amp using a pair of TZ40's

[Ali Tait]

Yes, that was a very nice sounding amp.

[Dave the bass]

Dave over on AT produced a very nice sounding amp using a pair of TZ40's

Just seen this, ta! It's been replaced with an 811a in A2, its both heavier and louder! Don't think its quite as good though TBH but I'll bring it to next DIY bash.

DTB

[Firebottle]

:stalks: A proper breadboard layout.

Some serious bits of kit there Nick :eek:

:cool: Alan

[Ali Tait]

You should see his 212's!

[Dave the bass]

:stalks: A proper breadboard layout.


Av' that! :)
http://i195.photobucket.com/albums/z109/davethebass1/CIMG0065-1_zpsdd68b98f.jpg

http://i195.photobucket.com/albums/z109/davethebass1/CIMG0063_zps322fb74f.jpg

DTB

[diymartin]

Found a picture of the 100th amp

http://www.lurcher.org/nick/Audio/100th/a2-set-1.jpg

I know it is an old tread but i am curious about the driverstage you used , it looks like el36 or e130l ?

[lurcher]

Yep, E130L, two per channel, one as a cathode follower, the other as a load for the cathode follower.

[Colin Wonfor]

Found a picture of the 100th amp

http://www.lurcher.org/nick/Audio/100th/a2-set-1.jpg

Very nice I like it. And no Heatsinks, cool.

[IslandPink]

I wish I'd heard that particular amp Nick, but I think the last A2 amp I heard would have been Ed's 572B amp (?) and of course we've had a couple of nice 833 amps in recent years running around 0V bias. Definitely a great sound to be had - but some high voltages usually. Well worth a chat with Nick & Paul B on Audio Talk .

[diymartin]

Yep, E130L, two per channel, one as a cathode follower, the other as a load for the cathode follower.

I will try to go for the E130l and a ccs with mosfets as tube load, the iron you used is from AE as it looks !
What kind of output transformer did you using (load , E or C core )
:):)

[lurcher]

Very nice I like it. And no Heatsinks, cool.

Well, there was for the two 5A heater supplies.

[lurcher]

I will try to go for the E130l and a ccs with mosfets as tube load, the iron you used is from AE as it looks !
What kind of output transformer did you using (load , E or C core )
:):)


12k amorphous core. I had them around originally for 211 use. Must build that (and several others) again.

[diymartin]

Can you discribe your schematic :)as opp points.
Got another 5 pairs of Mullard E130l so will go for the full tube cathode follower setup.
I go for Silvercore output transformers , he has lots of experience with A2 triodes....

[lurcher]

I don't have the circuit written, t was a breadboard some years ago from what I could find, think the driver stage had about + and - 250v rails. Driver was a ECC40 driving a 1:1 interstage to grid of cathode follower pentode, Other end of the secondary went to a potential divider to allow setting the grid bias of the th100, the cathode follower was directly coupled to the grid of course. About 1000v on the th100 anode. Thats about it. TBH, that should be enough info to build it.

[diymartin]

I don't have the circuit written, t was a breadboard some years ago from what I could find, think the driver stage had about + and - 250v rails. Driver was a ECC40 driving a 1:1 interstage to grid of cathode follower pentode, Other end of the secondary went to a potential divider to allow setting the grid bias of the th100, the cathode follower was directly coupled to the grid of course. About 1000v on the th100 anode. Thats about it. TBH, that should be enough info to build it.

Thanks ;)