I recently stumbled across https://hackaday.io/project/3297-low-voltage-all-tube-amplifier, which looks kind of nifty; it's a single-tube ECL82 (plus a rectifier in the power supply) amp that requires only 40 volts DC. I'm not sure how well this would work; most of the online reference designs require ~ 210 v. Does anyone have any experience with a design like this?

If this is actually a practical circuit, are there any design changes you'd make?

Finally, could you substitute a 48vdc switching power supply (maybe with a buck converter to provide the 6.3 volts for the heater), or would that be too noisy to be practical?

Can't tell about the circuit. You have to be a member to see it.

Switching regulators can be fine IF they are properly designed. Anything cheap will probably be noisy unless you add your own shielding and filtering.

I'm not a member and was able to click and see the schematic.

Looks like seriously starved B+:


I would expect it to function, but not well. Not much voltage swing, and no negative bias on the pentode.

Bob

The ECL82/6BM8 is no stranger here.
Amps and transmitters....but with 40V?
Doubler at least.

I could see the point of that design if power was from 2 20 volt rechargeable batteries.

But assuming the power transformer primary voltage is 120 vac, it doesn't make sense to reduce the plate volts to only 40 volts, no need to except that the secondary is isolated.

Maybe this amp powers a set of earbuds?

No, it's supposed to be able to drive a speaker. Don't know how successfully.

I might have to try this just because; if I do, I'll let you know how it works out.

Thanks all for the info!

Most audio output tubes I've tried at low B+ levels will operate OK-ish around 50-60v, dunno about a 6BM8 never tried one... The low B+ types, such as 25C5, 25L6 etc have performed the best... There are 6v versions of these tubes but have a hogish 1.2A heater...

The above operated from a 25v xfomer & using a B+ doubler(approx 65-70v) makes a fairly good amp... Of course one needs a additional audio amp stage but assuming the xfomer has a CT, a 12AV6 can be powered from one side...

In the "old days" there were farm radios that operated on 32 VDC. Most of them used a special output tube but some of them used ordinary tubes. I've never heard one so I don't know how well they worked. There are special output tubes that can put out a fair amount of power on 26 V supply: 26A7 and 28D7.

There's a schematic in the specs for 26A7, guessing the 28D7 would perform equally well...

The 26A7 can handle 50v plate so 35-40v B+ from a 25v transformer would be OK, probably work fine with a 25/50L6 PP opt xformer... Specs given for 28D7 is 100v plate but in my experience those are very current hungry at more than 65-70v...

You're thinking of a single-ended output transformer. 2000 ohms plate to plate is much lower impedance, more like a transformer for push-pull triodes.

No I'm thinking of approx 4K P to P 25/50L6 xformer, at 40-ish volts vs 26 that xformer would likely be fine... If one has access to PP xfomer for 7695 use it, I'll use the 50L6...

In my experience far too much is made of having the "rated" load on most tubes... No I'm not saying one should use a 2K 50L6 xformer for a 10K 3V4 but anything halfway close works without noticable issues... I used the loudest taps on a universal xfomer in my TV tube radio that I had no spec... Found out later it was setup for approx 24 ohm speaker, lost probably 25% max volume setting it up correctly... No issue, I didn't need that 25% anyway...

Since I have 28D7, the interstage xfomer and a bag of output xformers, maybe I'll toss that 26A7 amp together... There's already a chassis around here with a Loktal socket & 25v xformer(yes sometime back I built a parallel version, didn't make enough boom for my taste)..

Be interesting to see how a low voltage 12AE6 drives the 28D7... In car radios those generally operated with approx 4.5v on plate...

Truth be told, I didn't think of car radios; I was thinking more along the lines of farm radios and such. I'll be anxious to see what you come up with.

Thanks for giving this some thought.

I have a stereo Philco amp with center channel that's driven PP... It uses a 12AU6 in one channel and 12AU7 in other channel for amp and phase inverter... One section of 12AU7 operates on 13v and other around 40v...


I'm gathering parts for the 28D7 amp, the chassis I was going to use has a different xfomer than I remember(and is a bit small for this project)... Another does have 25v xffomer plus 12AE6 already wired with gain control, I have to mount the 28D7 socket... Has fullwave P/S but I suspect the filter caps aren't beefy enough... Also rather smallish but I believe the interstage xformer will fit under chassis...

Well the verdict is in, it's a little weak on bottom end(may be the 100w rack system speaker) but otherwise impressive... It absolutely clobbers the 12K5 amp that was last project on this chassis... Only deviation from schematic is the gain control & I added a 1K resistor in 12AE6 plate supply as a safety precaution for interstage xfomer, don't have a abundance of those...





On 27v B+ it'll produce 500mw(least that's what the LED claims)...





Audio is clean not distorting at approx 80% volume, scope probe connected directly to output xformer... Signal supplied from FM tuner.. Before this is said & done I'll toss three or four more xformers at it...

I traced weak bottom end partially tothe 1K ohm resistor I installed in primary B+ feed to interstage transformer... Added a 1mfd electrolytic to junction, but resistor isn't necessary, current draw only 2ma.. If you don't have a 12AE6 a 12BF6 works just as well, 12AT6 & 12AV6 distort badly...

As a side note, RCA's notes on design states the 12AE6 was based on 12BF6...