PAM-1 filaments aren't lighting

I'm restoring a ST70 and a pair of PAM-1 preamps.

I have them on the bench right now, and the ST70 is all dialed in and biased and seems to be working well.
The PAM-1 does work, but the filaments on the 12ax7's aren't lighting at all.

Measuring the voltage coming off the sel rectifier and going to the filaments, I see only 6.7VDC.
This should be doubled and closer to 12.6VDC right?
Does this mean one of the two diodes in the sel rectifier is shot?

Any help, confirmation, or suggestions to further test/prove this would be awesome.

Thank you

antsap wrote:
This should be doubled and closer to 12.6VDC right?
Does this mean one of the two diodes in the sel rectifier is shot?

Yep, that would be the case on the bad diode, but not on the output voltage (under load) - that is, per the manual, 11 VDC. Replace both the caps and the rectifiers with silicon. I use 1N4007 as my general-purpose replacement in these circumstance.

There is a bit more to it, however. There is much less forward-drop with a silicon diode than with a selenium diode. Per the schematic, the OEM design had the filament voltage at 11 VDC under full load. If you are getting 6.7 V from one (1) diode, that suggests you would be getting something on the order of 13.4 V if both were working - not good.

a) Install the two new diodes - mind polarity.
b) Measure the actual voltage-to-ground with the tubes in place - for the few minutes necessary, no harm will come to them.
c) Using the calculator (linked), determine the ohms and wattage required to make the correct output voltage. Go UP to the nearest next watt. Don't go extra-heavy as that resistor will be a last-resort fuse if needed.
d) Note that you are drawing 150 MA per filament in series (per tube). This will help you calculate the resistor wattage.

I just went through this on a PAS 3x that recently crossed the bench, and has, now about 500+ hours on it since, playing in my office.

http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html  

Enjoy!

Peter, thanks so much for your reply!

I went ahead and wired up two 1N4007's across the selenium rect, and now she's putting out the expected 12.6 vdc on the nose!

Do you really think I need the dropping resistor? 12.6v is just about perfect for 12ax7's based on their spec sheet? I was thinking the 11v in the manual is just because of the extra drop from the sel rect but not a requirement.

I also have a low hum (not sure if its 60 or 120hz yet) coming out of the PAM. It's not effected by the volume knob at all. I'm assuming its the filter caps (can), even though it measured very well for ESR. Anything else I should suspect? Guess I'll bust out the scope and confirm the source etc.

thanks!

antsap wrote:Peter, thanks so much for your reply!

I went ahead and wired up two 1N4007's across the selenium rect, and now she's putting out the expected 12.6 vdc on the nose!

Do you really think I need the dropping resistor? 12.6v is just about perfect for 12ax7's based on their spec sheet? I was thinking the 11v in the manual is just because of the extra drop from the sel rect but not a requirement.

It is my personal belief that the small-signal tubes in Dynaco products last so long because they are run at slightly-lower-than spec. voltages. That 11 V is a fully loaded voltage, not free voltage. But, your system, your tubes. Not to worry.

I also have a low hum (not sure if its 60 or 120hz yet) coming out of the PAM. It's not effected by the volume knob at all. I'm assuming its the filter caps (can), even though it measured very well for ESR. Anything else I should suspect? Guess I'll bust out the scope and confirm the source etc.

Try it with shorted inputs - does that reduce the hum?

thanks!

Enjoy, in any case!