Posts: 10
Threads: 3
Joined: Jun 2022
City: Chubbuck
State, Province, Country: Idaho
Hoping someone can help with this issue. I have a 38-10 xf that I recapped and tested the existing tubes but failed to notice that the 6f6 was replaced with a 6v6. I have replaced the 6v6 twice before I realized the substitute, however my research has shown that a 6v6 is an acceptable substitute in most instances. Is this substitute not acceptable in the 38-10 chassis, or is there something else that I may have missed? All the voltages were within tolerance after the recap and I can not find any shorts. The 6v6 filaments do heat up but fail to produce sound through the speaker and fail on the tester. All other tube continues to test good.
Posts: 1,194
Threads: 46
Joined: Feb 2015
City: Roseville, MN
Hi J,
Welcome to the Phorum.
Your tube substitution should be fine for now. If you are looking for best performance you can replace it later.
Schematic:
http://www.nostalgiaair.org/resources/233/M0013233.htm
When you say the voltages test OK, does that mean you tested all the tube sockets or only B+? Testing the audio section, do you get a low level hum when the volume is turned up? How about when you touch the grid cap of the 6Q7? If you get no response from either of these then there's something wacky in the audio amplifiers. Double check all the components, even the ones you didn't change, someone else might have made a mistake just to throw you off.
(This post was last modified: 06-08-2022, 05:33 PM by
RodB.)
Posts: 10
Threads: 3
Joined: Jun 2022
City: Chubbuck
State, Province, Country: Idaho
Rod
The voltages tested were between the tube pins and chassis and were all +-10V from the 38-10 schematic. I do get a hum at high volume and when touching the cap on the 6q7. Just can not get any audio without changing the tube out. Gets kind of expensive to replace a tube every couple weeks. Everything I put in a new 6v6, I get great audio for a week or two depending on usage. Then I get no audio except the humming and the 6v6 will test bad. All of the other components appear to be originals with the exception of the caps I replaced as well as the power cord.
Posts: 1,194
Threads: 46
Joined: Feb 2015
City: Roseville, MN
The 6V6 is probably conducting a higher than normal plate current. Can you measure the voltage at pin 5 of the 6V6?
Posts: 10
Threads: 3
Joined: Jun 2022
City: Chubbuck
State, Province, Country: Idaho
Rod, I will definitely check that voltage as soon as I can. Just went through a move and the radio is currently in a different state. What would be considered to much on pin 5?
Posts: 1,194
Threads: 46
Joined: Feb 2015
City: Roseville, MN
Pin 5 is the control grid and should have a bias of -13 volts to -17 volts, never positive. This bias is developed in the power supply circuit.
Posts: 7,305
Threads: 270
Joined: Dec 2009
City: Roslyn Pa
Should see about negative 16vdc at pin 5 (control grid). As you get low (moving towards 0 and then positive voltage) in voltage it causes the plate current to rise. If there is positive voltage there it can really haul down some current.
Happy Troubleshooting!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
