I’m trying to repair a 63 year old record player. Opening it up, all of the electronics have aged badly, except for the drive motor, which is basic enough that it operates properly. I know enough about the resistors and capacitors that I can just get modern equivalents that will last much longer than their 50s counterparts, but what I’m concerned about are the vacuum tubes, one 35W4 and a 50C5.

enter image description here enter image description here

This image is of the tube’s socket pins. The top tube is the 35W4, while the bottom tube is the 50C5. You can see a bright bluish-white line of electricity connecting two pins together. It doesn’t seem to go away until I cut the power. The 50C5 seems to be experiencing spark gaps, if that’s the proper term, between pins 6 and 7, along with making an annoying buzzing noise. I don’t leave it powered on because I don’t know if it will start a fire, explode, etc. Tubes aren’t my specialty, but I know they’re dangerous. Does this mean anything is happening to the tube? Is it faulty?

Here is a picture of the record player schematic. enter image description here

The white powdery substance is just a 250 ohm Lectrohm resistor, I’m not sure why it uses powder, it is some 60 years old, but it isn’t functional and the resistance has dropped drastically.

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    \$\begingroup\$ @jonk Yes, see that now, wasn't visible on my phone. Shielded input to the grid at pin 2. I suspect the white powder indicates the bottom of a dried up chassis mount electrolytic. \$\endgroup\$ – AlmostDone Apr 3 '18 at 2:28
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    \$\begingroup\$ @toasthouse If you're sincerely trying to restore this, my advice is to first replace the parts that by inspection you know or suspect to be bad, including the tubes. The tubes should be available through antique radio sources and ham radio flea markets. \$\endgroup\$ – AlmostDone Apr 3 '18 at 2:34
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    \$\begingroup\$ Cannot tell if blue arc is through air or over the surface of the phenolic tube socket. A good cleaning (and drying) of the socket may help. 50C5 and 35W4 are intended to have filaments wired in series, along with a dropping resistor directly to 120V AC. Could that be your failing 250 ohm? \$\endgroup\$ – glen_geek Apr 3 '18 at 2:47
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    \$\begingroup\$ Just because nobody else has pointed it out, you need to be very careful with this circuit, even more so then with the usual sort of valve based thing. This one is LIVE CHASSIS so an isolating transformer is more or less mandatory if you are going to work on the thing. I am guessing that this was originally inside a wooden box with no access to the metalwork or any other connections? If the resistor R4 has really gone very low (Note it is shunted by the motor winding) I would expect the filaments to have died. \$\endgroup\$ – Dan Mills Apr 3 '18 at 9:50
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    \$\begingroup\$ Arc is between plate and screen grid, which should have similar DC voltage. Always replace electrolytic capacitors - in your case 3 are required: a low voltage 20uf, and two high-voltage 40uf, 60uf. \$\endgroup\$ – glen_geek Apr 3 '18 at 14:17

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