Sorry for the essay-length question, but it's kind of an essay length problem, at least for me.
I'm half-hacking half-designing a triode PP amp using a 21LR8 (triode + power pentode) with the pentode half triode strapped. Plan to use a LTP to drive it. There's got to be some kind of buffer in between, I already tested without it and the load from the triodes drags the LTP into bad distortion (no surprise I guess but this the first time I actually built something like this and measured it).
Using the triode half of the LR8 as a cathode follower, with a negative supply on the cathode load resistor, buffers well and lets me DC couple to the output tubes (well, it's all in one tube I guess...) and also provides a really easy way to set the bias by using the right value for the cathode load. It works and can drive 3 watts out with approx 1% distortion (no feedback), but it requires a 60V swing on the grids of the output tube.
Where to get that 60V from with something reasonable (say 1V PP) coming into the amp? The LTP has less than half the gain needed. It's built with a 6BH11, dual identical triodes and small signal pentode, pentode as CSS on the tail, and works nicely in isolation. I described it in another thread awhile back.
Am I better off trying to buffer between the LTP and the output tubes some other way, using the triode in the LR8 (less tubes, but probably have to toss the DC coupling idea), or adding a preamp before the LTP (more stages, more tubes). Maybe using a different phase splitter design? Looking for some hints or reference designs here.
Thanks....
I'm half-hacking half-designing a triode PP amp using a 21LR8 (triode + power pentode) with the pentode half triode strapped. Plan to use a LTP to drive it. There's got to be some kind of buffer in between, I already tested without it and the load from the triodes drags the LTP into bad distortion (no surprise I guess but this the first time I actually built something like this and measured it).
Using the triode half of the LR8 as a cathode follower, with a negative supply on the cathode load resistor, buffers well and lets me DC couple to the output tubes (well, it's all in one tube I guess...) and also provides a really easy way to set the bias by using the right value for the cathode load. It works and can drive 3 watts out with approx 1% distortion (no feedback), but it requires a 60V swing on the grids of the output tube.
Where to get that 60V from with something reasonable (say 1V PP) coming into the amp? The LTP has less than half the gain needed. It's built with a 6BH11, dual identical triodes and small signal pentode, pentode as CSS on the tail, and works nicely in isolation. I described it in another thread awhile back.
Am I better off trying to buffer between the LTP and the output tubes some other way, using the triode in the LR8 (less tubes, but probably have to toss the DC coupling idea), or adding a preamp before the LTP (more stages, more tubes). Maybe using a different phase splitter design? Looking for some hints or reference designs here.
Thanks....
Only the "concertina" and LTP are currently in favor. Search the archives for the rationale. A "concertina" splitter exhibits an insertion loss. Therefore, you must improve your LTP's behavior.
There is no getting around the 1/2 μ gain limit, when only 1 LTP grid is driven. However, you can approach that limit by using large anode load resistors. That, in turn, requires a very "tall" B+ rail voltage. You can use the concept of 2 rails from a single CT winding that I've uploaded to feed the LTP with a "tall" B+ voltage.
There is no getting around the 1/2 μ gain limit, when only 1 LTP grid is driven. However, you can approach that limit by using large anode load resistors. That, in turn, requires a very "tall" B+ rail voltage. You can use the concept of 2 rails from a single CT winding that I've uploaded to feed the LTP with a "tall" B+ voltage.
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How about turning the LTP to paraphase? It's only a matter of obtaining an inverted signal to drive the other triode's grid. An AC-coupled voltage divider from the plate of first triode should do it.
SY is reaching for the Fels Naphtha bar. 😉 Paraphase splitters are out of favor, for good reason. There are symmetry issues.
Keep the triode section of the 'LR8 as cathode follower grid drivers. You can get the gain either from an LTP made from small signal pentodes (6CB6, or 6SJ7) or use two triode LTP's: 6SL7 up front -> 6SN7 (2nd pre). Cascode 6BQ7s also work as an LTP, but the signal swing is rather limited, as this is a cascode connection, and less useful if you don't have the necesary DC rail.
The third possibility is to keep the 6BH11 as is, and add an independent gain module made from a 6SL7 or 12AX7 with gNFB to get the gain to where you want it, and to reduce the distortion.
Use a triple triode Compactron and you could use two triodes as cascaded gain stages with NFB, and the remaining triode as a cathodyne splitter to drive both sides of the 6BH11 LTP as a balanced-balanced gain stage, thereby doubling the voltage gain.
I've built floating paraphase, works as expected, no more symmetry issues then with any other phase splitters. Just use LTSpice to simulate and adjust certain R values.
Which can easily be solved with a cheap trimpot on the voltage divider? The only problem i can think of with paraphase is phase shift.
edit: ..and the inability to introduce negative feedback to the other triode grid.
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use a sand buffer or even a sand splitter. MOSFETs are a thing, and contrary to some beliefs they don't poison the well or leave your best heifer pregnant.
Frees up the triodes to be a gain stage. Configuration to suit your personal beliefs and predelictions although, as noted, paraphase is ickky.
Frees up the triodes to be a gain stage. Configuration to suit your personal beliefs and predelictions although, as noted, paraphase is ickky.
awww first do you have the 21 Volts Heater correct?
Why boot the pentode in 'triode' it is sure not meant to be used like that. You should use it in UL with a proper UL transformer,
Clipping distortion might not be caused by the LTP driver and simply by the 1 watt max power of the pentode in triode mode...
Before finding a fault in the LTP (which is able to drive lots of stuff!) check the distortion at the output of the LTP stage and not at the output of the amp.
added : IF is irrelevant, add a triode input stage tube (just one at 21 V heater ???) and bring the feedback there. LTP don't work without feedback and inserting feedback to them is a disaster sound guaranteed.
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Are you dead set on a triode LTP? Otherwise I would suggest a pentode feeding a triode cathodyne, followed by your triode cathode followers. The pentode would easily acheive the gain of 60 needed, and could easily be DC-coupled to the cathodyne. Very convenient with a triode-pentode like the ECF80, etc. Supply voltage would need to be realtively large to get 2x60V swing, but then you already have a negative rail available.
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The IRFBC20 MOSFET makes an excellent DC coupled "concertina" splitter.
Regardless of the phase splitter chosen, the OP has the option to "steal" a page from Williamson's book and add a differential gain block between the splitter and the "finals". The OP stated no loop NFB, which pretty much removes the key Williamson weakness (low freq. instability) from the equation.
Regardless of the phase splitter chosen, the OP has the option to "steal" a page from Williamson's book and add a differential gain block between the splitter and the "finals". The OP stated no loop NFB, which pretty much removes the key Williamson weakness (low freq. instability) from the equation.
(Merlinb)
Are you dead set on a triode LTP? Otherwise I would suggest a pentode feeding a triode cathodyne, followed by your triode cathode followers. The pentode would easily acheive the gain of 60 needed, and could easily be DC-coupled to the cathodyne. Very convenient with a triode-pentode like the ECF80, etc. Supply voltage would need to be relatively large to get 2x60V swing, but then you already have a negative rail available.
Not set on the LTP, I just built it first to see how well it would work using a pentode CSS on the tail. My last amp just used a resistor. Standing alone it's a wonder of balance and really clean, but in circuit not so clean. This is all a learning exercise, basically working though ideas I'm pulling from various books (including one I think you know well... 🙂.
I'll try your suggestion. I've got a cabinet full of compactrons, I'm sure there's something in there that would fit the bill.
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