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Philco 39-7 issues
#16

Hi Jeff,

If you have worked on any "post-war" transformer powered radios, you will notice some differences. In post war radios, the chassis is "B-" and tubes requiring bias are "cathode biased" by individual resistors between cathode and chassis.

In this set as in many pre-war sets, they referenced the cathodes of the tubes to chassis ground, but the chassis is positive with respect the most negative of the power supply, the HV secondary center tap. This is the reason that I wanted you to verify resistors #23 and 24 as they partially complete the circuit between the cathode of the output and, in fact, of all tubes and the power supply. If these resistors are bad, all bets are off. I would replace these with 2 Watt metal film resistors, whether good or bad, as they carry all of the current of the B supply. Since the voltage drop across these resistors is fairly low, the power dissipation is low, but pretty close to 1/2 watt.

The other "biggie" is a rather unusual feedback arrangement where the secondary of the audio output transformer is between ground and the output tube cathode. This provides a feedback path (I would assume negative feedback, but it could be positive feedback to enhance bass response). If this connection is not made, no current will flow through the output tube and there will be no audio. If someone modified this and connected the output tube cathode (pin 5) to chassis ground, this would work, but if connected to the output transformer secondary but the other secondary lead is not grounded, than there will be no audio. I have never seen this method of providing feedback, but I also never saw the output tube plate to 1st audio tube screen and input feedback used in the Crosley 5C2.

"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis

Best Regards, 

MrFixr55


Messages In This Thread
Philco 39-7 issues - by Jeff Lepore - 02-19-2026, 12:02 PM
RE: Philco 39-7 issues - by morzh - 02-19-2026, 12:56 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-19-2026, 04:06 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-19-2026, 07:05 PM
RE: Philco 39-7 issues - by Joe Rossi - 02-19-2026, 08:37 PM
RE: Philco 39-7 issues - by morzh - 02-19-2026, 09:35 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-20-2026, 10:25 AM
RE: Philco 39-7 issues - by Jeff Lepore - 02-20-2026, 04:39 PM
RE: Philco 39-7 issues - by morzh - 02-20-2026, 04:49 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-20-2026, 05:53 PM
RE: Philco 39-7 issues - by morzh - 02-20-2026, 10:22 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-21-2026, 09:46 AM
RE: Philco 39-7 issues - by Jeff Lepore - 02-21-2026, 11:21 AM
RE: Philco 39-7 issues - by RodB - 02-21-2026, 12:18 PM
RE: Philco 39-7 issues - by morzh - 02-21-2026, 12:37 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-21-2026, 02:09 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-21-2026, 06:48 PM
RE: Philco 39-7 issues - by morzh - 02-21-2026, 07:13 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-21-2026, 09:25 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-21-2026, 09:51 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-21-2026, 10:57 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-22-2026, 10:22 AM
RE: Philco 39-7 issues - by MrFixr55 - 02-22-2026, 10:49 AM
RE: Philco 39-7 issues - by RodB - 02-22-2026, 11:28 AM
RE: Philco 39-7 issues - by morzh - 02-22-2026, 02:02 PM
RE: Philco 39-7 issues - by RodB - 02-22-2026, 02:49 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-22-2026, 07:03 PM
RE: Philco 39-7 issues - by morzh - 02-22-2026, 07:26 PM
RE: Philco 39-7 issues - by morzh - 02-22-2026, 07:31 PM
RE: Philco 39-7 issues - by RodB - 02-22-2026, 09:07 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-23-2026, 06:54 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-24-2026, 07:32 PM
RE: Philco 39-7 issues - by morzh - 02-24-2026, 08:35 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-25-2026, 12:44 PM
RE: Philco 39-7 issues - by morzh - 02-25-2026, 02:18 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-25-2026, 06:34 PM
RE: Philco 39-7 issues - by morzh - 02-25-2026, 09:08 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-25-2026, 09:31 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-26-2026, 12:45 PM
RE: Philco 39-7 issues - by MrFixr55 - 02-26-2026, 03:02 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-26-2026, 03:24 PM
RE: Philco 39-7 issues - by morzh - 02-26-2026, 03:50 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-26-2026, 06:25 PM
RE: Philco 39-7 issues - by morzh - 02-26-2026, 09:50 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-26-2026, 10:36 PM
RE: Philco 39-7 issues - by morzh - 02-26-2026, 10:49 PM
RE: Philco 39-7 issues - by Jeff Lepore - 02-28-2026, 12:34 PM
RE: Philco 39-7 issues - by morzh - 02-28-2026, 02:21 PM



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