I finished TSE-II 300B Monoblock #2, sounds great as expected, so on to the next Tubelab Project; one of the following:
(1) TSE-II (Stereo) with 2A3s or 45s.
(2) Universal Driver P-P monoblocks with 6L6GC/KT66/KT88
(3) Universal Driver P-P with 6AV5s or 6BQ6GAs; mono or screen drive (?)
(4) TubeLab SSE with 6AV5/6BQ6GA in SCREEN DRIVE
That last one popped into my head because I have a well-abused breadboarded SSE board already equipped with outboard driver sockets.
Plus, several dozen 6AV5s and at least a dozen of 6BQ6s (their top-cap cousin).
Maybe a nice fill-in project...
However, I've NEVER SEEN a single-ended screen-drive, and don't know if it's even POSSIBLE.
With the breadboard SSE, a screen supply is pretty easy.
If Power Drive (with MOSFET) is required, I could just INSERT it in place of the coupling cap; already have a prototype Power Drive board built & tested in my old Point-to-Point TSE clone that I'm not using.
Is SE Screen Drive possible?
Is it worth doing, or should I just work through the UD P-P progression, i.e. 6L6/KTs/Sweep Tubes?
(1) TSE-II (Stereo) with 2A3s or 45s.
(2) Universal Driver P-P monoblocks with 6L6GC/KT66/KT88
(3) Universal Driver P-P with 6AV5s or 6BQ6GAs; mono or screen drive (?)
(4) TubeLab SSE with 6AV5/6BQ6GA in SCREEN DRIVE
That last one popped into my head because I have a well-abused breadboarded SSE board already equipped with outboard driver sockets.
Plus, several dozen 6AV5s and at least a dozen of 6BQ6s (their top-cap cousin).
Maybe a nice fill-in project...
However, I've NEVER SEEN a single-ended screen-drive, and don't know if it's even POSSIBLE.
With the breadboard SSE, a screen supply is pretty easy.
If Power Drive (with MOSFET) is required, I could just INSERT it in place of the coupling cap; already have a prototype Power Drive board built & tested in my old Point-to-Point TSE clone that I'm not using.
Is SE Screen Drive possible?
Is it worth doing, or should I just work through the UD P-P progression, i.e. 6L6/KTs/Sweep Tubes?
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I've heard of screen drive and crazy drive, and some others, but I've never really taken the time to learn anything about their topology.
I still have gobs of difficulty with ordinary control grid drive.
Maybe if this thread grows some legs, they can be explained herein. So I hope it can be done on a single ended stage!
I still have gobs of difficulty with ordinary control grid drive.
Maybe if this thread grows some legs, they can be explained herein. So I hope it can be done on a single ended stage!
EAR 859 (YIKES)
Here's the schematic...
Holy Sheit/Merde/[insert your language here]
47uF/630V caps. A walk in the park.
Oh, probably jumped the gun on that; see Post 38 here:
https://www.diyaudio.com/forums/tubes-valves/27132-ear-859-questions-4.html#post770942
Wacky (and possibly "seeded") component values, plus a specially-wound OPT.
"Eccentric" is a word frequently used in EAR 859 posts.
Was hoping for something a little more conventional (read: CHEAP); ideally using iron and tubes gathering dust in the back of a shelf.
This is the TubeLab forum, after all.
Here's the schematic...
Holy Sheit/Merde/[insert your language here]
47uF/630V caps. A walk in the park.
Oh, probably jumped the gun on that; see Post 38 here:
https://www.diyaudio.com/forums/tubes-valves/27132-ear-859-questions-4.html#post770942
Wacky (and possibly "seeded") component values, plus a specially-wound OPT.
"Eccentric" is a word frequently used in EAR 859 posts.
Was hoping for something a little more conventional (read: CHEAP); ideally using iron and tubes gathering dust in the back of a shelf.
This is the TubeLab forum, after all.
Attachments
It is possible to build a single ended screen driven amp. There have been a (very) few scattered through the tubes forum on diyAudio over the years. I seem to remember one using the 4D32 transmitting tube, but maybe I'm not remembering correctly
The main issue is that a screen driven tube has a very high output impedance. In some cases it can be higher than the same tube in grid driven pentode mode. This leads to a near zero damping factor unless a pile of GNFB is used, kinda defeating the purpose of an SE amp to begin with.
The only positive reason is that a sweep tube can be used due to its sensitive screen grid, the negative reason is that a sweep tube can be blown up by overdriving that sensitive screen grid.
The whole UNSET thing came about over several years of experimenting to run sweep tubes in a triode like mode without blasting the screen grid......
My fascination with screen drive started in 2007 with the 6AV5 push pull design and peaked shortly after that. Many of the experiments played out in these old threads. Here some cheap TV tubes made stupid amounts of power, but I discovered the fatal failure mechanism in all screen driven designs. It's discussed somewhere in one of these threads. After blowing up a box full of tubes I began to search for a better way.
Tube sale at AES
Screen drive and other P-P experiments
Show me your screen drive circuits
I built the first working Dual Drive amp in posts #29 and 30 here (also called crazy drive):
G1=G2/mu Scaled Drive Strawman Design
The main issue is that a screen driven tube has a very high output impedance. In some cases it can be higher than the same tube in grid driven pentode mode. This leads to a near zero damping factor unless a pile of GNFB is used, kinda defeating the purpose of an SE amp to begin with.
