Antique Philco 37-665 audio output problem
 

I'm working on a tombstone Philco radio model 37-665. (1937 vintage) I've completely recapped the radio, replaced both audio output tubes, and others. Speaker was sent in and re-coned, and works fine, as long as only one 6F6 (the top one on the schematic is installed and the other removed. If I put in both 6F6s the volume will increase slightly, but as soon as I turn up the volume to a normal level the sound has very loud popping noise, especially with music playing. The schematic doesn't make much sense to me. The grid of the top 6F6 the signal is applied direct from the 1st audio tube 6K5 through a .03uf capacitor. That is the tube that will work with the other one pulled out. The resistor network which consists of a 490K and a 330K the center of the 2 resistors is connected to the center-tap of the power transformer winding's that that supply the B+. This basically kills the audio signal to the grid of the lower 6F6. Plate and screen voltages are within range for both tubes. I also have apx -15 volts at both control grids. There is also a .01 capacitor between the screen grid of the top tube (6F6) to the control grid of the bottom 6F6. I assume the center tapped output transformer is suppose to provide a push-pull action. I also tried reversing the leads going to the 2 ends of the audio output transformer and it sounds the same, so it's not a matter of reversed leads. I am completely perplexed with this old radio. Attached is a PDF of the schematic. ps, you may have to rotate the view to see the schematic properly.

The set in question uses Philco's famous funky Phase inversion circuit.
The center tap of the power transformer is above ground, to furnish grid bias for a few of the circuits including the audio output tubes, -15 volts sounds about right.
I know, Philco used that circuit in several models, that one being one of the first. I recapped a few of the sets using the circuit, but never had that problem. :scratch2:

An update that I found, if I disconnect C60 .01uf (mistakenly labeled on the parts list as #59) the distortion, and popping disappears however the volume is lower. and the lower 6F6 (on the schematic) really isn't doing anything, other than conducting and getting warm (not overly warm). Seems as if the signal for the control grid of the lower 6F6 is getting it's signal from the screen grid off the upper (on the schematic) 6F6. Bias at each control grid of each tube runs just over -3 volts, which looks about normal to me.

Actually about -3 volts on control grids
 

I was wrong when I posted -15 volts. I just got done measuring it and it's a bit above -3 volts. Maybe I have a power supply problem. Will do some more checking.

The Philco 37-665B is home and in service playing better than factory. Thanks Walter: good show!

I think you meant Philco's phamous phunky phase inversion circuit. :D

I never thought of phrasing it that way. :thmbsp:

:lmao:

If it were my radio, I'd modify it so it uses a real phase inverter. Change the 6K5 to a dual triode tube, maybe a 6SL7.

Or maybe a subminiature triode tube hidden under the chassis.

Or you could do it even simpler, using a grounded-grid, common cathode setup for phase inversion. Did that a few times on homebrew stuff back in the day. Worked great. Output tubes were typically 6V6 or 6L6 :nerd: