modification on a Leslie 122 amp

[ebel]

I've ssen a couple of threads on this subject, and some great comments.

Problem:
The instrument I'm using (a Muse Receptor pro2 w/ VB3 VSTi) can't really drive this 122 amp to its full screaming potential.
This amplifier will need something in the range of 2.5Vrms grid-grid to unleash the beast.
I measured a max 0.7V from the instrument using a digital meter making a "big" chord. The internal Receptor mixer meter displays ca 0db.

I know the Leslie can go louder, I measure about 15V across the 16 ohm speaker terminals. The amp is rated 40W rms.

Original ciruit design of the 122 amp::
http://www.captain-foldback.com/Leslie_sub/Leslie_schematics/122.GIF

Now, I have already modified this amp by removing the motor control circuit, a relay driven by half of the second 12AU7A intentionally controlled by a DC signal modulated on the tonewheel out signal. I have made a more suitable speed control integrated in the swell pedal.

This leaves an unused 12AU7A to help drive up the signal!!

My initial idea here is to wire this second triode similar to the first differential stage. Which has a gain near 15 or so.

http://www.pentodepress.com/home/amp-technology/balanced-amp/
gives an idea of the benefit of such stages.

My problem is to calculate a good value for the plate and cathode resistor. I though maybe to put in a 1 k pot at the cathode, and keep the 56k at the plate side.

Any comments are welcome.

 

[PRR]

K.I.S.S.

 

[Biasrocks]

Or even simpler, modify your pre-amp for more gain.

Mark

 

[rob61]

If I may ask a question???  I have a model 122 Leslie speaker for a Hammond B-3 organ. When I switch the motors from fast to slow, I get a pop in the speakers. Anyone have an idea what would cause this, and is there a simple fix? (replace something I suppose). I don't mean to hijack this thread, but it seems there are some knowledgable folks here and with this talk about the 122 Leslie amp I thought I'd ask. Thanks.

 

[ebel]

If I may ask a question???  I have a model 122 Leslie speaker for a Hammond B-3 organ. When I switch the motors from fast to slow, I get a pop in the speakers. Anyone have an idea what would cause this, and is there a simple fix? (replace something I suppose). I don't mean to hijack this thread, but it seems there are some knowledgable folks here and with this talk about the 122 Leslie amp I thought I'd ask. Thanks.

Hi, the reason for the pop is due to a transient (spark) in the AC relay. If you look at the relay in the Leslie Amp (if you have the original wiring), black terminals indicate this. The way to deal with this is to add a simple RC filter across the AC relay or across the motor inside the Leslie cabinet. Or use a varistor (which is a new invention since 1960

It's ok to use this thread to clarify questions for the 122 Amp as long as we do not overshadow my intention.

 

[ebel]

K.I.S.S.

Yeah...thx,... I will  And adding 5 resistors, two caps and a tube to this beauty is indeed as simple as it can be.  ;D

I also though about this solution of adding a front stage instead of a mid stage, but was a bit uncertain if it was good/ necessary to keep the 10k+10k series in the mid stage when the 1 Megs are there (keeping the rule of following stage to have a load a minimum of 4 times the plate resistor). But with  0,5mF caps the knee should be around 30Hhz. And the 100k should be fine to trim the gain (instead of a making a long-tail - like 220 ohm in series with 1k pot). The differential design should take care of noise blocking (no grid blocking resistor needed).

I'm using a  Lundahl 600:600 ohm line transformer between the instrument to keep it nice and balanced, so at first look I guess we could do without the extra 20k at the input. The only thing is the original 8k treble roll off' which the grid-to-grid cap of 0.001 creates with the 2x10ks. This together with the 390k/0.005 feedback to the 6550 grid/plate is part of this amps all over sweetening to reduce the highs and deal with the overdrive treble harmonics. With as little as 600 ohms there is no longer any use for this cap. Comments?

