Reducing Philco Model 650 Push Pull AF Output

[Paul Smith]

Hi all

I am busy restoring this fine radio which runs two type 42 tubes in push pull for the output stage.
Looking at the specs I see that the plate and screen voltages are both supplied with a nominal 300V. While this is fine for the plate I would like to reduce the power dissipation in the stage in an effort to prolong the life of these increasingly rare tubes. Manufacturer’s specs for the screen is max 250V.
I have added screen dropping resistors in single ended output stages in the past for the same reason but this is the first time I am considering it on a push pull stage.
Can I use the same approach here? Any suggestions are welcomed.
I have tried to attach a copy of the service manual for info but no luck so far.

Thanks, Paul

[Jim Sirtonski]

Hello Paul. What are the actual voltages and current that you’re seeing at the plate and screen pins?

The Philco 650 took on many cabinet styles, both tabletop and console. What variation is yours? See if you can attach an image.

Jim

Attachments:

1. 

   Philco-650.pdf
   

[John Greenland]

Paul,
If you really do not wish to permanently modify the radio and have a bit of room above the type 42 tubes you could make an adapter socket and use 6F6’s instead.
According to info, the 6F6 is the same tube but with an octal base. In the even that you did decide to sell the radio , replacing the adapters and 6F6’s with 42’s returns the radio to original condition.

Food for thought.

[Paul Smith]

Hello Jim

I have the same Rider manual you attached and have been using the schematic as it stands without running into any differences, so far at least.
The set is apparently a Canadian model with chassis number appearing as 3650. There is no other model ID visible anywhere.
I am busy recapping etc and not yet able to power the chassis up so I am not able to give you any real world measurements yet.
From a general standpoint though I would like to know if I can reduce screen voltage on each tube to throttle back the output and reduce strain on the tubes.
I have a couple of pics and will try to attach here.

Thanks, Paul

Attachments:

1. 

   Philco2.jpg
   

2. 

   Philco1.jpg
   

3. 

   Philco3.jpeg
   

[Paul Smith]

Thanks John
That is a good suggestion given that the 6F6 is electrically identical to the type 42.
I used that socketing approach once before with hard to find rectifier tubes like the type 80.

Paul

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