Leslie 122 Line Preamp - Updated - Hum/Buzz

[Nishmaster]

Hey gurus,

I've got a Hammond BV with a Leslie 122 in the studio that I'd like to plug additional instruments into such as a guitar, Rhodes, or other synth. I've done some research and here's what I've found so far.

The input to a Leslie 122 is a six pin connector, with two pins for AC, two pins for balanced input, and a pin for ground. Another pin sends 300V (200?) B+ back to the organ preamp for supply power. What I am unsure of is how the fast/slow switch operates.

I'd like to put together a simple tube circuit for this task. The Leslie amp seems to be about a 40W unit. I have very little design skills, but I was thinking maybe a simple SET type low wattage device, or maybe a single 12AX7. While I don't expect someone to put together a schemo for me (I'd prefer to work it out and teach myself something), I'd love if some of you all could give me some pointers/directions/laugh and point until I get it right.

One thing that strikes me right off the bat is that obviously guitars will need different input impedences than other line inputs. Will I need two input trannies, etc. I'd also like the thing to have enough drive to get the leslie amp a little dirty if need be, as well as having a switch for rotor speed.

I realize the BV has some Hi-Z input terminals, but I really like to put this all into a self-contained box since the 122 is obviously mobile (even if only dubiously), whereas the Hammond is stuck right where is it.

Thanks in advance to any and all that lend a hand. I really appreciated the responses in my thread regarding the oil dummy light for my bro's snowmobile. We went with ignoring the float bounce issue in favor of device simplicity, for those who were following. Works great.

Schematics of the units in question:
Leslie 122 Amplifier
Hammond BV Preamp Section

 

[Nishmaster]

Well, I see nobody has chimed in as of yet. 'Tis ok. :wink:

I've been doing some investigation, and from my reading of the schematic, it appears that the relay of the fast/slow motor speed switch is done using a DC signal across the pair of input pins, which are pins 1 and 6. No indication of what voltage the relay requires, that I can see. Looks like the DC is blocked with a pair of caps before it hits the 12AU7.

 

[stickjam]

A recent thread about this will help: http://www.groupdiy.com/index.php?topic=25800

I posted a link from bentonelectronics in one of the replies halfway in. It's got the scoop on the DC switching voltage along with a schematic to minimize noice and accommodate failure modes. It's 50-60 volts BTW.

--Bob

 

[Nishmaster]

Thanks a bunch for that Bob. That helps me out immensely.

No pointers on the input stage? Anyone? Where's PRR when you need him?

 

[stinky]

I can't get you all the way there, but I don't think you'll need two transformers. I'd use a tube. The tube is Hi-Impedance and a guitar/bass will play with it nicely. Just a switching jack after the secondary of the input transformer will make it easy to plug a HI-Z input in.

You'll need to do some math, but It should be fairly simple to to use the return B+ Voltage from the Leslie to generate your plate voltage for your preamp. That might save you some bucks if you only need to generate the heater voltage. For studio use you might consider a DC heater voltage.

I believe that all 122 kits use a transformer to supply that switching voltage and the signal. The secondary of the transformer balances the output of the organ, and it has a center tap which is where they apply the switching voltage. Most clever. You could substitute capacitors to save on cost.

I may spend some time tonight dabbling with my own design for this purpose. Hope I've helped somehow, otherwise, disregard.

 

[Nishmaster]

Stinky,

I appreciate the nudge in the right direction. Hammond's use of an iso transformer in the 122 kit seems a bit overkill, and even more so when you consider that the balanced out from the organ preamp just came from a tranny.

I've found a site with some other helpful pointers as well as a tranny-less switching schemo.

Hammond Wiki

I'll probably use the B+ return to power the pre. I'm going to try and come up with something soonish, perhaps tonight or tomorrow.

 

[pucho812]

look for captain foldback

on his site are schematics for a leslie preamp. It is all transistor but allowes for any 1/4 device to go into a lesllie speaer. Can be built on the cheap too. I did one for a client. Not only can his guitar go into a leslie, it also has the switches on it for fast, slow and break. Pretty nice.

 

[Nishmaster]

So, after hours and hours of reading and getting some books on the subject, I think I may have, at least preliminarily wrapped my head around basic tube design. Here's what I'm thinking of putting together:

It should put out about 7-8V Peak to Peak, if I'm doing my math right. I've been told that should be adequate to drive the Leslie power amp, but I'm going to have to breadboard it to make sure.