Thanks anyways PRR, either way I think this is going to work, let's fire up the iron....
This could hopefully be the best improvement to make this superb speaker system more adaptable to low-amplitude signals. There are plenty of people who uses this amp with something else but an old tonewheeler with it's fine tuned preamp. And with no Hammond to company, the motor circuit has to be dealt with anyways. So why not use it.

And adding yet another external preamp unit or modify the instrument output stage is definitely not a k.i.s.s.

-Eystein

 

[Biasrocks]

And adding yet another external preamp unit or modify the instrument output stage is definitely not a k.i.s.s.

-Eystein

If you say so....

http://www.btproductions.com/Gear/Preamps/Hampton/hampton%20preamp.html

Mark

 

[Michael Tibes]

You could probably also use a step up transformer if you have anything lying around. Since distortion is the goal, I'd try anything which might help, that's a quick shot to see how it sounds. I remember I always used a V76 as a booster when I was still working in a studio which had this type of Leslie, this worked for sure ...

I've recently replaced the relay in my Leslie (not a 122) with zero crossing solid state relays, and I love it. No more clicking noise and very little current needed to light the LEDs, safely supplied from the heater voltage so that I can use any footswitch without extra electrical safety considerations (before my Leslie used mains voltage to switch the relay...).

Michael

 

[PRR]

> instead of a mid stage

The existing 12AU7 stage is the Driver for the power amp. Hammond designed it for the job. You want to leave that much intact.

You want more gain. At least 4, maybe more. But nowhere near the 100+ of a mike preamp. 

The 10+ of that synth/guitar booster would be a sweet ready-made fix.

But in a non-Hammond world, it seems appropriate to bring the 122 sensitivity up to "common level", and to do it with Hammond designs. That driver will also work at lower level.

The odd 1-pot balanced attenuator is a hassle. It needs much less than 100K source impedance for decent minimum loss. The 12AU7 stage will do that. But the coupling caps must be sized for around 20K each side (10K plate impedance, 10K resistor) to get full bass at low gain. Hence 0.5uFd.

Gain before attenuation asks for trouble. However the 12AU7 stage can take 3V each side, 6V across the inputs, which will be ample.

Alternatively you could go attenuator right off the input jack, then two stages of 12AU7. This avoids the 0.5u caps. Attenuator before gain asks for noise. However the sensitivity at the first 12AU7 is 0.16V RMS, noise may be 5uV, S/N is 90dB which is ample for performance.

A 1:4 or 600:10K input transformer would get your gain, barely, IF your source drives 600 ohms well. The Receptor website is light on tech details. When in doubt, I'd rather present >10K to the source.

 

[ebel]

Since distortion is the goal....

I've recently replaced the relay in my Leslie (not a 122) with zero crossing solid state relays,

Michael

Michael,
- Goal is to bring signal level up to a "hammond-leslie" level - not adding more distortion than absolutely necessary!
- Yes, those zero-crossing relays are great. My swell pedal has a right and a left momentary foot switch that controls speed and brake.
Which I use to drive two AC impulse (toggle) relays with RC filters added. The 24V signal voltage for these relays is picked up from the 122 using an adapted resistor. My left hand free man...
- Eystein

 

[ebel]

> instead of a mid stage
The existing 12AU7 stage is the Driver for the power amp. Hammond designed it for the job. You want to leave that much intact.

Yes, this is why I suggested to copy an extra stage 100% equal  - except for the 10ks in front of the de-coupling caps, and the grid-grid cap. And leave these as-is for the front stage.

> instead of a mid stage
You want more gain. At least 4, maybe more. But nowhere near the 100+ of a mike preamp. 
The 10+ of that synth/guitar booster would be a sweet ready-made fix.
But in a non-Hammond world, it seems appropriate to bring the 122 sensitivity up to "common level", and to do it with Hammond designs. That driver will also work at lower level.

Exactly to the point!

The odd 1-pot balanced attenuator is a hassle. It needs much less than 100K source impedance for decent minimum loss. The 12AU7 stage will do that. But the coupling caps must be sized for around 20K each side (10K plate impedance, 10K resistor) to get full bass at low gain. Hence 0.5uFd.

