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Philco Model 37-38B Questions - Printable Version

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Philco Model 37-38B Questions - jjfritz5528 - 10-14-2023

Just starting the restoration on this DC set.
1. The connection to the A+ from the Volume Pot is not there. I think it needs to be, but just want to confirm. Also the #23 capacitor on the wiper is 0.015mf on the schematic, but 0.005mf installed, and appear to be original.
2. The Audio Output transformer has one of the Primary halves open. Anyone have a suitable replacement? Push-Pull, 3 Watt, 4 ohm output, ~350 ohm DC resistance. Theory question - Why are the 2 halves of the Audio Interstage & Output transformer Primaries unequal resistance?
   

RE: Philco Model 37-38B Questions - morzh - 10-14-2023

1. Check if your ground on the vol pot is not done through the case bolted to the chassis, one of the ways it used to be made.
2. Uneven resistance is simple
: the same number of turns but different diameter (an outer turn is longer than the inner; hence the higher resistance).
For replacement, look at the Hammond 125 series; pick one suitable by the size and power. Make sire it is 125, nogt 125SE (for Single-ended).

RE: Philco Model 37-38B Questions - radiorich - 10-14-2023

Hello john,
My Philco 38-10T that I am restoring also has that same .015ufd going to the volume changing it to something that small will effect the frequency response .
Sincerely Richard

RE: Philco Model 37-38B Questions - jjfritz5528 - 10-14-2023

Richard & Morzh -
Great Information! Thanks Guys!
Yes that lug on the Volume pot IS connected internally to A+. I saw the glob of solder on the lug and "assumed" that it used to have a connection to it.
I also looked again at the Cap on the CT of the Volume pot. It must have been replaced at sometime way in the past. It has a Solar Brand cap and all the others are Philco Brand. Probably just picked one that seemed to work, in the past. I will replace it with the original value.
And finally, yes the added length of the outer windings would certainly add to the resistance! I thought it might be something much more technical. Keep It Simple!

RE: Philco Model 37-38B Questions - morzh - 10-14-2023

There are various ways to keep the halvess identical or close to it. Plus achieving other things along the way.

Sectioning the windings, interleaving, etc.
Radios are not hi-fi devices, and difference in ohmic resistance in the pushpull stage was not that important.

RE: Philco Model 37-38B Questions - jjfritz5528 - 10-19-2023

Additional questions related to the Output transformer. I purchased a Hammond #125A for Push-Pull output, 3W. The DC resistance is a fair amount lower than the original. 115/165 ohms vs. 320/350 ohms on the original.
I'm concerned that this would exceed the Plate Current rating of the 1J6 tube (50ma). Should I add a resistor to each leg to be similar in total DC resistance for each half of the winding (185/205 ohms)?
Not sure what the original impedance was for the Philco transformer, the Voice Coil is stated to be 5 ohms, so I will plan to use the 4 ohm output on the replacement. The Primary also has choices. The 1J6 states a 10k Plate-Plate effective resistance. The closest to that for the 4 ohm output is either 11.2k or 8.8k. So I'm thinking
11.2k would be safer.
Thoughts?

RE: Philco Model 37-38B Questions - morzh - 10-19-2023

Do not worry about it.

The plate current is NOT definecd by the load DC resistance. So you will be OK.

As for the impedance, chose 8.8KK, as the P-P load is 10K for the tube, and with 25% load increase the P-P will become 11K.

RE: Philco Model 37-38B Questions - Radioroslyn - 10-20-2023

For testing purposes what you could do is use the opt as is. Since the stage uses triode sections of the opt tube there is no issues with damaging the screen grid thru over dissipation. There will be a slight drop in output about 15% but 19 is a pretty stout tube.

This holds true for triode connected tetrodes or pentodes. Like the Philco Super Class A output stage (P/P 42's w/the screen grid connected the plate). Bob and I did it on a 116B w/a 1/2 open opt. 

What you don't want to do is is operate a set w/1/2 an opt and tetrodes or pentodes that the screen grid are connect back to hv line. In this configuration there is no plate voltage on the plate. So, the screen acts as the plate would but it can't dissipate the current that the plate can. In some cases, the screen will light up and burn up and tube's gain drops to 0. I think I hear Chopin playing in the background.    (No Copyright Music) Funeral March (by Chopin) by Chopin - YouTube

File this under cheap tricks. [Image: https://philcoradio.com/phorum/images/smilies/icon_lol.gif]

RE: Philco Model 37-38B Questions - jjfritz5528 - 10-20-2023

Thanks Morzh!