Differential Diagnosis - JTM-45 Offset Panel Reissue

[Wilder Amplification]

Here's one for the pros. The amp has already been diagnosed and repaired, but I wanted to run the symptoms by all of you and see how many theories we can all come up with before I reveal the positive diagnosis -

Amp is a JTM-45 offset panel reissue. With no valves installed except the rectifier valve, everything comes up fine and all unloaded voltages are right where they should be. As soon as you install any valve (you only have to install one and it doesn't matter which one you install), the rectifier valve flashes brightly and the mains fuse blows immediately. The HT fuse never blows.

Post up what ya got.

[Gtr_tech]

Odd problem....never dealt with anything like that. Sounds like something related to the heater circuit since the preamp tubes don't draw enough plate current to cause this. Plate supply getting on the heaters somehow?

[vavlehead]

Hard to say without schematic.. But I bet a faulty PT or some how the HT or heater loops were crosswired with the Rec heater....

[Wilder Amplification]

You guys were definitely very close on this one!

On a rec valve amp, the 5V winding is referenced to the HT rail by default due to one side of the valve heater being internally tied to the cathode (or the heater itself being the cathode on a directly heated type). The 6.3V winding has its center tap referenced to ground. We haven't figured out which windings shorted yet but basically some of the windings shorted internally in a way that was allowing a heater on the 6.3V winding to complete a circuit that shouldn't have been there, created by the internal short in the PT.

This was an issue with an amp that a friend of mine was working on. I had never seen that happen nor had he, but when the symptoms fit the diagnosis we had no choice but to go with it.

To test our theory prior to ordering up a new PT, I had him completely disconnect the 5V winding, then strap SS rec diodes from pins 4 to 8 and 6 to 8 and leave the rec valve out. Sure enough, with this configuration the amp worked flawlessly, which supported our diagnosis. A new PT fixed her right up.