Gain before attenuation asks for trouble. However the 12AU7 stage can take 3V each side, 6V across the inputs, which will be ample.

Alternatively you could go attenuator right off the input jack, then two stages of 12AU7. This avoids the 0.5u caps. Attenuator before gain asks for noise. However the sensitivity at the first 12AU7 is 0.16V RMS, noise may be 5uV, S/N is 90dB which is ample for performance.

Which is why I wanted to keep this odd 100k pot and the 10ks in the front, with a mid stage that just is a straight forward copycat differential stage, like you say here.

A 1:4 or 600:10K input transformer would get your gain, barely, IF your source drives 600 ohms well. The Receptor website is light on tech details. When in doubt, I'd rather present >10K to the source.

I measured the signal on secondary side, the Receptor has no problem driving this, signal drops a few mV. Most likely its has a very standardized  low impendance balanced output.


Thanks again for very good comments. Work will start next week!

- Eystein

 

[ebel]

ok, after a postponement, I finally got down to it!
Now, keep in mind that the goal of this project was to make the amp more sensitive to lower signals than the Hammond (B3 etc) amp is able to output, without messing up the original tonal qualities. So, the impedance must not change, as well as the overall frequency response.

I soldered in the 7 extra components (2x56k + 2x1M + 1,2k + 2x0.1uF) needed. I moved the 0.1 uF caps from the first 12AU7 plate to the second and new stage, same place (plate). And replaced them with the two new caps. The rest is just adding in the resistors. After approximately 1hour wiring, bingo!

Result:input sinus 1k signal 100mV (measured between the 10ks at the front).
The 100k input pot is turned all the way up (no attenuation).
A 16 Ohm semi-inductive  load was attached to speaker output.
NB all measures are done between the (plate) terminals, and NOT from ground to terminals. This is a balanced amp (push pull stage) all the way through making it have excellent common mode rejection (very silent, no hum).

Plate one: 1,02V
Plate two: 9,6V
Plate three (6550): 160V
SP Output: 8,7V

input signal 265mV (at boarder of 6550 going into saturation/ distortion):
Plate one: 2,87V
Plate two: 27,5V
Plate three (6550): 370V
SP Output: 21V

Now, 21V over 16 Ohms gives you 27W...the amp is classified as a 40w rms. I found the voltage from the power supply to be 20volt under the correct levels. Also, the plate voltage at 6550 was 398V and not recommended 415. So this also explains some of the drop in power.

At this point, neither of the 12AU7 is even close to saturation. I want to make a point that the 10k from the PS in front of the plate resistors is too high to keep all four plates biased at 130V. The voltage dropped to around 100V. However, since the second stage starts to saturate the 6550 already at 30V, headroom is plentiful. If more gain (headroom) was needed, I could have soldered a 10k (1W) in parallel, which would have brought the plates closer to 130V. Less bias current is less heat

So, the new stage is boosting the signal by 20dB (gain = 10). And saturation at a 1/4V makes the amp a bit sensitive for my instrument. On the other hand, the input pot now comes in very handy, and I can leave the instrument mixer with much more headroom. The VB3 VSTi has a tendency to boost the lower keys (C1-D2) when the 16" is all the way out. It actually emulates the B3 very accurately. So more headroom means no digital clipping and the 122 will now easily make the organ scream when the swell is kicked down.

Since stage 1 and 2 are identical, I have tried not to induce a new design to this classic amp, just repeat how the signal is treated at first stage with a 100% equal second stage. I could have made a long tail design by adding in a pot in series with the cathode resistor. This would make up for a missing gain control. I was debating weather the gate resistors of the second stage should be 1M or 220k. 200k would have made the first stage "see" a more similar  impedance as represented by the 6550 stage. However, I don't think it would make much difference, perhaps adding a bit more noise with 1M.


Conclusion: I consider this tiny modification to be very successful, I can recommend this for other 122 owners that is not using the original wiring to control the switching of the Leslie motors. Upside: No need to external preamps to boost the signal. The design can be accommodated to accept guitar levels since the 12AU7 stages can give even more gain.