Mystery RCA Mixer-Amp to Mic Preamp?

[doorunrun]

You want to supply V1 with a voltage that is as clean as possible, so I would make the following change, which will give you more ripple filtering for V1.

The additional 1M resistor will keep the potential stabil on V2 grid even if VR1 gets old and corrosive. Not absolutely necessary, but good practice.

Thanks! That's a "grid leak" resistor, right? I read about using one in a related thread here.

 

[rock soderstrom]

Thanks! That's a "grid leak" resistor, right? I read about using one in a related thread here.

This resistor could be called this way, but it is not so much a gridleak (bias) resistor here IMHO but rather a parallel resistor for the potentiometer, so that the grid of V2 always sees a defined potential, even if the potentiometer wiper loses contact (due to corrosion or whatever).

 

[MagnetoSound]

@rock soderstrom good call on the B+ filter mod.

I think 82k at R3 is a good move. I’d be looking to increase gain in V1 and reduce gain in V2.

If R2 is 10k and R6 is 47k, what plate voltages do you see?

 

[doorunrun]

I'll do some rewiring with resistors I have on hand and see what we get!

RCA originally used 1W resistors in the plate/screen wiring of each input (6SJ7). But the 100k plate resistor for V2 (6SF5) is 1/2W. Do you see any problem using 1/2W resistors for V1, or elsewhere for that matter?

Thanks!

 

[emrr]

The 1w is because higher rated carbon has less noise

A volume pot should always be log. I hadn’t noticed it was 1M. That is to maximize gain. Could be 100k with that cap. No more than 250k. Otherwise you lose treble with lower settings.

V2 could be 6c5 or 6j7

 

[doorunrun]

I hadn’t noticed it was 1M. That is to maximize gain. Could be 100k with that cap. No more than 250k. Otherwise you lose treble with lower settings.

The original design uses 500K input pots with a master gain pot of 100K. I can move things around to make the switch.

V2 could be 6c5 or 6j7

The way V2's socket is installed in the chassis its heater pins are very close to the rear panel barrier strip's 6.3V location. Again, with some rewiring I can swap tubes, which I think is probably a good idea and I happen to have the tubes.

 

[doorunrun]

Here's the revised version of the schematic incorporating the suggested changes. V2 is now going to be a 6C5/6J5. I decided to reverse the orientation of V2's socket making its orientation the same at the others on the chassis.

At this time I'd like to keep using the 500k pot. @emrr would it then be better to change C4 (and C9) to 0.5uF caps?

I only have V1 voltages so far. With 220V B+:
V1 Plate: 60V
V1 Grid1: 0V
V1 Cath: 2.3V

Just a bit more wiring to do and see how voltages stack up and hopefully how it sounds.

 

[emrr]

At this time I'd like to keep using the 500k pot. @emrr would it then be better to change C4 (and C9) to 0.5uF caps?

No, too large a coupling cap may be why there’s motorboating. 0.1/100k is commonly seen. 0.22/100k can motorboat on some circuits. The suggested parallel safety resistor is also never seen in old circuits, still, you can use parallel 500k with 0.1 just fine.

 

[rock soderstrom]

At this time I'd like to keep using the 500k pot. @emrr would it then be better to change C4 (and C9) to 0.5uF caps?

The potentiometer and the grid resistor work together in parallel. As it is now, the resulting total resistance is 500 II 1M = 330K. That is why I suggested a 1M potentiometer to maintain the original circuit at this point (1M II 1M = 500K).

here is an online calculator if needed.

http://www.sengpielaudio.com/calculator-paralresist.htm
However, Doug is right, too high resistors will potentially create the problems he mentioned.

A total resistance of 250k is quite common (and better!) for this type of circuit in this position.

 

[doorunrun]

No, too large a coupling cap may be why there’s motorboating. 0.1/100k is commonly seen. 0.22/100k can motorboat on some circuits. The suggested parallel safety resistor is also never seen in old circuits, still, you can use parallel 500k with 0.1 just fine.

Oops, I goofed on the value. I meant to put 0.05uF. I recall from your schematic of the MI-12241 the coupling cap could be 0.1 or 0.05uF.

Sorry 'bout that!

 

[doorunrun]

Finished wiring it up. Here are the voltages, B+ is 220V
V1 Plate: 60V .......... V2 Plate: 84.5V .......... V3 Plate: 162V
V1 Grid: 0V ............. V2 Grid: 0V ................ V3 Grid: 0V
V1 Cath: 2.3V .......... V2 Cath: 3V ................ V3 Cath: 6V

Gave it a quick mic test and it sounds good, no motorboating with VR1 full open.
Note: VR1 is 500K, C6 is 0.05uF, plate resistors are older carbon comp. 1W

I came across a couple of 6J7's in my tube stash. Down the road I thought I might wire one of the other unused input channels for it and see how that sounds.

Thumbs up!