V76 gain Question

[DaveP]

Looking back at the feedback issue discussed by CJ and ruffrecords here:-

http://groupdiy.com/index.php?topic=59609.msg757475#msg757475

CJ said the tone changed because of the 50uF cap, but Ian disagreed.

I have just made this circuit and when I checked the frequency response,  it does indeed change with different gain settings and I think I know why, here's the circuit for reference:-



Ignore all the circuit after the 400H output choke as it's not relevant.  The resistor string on the gain switch adds up to 70k and that is the load to ground on V2.  The 50uF cap P13 acts like the wiper on this 70k "Pot" and sends the selected signal back to the V1 cathode resistor as +ve feedback (acts like -ve with respect to grid).  the reactance of the cap is 170 ohms at 20Hz and 17ohms at 200Hz so its effect on the feedback string is negligible as the lowest resistance it encounters is 40k in the lowest gain settings.

However, when we look at its other function, that of a variable cathode bypass, we can see that its reactance is greater than the 98 ohms resistor at the maximum gain setting and almost equal at the second highest setting at 20Hz, so gain in the bass on the higher gain settings is relatively lower than mids and highs and the frequency response is impaired.  Now this pre was used with bass filters in its intended TV studio role so maybe it was not an issue. but for those of us who want 20~20kHz studio roles then we need to put a much bigger cap in place than 50uF, more like 470 -1000uF I guess.

best
DaveP

 

[PRR]

Values stolen directly from EF804S data, Telefunken 1963, Page 2, 100V 0,2Mohm column. Rk is 3K, 50uFd bypass.

I do not know where to find Notes for Telefunken R-C tables. US tables were sometimes for 50CPS @ -2dB.

V76 differs in having more than(?) 100V supply, but that makes little difference except for peak output. It also stands on a 12K resistor, which nominally suggests a little less bypass, except it is a bootstrap.

If you think part 13 is short, increase it. 100uFd seems just-enough for 20Hz. 220uFd costs only pennies. With modern caps, electrolytic leakage will not be an issue.

You sure did not want extended bass in a TV studio. Certainly not at high gain. Subsonics from large blowers and rumble from the camera dollies (more like lorries). And no TV set reached 50CPS. As this was probably very-best parts in a too-small chassis, 50uFd seems ample for the original design brief.

 

[ruffrecords]

NIce catch Dave, My Eurochannel mic pre is inspired by the V76. My max gain resistor is 430R and I use a 100uF caacitor in the equivalent position to the 50uF in the V76 so I guess for a modern day frequency response, something like 4 times that value would be necessary for a 98 ohm resistor.

Cheers

Ian

 

[abbey road d enfer]

However, when we look at its other function, that of a variable cathode bypass, we can see that its reactance is greater than the 98 ohms resistor at the maximum gain setting and almost equal at the second highest setting at 20Hz, so gain in the bass on the higher gain settings is relatively lower than mids and highs and the frequency response is impaired.

Dave, you're absolutely right, but OTOH, the overall NFB counters that, and at the highest gain settings, the NFB signal is high-passed to the cathode. This tends to create a complementary bump.
I'll simulate the circuit as soon as I'm back at my desktop PC and check the evolution of opene and close-loop vs. Ck.