Pentode driven 2A3 ?

[The Shadow]

Hi Guys,

I'm at it again!

I want to build a nice little 2A3 amp for a small system in my study that will simply connect to a line source or an iPod to provide high quality sound whilst working in there.

I could simply re-build my parts raided Bluebell Loftin White amp on a smaller chassis and be satisfied with that but you all know me well enough by now to realise it's not going to happen quite like that.

I've got a couple of possibilities in my head

Use 6c45s as drivers

Go down the pentode driver route.

The pentode route I feel would teach me a little bit more about amp design and expand my experience much further than just simply treading the same old triode ground again.
I've been there, done that with the EL34 amp (and very good it is too, not knocking it)

So if anyone could give me a few preliminary ideas to start me off I would be very grateful indeed.

Steve

[andrew ivimey]

I'm at work at the mo' so no schematics but driving a 2A3 is easy; half a 6SN7 does the job easily - but you have already been there! You don't need a pentode but yes, OTOH, you can play around and learn what they do. Angela.com not only do their version of WE91 (with 300b) but also pentode driving 2A3, just for the hell of it. If you google angela.com you will find (your way around a huge site) to the circuit I am chuntering on about.

[pre65]

Hi-i believe one member used a C3m (or C3g) valve to drive his 2a3.
these were made for the German telecoms system and are of high quality and high gain.only downside is you cant see the valve,it inside a metal canister.
ive just found the circuit diagram that i downloaded at the time so if you want to PM me with your e-mail address i can scan it for you.still cant remember whose circuit it was though.


Philip

[andrew ivimey]

No probs with What Philip says but the Angela uses 6Sj7, which A) is very good and b) can be picked up on ... ebay, for pence.

There are lots of pentodes out there. A nice mullard ef86, e180F, um endless ... C3m would, I am sure be fine.

[James D]

The gain of the pentodes such as C3m or C3g can be anything - it doesn't have a fixed(ish) mu like a triode so there is not really such a thing as a low or high gain pentode.

The metal case on the C3x series can be carefully removed exposing very pretty valves underneath.

I designed and built the first C3g driven 2A3 based on the WAD 2A3PSE chassis and published the circuit on the old WAD BB. Richard Higgins built one using the C3m valve and that thread has my circuit published here. A couple of others have built it or are in the process of building it.

I wouldn't use a low Gm pentode like an EF86 to directly drive a DHT, it needs to be a meduim to high Gm pentode to allow enough current drive to prevent slew rate limiting. If you want to use an EF86 or something like it then use a pentode Aikido like Dave Dove uses on his 300BXLS amp.

The VR tube decoupling/shunt regulation of Vsg is a vital part for extracting the best out of the pentode. Without it the sound is less open and the bass is woolly.

Any questions ask away.

James

[The Shadow]

Thanks James

I still have not decided what to do yet as it is very early days in the process.
I have downloaded your circuit. It looks really interesting and best of all it looks straightforward to implement.

I've got the loadlines valve data and the calculator out right now working up a grounded cathode driver cascaded to a cathode follower to get a bit of design practice in.

If I PM the circuit to you later on, would you give me your opinion?

Steve

[IslandPink]

Steve,
I had a mod of the WAD 2A3 amp going for a while with bog-standard russian EL84s as drivers and it was beautiful - much better than 6SN7s . I'm sure after the 12GN7 experiments on my VV32 amp that James's recommendation of those or C3g would be even better and would be a lovely amp . Only one coupling cap ; use the VR tube screen mod to get even more transparency and better bass. Check out Thorsten Loesch's comments on the 'Legacy' 300B amp for some background .
Mark

[The Shadow]

Cheers Mark

I'll do that.

I must admit Those OA2 gas regulator tubes look dead cool too
Pentode drive is getting more and more tempting.

Steve

[andrew ivimey]

I have just spotted my mistake. Angela.com use 6SJ7 (Pentode) to drive either 300b or 2A3 and verygood it sounds too.


How about this Shibuichi amp. I know a vt25 isn't a 2A3 but surely it is in the same ball park ?? sorry can't upload it as I don't know how 're-size it' its only a gif !!!!

[James D]

Oh dear, I'm going to upset the applecart.

I don't think the 6SJ7, 310a, EF86 etc. can drive a DHT properly... They haven't the current drive or current gain to do the job properly and they slew rate limit into the DHTs grid... Mind you most people won't hear that and recognise it as slew rate limiting and it only occurs on loud peaks with a lot of hf content - but it is stil there waiting to catch out the unwary...

If you have speakers with greater than 100dB/1m/1W sensitivity then you can get away with it as you won't use more than a few 100mW anyway but anyone with less sensitive speakers is going to hit the slew limit...

Andrew, Don't know the Shibuichi amp - do you have a URL for it?

Steve, send it through whenever your ready.

James