How about buffering the pentode with a MOSFET source follower? Having some success here with a 6SJ7 feeding a 2SK2717 follower to drive triode strapped 6AV5GA, audibly superior to r-c coupling pentode directly to 6AV5 grid, no significant HF rolloff, measures nicely, -3dB point is ~60kHz if I can trust the stability of HP200CD generator. Yes, I have been reading many threads re the 6AV5 and MOSFET followers, many thanks to George (Tubelab) for the interesting posts about those.
hey-Hey!!!,
While it isn't octal, have you considered the EL84?
George, thanks for that mention of using the 6V6; I have an amp running a 6AC7 LTP up front and modifying it to take 6V6 would be not so difficult...
I don't think the 6AC7 is a bad one at all; if the 6V6 is better, than get outta my way!!
cheers,
Douglas
While it isn't octal, have you considered the EL84?
George, thanks for that mention of using the 6V6; I have an amp running a 6AC7 LTP up front and modifying it to take 6V6 would be not so difficult...
I don't think the 6AC7 is a bad one at all; if the 6V6 is better, than get outta my way!!cheers,
Douglas
I do like 6V6 as a driver. I haven't found a compelling need for high transconductance in a pentode driver yet (driving another pentode). What's the issue with Rp of a pentode? don't you have a load resistor in the circuit? Are you building a single stage driver for a low mu triode?
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Funny. I just found myself in the same situation yesterday. I ended up adding a source follower a la tubelab's PowerDrive. Yeah, yeah.... I know. Sand in the glass... But for a low output impedance driver it's pretty tough to beat. Only takes 2~3 components.
6J5 + source follower + 300B = Audio Nirvana.
~Tom
6J5 + source follower + 300B = Audio Nirvana.
~Tom
No, what I'm doing is designing a universal driver very similar to P. Millett's recent pentode driver, only with octal tubes.
http://www.diyaudio.com/forums/tubes-valves/203110-mono-push-pull-driver-pcb.html
My first version will also have a high gain first stage (before the driver) and very high NFB. Just experimenting with things.
I think I am working on my 10th or 11th universal driver. The first one used a 5751 LTP driving a 6CG7 LTP feeding mosfet followers. It begat an octal sibling using a 6SN7 or 6SL7 LTP driving a 6SN7 LTP with mosfet followers. It is one of the best and still a front runner.
After spending some time with Pete's red board and making some big power amps (up to 250 WPC) with it, I liked a lot, but found it short of gain to drive the big stuff, and couldn't handle grid current for AB2 or screen drive. That led to several boards using combinations of single or dual 9 pin miniature tubes with a pair of triodes in LTP driving a pair of pentodes in LTP feeding mosfet followers. They are all good chioces.
I also laid out two boards with a 6SN7 or 6SL7 driving a pair of octal pentodes with mosfet followers. One board works with a 6SJ7, 6AC7, 6AG7, and similar. The other uses a 6V6 or 6K6. Neither has been built or tested yet.
Soon as you do, I'll buy a half dozen
My thinking is 6SL7 LTP into octal pentode LTP.
A 6SL7 feeding a pair of 6SJ7's may have too much gain unless a ton of feedback is used. I have a board that used a 5751 driving a pair of 6EJ7's. It has too much gain. It works best with a 6GC7 for the input tube. A 6CG7 is a 9 pin 6SN7.
I started tinkering with the universal driver concept back in 2007 with some screen driven sweep tube experiments. There were several versions of the same circuit using 9 pin miniature tubes.
In 2008 a thread got started that forced the development of the triode LTP feeding a triode LTP feeding mosfet followers. That thread may go dormant for a while but it's back to life again. It does chronicle the development of the 6SN7 or 6SL7 feeding a 6SN7 feeding mosfet followers driver design and a complete amplifier. It is here:
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html
During the 2008 to 2011 time period I was dealing with two close relatives with terminal illnesses. This forced my wife and I to spend a lot of time travelling and sitting around in places like hospitals, assisted living facillities and doctors offices in distant cities. I carried a laptop and spent my time committing ideas to PC board layouts. I still have several PC board layouts that have not been made yet.
This year should have less of life's distractions, but only time will tell. My next step is th get the breadboarded amp designed in the AB2 thread back out of the closet and fire it up. After that????????
Yes and I do have an application that requires tons of FB for speakers that require heavy damping. But 6SL7 can easily be replaced with 6SN7 for less gain. Not sure how well a 6EJ7 LTP pair can feed a PPP final stage even in AB1 without followers, thus my search for a "heavier" octal pentode driver.
I did follow the AB2 thread, that was a very good one, thanks.
Edit: sorry, you did say 6EJ7, I was thinking 6SJ7. Yes, if I could find an octal 6EJ7 that would be perfect.
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PPP, building something BIG???
Another long thread to follow for some educational value is Pete's red board thread:
http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html
I was doubtful of the ability of a lowly 6CB6 (wimpier than a 6EJ7) to drive some really big sweep tubes into stratospheric power levels without followers, but I bought a board with the statement that I would be dissapointed if I didn't see 50 WPC. That turned out to be a big understatement. I have reached a plateau at 250 WPC, and will probably stop there. That seems to be about the most I can squeeze out of a pair of sweep tubes without the red glow of death. The issue is not lack of drive capability but lack of gain. I have connected about 25 different types of tubes into the red board and learned a lot, so...
I still think followers would help the distortion in areas where the grid gets close to or crosses into positive voltage. Mosfets make the best followers, but pentodes can be used if done right. This isn't needed with high perveyance sweep tubes, but can squeeze power out of DHT's, DHP's and even 6L6GC's. I also think another stage is needed, but I would rather have a fully differential driver, so there is cascaded LTP's. All of my universal driver experiments have been using this topology since I made the 300Beast P-P amp 10 years ago.
This had led me to the circuits discussed above. The choice of octal VS miniature tubes is purely asthetics but there are more high Gm pentodes available in the 9 pin flavor. Petes red board proved that medium Gm pentodes are quite good in LTP.
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