Long Tail component values

[EETStudent]

How about some opinions on the different resistor/capacitor values in a long tail phase inverter. Specifically for a 12AX7 tube. Ive generally seen the two
1M grid resistors, however the tail resistor anywhere from 10k-47k and the bias resistor anywhere from 470-1500 ohms. Ive also seen various 'presence' circuits and input/grid coupling caps. I havent read a clear article as to what different values achieve tonally.

[tubeswell]

I did some digging around about this a couple of months back and found some useful resources

I think it was on this thread

http://music-electronics-forum.com/s...ead.php?t=7552

Geoff had a really good article he could email you and I've probably got it if I dig around a bit more

[MattT]

I'm not sure I can answer exactly what you're asking but here's my 2 cents (I'm leaving B+ voltage considerations out of this...but the B+ voltage will affect all this too):
The cathode resistor sets the bias for the PI...i.e. the idle current through both triodes. All of that cathode current also goes through that tail resistor, so you get a voltage drop there (which is the whole reason it's there in the first place). If you have a low value cathode resistor, like 470ohms, then you have to use a lower value tail resistor (like 10k) otherwise all that current would create too large of a voltage drop across the tail resistor. So if you used the 470ohm cathode resistor but then used the 47k tail you'd have way too much of a voltage drop across the tail and you'd lose headroom (or rather 'footroom' if you like).

EL84 power tubes require much less signal to drive them than most power tubes. That PI cathode resistor also affects gain just like the cathode resistor does in our 'regular' gain stages...so that's why you often (usually?) see the high-ish value 1.2k cathode resistor there with EL84 power tubes. And since the 1.2k cathode resistor sets the idle current sort of low, they use the high-ish value 47k tail there to get the tail voltage up to where it should be. You'll often see the combos of 470ohm/10k (Marshalls and some Tweed Fenders), 820ohm/22k or 24k (BF Fenders), and 1.2k/47k (Vox).

A higher tail tends to help balance the PI...at 10k it's kind of unbalanced (even with 82k/100k plates) and at 22k it's reasonably well balanced (with 82k/100k plates). You can also play with the plate values to achieve even better balance...like maybe using a 91k instead of one of the 82k/100k...or maybe a 110k. I've installed a pot there (Dumbles have a trimmer there) and to be honest it doesn't seem to make all that much difference within a reasonable range.

I would encourage you to install some pots at the cathode and tail resistor spots (I like to use a pot + resistor so I never go all the way down to 0 ohms) and a 25k (or 10K) trimmer in series with the 82k plate resistor and tweak away. You'll notice some difference but IME once you get in a reasonable ballpark it all sounds pretty similar. Make sure when you listen that you push the amp hard...a lot of the subtle differences will show up when you push the amp.

I always seem to prefer input caps that are not too big. .022uF and .01uF are often my choices. The cap on the second grid just needs to be sufficiently large...I just stick a .1uF or .22uF there and forget about it.

HTH.

[martin manning]

Geoff had a really good article he could email you and I've probably got it if I dig around a bit more

Can you link to it or repost it? I'd be interested in reading too.

MPM

[tubeswell]

Can you link to it or repost it? I'd be interested in reading too.

MPM

Yeah I'll have a look

[tubeswell]

Can you link to it or repost it? I'd be interested in reading too.

MPM

Ta-Da! Found it! - The thread is attached as a word doc

[martin manning]

Great, thanks!