Advise needed on Leslie "tube flavor" unity gain s

(If you feel a vague sense of deja vu reading this, I made a similar post in the Lab yesterday. I have clarified it a little and moved the topic to the Drawing board, as it's more appropriate, as it really is at the "drawing board" stage than a project under construction.)

I'm just getting back into tube work after a 35 year hiatus! I'd really appreciate the input of PRR and all you other hollow-state dudes to as to how I can improve this design (make it work right). I'd also like to validate that I'm generally on the right track as well. No offense will be taken, as the component values are shots in the dark and my tube design chops are extremely rusty.

I'm working on a circuit modelled on the Leslie 122 amp to essentially be a unity gain device to insert beween the output of a Hammond X-5 organ and a Leslie 770 speaker. The idea is to add the distortion character of the 122 amp to an otherwise too-clean solid state signal chain.

I tried a simple single-ended transformerless 12AX7 design and ended up with more of a crackling farting sound than anything else. I got basically the same tone from the real circuit as did LTSpice--disappointing, but a pleasant surprise that LTSpice predicted it so well.

As I understand, the Leslie 122 amp gets its distortion characteristics from the combination of balanced 12AX7 amp feeding into a 6550 push pull amp. This is what I've come up with so far, grafting the 122 input stage onto a 12BH7A PP preamp output stage I found somewhere else.

Version 1 schematic here

The LTSpice model that produced the schematic above actually does pass a signal to the output -- one heck of a huge signal--definitely not unity gain, however, I'm sure the transformer model isn't right. :?: What is a decent, but affordable output transformer make and model for a design like this and how would one model such a transformer in Spice before I blow up a real one? (safety goggles on!)

FWIW, here's the LTSpice file for the above too.

Thanks!

--Bob

google Doug Hammonds firefly amp for a self split amp

FWIW I built a guitar splitter box for a friend I used a 356 opamp into a 1:1 modem transformer for one output that side sounds nice, something with the cheap iron? I set the opamp at a gain of two (min noninverting) I can't find the schematic but I used a series resistor to the primary that seemed to help.

I have been meaning to do more tests/experiments with pushpull low power circuits(opamps etc) into Si steel transformers for thing like bass DI etc.

I am thinking about a small 100millwatt or so amp into a transformer and a speaker (4 inch or so)inside a sealed box with the output off the speaker terms.

Some bass player use tube amps to a big solid state amp but there is something about going thur a transformer that seems to add something .

> the Leslie 122 amp gets its distortion characteristics from

I really hate to say this, but: BanjoCenter has $99 boxes for guitarists that will digitally emulate 99 Great Amps Of The Past. One of them may pass for an L122. You may have to pad the input and boost the output, and hold your nose at the thought of what is happening inside, but this IS the "logical way to go".

If you must have tubes: When playing with gain and distortion balance, be sure to try dropping the B+. It can vary over a wide range with small change of gain, and THD is nearly constant IF the signal level is proportional to power voltage. If signal level is fixed, increasing the B+ reduces THD, decreasing B+ raises THD.

Also, thow in some adjustable unbalance. Even if your idol seems balanced push-pull, odds are that balance was never perfect and this may be part of the "sound".

Try a cascade of JFET voltage-amp stages. I think this is part of the sound of the Yamaha Electric Grand (whatever happened to those?). Each stage was quite low gain, 5 to 15, with various tone and gain pads between, so the distortion was layered. If you are sure you like the 3rd, duplicate the final stage but with attenuation between so both stages work at the same signal level: not quite the same as push-pull but close as far as linearity.

[quote author="PRR"]I really hate to say this, but: BanjoCenter has $99 boxes for guitarists that will digitally emulate 99 Great Amps Of The Past. One of them may pass for an L122. You may have to pad the input and boost the output, and hold your nose at the thought of what is happening inside, but this IS the "logical way to go".

If you must have tubes: When playing with gain and distortion balance, be sure to try dropping the B+. It can vary over a wide range with small change of gain, and THD is nearly constant IF the signal level is proportional to power voltage. If signal level is fixed, increasing the B+ reduces THD, decreasing B+ raises THD.

Also, thow in some adjustable unbalance. Even if your idol seems balanced push-pull, odds are that balance was never perfect and this may be part of the "sound". [/quote]

I already went down the Banjo Center box path--from digital modeling to those with a "Genuine 12AX7 tube"--okay in some cases, not so great for others. Considering the things I'm doing to this organ, "logic" is pretty much out the window in favor of "fiscally-constrained insanity!" :grin:

PRR, should I read by your reply that, other than adding a balancing trimmer to the PP cathodes and possibly bringing down B+, what I threw together on "paper," component values and all, are reasonable enough to try to actually breadboard?

