• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need help with new amp design

Hi. A couple years ago I did a little "antique radio" podcast player project around a Beaglebone Black single-board computer. The original prototype used a bunch of off-the-shelf components and was hastily put together.

I since developed an add-on board for the BBB that integrated all the functional blocks in the original and improved the audio quality.

But to really bring the project home, I want to develop a tube-based amplifier section. I've been looking at various circuits online, but none of them really meets all of my needs:

  1. Monophonic
  2. Lots of tubes (3 or more; many designs have only one)
  3. Glowing tubes
  4. Small or no transformers
  5. High fidelity is NOT a requirement
  6. High power is NOT a requirement (20 W max)
  7. Ideally, can be powered from 12 - 15 VDC source (for battery operation)
  8. All components on a single PCB, including DAC and other non-audio interface electronics

I came across this Velleman kit that seems to meet the first two requirements, but is a bit excessive. I think the output stage of four tubes can be reduced to two tubes (which helps with #6), but it still requires fairly high power (200+ W) and AC power in places. I know some old radios were battery-powered, but it seems battery-powered tube amps are now hard to find.

Anyway, I'm looking for design guidance and help. Thanks!
 
Are you looking for this to drive a speaker? If so, you're going to have trouble with #4 and #7 (unless you use some kind of boost converter). 12-15VDC is enough for a small line level tube (keeping in mind #5), but not enough for an output stage for a speaker. That Velleman is going to be using much, much higher voltages.

Battery power and tube power amplifiers are not something you'll find everyday (and for good reason).
 
Hi Sodacose, yes, this would power a speaker.

I've seen discussions online about 12 V-capable tubes for automotive use (although one said they work better at 24 V). There was also mention of a "vibrator" for generating the high voltage. Given this discussion, I'd probably generate that electronically.

The KT88 Velleman circuit seems to want 200 W for the plates on the output stage. That's over 16 A at 12 V, which is substantial. Now, since I was thinking of building only half that output stage, I only need half the power, and really I need less since I'm looking for a smaller maximum power output. So maybe I can successfully generate the high voltage from a digital circuit.

I wonder how much noise a DC boost regulator would introduce into the output audio, and could I filter that out?
 
Your list of requirements is pie in the sky stuff.

Can you be more specific? Can you elaborate on what I can reasonably expect to have?

Given that old radios operated from 6V batteries, I'm pretty sure I can find a way to make this work. In fact, as I do more research, I'm getting more confident that it will be possible (a 300 VDC boost converter is not terribly difficult).
 
Forget 15 V. If you are willing to go to 45 V., the 3S4 "power" pentode can be your "salvation". Milli-watt power is what you'll get. You will need an O/P transformer and Edcor's XSE10-8-8K is more than hefty enough for the job.

A series wired group of 5X 9 V. alkaline radio batteries will last for quite some time.

Thanks, Eli. I think I'll focus on finding a way to provide the high VDC necessary.

Does anyone know if it's okay to drive filaments with DC?

Thanks!
 
Hi Sodacose, yes, this would power a speaker.

I've seen discussions online about 12 V-capable tubes for automotive use (although one said they work better at 24 V). There was also mention of a "vibrator" for generating the high voltage. Given this discussion, I'd probably generate that electronically.

I've heard of these as well. Maybe someone else will be able to suggest a suitable schematic. I haven't personally played with these, but some have been popular (6GM8) at one point or another.

The KT88 Velleman circuit seems to want 200 W for the plates on the output stage. That's over 16 A at 12 V, which is substantial. Now, since I was thinking of building only half that output stage, I only need half the power, and really I need less since I'm looking for a smaller maximum power output. So maybe I can successfully generate the high voltage from a digital circuit.

The Velleman may dissipate 200W in the tubes, but it will be at high voltage. Unfortunately, the tube chooses the proper voltage. We can't just pick our operating voltage and find the current to make it fit. Tubes operate in mA.

I wonder how much noise a DC boost regulator would introduce into the output audio, and could I filter that out?

This guy might be handy:

10V B+ Flyback Supply
 
The Velleman may dissipate 200W in the tubes, but it will be at high voltage. Unfortunately, the tube chooses the proper voltage. We can't just pick our operating voltage and find the current to make it fit. Tubes operate in mA.

Well, 200 W is 200 W. The reason I'm concerned is that in a boost converter, the low-voltage side operates at much higher current. To provide 200 W at 12 V, I need 16.7 A.

I'll check out that flyback supply. Thanks!
 
Yes, you can definitely power filaments with DC.

Sorry, I missed that you were already considering the boost converter and thought you were going to just feed tubes 200W at whatever voltage you had available.

With a 10W boost converter, you might be able to get 1W of audio out of a tube design. I'd suggest looking at the 12B4A if going that route. An output transformer will still be necessary.
 
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Original requirements are three tubes per channel, Bigun. Figure that out :)

But there's so many ways to use up tubes at the front end !! - and the poster said '3 or more' so it could be more !!

I'd narrow the search down to tubes happy to run off low voltage so that we don't need to do a lot of boosting here.

One of the best options is to find a low-mu tube (because grid is further from cathode so grid current at low plate voltage is minimized) and a low power tube (because we don't want to draw too much heater current). Doesn't the 12AU7 fit this description ?
 
Sounds like Aikido talk to me :)

Why not? :)

One 6J1P or 6J2P, even 6J32B, triode strapped, loaded on a Gyrator using one MOSFET, second MOSFET as a source follower loaded on one more CCS. 50V of B+ would be more than plenty. Use a boost - converter (available cheap on ePay) to convert 12.6V to 50V, and power everything (including filaments) from the battery.

Or, use 42KN6 tubes with 42V B+, OTL, instead of 2'Nd and 3'Rd MOSFET ;)