6v vs12v heater

[drewl]

Totally reworked a Silvertone 1474 and rebuilt the reverb circuit to use a standard tank with a transformer driving it.
I rewired the first stage to a standard heater configuration, and used a 6CG7 as the reverb driver and return stage.
Now, stock that stage was a 12AX7 for send and return of the crystal reverb "tube"
Any idea why they wired that stage to run a 12AX7 with just 6v to pins 4 & 5?

I like the way the amp/reverb turned out, but I was experimenting with some other tubes like a 6AQ8 and 6Bq7 and think that stage might have sounded better if I had kept a 12AX7 there and rewired it for a standard heater layout.
It doesn't want to conduct properly with 6v between 4 & 5.

[Enzo]

All the tubes should be wired for 6v operation. Regardless of what is sitting there now.

[drewl]

Let me rephrase that.

I was curious as to why on that stage they would wire the 6volt heater voltage to pins 4 & 5 only and not between 4/5 to 9.

[Bruce / Mission Amps]

Which tube(s) has it's heater string "biased" by the "cathode biasing" resistor voltage divider on the power tubes?

[52 Bill]

My vote would be that the amp didn't have a 12AX7 in that position from the factory. More likely a 6CG7, which they used for the reverb driver in a lot of their amps and has the filaments wired to pins 4 and 5.

[drewl]

Actually it did have a 12AX7, I've seen later models where they changed it to a 6CG7, sometimes they just put a sticker over the label.

And Bruce, this stage V2, and V1 were using the DC cathode voltage for heater wiring, I forgot (DOH) I rewired them to AC.

[Enzo]

I'm with Bill, the socket was wired for a 6CG7. They would not have wired a 12v heater for 6v. The 6CG7 has the same pinout as a 12AX7 except for heater. Maybe someone stuck the wrong tube in there, maybe someone rewired for the other tube. Hard to say how the combination of factors you see now came to be, but they just simply had no reason to wire a 12AX7 up for half voltage on its heater.

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