Hello, Hoping that you can help me with a couple of questions regarding the 37-650. There is a resistor soldered between the Pin 5's (Grid 1) of both 6F6's. It has an Orange body with a Yellow stripe and measures about 400Kohms. It doesn't show on the schematic! Can anyone confirm the appropriate value of this resistor? Second, there is a Bakelite cap (8035-DG), containing two 110pf caps in the 2nd Detector/AVG (6J5) circuit. Philco rated them at 1000V. I don't see how there can be more than 300V in this circuit, but I could be wrong. Can I use 500V rated caps here? I appreciate your assistance on this. Thanks,
Hi
First of all, the resistor in question is color-coded using the old
body-tip-dot format. In other words, the
body of the resistor is the first digit, the
tip (one end) of the resistor is the second digit, and the
dot or single stripe is the multiplier.
More here:
http://www.philcoradio.com/tech/resistor.htm
Your resistor has an orange
body, no
tip color, and a yellow
dot (or, in this case,
stripe).
Orange body = 3
No tip = Same color as body = 3
Yellow stripe or dot - 0000
330,000 ohms or 330K.
I am not at home and not near my schematics, but check the
Philco Changes In Models data (available as a free download
here) first. Then, if that resistor is not in the schematic or in the Changes In Models data, get rid of it. If it is in there, replace it because it is out of tolerance.
Now, regarding the 110 pF caps: Yes, replace them with your 500 volt caps and you will be fine. These, as you correctly point out, will never see any high voltage under normal conditions.
I just looked at the schematic on
Nostalgia Air.
There is a 330K resistor, part (46), connected to one of the 6F6G control grids. But the other end does not connect to the other 6F6G grid - it connects to B-. A 490K resistor, part (53), connects from the control grid of the other 6F6G to B-.
Perhaps this will help...
Thanks Ron! I probably should have mentioned that the yahoo who owned this subbed 6V6's for the 6F6's. Can't be sure that the extra resistor is a circuit mod for the new tubes, although looking at both tubes characteristics I don't see the need. Thanks for the forum, this is great to know there is a knowledge base to draw from (and hopefully add-to).
Although biasing requirements for the 6V6 are somewhat different from the 6F6, you can usually get away with using 6V6 tubes in place of 6F6.
Regardless, there should be no single resistor connecting both of the 6F6 control grids together. Follow the schematic.
Hello, Thanks for all the help. I've just completed the recapping and re-resistoring. Along the way the 5Y4 burned up and I created a socket adapter and have a 5U4G working. The 37-650X sounds great; the speaker gives a screech sometimes at high volume, but overall everything is great. I'll purchase NOS 5Y4 and 6F6's to get the tubes back to original. Next job: Renovating/restoring the cabinet...thanks again!
Ron, Final (I think) post on this one. I found a phorum question (posted by Robbie Roberts) from 2006 on this same model with the same question about a 330K resistor between the 6V6 tubes. I wonder if Philco published this as a mod when using the 6V6 in place of the 6F6. The resistor was a Philco product. Anyway, thanks again for the help.