Most likely the most insane question you'll ever hear...

[Cliff Schecht]

That thread was a great read while trying to stay awake in Engineering Analysis today (nice mix of differential equations and linear algebra). Thanks for the post!

[Jana]

Gingertube,

Isn't it possible there is some heater current for the rectifier tube? Pins 2 and 7 are normally the 6.3vac heater current for the power tube. With a rectifier tube plugged in, pin 2, one of the rectifier heater pins is connected to half of the 6.3vac. Pin 8 is grounded on a power tube, that is now grounding the other pin of the rectifier filament. IF the 6.3vac in the amp is center tapped and the center tap is grounded, then there is 3.1vac across the 5 volt rectifier filament. That might be enough to at least allow partial conduction.

[NickC]

Many of you may already know (Howdy Architect!) of this "classic" thread, but for those that don't ..... it's required reading:


http://recording.org/pro-recording-foru ... setup.html


Laughter is the best medicine.

[patrick620]

Mostly I just lurk around on here but this thread made my morning. Thanks

[VacuumVoodoo]

The diodes, (tube rectifier is 2 diodes) one or both will be reverse polarized by SG supply voltage. Both if SGR is installed between pins 4&6 on the socket. If there's a 1 Ohm bias check resistor between pin 8 and ground then it will end up in series with rectifier tube heater and may or may not get fried. If that resistor is common to both power tubes the amp will go silent when it fries.
Repair charge:
resistor 0.1$
time 10$
penalty for being an idiot: 500$

[Firestorm]

Gingertube,

Isn't it possible there is some heater current for the rectifier tube? Pins 2 and 7 are normally the 6.3vac heater current for the power tube. With a rectifier tube plugged in, pin 2, one of the rectifier heater pins is connected to half of the 6.3vac. Pin 8 is grounded on a power tube, that is now grounding the other pin of the rectifier filament. IF the 6.3vac in the amp is center tapped and the center tap is grounded, then there is 3.1vac across the 5 volt rectifier filament. That might be enough to at least allow partial conduction.

A 5U4 will run happily on 3 volts; it isn't until filament voltage drops below 2 that emission crosses the "reject" line. (Not great for tube life though.) But the Kustom Amp in question uses some kind of hybrid fixed/cathode bias scheme, so the question is how big is the cathode resistor? Probably too big to let much of the 3.1VAC flow, but if it's smallish a modest amount of DC will flow from Pin 8 through the 5U4 filament to the 6.3VAC supply ground (center tap or artificial) so the 5U4 could conduct. In which case, what burns up first?

[VacuumVoodoo]

There won't be any conduction because rectifiers cathode will be at higher potential than its anode. See my post above. Or draw a rudimentary schematic.

[Firestorm]

There won't be any conduction because rectifiers cathode will be at higher potential than its anode. See my post above. Or draw a rudimentary schematic.

Hypothetically (not knowing the exact circuit of the amp in question) the rectifier cathode/filament is at ground potential (owing to the center tap connection of the 6.3VAC supply) and heated to some unknown degree by DC current flowing from the 6V6 cathode through the filament to that same ground. Rectifier anode(s) (one or both) are at Vscr (initially maybe 350-400 VDC). How can that tube NOT conduct?

Electrons would flow from cathode to anode as in any diode, but no V+ would develop on the cathode because of the ground connection. That same flow (V-, in effect) into the electron-depleted screen supply should decrease Vscr (limited by series resistance in the power rail) to some equilibrium. The rectifier tube would become a sort of Frankenstein version of a voltage regulator.

[VacuumVoodoo]

Sorry, You are correct, I'm having a seriously bad diabetes day....brain works in reverse mode.