I'm sure I'm doing myriad boneheaded things in this schemo, if anybody has any thoughts I'd appreciate it. I'm not sure entirely what kind of output line devices usually give, so I have it set up this way as it seems to me I'm not going to need nearly as much gain for a line device. The other thing that strikes me is that the volume pot may be a bit interactive, frequency-wise, but from what I can tell not terribly so.

-Matt

 

[PRR]

> Where's PRR when you need him?

You do not want to know.

> myriad boneheaded things in this schemo

Not myriad.

TR1 Sec to V1A plate is bonehead. Will not work. IAC: I'm not sure what TR1 does for you.

I have no clue what the Presence pot does..... ah, you meant to tie the wiper to an end. I'd call that a Treble Rolloff knob, but whatever.

Given 750K volume control, you would often load V1A in some higher value, 47K or 100K. Nothing wrong with 14K. But did you cheat with a Resistance Coupled Amplifier table? Those give good starting values. Leo sold a few amps with values stolen directly from GE's tables.

The 12AU7 preamp is so NOT boneheaded. Without doing math, you may have less than 200V B+ supply out here (especially with the heavy load of 12AU7). A high-gain 12AX7 would overload in the first stage. The 12AU7 won't.

Ah... R10 R11 1K will load heck out of V1B C6 C7, What is the relay amp input.... well, it is infinite. Even if relay tube goes short, it is 1Meg. R10 R11 should be over 100K, not 1K. Frankly, whatever yellow-stripe resistor you got too many of will do.

I'd pencil the TR2 tranny as 1:1CT. 10K:10KCT is readily available. Lower Z means less HF trouble. The 2:1 loss of gain is usually small-change.

Lessee. 6550 cathodes at 25V. 12AU7 gain near 14, so to get MAX roar we need 1.8V peak or 1.3Vrms each side of the input socket. 2.5Vrms across the two pins. Assume 1:1 TR2 we need 2.5Vrms at V1B plate. Input to main amp is as low as 20K when turned-down, but will normally be like 110K. We also have the 14K across TR2 primary. So V1B is loaded in 12K, assume V1B Rp is 6K, we get gain of 2/3 Mu or 6. V1B grid signal must be 0.4Vrms. Assume Pres and Vol full-up, that is V1A plate signal. V1A may have gain as high as 16 (though much less with 14K load), grid signal needs to be 25mV. A good target for full-up full-power sensitivity is 20mV, so you are spot-on, if V1A is tweaked to a more volt-amp (higher impedance) condition, or you strum hard. As shown, V1A gain is 11 (not bad for such an unusual load), input sensitivity is 36mV, on the high side but perfectly playable in older styles or with hotter pickups. Maybe you do want the 1:2CT tranny at TR2, it gives really good nominal sensitivity numbers.

It looks bass-heavy. Flat to 12hz while lesser loudspeakers usually like a slight bass-shave to cut their bottom-octave mud, treble-cut where e-lectric gitar usually wants a bit of treble-rise. If the sensitivity turns out good, you might just trim the 47uFd cathode caps to 10uFd or 2uFd.

Geetar amps traditionally omit C1. A wound pickup can't dump DC on the grid. Having C1 and overdriving the input makes a different sound: either less popular or not worth the extra buck. Perhaps not an issue since absurd 5V peaks are not grossly overdriving the 12AU7 (tho it is 9%THD), and atypically large 1V peaks are far below that.

 

[Nishmaster]

>TR1 Sec to V1A plate is bonehead. Will not work. IAC: I'm not sure what TR1 does for you.

Yeah, I noticed how colossally dumb that was about 10 minutes after I put up the schemo. The reason for TR1 (in my head) was to provide a better input impedance to drive a synth from. Is a synth going to be ok with the possible 1Meg input impedance of V1B? I figured that a 1Meg impedance on the secondary would be reflected as a 10K impedance on the primary then. I'm sure I'm misunderstanding how this all relates, as grid impedance in this situation is near infinite anyhow.

>I have no clue what the Presence pot does..... ah, you meant to tie the wiper to an end. I'd call that a Treble Rolloff knob, but whatever.

That is correct. I figured some of the ugliness of the super highs of a guitar could be pulled out with that adjustment, although you say further on that guitars like a treble-boost, so perhaps I have some misconceptions here.

>Given 750K volume control, you would often load V1A in some higher value, 47K or 100K. Nothing wrong with 14K. But did you cheat with a Resistance Coupled Amplifier table? Those give good starting values. Leo sold a few amps with values stolen directly from GE's tables.

