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Old 19th August 2012, 03:13 AM   #11
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Hi again
I made mistake. I wired pentodes in triode mode. Stupid me. Now I corrected it and amplification is 20 without GNFB and 16 with it. Not much but I can hear the difference. Sound seems to be more dynamic. As to my scope it is still piece of junk. Switches are worn out and need tapping to keep the waves from jumping up and down the screen. One of these days I'll tap the whole thing with the sledge hammer and fix it for good.
On the picture You can see where I made mistake.
Cheers again
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Old 19th August 2012, 05:44 AM   #12
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Hey voyciek,

Can you explain the necessity of the 1Meg/4,7u in the feedback loop?

750ohm at the Sg seems high, any reasons for that? In the ballpark of 100ohm normally works best.

NFB should be about 1db with the values you have given, 47ohm-4.7k. Some of you guys miscalculated with a factor of 10. You need for an acceptable result with UL wrt dist and Zout. Triodestrapped you shouldn´t need any NFB at all.

Last edited by revintage; 19th August 2012 at 06:11 AM.
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Old 19th August 2012, 06:43 AM   #13
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Quote:
You need for an acceptable result with UL wrt dist and Zout.
That went wrong. Should be:

"You need more NFB for an acceptable result with UL wrt dist and Zout."

Also you shouldn´t calculate for more than just over 30dB(ca 30x) gain in the SRPP LTP.
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Old 19th August 2012, 11:32 AM   #14
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There is a danger with your Bias circuit. If or when the wipers on the 100k pots go intermittent then you'll lose bias completely to the relevant EL34. I would swap the pot & the 100k fixed resistors positions. Ground the wiper with one end of the pot. The 100k resistor should then be split in two, choose values to give maximum bias and minimum bias dependent upon the pot wiper position. Other possible ideas are also available on here if you care to look around.
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Old 19th August 2012, 12:41 PM   #15
rongon is offline rongon  United States
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Quote:
Originally Posted by revintage View Post
Hey voyciek,

Can you explain the necessity of the 1Meg/4,7u in the feedback loop?
That was not answered. I've never seen that before. What is the purpose of the 1M/4u7 in the feedback loop, on the grid of the 12AX7?

thanks
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Old 19th August 2012, 05:11 PM   #16
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Default Don't know

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Originally Posted by rongon View Post
That was not answered. I've never seen that before. What is the purpose of the 1M/4u7 in the feedback loop, on the grid of the 12AX7?

thanks
--
I see it this way in differential amplifiers but I don't why it is this way. I was hoping that someone can give me some idea what to do with them apart from sticking them where sun doesn't shine.
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Old 19th August 2012, 05:18 PM   #17
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Default Good idea

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Originally Posted by payitforwardeddie View Post
There is a danger with your Bias circuit. If or when the wipers on the 100k pots go intermittent then you'll lose bias completely to the relevant EL34. I would swap the pot & the 100k fixed resistors positions. Ground the wiper with one end of the pot. The 100k resistor should then be split in two, choose values to give maximum bias and minimum bias dependent upon the pot wiper position. Other possible ideas are also available on here if you care to look around.
Thank You for suggestion. It makes sense to me. When I get some time I will rewire BIAS circuit his way. On diagram I wrote optional values in red.
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Old 20th August 2012, 04:30 AM   #18
AJT is offline AJT  Philippines
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i am not too bothered by it since the 4.7uf effectively makes it a short at ac...
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Old 20th August 2012, 06:10 AM   #19
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Unfortunately this amp needs a total redesign due to too low gain. The 12AX7 SRPP LTP hasn´t gain enough. Without GNFB 3H will be unacceptable.

The idea is good, but then you´ll need a driver like 416C and something like a sand mu-follower instead of SRPP.
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Old 20th August 2012, 07:32 AM   #20
AJT is offline AJT  Philippines
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my thoughts too....could do a cascode type using 6dj8's instead...lots of fine examples in this forum.....

miles prower and Bandersnacth have such designs.....
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