[Patrick]

Just converted a 54C using Ron's instructions. Radio plays really well when I touch the metal 12SQ7. But as soon as I take my finger off of it, the sound is garbled. I checked all the solder joints, the socket itself, and tried another tube. I also grounded pin 1. Still garbled unless I touch the tube. Any ideas?

Pat

[Image: https://i.imgur.com/RtxQcly.jpg]

[Ron Ramirez]

It sounds like something (that is, an AVC or B- connection) is floating that shouldn't be.

As I recall, you were looking for an antenna coil for this set. Did you find one? Are all the windings good?

Are the windings of the IF transformers (especially the secondaries) ok?

Did you ground pin 1 of the 12A8GT and the 12SK7 as well as the 12SQ7?

[Patrick]

Hi Ron, thanks for responding. I actually have 2 54C's, so I used the antenna coil from the other one. IF's are both good, and I did ground pin 1 of the other tubes as well.

[Ron Ramirez]

Only other thought that comes to my mind: Double-check your wiring.

Terry (Radioroslyn)? Mike (morzh)? Feel free to jump in here, guys...

[morzh]

Q: was the original tube glass envelope or metal type? If glass, is the metal case (pin 1?) grounded? Judging by the "I touch it and it is good" it is not.

[Patrick]

I'll triple check the wiring. And Morzh, the original was a glass 75. The case of the metal 6SQ7 IS grounded. Touching ground next to the tube does nothing. Just a light touch on the case, and the garble goes away. So am I somehow adding capacitance to the tube by touching it?

[Radioroslyn]

I would think that you would want to connected pin 1 to B- and not chassis ground. 
In this configuration B- is 150,000 ohms above chassis ground to lessen the shock 
hazard. This could allow some small amount of ac to be present on the tube's
shield. It's such a small amount that when you touch it bleeds off. That ac voltage
could be the cause of your issue. Oh, B- is easily found at the - side of the filter caps.

Or try adding a .2mf at 630v across the 150,000 ohm resistor.

Or just disconnect the connection from pin 1.

Or replace the faulty 12SQ7 maybe.

[morzh]

Terry

C7, a 0.22uF cap is already placed across thaqt 150K resistor, (well, from B- to GND which is the same).


Patric,

You could do what Terry suggested. The only thing is, the B- is an AC outlet pin, and so if it goes to Hot and not Neutral, it is still a shock hazard.
But do experiment with caps, and make sure the GND is real. Or add a cap from that pin to B- instead of GND it. Or in addition to....

I am sure somehow, your trouble is a (the?) result of subbing metal tube for the glass one.

[Radioroslyn]

Tnx Mike,

Sometimes I get a little myopic.

[Ron Ramirez]

Mike, Terry,

I originally performed this modification to a model 54 which I own, and wrote it up on my blog. It works perfectly - which is why I am puzzled as to why Patrick's does not work properly.

Pin 1 of the 12A8GT, 12SK7 & 12SQ7 are all connected to ground (chassis). The 12SK7 & 12SQ7 tubes are metal, to compensate for the lack of shielding - glass tubes, as you know, are not shielded unless a metal shield is placed over the tube and connected to the chassis.

[morzh]

Ron

This is why I think first we need to establish that the GND that comes to the Pin1 is good and not some convoluted meandering wire that goes past gosh knows what kind of noise source. Then we can look further at it.

The fact that touching GND near the tube does nothing, but touching GND-ed tube does something tells me there is some bad GND-ing somewhere.

BTW, I would try another tube if possible, after having established that the GND-ing of the pin1 is good,. just to make sure there is no disconnect in the tube itself between the pin and the case. Or at least try to ohm out the pin to the case.

[Patrick]

Good info from everyone. I'll hit it again in the morning. Had to watch the 49ers cage the Cardinals (I'm from SF).

[Patrick]

Well, Terry had it. I connected pin 1 to B- instead for ground. I left pin 1 of both the 12A8 and 12SK7 grounded. No more garble. But instead of directly to B- from pin 1, should I add a cap?

[morzh]

Yeah, try the cap. Use the smallest value that works (that is, if it works).

[Ron Ramirez]

> I connected pin 1 to B- instead for ground.

Patrick, do you mean you changed pin 1 of the 12SQ7 from ground to B-, or from B- to ground? Please clarify.

In my 54C, my 12A8GT, 12SK7, and 12SQ7 tubes all have pin 1 connected to the chassis, and the radio plays fine. So I really don't understand what could be going on with yours.

Another thing to consider - I changed my set to use a PM speaker. It would not work correctly with the old field coil speaker after the modifications were done. However, it is great with the PM speaker.