using obnly one half of a 12AX7?

[Bluefinger]

I am thinking of building me a 5F4 based amp with a bias trem like in the Princeton Reverb. In theory this should be relatively easy. For the bias trem I need only half a tube. Is it ok to simply not connect the other half? I have no use for it so I wonder if this could be a problem ... weren't there old tweed amps that used only half a tube? I can't recall which one it was ...

thanks!

[Enzo]

Yes there have been amps with an unused triode left unwired.

SOme amps with unused triodes strap the unused pins to ground.

[Bluefinger]

ok, thanks. That's what I wanted to know

[Amp Kat]

The fender bassman AB165 does this and they just leave it unconnected.

[loudthud]

The fender bassman AB165 does this and they just leave it unconnected.

In some Blackface amps I've seen, they didn't connect the heater to the unused side.

[NorCalTuna]

way easier? wiggle the bias with an IRF820 or some other 500v n-channel mosfet! I find it more stable anyway and has no perceivable difference in sound. Oh, and they're a buck.

[Bluefinger]

way easier? wiggle the bias with an IRF820 or some other 500v n-channel mosfet! I find it more stable anyway and has no perceivable difference in sound. Oh, and they're a buck.

Interesting ... Would that be as smooth as a real tube tremolo? I have no experiance with mosfets ... are there any schematics how to wire such a thing in a tremolo circuit?

thx!

[imaradiostar]

Search here for "mosfet follies" or IRF820 and you'll find lots of results. I doubt you'll be able to tell the difference. Save your tubes for fun stuff like gain/distortion.

jamie

[Wombaticus]

Interesting ... Would that be as smooth as a real tube tremolo? I have no experiance with mosfets ... are there any schematics how to wire such a thing in a tremolo circuit?

thx!

+1 on the recommendation to do the trem with solid state - there is pretty much zero reason to do this with tubes.

My preferred configuration is to use a LND150 high-voltage depletion mode MOSFET for the phase shift low frequency oscillator, and put the output of that into a direct coupled source follower using a standard IRF820-class enhancement mode MOSFET. Done this way, you barely need to modify the classic oscillator/cathode follower tube circuits used in countless amps.... Pretty much just a smaller bias resistor for the LND150 than you would use for a tube, and usually more attenuation on the output are all that needs to be changed.

[Gtr_tech]

With an unused triode section (assuming the heater is running), I'd prolly float the cathode and ground the grid. Wouldn't be a bad idea to connect the plate to the b+ rail as well. Since the cathode is still throwing off electrons, b+ on the plate will give them somewhere to go.....IE get rid of the space charge. The theory being that a tube left in a standby state for long periods of time will get "cathode poisioning" and lose its ability to operate normally should you swap the tube to another position.

[Wombaticus]

With an unused triode section (assuming the heater is running), I'd prolly float the cathode and ground the grid. Wouldn't be a bad idea to connect the plate to the b+ rail as well. Since the cathode is still throwing off electrons, b+ on the plate will give them somewhere to go.....IE get rid of the space charge. The theory being that a tube left in a standby state for long periods of time will get "cathode poisioning" and lose its ability to operate normally should you swap the tube to another position.

Just to finish my thought from before... If you do decide to go tube on this, then I suggest you use the other triode as a cathode follower following the oscillator. The follower isolates the triode used as the oscillator from the load, and there is a consequent improvement in performance - there is usually less of tendency for the oscillations to die out at certain settings. Best results with tubes would probably be obtained by including the follower in the phase shift loop (as is evidently common practice with solid state oscillators), so that the low impedance output of the follower can be used to good effect driving the loop. The only commercial amp that I'm aware of that actually does this with tubes is the Custom Shop Vibrolux Reverb. It's certainly worth taking a peek at this schematic.

[loudthud]

The only commercial amp that I'm aware of that actually does this with tubes is the Custom Shop Vibrolux Reverb. It's certainly worth taking a peek at this schematic.

Very interesting schematic. A little different than your typical Fender. Anyone know who "Zinky" is? That name appears in the title block as the engineer.

[JoeM]

Very interesting schematic. A little different than your typical Fender. Anyone know who "Zinky" is? That name appears in the title block as the engineer.

Bruce Zinky

http://www.zinky.com/index.html

[Alan0354]

Just to finish my thought from before... If you do decide to go tube on this, then I suggest you use the other triode as a cathode follower following the oscillator. The follower isolates the triode used as the oscillator from the load, and there is a consequent improvement in performance - there is usually less of tendency for the oscillations to die out at certain settings. Best results with tubes would probably be obtained by including the follower in the phase shift loop (as is evidently common practice with solid state oscillators), so that the low impedance output of the follower can be used to good effect driving the loop. The only commercial amp that I'm aware of that actually does this with tubes is the Custom Shop Vibrolux Reverb. It's certainly worth taking a peek at this schematic.

I was just thinking about using the unused half as cathode follower, but for signal path like the Marshall to give a beefier sound.

I did looked at the Vibrolux schematic. I used to have a BF Vibrolux Reverb from 1968 and I sold it after I got the Marshall Plexi. It had a rectify tube, not SS rectifiers. That's cheating. That's the best part of the Vibrolux and Deluxe!!! Oh I miss that Vibrolux!!!

[Gingertube]

If you want to use that other 12AX7 half then look at Fender schematics from the Rivera era.
He routinely used parallel 12AX7 triodes as the input stage. Benefit? twice the gain. 0.7 (root 2) of the noise. Ideal for that 1st stage of any git amp.
schematics for Rivera era Fender amps
Cheers,
Ian