Troubleshooting Fender amp

[ethering]

I am wondering if anyone can offer some advice.

Fender "the twin" (I know, no one likes them). A subtle, ring modulation effect on some notes, and a little bit distorted. I replaced all the electrolytics (they were 20 yrs old) but it didn't change anything. Measured the voltages on all pins and found that the heater voltage is pretty much zero. Is this a bad power transformer, or is there some other possibility?

[HotBluePlates]

... A subtle, ring modulation effect on some notes, and a little bit distorted. ... found that the heater voltage is pretty much zero.

You can't have it both ways.

Either there is no heater voltage, or there is heater voltage and you get sound.

Since you hear something out of the amp, you must have some amount of heater voltage. I recommend starting over with the voltage readings, and be careful that you have your meter on the proper range (if not autoranging). If you try to measure 3-6v with the meter set on a 200v range, it may not register or may look like almost zero voltage. This is especially true for analog meters.

[ethering]

I had sound yesterday but checking again, there is no sound. 

[HotBluePlates]

Here is a schematic for The Twin.

Make sure the connectors for cabling running between boards and boards/transformer are snug.

[Zipslack]

May be just my inexperience, but when I hear "ringing modulation with distortion", I automatically think 1-tube going bad, 2-microphonic tube, 3-oscillation.  If you have no sound now, I would focus on #1.  Are all the tubes lighting up - mainly the preamp and PI tubes?  BTW, for measuring heater voltages, I like to just measure directly across the heater pins instead of referencing to ground.

[HotBluePlates]

Which channel and which jacks are you plugged into when the amp is acting up? All of them?