Push Pull 2A3

paulbottlehead

Active Member
A Hammond 193Q would work as L1 in your choke input supply power supply.

For the cap input supply, instead of using a 10H and a 1.5H, I would model it with two 5H chokes to see if things look any better.

Since you are getting 330V, you would still want the fixed bias supply, not R4/C5, or you could increase the size of the first cap to bring the B+ up closer to 400V.
 
A Hammond 193Q would work as L1 in your choke input supply power supply.

For the cap input supply, instead of using a 10H and a 1.5H, I would model it with two 5H chokes to see if things look any better.

Since you are getting 330V, you would still want the fixed bias supply, not R4/C5, or you could increase the size of the first cap to bring the B+ up closer to 400V.
Thanks Paul, just the advice I need

Ill rearrange the first capacitor and chokes and see if I can get around 400v

edit: pretty close, what do you think?

capinputPSUv2.jpg
 
Yeah, that will work with cathode bias, and there's way less ripple.
Thanks! does this Psu look like a good bet?

I saw it has a lot less ripple but the same amount of H (less actually) , will try and do some reading and find out why.

Cheers
 
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paulbottlehead

Active Member
Yeah, at some point you're going to have to build the thing. For a power supply like that, I would buy a 4.7uF cap and a few 1uF caps and just get it built out with the 4.7uF cap, then add caps as necessary to bring the voltage up (you'll need the 2A3s drawing power to know exactly what the B+ will settle to).
 
Yeah, at some point you're going to have to build the thing. For a power supply like that, I would buy a 4.7uF cap and a few 1uF caps and just get it built out with the 4.7uF cap, then add caps as necessary to bring the voltage up (you'll need the 2A3s drawing power to know exactly what the B+ will settle to).
Ace, I will order the pt and chokes when the shop opens in the morning, I already have a few caps in around 3 - 10uf values that should be good.

Thanks again for your help.

Ill update again once I've made some progress.
 
The power transforners have arrived but the chokes may not come due to Brexit so I need to hold on a week or two more and if the guy cant get them ill prob order from Sowter.

Ive been a bit preocupied with job interviews, having finally finished at law school (thought it would never end, actually im still working towards a Masters then Doctor in Law) I have my fingers crossed ill hear about a job id really like early next week🤞🤞🤞🤞
 

Salectric

Senior Member
The power transforners have arrived but the chokes may not come due to Brexit so I need to hold on a week or two more and if the guy cant get them ill prob order from Sowter.

Ive been a bit preocupied with job interviews, having finally finished at law school (thought it would never end, actually im still working towards a Masters then Doctor in Law) I have my fingers crossed ill hear about a job id really like early next week🤞🤞🤞🤞
Good luck with the job search! I’m curious about the Masters and Doctors programs. Here in the US nearly all law school graduates stop with the Juris Doctor which is a 3 year program. There are a few people who go after a Masters in Taxation but that’s the only post-JD program I know about. Is there a different track in UK?
 
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Good luck with the job search! I’m curious about the Masters and Doctors programs. Here in the US nearly all law school graduates stop with the Juris Doctor which is a 3 year program. There are a few people who go after a Masters in Taxation but that’s the only post-JD program I heard about. Is there a different track in UK?

Yes, its just like any other degree here. You can persue it to master then doctor level. The only reason im doing it is that it dosent cost any extra, well that and i did quite like stydying it also I can do it part time.

I did the law degree then the Legal Practitioners Course which is the couse that allows you to become a solicitor, thats where most people stop and then do a training contract with a law firm
 
Im back on track! Started a new job in January and moved flat so havent had much time even think about these amplifiers but now things are more settled im keen to get back to this.

Chokes are on their way, I just need to work out the 2A3 heater filament...

After talking with they guy who sold me the chokes he thinks i should have a transforner for each 2A3 so each can have a hum pot then a shared balance pot, he thinks this will be much better than a shared transformer and shared cathode and capacitor. Or, is he just trying to sell more transformers?
 
