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Flea-power class-A2 using a power opamp driver

classa2.jpg


I always wanted to try using a power opamp as a class A2 driver. I finally got around to some experiments. I managed to have some sucess, especially using a 1636 (ala Darling amp) to get ~1.2W out. Still experimenting - I need to try push-pull, and some different tubes. Here's the info:

Class-A2 and a power opamp driver

Let me know if you can think of any candidate tubes. The power opamp can only put out about +/-36V @ 50mA, so stuff like 811's are not workable. So far I've tried the 1636, 2E26 (OK but not a lot of advantage over A1), and 6V6/6L6/6W6 (worse than A1). Seems like some of the newer semi-useless 9-pin-mini RF tubes might do well here?

Pete
 
Small power valves were usually either:
- not intended to take grid current, so the grid may overheat
or
- intended for Class C, so not especially linear

Exceptions are QQV series tetrodes (e.g. 6360 in USA), which have a rugged grid for Class C but are also intended for linear applications (mainly AB1). Another possibility is 5B/254M, but I would expect that to behave like a 6L6 or 807.
 
Looking at the grid drive 'scope picture on your website makes me wonder if the TI amplifier chips would have an issue with asymmetrical power supplies. If the supplies don't have to be symmetrical the supplies could be optimized for the tubes needs.

Good point - the IC doesn't care where ground is, so that would work well.

Pete
 
The 3C24 looks like a great candidate. It could be zero-biased with symmetric grid swing, 550V B+ or so, and putting out 4-5 W Po, but needing an 18K Zpri. It may still want some feedback, but that should be easy to arrange with the opamp. The quiescent power at ~14W should light up the anode a little.

Cheers,

Michael
 

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Tried a few more tubes in SE this morning:

2C34 / RK34 / DET19 (both sections in parallel): Very promising... B+ 200V, VP ~180V, IP ~ 60mA, zero bias, 2.5k plate load - 1.4% THD @ 1W, 3.3W at 5% THD. I tried cathode bias and upping B+ but performance was worse.

3C24: Need higher impedance OPT and maybe higher voltage, but... at B+ = 400V, IP ~ 30mA, zero bias, 7k load - about 4% THD at 1W.

703A: Can't believe it can really live long at 20W dissipation on that tiny plate... B+ = 300V, IP ~ 70mA, zero bias, 7k load - 3.3% THD @ 1W, 5% @ 2.2W

6360 (triode connected, both sides in parallel): Needed to add 500 ohms cathode bias... B+ = 300V, IP ~ 50mA - 1W THD 2.8%, 5% @ 2.8W
 

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[IM[/IMG]

I always wanted to try using a power opamp as a class A2 driver. I finally got around to some experiments. I managed to have some sucess, especially using a 1636 (ala Darling amp) to get ~1.2W out. Still experimenting - I need to try push-pull, and some different tubes. Here's the info:

Class-A2 and a power opamp driver

Let me know if you can think of any candidate tubes. The power opamp can only put out about +/-36V @ 50mA, so stuff like 811's are not workable. So far I've tried the 1636, 2E26 (OK but not a lot of advantage over A1), and 6V6/6L6/6W6 (worse than A1). Seems like some of the newer semi-useless 9-pin-mini RF tubes might do well here?

Pete
Hello Pete.
If you want Flea power in a cool old bottle try a 6G6G.
DT
All just for Fun!
 
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Tried push-pull-parallel 2C34/RK34's. One opamp set up inverting and the other non-inverting, with gains of 24. Zero bias, 175V on the plates for about 10W dissipation per tube... 8k plate-to-plate (it's what I have sitting around).

Without making any effort to balance or match tubes, 0.5% THD @ 1 watt and about 7 watts at clipping (output tubes, not opamps were clipping at this level).

I need another power amp like I need a hole in the head, but I think I will have to build a real amp using this and a 1641/RK-60 rectifier (another 2-cap tube). Maybe I should call it "Mickey"...

Using NOS plate caps and sockets, the caps and sockets will cost double what the entire tube compliment costs. RK-34 / 2C34's are going for $4.00 each...

BTW, data is here http://www.mif.pg.gda.pl/homepages/frank/sheets/081/r/RK34.pdf and here http://www.mif.pg.gda.pl/homepages/frank/sheets/139/d/DET19.pdf
Pete
 

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