Running a 2A3 at 5K

[Cressy Snr]

This has been covered a thousand times but I'll talk about it anyway.

I'm currently experimenting with a couple of Shuguang black plate 2A3Bs,
driven by 6AU6s, on the the monoblock chassis I've built.
I read an article by Thorsten from a decade ago by now, where they were playing around with 2A3s at high Va (around 300V) with lowish current (45-50mA) into a 5K transformer load and reporting very good sound.

Most people run them by the book at 62mA and 250V into 2K5 outputs.

I am currently running a pair at 345V B+ with 58V cathode bias at 48mA into the 5K winding of my James OPTs and finding that they sound a fair bit better than any 2A3 amp I've built. They are dissipating approx 14W and putting out a respectable 4W per channel.

I know Don Garber runs 2A3s like this in his well regarded Fi 2A3 SET.

I normally find 2A3s a bit hard and forward in the mids when done by the book, but running with these conditions they are quite neutral with no hardness that I can hear.

Could be a combination of things. I mean the regulated PSU is easily capable of 400mA, nearly ten times the current the 2A3 amp is drawing so that could well be the biggest factor.

This might convince me to give DHTs another go; see if I can make one that does it for me.

Anyone else tried 2A3s at high B+ with 5K OPTs or could provide further enlightenment?

Steve

[Paul Barker]

No I haven't tried it, but the 2a3 and it's types (6a3,6b4g,) and the indirect 6A5G, are extremely solid performers. non of the difficulties the 300b can present, better power than the 45 with very little (though some) loss of tonal quality (relative to the 45) but not as bad a compromise as the gulf from the 2a3 to the 300b. Though this gap is simply closed by operating the 300b as a 2a3.

Perhaps one element of what you find is that operating the 2a3 more like a 45 restores the 45 tone.

sounds like fun anyway.

Ducks wearing hard hat! (considering all the 300b builds going on)

[Paul Barker]

there is an interesting "house sound" or family sound which the 45 2a3 and 300b share to a degree.

then if you take the transmitters such as the solid performing GE vt4c (don't bring the chinease into this discussion they are in a different post code, nowhere near the same construction quality making for distortion and vagueness. Horrible valves.) You get into a totally different family tonal quality. In many ways preferable, but sadly not perfect. Silky midrange excellent open tops characterise this "house" sound spacial sound srping out from the speakers which sounded like closed boxes to the former group. But sadly the bass is thin. like so many things we build, it is a tone control. Whether measureble ? I haven't a clue, perceptible yes. I hold to the idiom my ears resolve better the differences than any measurements you can show me. I need no explanation of that, if you won't believe without understanding the explanation I am sorry for you, but I cannot help you.

Other members of this group we will call the transmitters are the audio marketed Svetlana 8 and 5 prefixed valves. I only have the 811 10, but it sticks hard in my memory how far superior to my hearing it was to the JJ300b I had at the time. It was like breaking through a glass ceiling. What a shame we no longer have Svetlana to buy from. we witnessed the beginning of the end when they sacked Eric Batbour, we should have realised and bought as many of their valves as we could have afforded. It's too late now.

But like capacitors these different valve groups have their tonal contribution, some prefer one tone over another sufficient to not mind the accompanying compromises of tone controls. But the compromises between valve choice are less destructive to enjoyment that the compromises of passives such as transformers, resistors, capacitors, wire, solder, potentiometer, and their interaction (circuit design).

[Cressy Snr]

Interesting bit there Paul about operating the 2A3 more like a 45.
It certainly has a lot of similarities to the tone of the 45 pp amp that Mr I brought to Owston a couple of years ago, before you returned to the scene.

[pre65]

I run my d3a + 6B4-g at 5K.

Speaking of which, the d3a is due to be replaced in a day or two. I've ordered some octals and a few resistors from HFC to try something a "bit different".

If it works I'll show it to a few friends next weekend.

I'm not unhappy with d3a, just wanted to do something different.

[Nick]

I think you may find that the regulator is adding (or not adding) a lot more to what you are hearing than you might expect. Try a 300b with a good driver (say your existing 2a3) off the same power supply and you may hear much of the same goodness.

[Cressy Snr]

I think you may find that the regulator is adding (or not adding) a lot more to what you are hearing than you might expect. Try a 300b with a good driver (say your existing 2a3) off the same power supply and you may hear much of the same goodness.

Well I know Nck that you have, for a long time, promoted the use of regulators
in SET power supplies and it certainly is a lot better than previous unregulated iterations of 2A3 amps I have built.
Now if I can get those globe meshplate 300Bs off our Anthony, I might look into doing something with them, off these power supplies.
May have to finally invest in some decent inter-stage transformers.

