Unused triode

[Sir Cuitous]

Howdy again folks. I have a question regarding an amp with two 12AX7 pre-amp tubes. One of the triodes is not used in this amp. Strangely though, the grid splits from the volume and goes on this unused triode as well as the rest of the actual signal path. This triode has plate and cathodes wired and the cathode is bypassed (!?!). I just disconnected the grid and the amp still passes signal on my scope. Since there is no signal following the triode, does it matter if the grid is disconnected? Can I leave it that way?

[Chuck H]

If the tube is wired up I would think it's doing SOMETHING? Do you have a schematic??? I'd be interested in seing it before giving a response to this one.

On the other hand...

I've seen amps wired up "as modded" from other designs. Meaning that there are useless circuits doing NOTHING in there that were simply rewired "rote" because the builder or maker didn't know any better.

Chuck

[tubeswell]

Is the plate wired up to anything?

[Sir Cuitous]

I got this amp directly from the maker whom I am acquainted with and would not want to reveal. It is a highly regarded amp company though. So this amp was not modified or at least it came from the manufacturer that way.

I do not have a schematic, but I thought the same thing. Still, I have looked at it carefully and there is nothing following the plate or cathode. With three triodes, the gain is already over the top, so my guess is that the extra stage was unecessary. I suppose it could have been used as a cathode follower, but was not. Since I disconnected the grid, I just want to know if I can leave current running through this triode or should I ground the grid through a resistor.

[Sir Cuitous]

Tubeswell, I jumped your post. Yes the plate is wired to voltage, but that's all.

[tubeswell]

So is the plate wired to the other plate, the grid wired to the other grid and the cathode wired to the other cathode? (or not quite?)

[Chuck H]

Tubes is already onto my next line of thinking. Running triodes in parallel helps reduce noise and output impedance. I've seen designs that use different values for the plate and cathode R's for each triode but still have them in parallel.

Chuck

[Sir Cuitous]

Not parallel. Separate plate resistor and separate cathode resistor/cap and separate grid stopper. In fact, when I purchased the amp, I asked the designer (long before I looked inside of the amp) if there were any paralleled tubes - he said no.

I disconnected the grid wire and the amp still passed the signal on my scope. Assuming that it is just a superflous part of the circuit, I just want to know if I can play it for a few minutes without blowing the tube. Then it's on to some tone modifications. If I leave the grid floating will that be a problem even if the triode does not pass any signal? Should I ground the grid through a resistor?

[Chuck H]

If the tube still has plate voltage and cathode bias, then yes. The amp shouldn't even be run on the bench unless the grid of that tube is grounded. Through a resistor or otherwise, it doen't matter. Without the grid referenced to 0V the tube is a diode and will "run away" unchecked trying to draw more and more current until it dies.

Chuck

[Sir Cuitous]

Thanks Chuck, I had suspected that, but I wanted to make sure. I want to see exactly what sound I will get without this part of the circuit. If the amp sounds like it did before I disconnected it, then I may wire it as a CF before the tone stack.

[Enzo]

I don;t think the 12AX7 will run away, after all, it has the plate load and cathode resistors. If the tube became a dead short, even with 400v B+ and a 100k plate load, the thing only conducts 4ma. And I'd figure less than that in real life. Power tubes will run away because their load is a reflected one.

Passes signal is not the test here. With the grid floating, there is no reason the other half wouldn;t pass signal. What could happen though is odd instabilities or parasitic crap. Grounding the extra grid would eliminate all that.

[Sir Cuitous]

I did ground the extra grid and fired it up. The amp sounds exacly the same as it always did. Again, looking carefully, I see no connection to this triode for this circuit. It does seem odd that it would be wired up with components to the plate and cathode.

I also see no reason why it could not be wired as a CF stage before the tone stack. Right now the amp is too dark sounding tone wise, which I am seeking to correct. I thought it was just me until one of my coworkers played the amp. He turned the bass all the way down and the treble all the way up which is exactly how I play the amp. I would prefer to not have to use the tone controls that way. I have already changed the coupling caps from .022 to .01 which made a small by noticable improvement. I suspect that there is a limitation on how much lower the coupling caps can go without instability so I believe I will keep the .01 caps.

A CF stage will lower the impedance going to the tone stack, right? How will that affect the tone controls (1meg bass and treble pots)? Will they be more responsive? If I am going to use the triode as a CF then I think should do that before making other tone mods. Here's the basic flow:

input -> V1a:gain -> volume -> V2a:gain - > tone controls -> V2b:gain -> master volume -> EL84 -> OT

Wiring V1b as CF before the tone stack would place it out of order in the signal path, but physically the tube sockets are oriented above/below each other on the chassis wall. In other words, close to the same components. Is there any reason I should not wire the V1b triode after the V2a triode?

[black_labb]

i've heard that cathode followers can make a tone stack less trebly, but that may just be people comparing blackface and tweed amps without looking at everything. you could parrallel the the stage before the tone stack to halve the impedance, but i'd probably suggest the CF. have you considered modifying the TS to get more treble?

[Chuck H]

Go ahead and wire it as a CF. IME the CF feeding the tonestack will raise the overall freq. response. Probably due to the lower impedance. Don't worry about instabilities until they happen. Just try to practice the best lead dress you can under the circumstances and be sure to ground everything in the correct place and it will probably be fine.

Chuck

[Sir Cuitous]

I think I will try wiring the CF. I could always change it back. I realized soon after acquiring this amp that it would most likely end up as a platform to try some modifications. Yes, I have thought about modifying the tone stack. I am not looking to the CF to completely fix the tone issues, but I do want to hear what it sounds like.

If the CF works well, the tone stack will be likely my next modification. At this point, however it may be two weeks before I can get back to this project. I will post here in case anyone's interested. Thanks, as always, to all for sharing knowledge and suggestions.