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Single ended triode strapped pentode amplifiers.

Lavane

Lavane

Lunatic Member
I read a thread a few days ago on another site about the use of UL with single ended amplifiers and triode strapping pentodes in PP amps. I just went back trying to find that thread and can't find it.
Anyway this person did some testing and found single ended UL amps were bad idea. The test results showed it just added distortion. Also his tests showed using triode strapped pentodes in PP amps was detrimental to the performance of the amplifier as well. I've never liked triode strapped PP amps. I have several with triode/pentode switches. So I gave up on triode strapped pentode amps altogether.
However triode strapped pentodes in single ended amps apparently is another matter. The frequency sweep and distortion tests this person did showed triode strapped pentodes in single ended amps can perform and sound very good.
I'll keep looking for that article with test results and post it if I do.
So now I'm wanting to try a single ended amp using pentode power tubes in triode mode to see if I like it compared to a triode mode PP amp.
What I have on hand is an Antek power trany with dual 270v-0v secondaries that can be wired for 270v-0-270v as well. Since the primary is 115V and I would use a SS rectifier I would get a little more B+. I also have some 6L6GC, KT-77, 5881, and Russian 6P3S-E power tubes along with various 6N1's, 6N2's, 6N3's, 12AU7's, 12AY7's, 12AT7's, 12AX7's, 6922's, 12DW7's, 6SN7's 6SL7's, etc for drivers.
I've done some searches for triode strapped schematics and haven't come up with much.
Does anyone happen to have, or know of any schematics I could look at with the power transformer and tubes I have on hand?
Thanks,
Yet another rabbit hole to jump into.:)
 
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Lavane said:
I read a thread a few days ago on another site about the use of UL with single ended amplifiers and triode strapping pentodes in PP amps. I just went back trying to find that thread and can't find it.
Anyway this person did some testing and found single ended UL amps were bad idea. The test results showed it just added distortion. Also his tests showed using triode strapped pentodes in PP amps was detrimental to the performance of the amplifier as well. I've never liked triode strapped PP amps. I have several with triode/pentode switches. So I gave up on triode strapped pentode amps altogether.
However triode strapped pentodes in single ended amps apparently is another matter. The frequency sweep and distortion tests this person did showed triode strapped pentodes in single ended amps can perform and sound very good.
I'll keep looking for that article with test results and post it if I do.
So now I'm wanting to try a single ended amp using pentode power tubes in triode mode to see if I like it compared to a triode mode PP amp.
What I have on hand is an Antek power trany with dual 270v-0v secondaries that can be wired for 270v-0-270v as well. Since the primary is 115V and I would use a SS rectifier I would get a little more B+. I also have some 6L6GC, KT-77, 5881, and Russian 6P3S-E power tubes along with various 6N1's, 6N2's, 6N3's, 12AU7's, 12AY7's, 12AT7's, 12AX7's, 6922's, 12DW7's, 6SN7's 6SL7's, etc for drivers.
I've done some searches for triode strapped schematics and haven't come up with much.
Does anyone happen to have, or know of any schematics I could look at with the power transformer and tubes I have on hand?
Thanks,
Yet another rabbit hole to jump into.:)
Click to expand...
I live for rabbit holes and I'm multitasking and have 4 dug so far. :D :banana:
 
I was part way through one and started on another, so I can echo this sentiment. Transformers for one are in primer now, the other chassis rode around in the back seat for several months after doing some work on it and I realized I clocked one of the transformers wrong. Its been long enough that I don't remember which one. Need to finish one, then get back to the other to see what I goofed on, if it matters, and then how to fix it if needed.
 
radioactive said:
Yeah don't want them caving in on each other.My problem is them connecting to each other and branching off to another one. :thumbsup:
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Right? That's how you wind up with a bunch of random parts and iron over time.
I've gotten half way through builds only to shift gears and head in a totally different direction, or make changes to what I was building. I'm just trying to use up some of the accumulated stuff I have sitting around like an EL34 SEP amp I recently built. It started out with a PCB I had bought and forgot I had and happen to find while looking for other things. There were only so many mods I could make to it to make it a better performing amp though.
After reading through the article I posted the link to I want to try a SE triode strapped amp.
 
