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ECL85/6GV8 PP Supertriode
ECL85/6GV8 PP Supertriode
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Old 7th March 2018, 08:45 AM   #1
tikiroo is online now tikiroo  New Zealand
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Default ECL85/6GV8 PP Supertriode

I thought Id share some simulation and experimental results for a PP supertriode amp using ECL85/6GV8. I believe the supertriode connection was developed by the Japanese and has popped up from time to time on diyAudio. John Broskie has explored the concept at length:

SuperTriode
Single-Ended Amplifiers & Super-Triodes

My aim (bearing in mind that Im doing this for fun / intellectual challenge and not in pursuit of some sort of audio nirvana) was to use a pair of triodes and high voltage mosfets in supertriode connection in a fairly conventional push-pull arrangement with a mains toroidal used as an output transformer. Schematic for amp and power supply below. This was for proof of concept, and is supplied with a balanced signal (I used a cheap DRV134 board). +/- 1.5Vrms for 50W into 8 ohms at around 0.1% THD (sim) with 12 dB gNFB.

ECL85/6GV8 PP Supertriode-ecl85-pp-supertriode-jpg
ECL85/6GV8 PP Supertriode-power-supply-jpg

Comments on the schematic:
Stability networks actual values after squarewave testing. Zobels on Output Tx primary gave a slightly cleaner squarewave on 4 ohm loads but probably not necessary
Diodes D1-D4 and D5-D8 allow class AB operation of bias servos.
12V zeners D9/D10 stop mosfets from potentially destroying themselves while C4/C5 charge at start-up (I blew one when I turned on B+ with valves warm and a longer RC time constant)
ECL85 output bias voltage (-12V in schematic) adjusted to give balanced total current through each half of transformer. In practice not too sensitive and very stable.
Mosfets dissipating about 21W at 136 mA , set by 5 ohm resistor. 25W/170mA would be a bit lower distortion.
Power from 115V stepdown or isolation transformer (cheap!) with doubling to provide B+ to drivers. Circuit from the best voltage doubler -the 4X8!

Im using a cheap 120VA 115+115:30V (32.5V unloaded) toroidal as a 400:8 transformer, which is theoretically good for 112W into 8 ohms @50Hz, and allows decent power at lower frequencies.
ECL85 used because I had some, the pentode section looks very good triode strapped (see SPICE Design of 6F5P SET Amplifier and attachment), and it was easy to get the voltages right for the mosfet drive. 2SK3746 is a 1500V mosfet, chosen because I can keep within the DC SOA and bias current doesnt move around with temperature (at least according to datasheet). Scope pictures to follow.
Attached Images
File Type: jpg ECL85 PP supertriode.jpg (125.5 KB, 605 views)
File Type: jpg Power Supply.JPG (86.2 KB, 591 views)
Attached Files
File Type: pdf ecl85pentode_as_triode.pdf (21.7 KB, 34 views)
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Old 7th March 2018, 08:47 AM   #2
tikiroo is online now tikiroo  New Zealand
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Power at clipping about 70 watts into 8 ohms (sorry no picture, my test load cant take this for more than a few seconds). -3dB about 50 kHz open loop with no compensation. After compensation and 12 dB feedback, -3dB at 55 kHz (could push this higher if I was less fussy about squarewave response). I need to get a decent test load but at 30 Hz I was approaching 50 watts before visible waveform distortion. Signal generator and oscilloscope both from 1960s, 1kHz photo is identical to what was present at generator output.

Next a schematic for a folded cascade phase splitter that looks promising in the simulator.
Attached Images
File Type: jpg 1 khz sinewave 10 watts.jpg (118.1 KB, 288 views)
File Type: jpg 1 kHz squarewave.jpg (115.3 KB, 34 views)
File Type: jpg 5 kHz squarewave.jpg (117.6 KB, 29 views)
File Type: jpg 10 kHz squarewave.jpg (110.6 KB, 29 views)
File Type: jpg Supertriode breadboard 2.jpg (171.5 KB, 47 views)
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Old 7th March 2018, 08:51 AM   #3
tikiroo is online now tikiroo  New Zealand
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Folded Cascode idea from Rod Coleman, with modifications suggested previously by PRR. I incorporated some Aikido-like PSRR improvement. Not built yet.
Attached Images
File Type: jpg Folded Cascode supertriode.JPG (120.0 KB, 101 views)
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Old 7th March 2018, 08:55 AM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Super triode?
It is a triode-pentode with the screen grid connected to the anode of the pentode for lower gain in triode mode as a standard cathode follower driving a pair of FETs
No such thing as a "Super Triode".
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Old 7th March 2018, 12:02 PM   #5
ballpencil is offline ballpencil  Indonesia
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Quote:
Originally Posted by JonSnell Electronic View Post
Super triode?
It is a triode-pentode with the screen grid connected to the anode of the pentode for lower gain in triode mode as a standard cathode follower driving a pair of FETs
No such thing as a "Super Triode".
It would be a standard cathode follower had the pentode's anode connect to HV+ instead of the OPT.
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Old 7th March 2018, 05:24 PM   #6
tikiroo is online now tikiroo  New Zealand
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See below for the difference between the two topologies.
Attached Images
File Type: png Not So-Super Single-Ended Amplifier.png (19.8 KB, 83 views)
File Type: png Super-Triode Single-Ended Amplifier.png (19.9 KB, 59 views)
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Old 7th March 2018, 05:43 PM   #7
stocktrader200 is offline stocktrader200  Canada
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I did the same with bipolar NPN as opposed to mosfet. The sound is wonderful probably better then a power tube amp as the 6922 is more linear then a power tube.
12AX7 gain stage > 2N5551 splitter > 6922 + MJE13004 = 30w RMS from 150v B+ 4K transformer.
no plate feedback in fig.1 = awful sound quality and huge gain. Transistors do not have collector feedback like tubes

what is your idle current ?

Last edited by stocktrader200; 7th March 2018 at 05:47 PM.
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Old 7th March 2018, 10:37 PM   #8
tikiroo is online now tikiroo  New Zealand
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Quote:
Originally Posted by stocktrader200 View Post

what is your idle current ?
I was aiming for about 170mA (simulation shows this as the sweet spot) but got about 135mA as the actual beta and Vbe for the bias servo transistor was a little different to the simulation. I will reduce the value of the 5 ohm source resistor to increase the bias current. I want to keep the mosfet dissipation to about 25 watts per device to avoid massive heatsinks.
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Old 8th March 2018, 03:17 PM   #9
stocktrader200 is offline stocktrader200  Canada
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a 27 ohm in parallel with the 5 ohm should get you pretty close to 170 ma
I biased the BJT with a 1N4148 to prevent turn off and set bias to 80ma with the emitter resistor and 6922 cutoff voltage.
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Old 8th March 2018, 05:26 PM   #10
tikiroo is online now tikiroo  New Zealand
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Quote:
Originally Posted by stocktrader200 View Post
I biased the BJT with a 1N4148 to prevent turn off and set bias to 80ma with the emitter resistor and 6922 cutoff voltage.
Are you able to post a schematic or sketch of your design?
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