Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on February 08, 2013, 01:30:37 pm

Title: Using half of a 12Ax7
Post by: dude on February 08, 2013, 01:30:37 pm: Hello all,

Changing a circuit design I'm only using half of a 12ax7 the not used half, plate, grid and cathode are not hooked up. Been running this way for months now but after playing a while the amp cut off once. Before I trouble shoot, would the open half of the 12ax7 finally causing some problems?

I think I read somewhere to ground the unused half's cathode...?

Thanks as always,
al
Title: Re: Using half of a 12Ax7
Post by: Geezer on February 08, 2013, 01:58:26 pm: While an unused triode is a deadly sin. :cussing: , it won't harm your amp.........No, your problem is something else.

G
Title: Re: Using half of a 12Ax7
Post by: dude on February 08, 2013, 02:05:53 pm: Thanks, Geezer

Yeah, I'll find something for it, maybe tremolo... :think1:
Title: Re: Using half of a 12Ax7
Post by: alerich on February 13, 2013, 09:38:26 am: The unused triode is the ultimate exhibition of restraint.
Title: Re: Using half of a 12Ax7
Post by: sluckey on February 13, 2013, 10:19:22 am: Quote from: alerich on February 13, 2013, 09:38:26 am
The unused triode is the ultimate exhibition of restraint.
:grin:
Title: Re: Using half of a 12Ax7
Post by: eleventeen on February 13, 2013, 10:43:19 am: Perhaps the idle triode could be claimed as a dependent on your tax return, or maybe you could file an unemployment claim. Has it ever been gainfully employed?
Title: Re: Using half of a 12Ax7
Post by: Gary_S on February 13, 2013, 11:18:27 am: Quote from: eleventeen on February 13, 2013, 10:43:19 am
Perhaps the idle triode could be claimed as a dependent on your tax return, or maybe you could file an unemployment claim. Has it ever been gainfully employed?

:laugh: :l2:
Title: Re: Using half of a 12Ax7
Post by: Willabe on February 14, 2013, 01:13:30 pm: :laugh:
Title: Re: Using half of a 12Ax7
Post by: DummyLoad on February 14, 2013, 03:36:00 pm: the unused triode is a cathode follower waiting to happen...

--pete
Title: Re: Using half of a 12Ax7
Post by: tubeswell on February 14, 2013, 08:27:52 pm: Or a parallel stage
Title: Re: Using half of a 12Ax7
Post by: dscottguitars on February 15, 2013, 02:39:01 pm: I'm only a novice builder, but I would wire it in parallel like 'tubeswell' suggested. I do that on all my preamp stages because of what I read in a book by Gerald Weber:

"This would give a cleaner tone with more substance. Also the output inpedance would be cut in half, thus providing a more stable source impedance for the next stage."

But the the plate and cathode resistors used need to be cut in half and the cathode bypass cap doubled.
Title: Re: Using half of a 12Ax7
Post by: HotBluePlates on February 15, 2013, 10:15:47 pm: Quote from: dscottguitars on February 15, 2013, 02:39:01 pm
But the the plate and cathode resistors used need to be cut in half and the cathode bypass cap doubled.

Only if you want to keep the exact same gain you had before paralleling.

You could halve the cathode resistor to maintain (almost) the same plate current in each triode.

Yes, plate voltage will drop because of more current through the existing plate load. But lower apparent internal plate resistance for the composite stage working against the existing plate load will yield more gain.