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Electro Harmonix 2A3 above 300V?

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I've noticed that the EH 2A3 specifications indicate a maximum plate voltage of 450v, well above the 300v limit for conventional 2A3s. If that's accurate, it should allow increased power output from a class AB1 push-pull pair, above the 15W or so possible with conventional ones. If the tubes were biased at 40mA and 360v, for instance, it seems like 20W might be possible, still keeping within the 15W plate dissipation (at quiescent, at least). I'm curious enough to buy a pair and try it.

But first, I'm wondering if anyone out there has experience with the EH 2A3 above 300v?

Thanks,
Scott
 
I got some EH 2A3's Cheap from Sovtek; made amplifier at 400volts kept current low so not to exceed 15 watts; Problem is the 2A3 EH has gone way out of my price range. I find 811a's sound far better any way, in push pull. Maybe Sovtek was getting rid of my 2A3 Eh cheap they are very unstable even in the box; every time I test them in the AVO I get different reading. The 2A3 Eh sound good.
 
It's encouraging that you successfully ran them at 400v.

In the US, there are both Sovtek and Electro-Harmonix branded versions of the 2A3, which are quite similar. There is a significant price difference, and the specs sheets are identical. The EH are claimed to have a gold grid, which may improve bias stability, I don't know. Do you know which you had? Thanks.
 
There was a lot of talk in the early '00s that the Sovtek 2A3 was a mini 300B in a smaller bottle, derated a bit but much more robust than original 2A3s. Uncle Ned said the 6B4G variant was good up to 30W.

The EH variants are pretty much the same tube to my knowledge, gold grids etc.

I use the Sovtek 6B4G's in PP fixed bias, they're tough. 325v, 60mA, so I'm at 19.5W dissipation. They chug along happily for years and sound great. I did have one with a grid short after a couple hundred hours, but that happens.
 
Hi

Max plate dissipation is still rated at 15watts on the specs sheet, and if you look at the plate curves, running them with the plate at 400v means you need to need to bias them down into the knees to keep within this limit.

Sure, you can bias them at higher plate dissipation, and get them back into some linear range. They're your tubes. You can also burn the rubber off the tires on your car if you wish.
 
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I guess I tend not to risk the rarer or more expensive tubes for experiments..


Well, people don't come any cheaper than me, and I'm very comfortable running them at 20W. While I don't put a lot of hours on these, I built the amps in 1999, and I'm on my 2nd quad (plus the pair lost with the short mentioned above).

I agree though, 400v is probably too high to keep it linear.
 
OK, thanks for all the feedback! I'm encouraged enough to give it a try.

360v and 40mA per tube seems like a reasonable bias point to me - I'll measure the THD and power at that point using a bench power supply. I definitely wouldn't risk that with the few precious NOS 2A3s I have....

Scott
 
The JJ 2A3 is essentially a 2.5V filament version of the 300B.
The plate is as big as a 300B, and the glass bottle is as big or bigger than a 300B.

Many other current production 2A3 are monoplate; but they do not have the big plate, and do not have the big glass bottle of the JJ 2A3.

Pick your tube . . . 2A3 15W plate; or JJ 2A3 40W plate.
 
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The JJ 2A3 is actually a pretty rugged tube, and pretty consistent as well. I had a push pull AB2 using them a while back, and 30W idle dissipation was no sweat for them, after several hours of daily use for two years the tubes still have plenty of life left, and sound great according to the current owner.
 
The JJ 2A3 is essentially a 2.5V filament version of the 300B.
The plate is as big as a 300B, and the glass bottle is as big or bigger than a 300B.

Yes, the JJ 2A3-40 is an excellent and robust tube and can easily take 300V at 65mA, however it is NOT a 300B. The tube uses a 300B plate but biases up as a 2A3 and the plate curves follow a 2A3. I have both the JJ 2A3-40 and the JJ 300B and each precisely biases up at the operating points of their archetypes the 2A3 and 300B.
 
I agree - I really like the JJ 2A3-40, too. It has the filament voltage and characteristics of the 2A3 with the dissipation of a 300B - the best of both worlds. I have a 30W push-pull class-A amp using this tube.

But I'm still curious about the Sovtek/EH 2A3, perhaps for a smaller, lower-power application.
 
Correct, the JJ 2A3 is not a 300B, but it also is not a 2A3. Curves are not the only criteria. There is nothing that says you need to bias the JJ 2A3 to the classical quiescent conditions. The plate is perfectly capable of dissipating up to 40Watts. Why not build an amp that uses it at 30, 35, or 40 Watts in the plate. You will get more output than a 15 Watt plate 2A3 amp.

No traditional 2A3 plate and glass envelope can dissipate 40 Watts for long (not even with forced air cooling). Let me know if you try that.

And as for the 15 Watt dissipation Mono-Plate Sovtek 2A3, I have used them, and they work good and sound good in classical 2A3 circuits at classical operating voltages and currents. There is no reason why you can not build and enjoy a lower power amp, i.e. a 15W plate 2A3. Well, you may have inefficient loudspeakers, a very large room, prefer bleeding ear drums, or all of those.

I like both the JJ 2A3 and the Sovtek 2A3. Enjoy!
 
I finally had the opportunity to try the Electroharmonix 2A3 in my amp, in place of the JJ 2A3-40, and make some detailed measurements. There's not much published operating point information for these tubes above the traditional 300v 2A3 limit, so I thought I'd post my results for others.

The first attachment shows the operating points I tried for both tubes, and the corresponding power output (at 1% THD and 1kHz). To keep the no-signal plate dissipation of the EH 2A3 to 15W, I used a 5R4 rectifier in place of the 5AR4 I usually use in my amp, which dropped the no-signal plate voltage to 400v. So the EH 2A3 could be biased at 37.5mA x 400v. I got maximum power around 23W for the EH and 30W for the JJ.

The 2nd attachment is a plot of THD vs. power for both tubes at a plate-to-plate load of 4130 ohms. The JJ is slightly better, of course, but the EH performs very well. These measurements are in an amp with NFB, but the comparison should be valid at any feedback level. I'm impressed enough to try the EH 2A3 in a smaller amp.

Scott
 

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