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Salectric

Senior Member
I borrowed a pair of PP 2a3 monoblocks a few years ago. Each had AC heating with a single transformer, and no hum at all.
 
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paulbottlehead

Active Member
Well, I've built push-pull 2A3 amps and push-pull 6B4G amps. The 2A3 amps were AC heated with one hum pot per tube, and I couldn't ever quite get them quiet enough, but I was messing with this a long time ago.DSCN0784.JPG
(The hum pots were on the bottom) This was 15+ years ago though, so maybe I could overcome that today.

Within the last couple of years, I redid some Heathkit monoblocks to use 6B4Gs (6.3V heated 2A3 in an octal base). By all accounts, the higher filament voltage should've made them quite a bit noisier, but I used one hum pot on both tubes and everything was OK.

If you wanted to be really safe, plan for DC heating of the 2A3s. This means separate windings, mostly because AC current is just so high in general. You can use a 6.3V CT transformer at about 4A to DC heat one 2A3 with a full wave rectifier, a small resistor, and a beefy 10,000uF cap. The small resistor value will be 0.07-0.1 ohms or so depending on the transformer winding and the ESR of the filter cap. If you want to heat both 2A3s off the same winding, then you'd need a 7+ amp winding and two of those power supplies. I can look up the diodes that I use if you want that don't require a heatsink. Most will.

With two separate DC filament supplies, then where our output tubes are sitting, tie the negative filament side of each supply to one end of your 10 ohm hum pot with a 22 ohm resistor, then do the same with the positive end. This will mean four 22 ohm resistors (1/2W is fine for a 2.5V filament) will be required to connect up the filaments to the single hum pot. The presence of the extra resistors will allow each supply to be a bit more independent compared to just paralleling them and crossing your fingers.
 
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Hmm sounds complicated, someone else mentioned using dc for the filament but said in push pull and for 2a3 its overkill.

So one hum pot per 2a3 shoukd do it then?

the transformer guy i spoke to didnt even mention hum. He said it was as the tubes, even when matched drift and you can balance to combat that. I havent come across anyone discussing this online (using a pot to balance the tubes).

Ill place an order for the filament transformers in the week, it might be better to go with Sowter as it would be much quicker. I would have gone for the Hammond 266M5 but there not in stock at the moment. Since sowter can do whatever is there anything i should ask for?

Thanks guys!
 

mred

Senior Member
If you use two heater transformers and wire them like the diagram attached they will be dead quite.
This what I am using on my build.
Have done on another PP 2a3 the same way also years ago.

20200301_133848.jpg
 
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If you use two heater transformers and wire them like the diagram attached they be be dead quite.
This what I am using on my build.
Have done on another PP 2a3 also years ago.

View attachment 20533
That diagram is just like what was suggested to me, minus the hum pots. What are the transformers you are using? Ive seen some pics of your pp2a3 amps, looked interesting, you are using splitter/interstage too i think?
 

mred

Senior Member
That diagram is just like what was suggested to me, minus the hum pots. What are the transformers you are using? Ive seen some pics of your pp2a3 amps, looked interesting, you are using splitter/interstage too i think?
Yes to the interstate, splitter. It is a Lindahl 1692A.
The two filament transformers are Hammond 166L2
 
Yes to the interstate, splitter. It is a Lindahl 1692A.
The two filament transformers are Hammond 166L2
Thanks. How do you like your amps? I think you have some altec 604s right?

Ill try with one transformer per per tube, since you are happy with that and they are not that expensive, i think i know soneone who might have some to borrow even.

Cheers.
 

mred

Senior Member
Thanks. How do you like your amps? I think you have some altec 604s right?

Ill try with one transformer per per tube, since you are happy with that and they are not that expensive, i think i know soneone who might have some to borrow even.

Cheers.
I have only one on a board right now, listening in mono for a lot of hours as I've made changes and tried many combinations.
I ready to build the two final iterations and call myself happy.
It is dynamic, with good solid bass, 2A3 midrange and extended highs.
Pretty much checked all the boxes I am looking for.
That last experiment was a shunt regulator on the driver tubes power supply.
Nice little improvent.
 
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