Minimum Heater Voltages

[bulldogguitars]

Whats the minimum heater voltages can go, running in parallel, before theres an affect on the tubes performance.

[R.G.]

There's not a hard and fast limit, and what would be a limit varies depending on the tube and use.

The nominal limits the makers set is +/- 10%, so for a 6.3V tube, that's 5.67V. Preamp tubes usually work OK a bit lower, maybe to 5.3 or so.

Power tubes need all the electrons they can get, so it's best never to get them below 10%.

The tubes will still do something at lower voltages, but may have a shorter life because of the inability of the cathodes to maintain a space charge cloud over the cathode.

[Amp Kat]

Most of the Fenders out there that were made in the 70 & 80's are running the heaters of the 12AX7/7025's with todays line voltage of around 124 to 124 VAC at about 7.2 to 7.5 VAC and they seem happy with it and of course as RG said it also is that same voltage on the power tubes which evens things out. Not sure how much extra wear on the tube it has but I'm sure keeping it in that 10 % can't be a bad thing but I rarely see it out there with todays musicians.

[bulldogguitars]

Thanks guys for the help. My heater voltages right now are at about 5.8v on the preamp tubes, but on the power tube its the same 5.8v. I assume with the power tube that low would that cause low output. I may need to replace the power tranny with one that can stand more draw. Its a valve junior power tranny and Ive added another preamp tube for an additional gain stage.

[Old Tele man]

just a suggestion

...you might try separating the preamp filament wires from the power tubes (assuming they're now all in one long sequential path).

...split out the preamp tubes on their own separate wiring from the power tubes

...make the "split" occur as close as possible to the PT-itself.

[bulldogguitars]

Thanks ole tele man Ill give that a try.

[bulldogguitars]

I got the heater voltages where they should be approx 6.1v and got my output where it should be. I during my design of this mod, forgot to put in the coupling cap from the plate of the new preamp tube to the grid of the power tube, oops. Got plenty of volume now,but dont like the sound. No definition and very farty sounding. Im going to try using the other half of the tube for a cathode follower. Am i right in thinking that Im driving the grid of the El84 to hard and need to hit it with unity gain.

[Bruce / Mission Amps]

...you might try separating the preamp filament wires from the power tubes (assuming they're now all in one long sequential path).

...split out the preamp tubes on their own separate wiring from the power tubes

...make the "split" occur as close as possible to the PT-itself.

They would all still be in parallel (constant current) though so the only advantage would be IR^2 losses of the actual wire.. or is that what you meant?