Cathode Bypass Capacitor Doing Nothing!?

[ruffrecords]

Look at the RCA 41-B for an even wilder example of 2 stage shared cathode resistance. It's the bottom of the two in my drawing, simplified from that hard to follow manual drawing.

A shared cathode resistance is more common than you might think. It is a neat way of extending NFB down to dc thereby improving stability. Pultec did it in their MB1 mic pre for instance.

Cheers

Ian

 

[emrr]

Bias supply. Puts the cathode voltage above the grids.

You should really spend time on the other posts.

 

[emrr]

A shared cathode resistance is more common than you might think. It is a neat way of extending NFB down to dc thereby improving stability. Pultec did it in their MB-1 mic pre for instance.

Cheers

Ian

We should make a list, I can't think of many in audio. MB-1 is certainly an outlier in historical audio, and a rare amp in context.

https://groupdiy.com/threads/shared-cathode-resistance-across-2-stages-of-amp.82754/
RCA 41-B
Pultec MB-1
I think this Collins 6R barely counts given the divider ratio, and the more I think about it, I think the schematic is correct. The 6P does the same thing for cathode metering.

 

[Mr. Moscode]

You should really spend time on the other posts.

I don't know how to take that. . . I'll assume you think highly of me and my posts. I just enjoy solving problems.

I haven't seen any other solutions to this person's question of how to get response to 20K but lot's of reminiscing about the good old days. What's your solution?

 

[Mr. Moscode]

I read every post. Which one gave a solution to flattening the f response? Sorry if I missed it.

Edit -I missed your post about metering. My apologies.

 

[Mr. Moscode]

@ emrr
I missed the cathode current meter post. Oops. Maybe because the rest of the circuit isn't there. I'm not obtuse on purpose. I made a mistake. I also edited all posts based on that mistake made on that missing info. I can see that in that date of design, meter pin would be in order. In 1945 it's appropriate but I've never seen a meter for small signal tube circuits before. Excuse my ignorance.

Be that as it may, my solution would still work to extend frequency response and keep metering intact.

Anything else? I'm open to criticism.

Any other ideas to extend the F response? The only other one I see would be to put a hi pass filter in the 2nd stage grid leak resistor net. Either way the sub 10k response will be down 5 db. More than likely, AND I'm guessing here, the inherent rolloff is 6 db/octave above 10k so 1 hi pass would do it.

 

[emrr]

The problem is in the transformers used in this bench version, or their application. We don't know what they are. That's the 'ideal' solution, be it finding a more optimal loading condition, replacement, etc. We don't know that the external parts have all been proofed for best practice yet. Frequently I see treble change drastically depending on loading. Actually, we don't know if the input attenuator is in use, its secondary load is present, how it behaves without it. How it behaves without the input transformer. That all seems right to me given the OP wants to experiment with various existing PP circuits. An EQ addition is the band-aid if all else is chased down and nothing is to be corrected. Yes, parallel cap resistor for a 1st order filter between stages, or 5dB NFB, see which sound is preferred. Loss of gain either way. Collins did a couple very low NFB tweaks in this era, so that wouldn't be out of context much.

Collins 6S is the console module version of this amp, what I have to look at. The attenuator is simplified to a 3 step, and the metering is changed to a series 31R5 wire wound in the B+ line, since the other console modules all measure from B+ across a resistance, measuring an entire module rather than individual tubes. Both cathode resistors join at B-.

 

[emrr]

I'm guessing Collins didn't know this and included their 20UF capacitors just because?

Hum reduction from heater/cathode is one purpose.

RDH3:

When a common cathode resistor is used for 2 valves in a push-pull circuit it is generally desirable to use a bypass C in order to bypass 2nd harmonic components of the plate current. The 2nd harmonic voltages would otherwise be fed back in phase to both grids and produce a secondary form of harmonic distortion in the output.