[Mike H]

I read an article by Thorsten from a decade ago by now, where they were playing around with 2A3s at high Va (around 300V) with lowish current (45-50mA) into a 5K transformer load and reporting very good sound.

FWIW that's the most obvious way I'd think of doing it as well. At some point mean to work up to trying it with 6C4C

NB: the "very good sound" may be because the DC current is now lower if using the same OPT, so that it's a better match to the core's air gap size, or is just a different and "better" OPT to get the req.d 5k primary imp., or if it still is the same one, the different impedance ratio = using all the winding layers or a different winding order that makes the coupling better.

All I'm suggesting is beware of attributing it sounding better to just one thing, viz, running a 2A3 at lower Ia, higher Va. Might still be true but my money's on a change to lower DC better matches the air gap so the OPT is just working better.

I know what I mean ...

[thomas]

think you may find that the regulator is adding (or not adding) a lot more to what you are hearing than you might expect.

I'll second that! Just swapped out an LCLC with a valve regulated power supply (Nick's schematic cribbed in its entirety off his 300b schematic thread) on my 300b amp- very interesting! I've persevered with these valves Lord knows why and this is about the first time they aren't just murky, muddy and un musical...!

[Andrew]

May have to finally invest in some decent inter-stage transformers.

Just to add to the mix and increase your options here's a thought....

Personally, I'm not convinced for SE that CL coupling isn't going to be just as good as an interstage, or even LCL. The added benefit of LCL or CL is you can control the quality of the 'small' cap, most interstages are bifilar which means a certain amount of C anyway. In fact isn't a bifilar TX effectively LCL coupling anyway

Andrew

[Cressy Snr]

Thanks guys

Yes this is the first SET that has really done the business for me and Lord knows I've built a few.

All of them, though seemingly excellent sounding at first just left me bored
after a few weeks of use, hence my move to the more exciting sound of SEPs.

There does seem however to be something about this particular SET that speaks "music" rather than the "recording" the other SETs told me about.
I mean the bass in particular, kicks arse like no 2A3 amp I've heard to date.

It is the 13E1 tetrode and PL509 amps that were instrumental in my development of the big half-amp power supplies.
Although the 13E1 was flawed in the end, it did educate me in the art of the regulated high current PSU.

This seems to be paying off with the 2A3 although at first glance it would appear that the PSUs
for these monoblocks are ridiculously over-engineered for the low power DHT they are feeding.

Let's just say I would never have built such big powerful PSUs if I had been designing a 2A3 amp rather than a big SE Tetrode.

[Paul Barker]

I read an article by Thorsten from a decade ago by now, where they were playing around with 2A3s at high Va (around 300V) with lowish current (45-50mA) into a 5K transformer load and reporting very good sound.

FWIW that's the most obvious way I'd think of doing it as well. At some point mean to work up to trying it with 6C4C

NB: the "very good sound" may be because the DC current is now lower if using the same OPT, so that it's a better match to the core's air gap size, or is just a different and "better" OPT to get the req.d 5k primary imp., or if it still is the same one, the different impedance ratio = using all the winding layers or a different winding order that makes the coupling better.

All I'm suggesting is beware of attributing it sounding better to just one thing, viz, running a 2A3 at lower Ia, higher Va. Might still be true but my money's on a change to lower DC better matches the air gap so the OPT is just working better.

I know what I mean ...

I know exactly what you mean, we are on the same song sheet.

[IslandPink]

"I never met a 2A3 amp that I didn't like" as Joe Roberts said
;o)

[Cressy Snr]

What I'm saying is I think this SET is "better" because of the flatter load line, higher voltage swing, better OPT coupling as Paul and Mike suggest
and the kick arse regulated power supplies. All in all a formidable combination
and a long time taken to learn how to do it

It's only now I feel I'm getting somewhere
and have broken through the glass ceiling as Paul puts it.

Somebody should have told me all this in 2005!

[Paul Barker]

May have to finally invest in some decent inter-stage transformers.

Just to add to the mix and increase your options here's a thought....

Personally, I'm not convinced for SE that CL coupling isn't going to be just as good as an interstage, or even LCL. The added benefit of LCL or CL is you can control the quality of the 'small' cap, most interstages are bifilar which means a certain amount of C anyway. In fact isn't a bifilar TX effectively LCL coupling anyway

Andrew

Yes I agree. Except the capacitance in a cap coupled amp acts as a high pass filter whereas the interwinding capacitance of the transformer is a low pass filter which sets the -3db point of the High frequency performance. So they aren't equivalents.

Then of course there is the trick of bridging a transformer with a capacitor to boost Hf performance.