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.
- 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.
I am planning to build a se amp with a 1619 dht in triode mode. The datasheet shows the positive grid curves for the pentode mode. I manually checked out the triode connection in the a2 region and it looks promising. A +_25v swing with 2k load goes from 135 ma to 20ma. 180v,0 bias and 70ma quiescent current.
Alfredo
Alfredo
Is that the same as a DET19 or 2C34? I have a few of those, on my list to try. They seem like they would work great in A2.
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
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
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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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
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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