Push Pull 2A3

Im repurposing this thread originally asking about the output transformers for my proposed 2a3 push pull amplifiers to cover the whole amplifier.

revised schematic for a monoblock:

img002.jpg

R1 : 100K
R2 : 1K
R3 : 10R
R4 : 500R 50W

C1 : to be calculated as this will detirmine the B+
C2 : 50uf
C3 : 50uf
C4 : 100uf electrolytic

V1 : WE417a / 5842
V2 : 2A3 x2
V3 : 5U4GB
V4 : 0D3 VR

T1 : Hammond 374BX
T2 : Hammond 266M5
T3 : San-Sui 1000A 6.6K
T4 : Dave Slagle Interstage/splitter 49% Nickel 45Hy 15ma

L1 : Hammond 5H
L2 : Hammonf 5H
 
Last edited:
You can use a filament transformer as a source of 60Hz AC which will allow you to measure the impedance of the transformers. The latter is the critical factor, not the DC resistance which you have already measured. Report back when you have the filament transformer and the method can be explained to you.
 

paulbottlehead

Active Member
I don't see anything concerning about those measurements. It would be a very good idea to know what the primary impedance of those transformers is before deciding that they are appropriate for use with 2A3s.
 
Thanks guys!

I believe the primary impedence is 6.6k

I have spoken with someone who made a 6b4g push pull amplifier using these output transformers and they reported that they work very well, I understand a 6b4g is essentially a 2a3 with a different filament voltage?

Here is a 6b4g version made by Min using the same transformers: HERE
 

mred

Senior Member
Those should be real good transformers.
I used a 6k6 pair of Fisher OPTs with 2a3 pp and had great sound.
 
Those should be real good transformers.
I used a 6k6 pair of Fisher OPTs with 2a3 pp and had great sound.
Actually I saw you had breadboarded a pp 2a3 but didn't see if you had posted a Schematic, how are you doing the splitting? I'm planning on using a SE to PP transformer like in the link I posted above
 

paulbottlehead

Active Member
I would still check the impedance. For many vintage receivers, there are different OPT versions with different primary impedances.

I hope Min didn't actually use the LL1635 in that amp. From the datasheet:
"NOTE: LL1635 is not suitable for SE to PP interstage."

If the 1000A transformers are 6.6K, I would run the 2A3s at 275V plate to cathode (not 275V on the plate) and about 55mA. A 500 ohm shared cathode bias resistor would be about right for that. If you plan to use new production 2A3s like the Sovtek, you could run a bit more current and extract some extra power if you wanted to.
 
I would still check the impedance. For many vintage receivers, there are different OPT versions with different primary impedances.

I hope Min didn't actually use the LL1635 in that amp. From the datasheet:
"NOTE: LL1635 is not suitable for SE to PP interstage."

If the 1000A transformers are 6.6K, I would run the 2A3s at 275V plate to cathode (not 275V on the plate) and about 55mA. A 500 ohm shared cathode bias resistor would be about right for that. If you plan to use new production 2A3s like the Sovtek, you could run a bit more current and extract some extra power if you wanted to.
Thanks for that, hopefully I'll be getting some help from a local guy as I have never done a from scratch build, this amp seems faintly simple though.

With regards to the interstage transformer I've heard Magnequest make an appropriate one or the lundahl 1660, who I'll prob go with as I'm in Europe
 
Last edited:

paulbottlehead

Active Member
Magnequest could certainly make you something, but I'm not 100% certain that he's winding anymore. Sowter can certainly crank out an appropriate transformer, and actually the big Hammond interstage transformers have a lot of fans as well. I looked more closely at Min's build and he used the LL1692, which is a much better choice than that original schematic.

The 5842 has four pins for the grid, and this is no accident. If you build out this schematic, I would recommend looking up "the cone of silence" for high gm tubes and follow that advice carefully.
 
"the cone of silence" rings a bell something to do with radar?

are you referring to adding a resistor to each of the 4 grid pins? the guy i spoke to mentioned that, also cutting 3 pins off but that he found he was ok with just the one
 

paulbottlehead

Active Member
Yes, a grid stopper on each pin with the body of the resistor right against the tube socket pin. If each body is sticking straight up, you can twist them together and then send your incoming signal to that junction.

I would also recommend a 0.1uF MLCC cap between the heater pins on the 5842 tube socket, and another 0.1uF MLCC cap between each heater pin and the chassis. These present a very, very high impedance to the 60Hz AC heating the tubes, but a very low impedance to any garbage that's up where oscillations are a problem.
 

mred

Senior Member
Actually I saw you had breadboarded a pp 2a3 but didn't see if you had posted a Schematic, how are you doing the splitting? I'm planning on using a SE to PP transformer like in the link I posted above
I am using the Lindahl 1692A interstage.
I have not redrawn my final schematic.
I would happy to send you copy when do... I need to not procrastinate and do it any how.
 
Last edited:
Ive just edited the first post with details of the amplifiers Im planning to start soon.

Attached a pic of the interstage/splitter next to one of the output transformers, they look quite cute imoIMG_20190921_091027.jpg
 
Last edited:

Thermionics

Post Whore In Training
How have I missed this thread?

Very interesting build you have here. Curious to see where it takes you.
I'm feeling the same way - especially since I have a PP 2A3 amp project that I've never been able to get quite right (e.g. too much hum). Wondering if this might be the magic bullet.
 
I thought there should be very little hum, the 2a3s have lowish heater filiments and the Push Pull part cancels out a fair bit of hum, I have no experience so this is going on what others have said...

I was wondering about seperate filiment transformers for each 2a3 as this seems like it can be the cause of some hum but I've not come across anyone doing that in a push pull design so doubt it's worth while. Paulbottlehead makes some suggestions for the heater pins if the driver tube but I think that might have more to do with oscillation
 

paulbottlehead

Active Member
I have push-pull 6B4G amps sitting in my listening room that are AC heated and nice and quiet. A single hum pot will do nicely for an amp like this. A single ended 2A3 amp can be built to be pretty quiet, and the push-pull output stage will knock it down more.

If you have hum, the first thing to do is make sure it's hum and not buzz. After that, the circuit can be split up a bit to see where it might be coming from.
 
Top