[Tubeglow20]

I have just finished restoring an RCA 1X591 Gladwynn model radio. New caps and resistors and all tubes checked and replaced as needed. Radio plays and sounds great with it's big 8 inch speaker. Rewired the dial lights so that they are in parallel instead of series. The radio comes with 2 Mazda 1490 bulbs wired in series and when the radio is powered up, the inrush current ususally blows one or both 1490's and can take out the 35Z5 tube. The radio might work for a while,  but eventually the bulbs and tube will blow.  Anyway, here's my problem....the 50L6 tube was changed out with a tested 50L6 that checked good with no grid leakage or shorts, (tested on a B&K 606 tester). When the radio is powered up, it hums very loudly so I tried another 50L6, ( tested good also). This one works with no hum. I knew the first one tested good, so I cleaned the tube socket to see if that would help....nope. Now, I carefully turned the chassis sideways the get underneath to probe connections to check for loose connections; used a plastic probe. I VERY CAREFULLY touched pin 5 of the 50L6 tube with my meter probe, the hum completely goes away and the radio plays very loud and clear. Remove the probe tip and the hum is VERY loud.  Found that if I attach a test lead to pin 5 and let it hang out from under the radio, it plays normal. Just don't connect it to anything, don't want to get zorched!!!! Now, if I swap a different 50L6 tube, some will play just fine without the wire hanging from pin 5. Any suggestions about what might be happening?

[Radioroslyn]

Open R-13 could be the cause.

[Tubeglow20]

Thanks for the reply. I installed a new R13 (150 ohm) resistor and the problem persists. It's weird,  I can use two different 50L6 tubes that both test good and one works perfectly while the other makes the radio hum/buzz. I tried the tube that makes the radio hum/buzz in another set with a 50L6 and that radio plays ok. I'm still stumped. The coupling capacitor between pin 6 of the 12sq7 and pin 5 of the 50L6 is a .0022 and that's what I replaced it with. I believe that adding the length of wire increases the capacitance; any thoughts?

[RodB]

Try pinching the tube socket pins to make better connection and reheat each tube socket terminal with a soldering iron. Be careful when pinching the pins.

[Radioroslyn]

My schematic sez r-13 is the grid return and not the cathode resistor. http://www.nostalgiaair.org/pagesbymodel...014815.pdf

[Tubeglow20]

You're right, I made a mistake. R-13 is a 470k ohm resistor. I replaced it because it was measuring about 560k.

[Tubeglow20]

Ok, checked the coupling capacitor between the 12SQ7 and the 50L6. It's value on the schematic is a .0022uf. When I hold the capacitor between my fingers, the radio plays loud and clear with 2 different 50L6 tubes, but when I release the capacitor slowly, the loud hum/buzz returns. What do you think??? Oh, what I believe to be the original wax/paper capacitor at .0022uf @400 volts was in the radio.

[Radioroslyn]

>I have just finished restoring an RCA 1X591 Gladwynn model radio. New caps and resistors
This would imply that the .0022mfd cap was replace right??

[Tubeglow20]

Yes, the .0022uf cap was replaced with a yellow tubular. 0022uf capacitor @630 volts.

[Radioroslyn]

Well if you think more capacitance will help go ahead add some.
A .005 or .01 is fine. There isn't much voltage there even a 100v
cap would be fine. The .002 might have been used as a tradeoff
to roll off some of the low frequency response. By increasing it
value may increase the hum level.
One other thing use a GT style 12SQ7. Have seen where pin 1
was used as a tie point, not a good idea when using metal tubes.

[Tubeglow20]

Ok, here's an update. I had a GT style 12SQ7 tube in the radio and just to see what would happen, I tried a metal 12SQ7.  The loud hum/buzz went away on most of the 50L6 tubes that I have. Actually the schematic calls for a metal tube instead of a GT tube and on this radio, pin 1 has nothing attached to it under the chassis. I have swapped metal and glass tubes on other radios, but there was never a change in how the radio played. The radio has been playing now for several hours with one of the 50L6 tubes that was causing the loud hum/buzz and all is fine for now. Thanks for all the suggestions.

[Paul Philco322]

As Shemp would say "strange happenings"!

[Image: https://i.pinimg.com/236x/f6/c0/d2/f6c0d...series.jpg]

Glad it worked out. 

Paul

[Tubeglow20]

Ok, new update....after playing for some time, the radio started humming and buzzing again!!! Voices started sounding garbled and I just put the radio up and took a break. This morning I decided to make some changes to my work. When I installed the new filter capacitors,  I used pin 3 on the 12SQ7 tube which is tied to ground along with pin 4. I removed the filter capacitors and reinstalled them grounded to the on/off switch and ran new wires to the rectifier tube and pin 4 of the 12SA7GT tube,(check the schematic). The B+line runs all the way to the 12SA7GT. When I plugged in the radio and turned it on, it came up clear and no garbled voices!!! Again absolutely no hum except when the volume is turned all the way down and I place my ear against the speaker. Little hum is normal with most radios I have worked on. Do you think that using ground on the 12SQ7GT tube was not a good idea? Well, the radio is working now so I'm happy!!!!

[Paul Philco322]

Good ideas are those that work create no electric risk and that you take credit for. Paul

[Eliot Ness]

Sometimes the grounds can be very problematic and even worse, intermittent.  Often a tube socket or other rivet is used for a ground that over the years becomes a bit, loose, corroded, or both. Hopefully you've got your problem solved