Understanding V72s and changing gain

[abbey road d enfer]

My input transformer doesn't have a CT where i could put a that 1u cap for HPF. Does this cap work with transformer DCR or in some other way? I can't see how it would form a filter with inductance here...

Look how this cap is in series with the xfmr primary. It forms an HPF where the series element is the 1uF cap and the shunt elements are the inductance of the primary in parallels with the grid resistance reflected to the primary (425k/20.20 => a little over 1k).

  It is probably good to limit lows right before the input, or not?

Yes and no. This method has a drawback; the impedance seen by the xfmr primary is somewhat higher than expected, which may be the cause of some distortion.

Many other preamps have coupling caps for this (or is it just "side effect"?), so i might do the same.

Since you don't have a CT, you could connect the cap in either leg of the primary, but this would impair the CMRR at HF, because of the parasitic capacitance of the leads. So you need to put two caps of double value, one in each leg.
BTW, the value of the caps depends on the desired cut-off frequency and the xfmr's primary inductance, so you'll have to experiment. With 425k and 1:10 ratio, the reflected resistance will be about 4k, so the response will be underdamped (hump right over cut-off frequency). You'll probably need 100-120k there.

 

[My3gger]

I just found this transformer in my closet, didn't even know i have it:
http://www.sowter.co.uk/specs/8650.htm
Datasheet says push-pull or single ended connection with CT in primary and no DC permited. It looks like it is good transformer i could use in V72 and probably many American vintage preamps that require higher ratios OTs. With primary in parallel it would give 4,5:1 which is useful for many other preamps.
I'm a bit lost with this 2k/100R graph is showing, response under this condition isn't as good as at 8k. Did they only want to show how it works with different sources? So it seems i can use it with tubes that have Ri from 2k to 8k more or less, higher impedances would give better response.
But what about pentode in V72 driving 9:1 or 11:1 trafo, isn't their Ri way to high for this ratios? I see Ri like 0,5M, does inductor change condition, so we can use this ratios?

 

[My3gger]

Hi guys,

i would be really grateful if someone looked at my questions. I know i can put it in circuit and hear it for myself but as there is graph and pretty good description i would like to understand what i'm doing. Hope this is right place to ask, please move this if it is not. Thanks for any help!

 

[CJ]

looks like it would work, 9:1 = 81 Z ratio, if set up for 600 out, you have 81 * 600 = 48,600 ohms to the plate, not quite the 75K used in the 11:1 V72 original, but about equal to the V76 OPT,

26 dbu on output is about 20 volts, so stepped up by 9 = 180 rms, so saturation should not be a problem as there is no DC in the V series outputs,

looks like the freq graph was done with a signal generator instead of a vacuum tube, thus, the 2K input Z,