Lavane said:
What I have on hand is an Antek power trany with dual 270v-0v secondaries that can be wired for 270v-0-270v as well. Since the primary is 115V and I would use a SS rectifier I would get a little more B+. I also have some 6L6GC, KT-77, 5881, and Russian 6P3S-E power tubes along with various 6N1's, 6N2's, 6N3's, 12AU7's, 12AY7's, 12AT7's, 12AX7's, 6922's, 12DW7's, 6SN7's 6SL7's, etc for drivers.
Click to expand...
Careful, you could be tube rolling forever. :D

A Tubelab Simple SE would be a good starting point, but wouldn't allow much tube rolling for the driver stage in stock config. Of course, you could change the front end anyway you like. I'm running 6P3S-E tubes in my SE-UL amp at the moment, in place of 6CA7/EL34s.

jeff
 
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Lavane said:
However triode strapped pentodes in single ended amps apparently is another matter. The frequency sweep and distortion tests this person did showed triode strapped pentodes in single ended amps can perform and sound very good.
I'll keep looking for that article with test results and post it if I do.

So now I'm wanting to try a single ended amp using pentode power tubes in triode mode to see if I like it compared to a triode mode PP amp.


Thanks,
Yet another rabbit hole to jump into.:)
Click to expand...
Lavane, you aren't saying people use ultralinear mode in a triode-strapped pentode, right? How would the wiring be possible? The screen is tied to the plate.

:idea:
 
you can't connect the screen to two things at the same time, it simply will not work correctly. Its not possible to do UL and triode simultaneously. Closest you get is a switch to go between modes.
 
gadget73 said:
you can't connect the screen to two things at the same time, it simply will not work correctly. Its not possible to do UL and triode simultaneously. Closest you get is a switch to go between modes.
Click to expand...
Thanks! That's what I thought. UL is possible for pentodes in pentode mode.

:thumbsup:
 
I've done both, I personally don't like how triode strapped beam pentodes sound, `kinda flat and soggy. If you want a triode, use a DHT. I quit putting triode/UL switches on my amps because of this. The triode strap tubes do test better THD wise with zero feedback and the UL SE needs some NFB of some sort to tame the distortion, but less than a pure pentode would need.

Also know a triode strapped pentode is gonna make a LOT less power. In this article he claims you only get 6-6.5W from a SE UL KT88 which is just not true, it's more like 10-10.5W when biased correctly. But he is right you get 5-5.5W when you triode strap it, which is about 1/2 the output power. My experience has also shown SE UL amps need to have the output tube biased pretty hot, over 85% with 90% seeming to be the sweet spot. Bias them cold and the THD goes through the roof.

It also looks like a schematic that spec'd a very specific Tango transformer with GNFB tuned to that transformer was built with who knows what brand OT and that was used for the test. I agree a low quality OT used in a SE UL amp can test very bad. 100V more B+ with a 500R cathode resistor and a 5K OT would be a very different story.

One thing I also do on a SE UL amp is use a screen resistor to lower the screen a little under the plate, usually a 1.2K does the trick to get it under the plate voltage. Having the screen higher than the plate just seems like a bad idea to me. As always YMMV.
 
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I expect there is also a load impedance mismatch thing going on here, in addition to the lower gain which drops any feedback level. I just can't help but think any amp that runs in both modes can't be optimized in at least one of them.

that said I'm building a pentode amp which happens to be pin-compatible with a triode and i may buy a set just to see what that sounds like. I have a feeling what I'm building will actually be more optimal for the triode, but we'll see. No switch though, it will be a tube swap. G2 just doesn't go to anything on the triode tube.
 
A few years ago a single ended, triode connected, directly heated HY69 amp consumer made in Japan showed up on epay with a very attractive price for a Japanese auction. It looked good when I got it, carefully made using a ready made chassis with Tango output transformers. I put it on the bench but was surprised to find it made less that a watt at full power. The tube complement was a 6BL8 with the pentode as a voltage amp into the triode section configured as a cathode follower. Fortunately, it came with a schematic. Right away I noticed the voltage dropping resistor to G2 was in the meg-ohms. I changed it to 390k and the amp worked perfectly putting out about 10W.

OK, once I got it working well, I ended up modifying it entirely, replacing the 100V power transformer with a small transformer to create a negative supply and off-loading the plate supply and filament transformers to an outboard supply (not shown on the schematic below). I listened to it for a couple of years enjoying the sound on Zu Dirty Weekend speakers.

