12AU7 --> 12AV7 on a (1)47 amp

[sarqoz]

Hi all!



I've been away from the forums for a while - much playing in the meantime and not so much noodling with equipment.

After our last gig I had my Leslie at home for a few days to do some boring maintenance. I read a few posts on this forum from people fitting a hotter tube at the input of a Leslie amp with various results and I decided to take the plunge.

My amp is an early 60s 47 amp which had still the original tubes fitted. I play a Nord C1 through it, and, although its high level output (14V RMS) is capable of driving the amp into a bit of overdrive, it wasn't that spectacular. I ordered some new tubes and replaced the standard 12AU7 with an RCA 12AV7 one and bam! - THE sound has arrived!

I feel that the V sounds a bit smoother and with nearly twice the gain of the U I can now push the amp into lovely saturation. I don't have to use the Zener OD or the onboard overdrive any more - it's now under my foot:-). I also have the impression that the transition from clean to overdrive is now more gradual. Revving the Leslie up and down with small swell changes (in e.g. Sylvia) sounds quite dramatic. Love it!


So, if you play a clone and you want to fatten up the sound a bit: this tube swap might do the trick!


Cheerio

[Hamman]

Watch that V21!! They don't take to those chopped waveforms very well!

[BlueOliveB3]

Not a good idea in a 147. The 12AU7 is the phase inverter as well as the driver, and changing it unbalances the phase inverter stage driving the output tubes unequally. This is not good for the amplifier and can shorten tube life. You can get away with this in a 122 as the circuit is balanced throughout, but not a single-ended-input amp like the 147 or 251 or so forth. You can damage the output transformer if you run it too hard like this, too.

TP

[sarqoz]

Watch that V21!! They don't take to those chopped waveforms very well!

No V21s are hurt. I use a different driver.


Not a good idea in a 147. The 12AU7 is the phase inverter as well as the driver, and changing it unbalances the phase inverter stage driving the output tubes unequally. This is not good for the amplifier and can shorten tube life. You can get away with this in a 122 as the circuit is balanced throughout, but not a single-ended-input amp like the 147 or 251 or so forth. You can damage the output transformer if you run it too hard like this, too.

TP

Thanks Todd. Could you please explain why this becomes unbalanced?

I read that the 12xx7 family is more or less interchangeable and that some guitar players replace the 12AX7 in a PI/driver application for a less hot tube to get more clean headroom. To my knowledge I'm merely changing the gain equally on both sides of the equation.

Thanks again.

[jhelm_waterw]

...To my knowledge I'm merely changing the gain equally on both sides of the equation.

I concur.
Short of pushing the v21 too hard (which you've already addressed) changing that tube to one with
higher gain should be no worse (from a tube abuse perspective) than just pushing the amp really hard by
cranking the sh*t out of a combo preamp.

[BlueOliveB3]

The phase inverter stage is designed around the gain of the 12AU7. Changing to any other tube would require the resistors comprising the signal voltage divider feeding the other 1/2 section to be recalculated, such is the nature of a split-load phase inverter. You can get away with this in an amp with a long-tail-pair PI like a Fender, but not a split-load PI.

TP

[Hamman]

Doesn't Pat in his B3 tips book address this? He suggests exactly what sarqoz is contemplating. I do believe it was a 147 he was suggesting this with as well....just trying to recall.

[sarqoz]

2923

The upper triode inverts the signal, which is fed back to the lower triode to invert it again. I guess the voltage divider resistor values are chosen to give it unity gain: upper triode drives the signal and the lower triode doesn't provide any gain - it only inverts. That way it should give the 6550s signals equal in amplitude but 180 deg out of phase.

When I fancy a beefier tube in there, I would guess that the upper triode drives the signal a bit more, and the lower triode - with the voltage divider not calibrated to unity gain any more - drives the signal a bit as well. Then the 6550s will get signals not equal in amplitude any more, hence driving one 6550 harder than the other, right? I think this is what Todd is mentioning.


If this is true, it can't be that good, but, the bottom line is: how bad is it?


Some guitar amplifiers seem to be designed to be driven as such - if I recall correctly the Dumble Overdrive Special even has a special knob to dial in some PI unbalance to taste. And then again: how balanced is the original 12AU7 setup?


I'm eager to find out what's written about it in that B3 & Leslie tip book! There are also some messages on the Hammond mailing list (archive) giving ambiguous advice, including some references that swapping tubes (in organs as well as Leslies) was done big time in the 70s. According to our man Geoelectro back in 2008:


Some have had good results with just changing the 12AU7 to either a 12AX7 or a 12AT7. That's very easy and doesn't potentially damage anything.

I'm losing my focus on this one...<l:0

[geoelectro]

Ugh, did I say that!?!

First of all I'm not a big fan of overdrive. I'm also not a fan of doing any damage to equipment seeing how I've spent most of my life repairing damage! I suspect my comment was in context of many options of getting distortion, some not very nice to the equipment. I've heard Leslies distort and thought, I know people who would pay good money for that sound!

The primary concern here in this thread is causing an unbalance in the output stage. As it was stated, a 122 type is safer because it stays in balance with any preamp tube. I tend to agree with that. Having your outputs way out of balance is somewhat dangerous to the output transformer. In guitar amps, they overdirve single stages in the preamp area, not so much in the output stages. The volume required to overdrive outputs would be hard to tolerate. Overdriving preamp tubes is low current and no transformers are involved.

Now I'm using the VB-3 software Hammond and the Ventilator for my Leslie. I can turn overdrive on in either place. Everytime I play with it I end up turning it off.

Geo

[sarqoz]

Thank you Geo for clarifying.

Your comment was on a post regarding getting overdrive at low volumes. In the end it was suggested to turn the Leslie down and use the internal overdrive of the organ concerned (the C1 has that too). On the C1 it not only introduces some distortion but is also boosts the signal. With the OD off I can drive the stock 147 amp to the edge of overdrive at band volume (chorus off sounds clean, chorus on starts breaking up gently) - with the OD on I can dial the 147 into some distortion, but it sounds a muddy since the internal OD-clipped sound is OD'd again!:eek: (I know someone who uses the OD on/ off button as substitute for the Hammond soft/ normal tab)

I am reasonably sure that my OT is safe - it doesn't get too hot when I push the amp. No red glow on 6550s and I don't have the impression that overall volume is different than with the 12AU7.

I do have quite some low end now. I will do some tube swapping to see if it's the tube, or the tinkering I did with some dampening stuff inside the cabinet, or the removal of the Zener OD from my signal path (note to self: not clever to do multiple "mods" at the same time)...

Thanks again!