Any suggestions on what kind of transformer I should look for the PP line output?

Thanks.

--Bob

> what kind of transformer

Our selection is limited. Try a 10Kpp:8Ω output from a very cheap guitar amp. TubesAndMore.com has some Fender-replica outputs. Also look at Hammond's Universal Replacement series; they may give more bass-tone than even a Fender, and have a wide selection of ratios in one part. Assuming you pull 100Vrms across the primary, that's 3V at the secondary, which may be enough.

So it's kosher to run an output transformer designed for an 8 ohm load into a 20K line input?

> kosher to run an output transformer designed for an 8 ohm load into a 20K line input?

In this case, it may be. The triodes provide some damping, and the voltages are low enough that punch-through may not happen. But I actually was thinking of a speaker-like load. Changing the resistance would shift the sound somewhat. Since the power is so very low, you could load it in a small "full range" speaker, with some lead glued to the cone to shift the main resonance down around 50Hz. Non-resistive loads like that will warp the distortion versus frequency curve, which may be euphonic.

On the AikenAmps site, referred to in the Film Capacitor Polarity thread, I serendiptously found this article:

Designing A Reactive Speaker Load Emulator

Would this concept have equal merit? (Obviously, I won't need to use such high-power handling components in this application)

PRR's refinement of adding mass to the cone might be the best way a small speaker might not have the lowend bump a full size cab might have. A small amp into a transformer and then into a speaker. You could use a microphone in a sealed box to feed the bigger amp or take the output off the speaker terminals.

The reactive load might work fine but a small real speaker is cheap.

This all seems overly complex for the end resault. Why not just copy the 12AU7 push-pull stage, put a 12a_7 stage before it, a transformer after it to get back the single ended and then into the other half of the 12a_7 used on the input. Put a volume pot there and turn it to unity.

Or since the 770 is pretty much the same cab as a 122 with a new amp and new speakers, why not just toss the solid state amp, and clone the 122s amp?

adam

> Would this concept have equal merit?

It might. The ear is the only judge....

[quote author="adamasd"]This all seems overly complex for the end resault. Why not just copy the 12AU7 push-pull stage, put a 12a_7 stage before it, a transformer after it to get back the single ended and then into the other half of the 12a_7 used on the input. Put a volume pot there and turn it to unity.

Or since the 770 is pretty much the same cab as a 122 with a new amp and new speakers, why not just toss the solid state amp, and clone the 122s amp?

adam[/quote]

Yes, the 770 actually has a volume pot right at the input. I'll give it a shot as soon--I got a line on an old guitar amp output tranny. If it works, I may swap in something better.

My "Plan B" IS to disable the SS amp from the 770 and install a 122 amp. I put some feelers out for used 122 amps--if for no more than the chassis and iron, as I'll probably need to replace tubes, caps & stuff.

Thanks

-Bob

> why not just toss the solid state amp, and clone the 122s amp?

That was my thought: that 2N3055 amp is unsophisticated even for 1969, and putting a flavor-box in front might be like trying to taste a plump steak through warped glass.

Agree that finding genuine Leslie iron (chassis and trannies) may be easier than a whole-cloth clone. eBay is always one source, though finding a hulk in a local closet would be better than bidding against a half-dozen others for a poorly described "vintage amp" and paying to ship it from far away.

I'm convinced now that Plan-B is my Plan-A now, especially since I just got a line on a local guy with a sizeable boatanchor boneyard--including stuff a former Hammond tech unloaded on him. There's gotta be at least one decent 122 amp in there--maybe even a real B3 vibrato delay line to hook up to Jurgen Haible's scanner design! :green: :thumb: :sam: :guinness: :sam: :guinness:

I'm sure I'll also come up with some constructive use for those nice vintage 2N3055's in the 770's amp. Four channels of N*E*V*E perhaps... :thumb:

Thanks

--Bob

Grab me a vibrato too, it does not need to be a B3 one, any will do :grin:

adam

The first thing i did when i got my Leslie cab was to remove the 2N3055 amp and throw it into the canal.

Then, I took a Fender Champ and later, a Gibson Satellite 7 w amp and drove it with that. And have never looked back.

It's a 5 min test. Whats the worse that could happen? You might love it.