HY69 stereo amp.jpgL1003521.JPG
 
I would have been tempted to make that 1K grid stop on the output smaller and see if it would run in A2 mode, drawing grid current. Its got a cathode follower which should have been able to deliver at least some power.
 
gadget73 said:
I was part way through one and started on another, so I can echo this sentiment. Transformers for one are in primer now, the other chassis rode around in the back seat for several months after doing some work on it and I realized I clocked one of the transformers wrong. Its been long enough that I don't remember which one. Need to finish one, then get back to the other to see what I goofed on, if it matters, and then how to fix it if needed.
Click to expand...
I still have a phono preamp thats been sitting on my bench for some time now
Ive had parts for along with some wiring changes that are needed.
 
gadget73 said:
I would have been tempted to make that 1K grid stop on the output smaller and see if it would run in A2 mode, drawing grid current. Its got a cathode follower which should have been able to deliver at least some power.
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That's an interesting idea. The unit has a cathode follower to supply the current swing needed for A2.

:idea:
 
That's an interesting suggestion. The plate curves look perfect in A2. I can put it on the bench to test how well it works.

I forgot to mention, Lavane, that you can use a 12AX7 (or 6SL7) voltage amp and a 6SN7 cathode follower along with the above schematic if you juggle some of the components. Probably a 5881 or 6L6GC would be you best bet with that power transformer. The negative supply would have to be handled by a small auxiliary power transformer. I was lucky enough to find a Stancor PM-8406 cheap on epay, but any small transformer that can output about 30 - 40mA @ 150V would do.

HY69 triode A2.png
 
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StepheK said:
I've done both, I personally don't like how triode strapped beam pentodes sound, `kinda flat and soggy. If you want a triode, use a DHT. I quit putting triode/UL switches on my amps because of this. The triode strap tubes do test better THD wise with zero feedback and the UL SE needs some NFB of some sort to tame the distortion, but less than a pure pentode would need.

Also know a triode strapped pentode is gonna make a LOT less power. In this article he claims you only get 6-6.5W from a SE UL KT88 which is just not true, it's more like 10-10.5W when biased correctly. But he is right you get 5-5.5W when you triode strap it, which is about 1/2 the output power. My experience has also shown SE UL amps need to have the output tube biased pretty hot, over 85% with 90% seeming to be the sweet spot. Bias them cold and the THD goes through the roof.

It also looks like a schematic that spec'd a very specific Tango transformer with GNFB tuned to that transformer was built with who knows what brand OT and that was used for the test. I agree a low quality OT used in a SE UL amp can test very bad. 100V more B+ with a 500R cathode resistor and a 5K OT would be a very different story.

One thing I also do on a SE UL amp is use a screen resistor to lower the screen a little under the plate, usually a 1.2K does the trick to get it under the plate voltage. Having the screen higher than the plate just seems like a bad idea to me. As always YMMV.
Click to expand...
I built that Novar spud amp using 6LR8's as a SUEL amp. It can be wired UL or triode mode using a jumper, or a switch. I just used a jumper for UL mode so I could get the full 5wpc.
I'm really looking for a schematic thats fairly simple. Maybe something with a single dual triode driver stage. I would really like to be able to use the JJ 6L6GC's, or the Russiqn 6P3S-E's I have on hand. If I remember right, I got around 290v to 295v after a couple added diodes out of that toroidal transformer I have. Thats unloaded though. There just doesn't seem to be that many schematics out there for this kind of amp that I could find. Especially using 6L6 tyoe tubes.
 
Palustris said:
That's an interesting suggestion. The plate curves look perfect in A2. I can put it on the bench to test how well it works.

I forgot to mention, Lavane, that you can use a 12AX7 voltage amp and a 6SN7 cathode follower along with the above schematic if you juggle some of the components. Probably a 5881 or 6L6GC would be you best bet with that power transformer. The negative supply would have to be handled by a small auxiliary power transformer. I was lucky enough to find a Stancor PM-8406 cheap on epay, but any small transformer that can output about 30 - 40mA @ 150V would do.

View attachment 3850945
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Thanks. That amp is a bit more ambitious for what I really want to do.
I'm just trying to use up what I hqve on hand without having to buy much more than a couple OPT's and maybe a choke.
I was looking for a schematic with just a single dual triode driver circuit to drive the 6L6's, or 6P3S-E's. I'm just not really finding any of the simpler designs and builds.
I built a pretty simple SEP EL34 amp with a single 6N2P-EV driving two KT-77's using a PCB I had bought maybe a couple years ago. I ran across it again looking for some parts and while cleaning up my parts bins. I made a few mods to it and it sounds pretty good.
 
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