Heater current

[Tom-s]

I'm considering the Stereomour II.

Question for PJ.

What are the max current ratings for the heater current?

I'm asking for both the 2A3's supply and the 12AT7's supply.

This is asked with tube rolling in mind. 

Edit, driver tubes in mind: http://tdsl.duncanamps.com/pdf/a2900.pdf

Edit 2: Can't find the output tubes anymore. But had a pair of 2A3 tubes somewhere that look like 45's plate, not like 2A3's. But since the 45 draw less current, they'll probably work.

 

[Paul Joppa]

The 6.3v heater winding is rated 1.5 amps.

The 2A3 filament windings are actually 3.15 volts at 5.5Arms. (The high current rating allows for DC heating, which is an optional upgrade for those with extremely sensitive speakers.) In both AC and DC cases, the filaments have small series resistors to fine-tune the voltage.

 

[Tom-s]

Thank you PJ! Figured the AC supply for the output was overbuild but unsure about the drivers.

Good to learn i can try the Stereomour II with those higher current "12AT7".

 

[Deke609]

i can try the Stereomour II with those higher current "12AT7".

I look forward to reading about your experiments -- and to poaching your successes  ;D

 

[Tom-s]

Succes with the GEC is to be expected (so the internet says).

Other higher current tubes i'm not so sure about, like https://frank.pocnet.net/sheets/009/e/E180CC.pdf

Or: https://frank.pocnet.net/sheets/137/5/5965.pdf

Edit another: https://frank.pocnet.net/sheets/129/6/6211.pdf

 

[Tom-s]

Order complete! Can't wait to try A2900's in the Stereomour II with my headphones!

Found the weird 2A3's again. The one below is complete with it's original "bande de garantie" dating 20 June 1936!

Hopefully it will still work!

 

[Doc B]

Thanks for your order, Tom, and thanks to the others who have ordered as well!

 

[Deke609]

Found the weird 2A3's again.

Very cool - 45 constructed 2A3! I haven't heard any NOS 2A3 tubes - just the Sovteks that come with the kit and my JJ 2A3-40s.

Really looking forward to your listening impressions with those old 2A3s.

If you're interested, once your amp is bunred in I could loan you my JJ's for comparison - which I understand are a 300B construction with 2.5V filament. I'd love to know whether some of the 45 magic I hear could be had from NOS 2A3 tubes.

cheers,

Derek

 

[Paul Joppa]

Those were the first 2A3s; lasted about one year before they converted to the "biplate" design. Very rare, quite valuable too!

 

[Tom-s]

Last week i've learned a thing or two about the monoplate.
Now i'm actually in doubt if they work well in the Stereomour.
Are these monoplates more fragile? Should changes be made to Stereomour II to use them safely?

As an aside, as much as the vintage single-plate 2A3s are heralded as the bee's knees by some folks, I have heard that some (perhaps many) modern 2A3 amplfiers operate under conditions that are too much for 'em. I got some empirical data on this when I tried one in a single Bottlehead Paramour. Oops

 

[Paul Joppa]

Stereomour, which is the same basic circuit and operating points as Paramour, runs the 2A3 at its maximum dissipation and maximum voltage. The normal biplates are quite tough, and of course the modern monoplates are mini-300Bs and most of them can handle quite a bit more power and/or voltage - at one point the Sovteks were said to be good for 23 watts dissipation.

I too have heard intimations that the original monoplate is a little fragile. I would not operate them in Stereomour if they were mine, out of caution for a rare and valuable tube. But I have no direct experience and no actual data; I don't think I've ever heard exactly what the problem is.

I can speculate that the originals were either difficult to make, or had a high rejection rate, or a high failure rate in service. There must have been some reason they were quickly replaced, after all.

One could probably operate them as if they were 45s, reducing both the plate voltage and current - but as I said, I don't know what the risks are, so I can't make any actual recommendation.

 

[Tom-s]

Thank you for the thorough reply!

From what i've read it has to do with the heater wires easily failing and shorting the grid wires.

Anyhow. That's left me with 0 options to roll for the outputs of the Stereomour, but have many 12AT7's to try.
Was happy with these tubes past week. Now they'll go back in stock to await a purpose.

Can't wait to start the build on SII.