Bad tube in old record player?

[rman]

Hi.

I found a crescent 1601 record player at the recycle depot so I naturally had to take it home. OK it is a cheap old p.o.s, but it looked so forlorn waiting for it's final demise. I managed to order a schematic with voltages marked. I tried it carefully tried it out. I can plug it in to a dead wall socket at my work bench and turn it on with the light switch. It hummed loudly and had low distorted output, so I changed the filter cap, hum is gone, but still low and distorted output. The circuit connections check out and resistance values are close enough, and voltages on the input stage seem OK. The output stage, (50B5 pentode) Has about 12 volts on the grid, it should be zero. I then changed the coupling cap thinking it might be leaky. No change. The grid is properly connected to ground by way of the 500k tone pot. Disconnecting the coupling cap also makes no difference.(to the voltage) I also checked for shorts in the tube with my meter. The grid voltage is actually higher than the cathode voltage.

Anyway, I was wondering if this could be the tubes fault as I have no other ideas left.

Cheers.
Rolf.

[planet10]

Schema?

It could well be a tube, it is likely 50 some years old. I have some 50C5, i'd be happy to send a couple for cost of post.

dave

[rman]

Hi Dave.

Thank's for the offer. I just looked in my tube manual. The 50B5 and 50C5 seem to be the same except for very different pinout. I'll see if main electronics has some.

I have no way to get schematic into computer, but it is pretty simple. I think I described everything relevant.

Thank's again.
Rolf.

[planet10]

Different pinout? I assumed 50A5, 50B5, 50C5 were all just revisions of the same tube...

I looked thru my box of 50c5, no errant 50B5/ Might be some in the unsorted tubes, but that is somewhere i don't have time to go.

you could use the OPT for 50c5 or 50eh5, but that would require more work.

dave

[Globulator]

Most likely to be a coupling capacitor IME.
There can't be many so you may as well change them all!

[rman]

Dave.
Main electronics in Vancover lists them on their site so should be easy to get.
Globulator.
You are right about there not being many. There is the input cap which can't be the problem and the interstage coupler I already changed.

I don't know how a tube could fail in this way. I was hoping someone could tell me if they can. It is not an expensive tube so I will try it anyway.

Cheers.
Rolf.

[Frank Berry]

The old tube is probably gassy.

[rman]

Frank.

Thank you. that makes sense. I will definitely change the tube.

[rman]

So I changed the tube and a cathode resistor that read 245 ohms instead of the specified 150. The amp seems to work fine now.
I wasn't expecting Hi-Fi but the sound is pretty bad. It was kind of fun to crank Willie Nelson to 78 rpm.
Since I have gone this far, I ordered two new needles. The cart flips over for 78's.

Cheers.
Rolf.

[rman]

Just to add some closure, With a new needle, it tracks and sounds much better.
It still sounds wretched but probably a level of wretchedness that was acceptable to a teenager in1956. Or even some today

Just wondering, it doesn't auto return but it is obviously a changer so it should right?

Cheers.
Rolf.