FX Loop

[zevoz]

Has anyone used the second half of the second tube as an effects loop driver?
I read a post that says the Ruby TW does, but I can't find any schematics for it.
I searched this site and Google and came up with nothing.
Any ideas?
Any schematics?
Thanks

[jazzyjoepass]

I'm curious as to how it is possible to do a loop with half an 12AX7 tube.

Anybody has the schematics? Or anyone knows how TR managed it?

[d2camero]

I have noticed that there are two camps in the effects loop world: those with buffered effects loops and those without. Some Fenders, Dr Z and others, simply take a jack off after the pre-amp, just before the PI input.

Aiken has an article on this in his advanced tech section. He really does not like unbuffered effects loops.

Also check out the AX84 Renegage, it has an effects loop circuit you may be able to adapt.


Don

[d95err]

I'm curious as to how it is possible to do a loop with half an 12AX7 tube.

Anybody has the schematics? Or anyone knows how TR managed it?

You can use it as a buffer for the FX send, and use a passive FX return. The send part is probably the most critical. Using a return is mainly necessary if you want to use low output devices (i.e. stompboxes) in the effect loop.

Another solution is to use a high-voltage MOSFET (solid state heresy I know) as the send buffer and use the tube triode for the return gain.

Kevin O Connor's book "The Ultimate Tone" has a very good chapter on effect loops. He discusses both tube and solid state designs, including those that use high-voltage MOSFETS that can run on the B+ supply.

EDIT: About using that leftover triode - has anybody tried other ideas on how to use it? Paralell gain stage perhaps?

[FUCHSAUDIO]

The Ruby "effects loop" is the return amp from a Dumble loop (with nfb loop), with a master volume after it. It is switched in and out of the circuit with the front panel switch. The "send" portion of the loop is passive, coming off the output of the third gain stage. Actually doesn't sound half bad when used as a master volume, although most Ruby's I've seen get unstable due to the excessive gain, and tend to get noisy or whistle when cranked up high and loop is engaged.

[David Root]

I'm going to use O'Connor's 12AT7 loop from TUT 1, page 7-11 on my next amp as it seems to cover all the bases. I do have two questions though, 1) does the buffer output cap have to be 6.8uF? and 2) how far above 340V (if any) should I push the B+ on the 12AT7, as the buffer triode B+ goes directly on the plate?

I have seen other designs eg Mesa Boogie Mk. II B at 0.1 uF on the output cap, but that's a much older and more primitive design. I think 4.7 should be OK. On the B+ issue, the Komet 60 puts just over 400V on V2b, which is a 12AX7, and the T7 is rated for only 300V vs. 330V for the X7. In any event I guess this tube will need to be the toughest NOS I can find.