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Theoretical question about interstage transformers
Theoretical question about interstage transformers
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Old 13th June 2018, 10:35 PM   #41
smoking-amp is offline smoking-amp  United States
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Blocking distortion

Bob Carver lately has been using a SS diode + resistor to a modest neg. V (2X bias V), on the grid side of the coupling cap, to symmetrically load the coupling cap against grid current effects. He called it a DC restorer. If you already have a neg. V around for biasing or a tail CCS, you are all set.
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Old 14th June 2018, 12:46 AM   #42
kodabmx is offline kodabmx  Canada
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I'd like to see a schematic, my brain doesn't follow how it would be connected.
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Old 14th June 2018, 03:25 AM   #43
smoking-amp is offline smoking-amp  United States
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The DC restorer.

-Grid bias is held 1/2 way between the grid conduction and -2x grid bias.
When an AC input causes grid conduction it also causes restorer conduction on the opposite AC peaks. Rr is sized to simulate the +0V grid conduction path resistance, for symmetrical cancelling conduction to the coupling capacitor C.
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Old 14th June 2018, 05:17 AM   #44
smoking-amp is offline smoking-amp  United States
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One thing (I don't think I've seen mentioned yet) for the interstage xfmr is that it blocks common mode voltages from a P-P driver. A balancing (ct'd) load inductor could do the same thing for P-P, where DC isolation isn't needed. A CCS tail under a differential driver stage would also block common mode voltages.

AC common mode voltages are a product of symmetrical P-P tube distortion effects, so removing them removes some of the distortions. (some odd harmonics) Bound to have some sonic effect. It doesn't remove all odd harmonics versus the input however.

Last edited by smoking-amp; 14th June 2018 at 05:30 AM.
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Old 14th June 2018, 05:32 AM   #45
Hearinspace is offline Hearinspace  Canada
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Theoretical question about interstage transformers
Another thing is, according to Bill Whitlock in his easy to read paper here "Although a transformer cannot have response to 0 Hz or dc, it can have much less phase distortion than a coupling capacitor chosen for the same cutoff frequency."
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Old 14th June 2018, 03:06 PM   #46
45 is offline 45  Italy
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Quote:
Originally Posted by Hearinspace View Post
Another thing is, according to Bill Whitlock in his easy to read paper here "Although a transformer cannot have response to 0 Hz or dc, it can have much less phase distortion than a coupling capacitor chosen for the same cutoff frequency."
Important also the way transformer low frequency distortion results in much lower IMD (measured with SMPTE method: 4:1 ratio, 60Hz and 7 KHz) than active circuits having the same amount of THD. About one order of magnitude (10 times) less. This with standard grade steel.....but someone is still worried about hysteresis...
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Old 14th June 2018, 08:25 PM   #47
DF96 is offline DF96  England
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Originally Posted by 45
Have you ever measured how much grid current a common EL34 can draw when approaching full modulation (Class AB1)? When a smallish grid current will appear things will be the other way around with the capacitor being anything but ideal or nearby. So one either uses a transformer or proper DC coupled stage that can handle that.
If you are expecting grid current then a transformer may sometimes be the best option. However, the cap is still more ideal as a component - but less appropriate to the circuit situation.
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Old 14th June 2018, 08:40 PM   #48
45 is offline 45  Italy
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Originally Posted by DF96 View Post
If you are expecting grid current then a transformer may sometimes be the best option. However, the cap is still more ideal as a component - but less appropriate to the circuit situation.
I always expect grid current. Even if one buys super-selected tubes with no grid current until 0V is reached the point where the inversion happens will change during tube life quite significantly and unfortunately there is no way to predict it in individual cases.
If the amp is more powerful than needed it might not be a problem of course but that's subjective...
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Old 14th June 2018, 10:59 PM   #49
popilin is offline popilin  Argentina
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Quote:
Originally Posted by 45 View Post
Important also the way transformer low frequency distortion results in much lower IMD (measured with SMPTE method: 4:1 ratio, 60Hz and 7 KHz) than active circuits having the same amount of THD. About one order of magnitude (10 times) less. This with standard grade steel.....but someone is still worried about hysteresis...
Let's advise people that must use transformers without valves.

More seriously, do you have measurements that support your claim?
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Old 15th June 2018, 12:11 AM   #50
45 is offline 45  Italy
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Quote:
Originally Posted by popilin View Post
Let's advise people that must use transformers without valves.
I would certainly not advise people to use a 12AT7 to drive a 300B for cancelling 3rd harmonic distortion!?!?!

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Originally Posted by popilin View Post
More seriously, do you have measurements that support your claim?
Not my claim. Seriously you should read more carefully everything.

Pag.10 of the link above.
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