Mixing Up Triodes

[Tim Hall]

Gearing up for my second build...I’m graduating from strictly-kit to modified-kit this time, and I want to do some experimenting. The amp uses all triodes, preamp and power. I was wondering if there’s any reason why I couldn’t mix up the triodes between tubes.

In other words: it seems that customarily if you had three gain stages and a concertina splitter, the first two triodes in the signal path would be housed in the same tube, and the last gain stage and splitter would come from the same tube. Is there any reason why the first gain stage and splitter couldn’t come from the same tube?

Or let’s say I had this scenario: If my preamp was made up of a 12AX7 and half a 12AU7, and my power amp was a 12AU7 (both triodes, push-pull), could I theoretically use half of the preamp 12AU7 for power? Would this possibly give me more asymmetrical clipping?

I’m doing my best to ask some strange questions here ...thanks for entertaining them.

[Alex/Tubewonder]

It's simple: keep the circuits blocks that are functionally close together as close together physically as possible. Running wiring back and forth between several "bottles" increases stray parasitic couplings between the functional blocks. You can lose tone clarity and/or run into howling feedback problems.

[Enzo]

Right, you can get crazy crosstalk problems. WHy would you want the high gain input stage to share a room with the high signal level of the PI?

[Tim Hall]

Right, you can get crazy crosstalk problems. WHy would you want the high gain input stage to share a room with the high signal level of the PI?

That's what I needed...someone experienced to say "Bad idea!" I had a feeling some sort of nasty distortion or something might be more likely to occur. Never heard of "crosstalk" before, so this is my amp vocabulary lesson for the day. Thanks - I'm still learning!

[Enzo]

Crosstalk is where one segment of a larger circuit picks up the signal from another segment one way or another. Tha name makes more sense if you think of a large mixer. Assume channel 3 has a very hot signal going on. If the mixer is not well designed, it is possible some of that signal will bleed over into channel 2 or channel 4. Hence the word crosstalk - one channel crosses over to the next.

In a high gain guitar amp, you could remove a coupling cap between a couple stages in the middle somewhere, and the later stages might have enough gain to pick up some signal from the first stages - crossing over the gap so to speak. The result will be weak and thin, but it happens all the time.

A lack of decoupling between stages can also lead to crosstalk of a sort. That is where a hot signal on the plate of one tube modulates the B+ with that signal a little. Then any other stage sharing that B+ will have that litle bit of signal added in via that B+. SOmetimes people report it as low output, when really it is a break in the signal path resulting in crosstalk instead of a real output.

[Tim Hall]

Thanks Enzo. Now I know what crosstalk is, and I learned something about the value of proper decoupling between stages.