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Old 8th August 2018, 10:08 PM   #11
6A3sUMMER is offline 6A3sUMMER  United States
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Eli Duttman,

A 6550 in pentode mode does not require a lot of swing or slew current from a 12AX7. But if Schade feedback is used the 12AX7 will likely run out of current. And the 12AX7 might be more challenged if the 6550 is UL or Triode wired. The effective control grid capacitive reactance is much lower in those cases. Then the MOSFET you mentioned should take care of those more challenging load conditions.

I should have said I would not use a 12AX7 to drive a 300B. Different output tubes, different results.

Yes, the 12AX7 can drive a MOSFET follower driving the 300B. But the 12AX7 might not have enough linear swing if the 300B is biased at 75, 80, or even 90V. And using the same 12AX7 plus the same MOSFET as a common source amp, could provide enough swing, but it would be bandwidth limited because of the current from the 12AX7 would not be able to slew the common source MOSFET gate capacitances.

Last edited by 6A3sUMMER; 8th August 2018 at 10:18 PM.
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Old 9th August 2018, 01:46 AM   #12
Bfpca is offline Bfpca  Canada
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@DAK808

I did use my Triode PPP KT88 as an integrated amp without feedback for several years. At the time I thought I would try it as a power amp with feedback. It didnít sound as good with feedback but the output Z went from 2.7ohms to .5ohms which was important at the time because I was running speakers that were 4 ohms and required damping from the amplifier.
I have much better speakers and front end now and my priorities have changed. I will try the Schade style feedback in Pentode mode at some point.

@6A3sUMMER

I would try to avoid 12ax7 if possible. With typically 1ma or less flowing its very likely to be slew limited driving a mosfet, even in source follower mode. I like the ECC99 for a nice linear driver tube that can swing a lot of volts and run at 10+ma. Thatís what I used in the LTP in my pp amp driving 2 pairs of KT88s. Without feedback that amp was pretty much flat to 20khz in tride mode with about -45v bias. So 90vp/p drive voltage.
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Old 9th August 2018, 05:47 PM   #13
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by Eli Duttman View Post
DC coupling a ZVN0545A source follower to the wimpy triode's plate makes short shrift of the matter.
FET gyrators are great way to improve amp. definitely worth it. it also increases gain significantly, for high mu-ra toobz. IMHO the jfet is a notch better soundwise. to Ale Moglia findings.

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Old 9th August 2018, 06:22 PM   #14
Bfpca is offline Bfpca  Canada
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Yes, a gyrator is another possible. Lots of experiments are possible. Right now the distortion is higher than I would like so first job will be optimizing the driver stage and Iím also going to try fixed bias. I am too busy right now to put much time in on this. I am trying to find some listening time each day to get familiar with the sound before I make changes.
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Old 9th August 2018, 10:02 PM   #15
6A3sUMMER is offline 6A3sUMMER  United States
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Bfpca,

I am sorry, I do too much "tongue in cheek" writing, and expect everyone to understand the the whole technical discussion, but that is filled with sarcasm. I need to change my style of writing (thanks for calling me out, I will try and remember to change).

If you use the right MOSFET, and the 300B grid bias is set to -35V, the 12AX7 just may barely be able to drive the capacitance and the required +/- 35V swing. Not very well, but it can do it.

I was trying to tell that to someone who 'suggested that the 12AX7 plus a MOSFET could be used as driver'. I wanted to show it depends on what tube you are driving, and what bias that tube is set to.

Just read completely through that last paragraph I wrote in Post # 11 You may then understand I do not like the idea of a 12AX7 driving a MOSFET.

I think I made that clear in my earlier post # 4. I mentioned the lack of current, so another post-er suggested the MOSFET as the solution. I should also have mentioned the lack of linear volts swing, regardless of required current, no MOSFET follower is going to change the lack of linear swing of a 12AX7.
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Old 10th August 2018, 02:19 AM   #16
Bfpca is offline Bfpca  Canada
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I don’t have any problems with anything you wrote. It’s always difficult to interpret the tone of responses and I did not detect any sarcasm aimed at me. There is a great diversity of experience and views on this forum which in my opinion makes for very interesting technical discussions.
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Old 10th August 2018, 09:46 AM   #17
6A3sUMMER is offline 6A3sUMMER  United States
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Bpfca,

You are correct. The many different posters makes it interesting, and educational.
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