The only positive reason is that a sweep tube can be used due to its sensitive screen grid, the negative reason is that a sweep tube can be blown up by overdriving that sensitive screen grid.
The whole UNSET thing came about over several years of experimenting to run sweep tubes in a triode like mode without blasting the screen grid......
My fascination with screen drive started in 2007 with the 6AV5 push pull design and peaked shortly after that. Many of the experiments played out in these old threads. Here some cheap TV tubes made stupid amounts of power, but I discovered the fatal failure mechanism in all screen driven designs. It's discussed somewhere in one of these threads. After blowing up a box full of tubes I began to search for a better way.
Tube sale at AES
Screen drive and other P-P experiments
Show me your screen drive circuits
I built the first working Dual Drive amp in posts #29 and 30 here (also called crazy drive):
G1=G2/mu Scaled Drive Strawman Design
I think Tim de Paravicini did indeed specify a "very good" OPT, which is always good, but I do not see anything "exotic" on the unit used (no added windings, taps, etc).
Thing is, there is not a lot of worked out schematics for single ended screen driven amps, so some experimenting will be necessary. Look at this thread EL509 (JJ) or PL519? for some additional schematics using tubes and mosfets to drive the screen.
In these amplifiers it is basically a voltage gain stage, than a current gain stage, and the output tube. The output stage has a high plate impedance, making it a transconductance amplifier. Do you want that? If not, some form of feedback can reduce impedance and distortion.
The El519/509 allows for a lot of plate dissipation and consequently output power, still a 6AV5 will also make a couple watts.
Thing is, there is not a lot of worked out schematics for single ended screen driven amps, so some experimenting will be necessary. Look at this thread EL509 (JJ) or PL519? for some additional schematics using tubes and mosfets to drive the screen.
In these amplifiers it is basically a voltage gain stage, than a current gain stage, and the output tube. The output stage has a high plate impedance, making it a transconductance amplifier. Do you want that? If not, some form of feedback can reduce impedance and distortion.
The El519/509 allows for a lot of plate dissipation and consequently output power, still a 6AV5 will also make a couple watts.
I found the 4D32 schematic that I was thinking of. It is actually a dual drive / crazy drive design. Cathode feedback is applied to the output tube to improve the DF.
See post #581 here:
Gold mine of DI¥ audio tubes schematics from Japan
Pete Millett has recently posted a 4D32 design using conventional UL drive, and I may get to light a pair of these tubes up in UNSET mode if things go right.
See post #581 here:
Gold mine of DI¥ audio tubes schematics from Japan
Pete Millett has recently posted a 4D32 design using conventional UL drive, and I may get to light a pair of these tubes up in UNSET mode if things go right.
I mocked up an AC coupled, cathode biased version of Michael Koster's Schadeode so that I could easily test different tubes by changing the cathode resistor and the screen grid zeners.
https://www.diyaudio.com/forums/tubes-valves/196532-hybrid-partial-feedback-se-amplifier-2.html#post2710121
The plate to grid feedback is 98% by my seat of the pants calculations. And the tests were performed with Bud Purvine 5k ohm impedance transformers.
I tried the Rev1 circuit with 6L6WXT+, 7581, Philips top cap 6BG6GA EH 6CA7, 6CB5A and 6P20S, 26LW6 and 4D32.
The lower PD tubes produced 1 watt with midband distortion at 0.15% or lower, 8 watts at 1% and 10+ watts at 3%.
The higher PD tubes all performed better than this. The 26LW6 produced 1 watt at 0.1% distortion and 14 watts at 3%. The 4D32 produced 1 watt at 0.08% and 15+ watts at 3%. I only operated these tubes at 80% and 75% PD respectively, because my bench supply only produces 550 volts and my transformers were rated for 80ma.
I also tried the 4D32 with a James Transformer that supported 120ma and got similar results with 3% distortion occurring at 18 watts.
After these tests I ordered a handful of 6LW6 tubes and intend to build a stereo amp with them. I intend to use the Rev2 circuit for this amp.
https://www.diyaudio.com/forums/tubes-valves/196532-hybrid-partial-feedback-se-amplifier-2.html#post2710121
The plate to grid feedback is 98% by my seat of the pants calculations. And the tests were performed with Bud Purvine 5k ohm impedance transformers.
I tried the Rev1 circuit with 6L6WXT+, 7581, Philips top cap 6BG6GA EH 6CA7, 6CB5A and 6P20S, 26LW6 and 4D32.
The lower PD tubes produced 1 watt with midband distortion at 0.15% or lower, 8 watts at 1% and 10+ watts at 3%.
The higher PD tubes all performed better than this. The 26LW6 produced 1 watt at 0.1% distortion and 14 watts at 3%. The 4D32 produced 1 watt at 0.08% and 15+ watts at 3%. I only operated these tubes at 80% and 75% PD respectively, because my bench supply only produces 550 volts and my transformers were rated for 80ma.
I also tried the 4D32 with a James Transformer that supported 120ma and got similar results with 3% distortion occurring at 18 watts.
After these tests I ordered a handful of 6LW6 tubes and intend to build a stereo amp with them. I intend to use the Rev2 circuit for this amp.
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