Half of a dual triode.....

[C Moore]

V2 would be the tube in question.
I recently rebuilt a friends Bassman. I kind of used a combination of the AA864 and the AB165.
Anyway......it just dawned on me that when I ran the heater wires, I connected the heater for the un-used triode. His amp seems to work fine......he is happy with it. But, do I have a potential problem.? Is it a problem to do what I did.?
Noise issue, heat issue, current draw worrries.....?
Should I get the amp back and disconnect that one side.?
What is the correct way to handle the heaters for an unused triode in a 12AX7.?
Thank You

[statorvane]

I had a similar situation when I added a trem circuit to a P1 eX. I left the extra triode alone - nothing wired there.

I think you are just wasting filament current. I'd unhook the unused triode.

[ToneMerc]

Anyway......it just dawned on me that when I ran the heater wires, I connected the heater for the un-used triode. His amp seems to work fine......he is happy with it. But, do I have a potential problem

I would leave it, ho harm no foul. As far as the current draw, it's the same gross filament string draw as before you modified it.

TM

[martin manning]

I'd leave it too. It might even be easier on the the tube structure from a thermal growth and stress point of view to heat both sides. 150mA of filament current? Meh.

[jjman]

There's only one filament no? 2 ends and one CT connection. To connect only one half you would need to use only one heater wire and ground the CT but that would create an imbalance between the heater winding halves, albeit small. Do any amps ever connect just one half of the filament in a 12a-7 setup?

[martin manning]

They're paralled in the usual arrangement. All you'd do is not jump pins 4 and 5, and connect whichever one of those two want (4 and 9, or 5 and 9). The CT of the filament (9) is not grounded.

[C Moore]

http://www.webphix.com/schematic%20heav ... _schem.pdf

Maybe it is just rote.....but the layout shows everything connected per usual.

[Lynxtrap]

(Edited due to useless comment)

[rdjones]

http://www.webphix.com/schematic%20heav ... _schem.pdf

Maybe it is just rote.....but the layout shows everything connected per usual.

I just looked at my Blackface Bassman and it has pins 4 and 5 tied together on V2.
I believe it to be unmolested in that area.

rd

[C Moore]

http://www.webphix.com/schematic%20heav ... _schem.pdf

Maybe it is just rote.....but the layout shows everything connected per usual.

I just looked at my Blackface Bassman and it has pins 4 and 5 tied together on V2.
I believe it to be unmolested in that area.

rd

10-4
Thank You

[rdjones]

I wonder why they light up the unused half ?

I just looked at a sampling of 12AX7s including RCA, GE, Sylvania, Telefunken, Mullard, Westinghouse and a few Japanese rebrands.
They all consistently have pin 4 powering the filament for triode 1-2-3.
I don't have any current production to check.

It seems that it's not an issue of not knowing which filament pin(s) to use.

rd

[martin manning]

My guess is it's about heating the structure evenly so that the plates and cathodes in both triodes grow a similar amount in length.

[C Moore]

Per Usual.....thanks for the info.
I suppose to leave "well enough alone"
Thanks Again

[Alexo]

You should ground the plate, grid and cathode in the unused triode if the filament is on, otherwise, it can wear them out over time.