What tube should go here? 1935 Oahu restoration

[gerrickreidenbach]

Have you gotten this old gem working?

It's up and running. there isn't much magnetic pull from the field coil but there is sound. it overdrives very well. the addition of a capacitor to ground where @printer2 recommended helped quite a bit, cleaned up some flutter in the output. I'm concerned that the field coil isn't working correctly and I've discussed this with the customer. Output seems a little low. The voltage on the plate of the 6V6 bounces between 210 - 270 V depending on what you are playing through it. Again, I'm not sure there is much I can change. I came to expect that the field coil would have a magnetic pull on objects when energized and it doesn't.

 

[jthomas666]

I know almost nothing about this kind of speaker. But looking around the interwebs, I found this. It might make sense to take the field coil out of the equation and see what kind of sound you could get.



Here is a video about testing field coils.

 

[gerrickreidenbach]

It might make sense to take the field coil out of the equation and see what kind of sound you could get.

He may want to at some point but I think he will be happier for now if he can retain the original speaker, so long as it is adequate. I did a couple more tweaks, added 68K grid stop resistors at the inputs and lowered the value of the 20K resistor to 10K. The change was slight but there is some difference.

 

[gerrickreidenbach]

I removed the two capacitors to ground on the a/c side of the power transformer and then reversed the 10k resistor back to stock value and removed the two grid stop resistors.
There was no real benefit to the mods and I suspect that they were creating some noise in the circuit after I removed the two capacitors mentioned previously.
These changes are not reflected in the schematic I presented earlier, which may contain other errors.