12av7 or remote cutoff tube guitar effect

[Austin Translation]

I like the way the 12av7 sounds in my guitar amplifier. It sounds compressed and almost like an octave effect when turned up. Anybody know of a way I could exagerate or enhance this quality or if positive feedback (instead of the usual negative feedback) might be useful for this? Maybe I could make a pedal from one or two of them. Thanks, ~Austin

[soundmasterg]

What position do you use it in your amp, and what kind of amp is it? Did you change the parts around the socket to bias the 12AV7 optimally or did you leave them alone so the bias isn't optimized for that tube?

Greg

[mashaffer]

I have kicked around the idea of running some positive feedback to the screen grid of a small signal pentode to modulate the gain with the input signal. Haven't tried anything yet. Another possibility is to use a remote cutoff tube in the preamp.

mike

[Austin Translation]

Thanks for the replies, the amp I was using it in is a THD univalve. Here is the schematic:


I used it in v2 and a 12ax7 in v1. Probably not optimal for the 12av7 but still sounded nice I thought. I was thinking maybe some tubes inherently produce more second harmonic distortion than others and that positive feedback would be a way to really bring it out for the purpose of effect. Thank you, Austin

[Geek]

Hi,

Got a higher res for us with old eyes?

Cheers!

[soundmasterg]

Yah that one is hard to see....

Greg

[Geek]

Found it!
http://freestompboxes.org/members/bajam ... 5B1%5D.pdf

Cheers!

** edit **

I can't see it in that section causing that due to circuit. What make is your 12AV7? Might be a bit of magic in that type of tube

[Austin Translation]

It is an old rca tube I got on ebay pretty cheap (maybe five bucks?). Ive tried a variety of tubes in this amp and while alot of tubes sound alike or vary mainly in gain, this one seems to have a noticable difference to my ears. It seems to roll off some highs which might be bad for hifi but adds compression and when pushed seems to add a nice harmonic richness without the harshness or brittleness of some tubes Ive tried. I would say it is worth pick one or two of them up if you enjoy tube rolling like I do. Thanks for the schematic link, I couldnt remeber where I found it.

[Geek]

RCA... that explains it.

Most small signal tubes of theirs have a sweet quality to them.

Drop a Sylvania in there.... tone will probably go off a cliff

Cheers!

[soundmasterg]

It probably has something to do with the fact that the first section in that spot is a cathode follower. I would have to do the curves on that tube and see where it specs out, but I would guess that it is biased such that it introduces a compression effect. The 12AV7 can pass a lot of current and generally likes values similar to a 12AU7 around it in a circuit. When used with resistor values higher than that, it shifts its operating point. If it works and you like the sound, then its fine. If you adjusted things to be more optimal for that tube, then it would draw a lot more current and lower your preamp voltages down a bit overall.

Greg