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Old 23rd December 2011, 11:58 AM   #1
kees52 is offline kees52  Netherlands
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Default cascoded gyrator loaded 6H9C to low idle.

Hi All

The problem is, the bottom tube don,t want get enough idle current, except if the grid negative is 0.5 volt ( ilde is then 0.3 mA,) but that is to low for input signal to prevent positive grid and so distortion.

the supply is 335 volt and it is divide so 100 volt acros the gyrator, the rest acros the two tubes.

the 6H8C (6SN7) do work with enough idle current, now 5 mA and enough high negative on grid, I was supprised about the sound it was very nice warm en deep, a lot of separation between instruments, unwhile on the internet the cascode is not a good way to amplifiy but for the russian ones it is, what I saw on multim, the distortion is morely second and a little thirth, the rest upwards is very low.

Maybe what I think is for the 6H9C the voltage get higher to 450 volts then I can idle it.

I did use this because of needed high voltageswing for a hybrid amp.

thanks for your help.

regards.
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Old 23rd December 2011, 02:52 PM   #2
TheGimp is offline TheGimp  United States
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Schematic please.
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Old 23rd December 2011, 03:14 PM   #3
kees52 is offline kees52  Netherlands
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Schematic and distortion simulation.

I have yesterday try a very low ilde current of plus minus 0.300 mA and it did sound very relaxed and still speed and deep stage, this tube in a SRPP sounds hars, very hars, so maybe a mu follower with this can sound good.

It seems that tubes has all there own livingroom, like people.

thanks for the help I appreciated it.
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Old 23rd December 2011, 03:23 PM   #4
TheGimp is offline TheGimp  United States
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How much voltage across each tube?

My guess would be the bias of the right triode of U7 is set wrong and is limiting current through the cascode. I've not studied cascode much so I'll defer to others on it.

The 470R grid-stop resistors should be in the 10K-47K range. 470R will have virtually no effect as a grid stop for a small signal tube.
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Old 23rd December 2011, 03:29 PM   #5
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Quote:
Originally Posted by kees52 View Post
The problem is, the bottom tube don,t want get enough idle current, except if the grid negative is 0.5 volt ( ilde is then 0.3 mA,) but that is to low for input signal to prevent positive grid and so distortion.

the supply is 335 volt and it is divide so 100 volt acros the gyrator, the rest acros the two tubes.
The 6H9C is equivalent to the 6SL7, and is not a type that's amenable to cascode operation, as you've discovered. You'd be much better off using the 6SN7. In cascode, it should give ab Av= ~80, and that should be more than enough gain for just about anything.

If you need more gain than that, then other good choices would be the 6BQ7 (the ones with parallel connected heaters, as the ones with series connected heaters tend to be cheaper, and very microphonic) or the 6DJ8 -- both of which were designed for cascode operation. The 6SL7 was intended as a low current, high gain small signal amp that doesn't swing big voltages into anything but a very Hi-Z, Lo-C load, such as a follow-up cathode follower.

Quote:
the 6H8C (6SN7) do work with enough idle current, now 5 mA and enough high negative on grid, I was supprised about the sound it was very nice warm en deep, a lot of separation between instruments, unwhile on the internet the cascode is not a good way to amplifiy but for the russian ones it is, what I saw on multim, the distortion is morely second and a little thirth, the rest upwards is very low.
I've had no problems with cascodes and sonic performance, but there isn't much about hollow state cascodes so far as audio amp use and sonic performance. In doing a design with cascoded 6BQ7s as an LTP gain stage/phase splitter, it was a case of try and listen. It performed very well indeed.
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Old 23rd December 2011, 03:55 PM   #6
kees52 is offline kees52  Netherlands
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It is a joy to test the tubes, I have get free a box of them, with telefunkes, pope, phillips etc, so I can play.

I use the casode for a hybride, she drive a 6as7 tube, ut this tube is not easy to idle, I have to use a cathode CCs for it then it can dc couple to the mosfets with bipolair supply, I don,t exact how much current I need for mosfets, the 6AS7 was for the design of a high power hybride with multiple mosfets paralleled and a story that the big tubes can drive mosfets more relaxed what give a better sound, what we all here try to get.

ohh yes I use the cascode for drive a 6n6p cathodefollower who has high z and low c.

The schematic of the cathode coupled amp from the aikido seems better for this 6sl7 tubes, srpp don,t sound wel for me. Put a gyrator or ccs in second tube plate resistor.. Afcourse we have to recalt it for the 6sl7.

thanks in advance
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Last edited by kees52; 23rd December 2011 at 04:03 PM.
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Old 23rd December 2011, 04:08 PM   #7
kees52 is offline kees52  Netherlands
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Quote:
Originally Posted by TheGimp View Post
How much voltage across each tube?

My guess would be the bias of the right triode of U7 is set wrong and is limiting current through the cascode. I've not studied cascode much so I'll defer to others on it.

The 470R grid-stop resistors should be in the 10K-47K range. 470R will have virtually no effect as a grid stop for a small signal tube.
I have the schematic from tubecad, and it is called super liniair the resistors are oke, except the grid I now but is in amp 1k not so important with simulation.

the voltage divider resistors of top triode is 50 procent of supply.
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Old 24th December 2011, 05:49 PM   #8
kees52 is offline kees52  Netherlands
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Hi All

I have record a pieace of music with the 6H8C casdode on 5 mA idle and gyrator loaded, I did use Wavebourn design of gyrator I hope I was elowed to do ;-) and thanks.

I go put here some samples I have 6N1P, 6N2P, 6N6P, ^6H13C but these are not suitable for use as driver for mosfets, the mu is to low and not easy to idel it properly for mine purpose.

Let me now how the sound is, and know it is compressed in youtube, I have no place for high quality hosting. one drawback, I have some noise and rattle in speaker, but only hear that with
my ear closest to speaker, not in sound, plus if I toch the 6H8C I hear the brrr signal like on input, so cascode is sensitive, or the tube has russian sensivity like my old girlfriend who was russian..

6H9C-cascode-sound.mpv - YouTube
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Last edited by kees52; 24th December 2011 at 06:01 PM.
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Old 25th December 2011, 03:49 PM   #9
kees52 is offline kees52  Netherlands
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Here a piece of music with 7mA of idle in tubes.

6H8C-7mA idle.mpg - YouTube
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Old 25th December 2011, 06:16 PM   #10
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Quote:
Originally Posted by kees52 View Post

I have record a pieace of music with the 6H8C casdode on 5 mA idle and gyrator loaded, I did use Wavebourn design of gyrator I hope I was elowed to do ;-) and thanks.
I have noting against gyrator, but I don't understand why to torture the tube (6H9C) that wants much more voltage to sing nice?

Also, while gyrator allows you to minimize 2'nd order harmonic of the upper tube, the lower tube works in conditions that maximize it. With such design goal in mind I would use instead 2 stages loaded on plain resistors.
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