I haven't taken a look at the tables. The nominal 12AU7 plate resistance seems to hover around the 7-8k range, so I figured loading it 2x would put me about 14K. Maybe not quite enough oomph for a guitar, might be too clean, perhaps also too quiet. This is also my first tube circuit (or amp circuit of any kind for that matter), and I tried to do it with as little reference as possible, save the tube sheets.

>The 12AU7 preamp is so NOT boneheaded. Without doing math, you may have less than 200V B+ supply out here (especially with the heavy load of 12AU7). A high-gain 12AX7 would overload in the first stage. The 12AU7 won't.

That's what I came up with as well. Additionally, I prefer the sound of the 12AU7 a bit more. The distortion seems less harsh, to my decidedly un-golden ears.

>Ah... R10 R11 1K will load heck out of V1B C6 C7, What is the relay amp input.... well, it is infinite. Even if relay tube goes short, it is 1Meg. R10 R11 should be over 100K, not 1K. Frankly, whatever yellow-stripe resistor you got too many of will do.


Not sure how I missed that one. I'll fix that for sure.

>I'd pencil the TR2 tranny as 1:1CT. 10K:10KCT is readily available. Lower Z means less HF trouble. The 2:1 loss of gain is usually small-change.

Yeah, I originally had it at 1:1+1, but as you can see, changed it in the schematic to 1:1CT, but forgot to change the label. I'm not sure I can afford the loss, though, looking back at it, as I don't actually know what kind of output the Leslie is expecting, and I'd rather have more then have less. Additionally, I'm having trouble finding a decent tranny for this app that can handle the DC on the primary that isn't a Sowter or a Lundahl and therefore 100+ quid. As you can see, however, my transformer knowledge is lacking in, well, the knowledge department.

>...input sensitivity is 36mV, on the high side but perfectly playable in older styles or with hotter pickups. Maybe you do want the 1:2CT tranny at TR2, it gives really good nominal sensitivity numbers.

That's what I was thinking. I could also increase Rp of V1A and corresponding bias I suppose for a little extra kick, if you think it might be needed in this case. Might I ask the math you are using to come to your sensitivity numbers?

Also, is the effective plate resistance of V1B 14K paralleled with the 10K of the proposed tranny? Again, to me it looks as if the input impedance of the Leslie is 1Meg, so how is this reflected back to the tranny here?

>It looks bass-heavy. Flat to 12hz while lesser loudspeakers usually like a slight bass-shave to cut their bottom-octave mud, treble-cut where e-lectric gitar usually wants a bit of treble-rise. If the sensitivity turns out good, you might just trim the 47uFd cathode caps to 10uFd or 2uFd.

My calculated -3db points seem to correspond also. I think 12hz-22k or whereabouts, depending on the position of the volume and rolloff pots. I may reduce the size of the cathode caps to kill some of the super low, although I'm not sure the Leslie speaker can't handle it, consider how low a B3 goes (although perhaps not quite that low). Lowering the size of the cathode caps will slightly lower my gain also, due to the feedback, correct?

>Geetar amps traditionally omit C1. A wound pickup can't dump DC on the grid. Having C1 and overdriving the input makes a different sound: either less popular or not worth the extra buck. Perhaps not an issue since absurd 5V peaks are not grossly overdriving the 12AU7 (tho it is 9%THD), and atypically large 1V peaks are far below that.

I'm being slightly paranoid with that. In case I do something stupid while wiring this thing up, I'd hate to have DC going back into the instrument.

 

[Nishmaster]

Bueller? :wink:

 

[dramadisease]

I thought I'd chime in a bit since I'm sorta in the same boat right now, but with a 145... a slightly different amp

Does yours have the console load switch? We used the 8ohm load and ran a Fender brown Deluxe into it and it sounded like god with guitar. When it pushed into distortion it sounded really good. not the optimum way to doit, but we tried using a UA 1108 and some old calrecs and all sounded good, but the brown deluxe beat everything hands down...

edit: (the 145 is different than the 122. The 122 is looking for the "G G" balanced input and the 145 is unbalanced.)

 

[Nishmaster]

That is indeed the recommended practice for the 145. Be careful with how much of an amp you plug into the 145, however, as the input resistors when working with the load switch are only 10W. It's pretty easy to burn them out with a Deluxe at higher volumes. Obviously, a 10W head would be the perfect match in this case.

 

[dramadisease]

It's a cool setup...

What I'm thinking about doing is installing some sort of auto-transformer + speaker motor system that would allow us to select a "load impedance" so that we could plug any type of amp into the leslie. Also I was thinking about maybe using a speaker motor for an inductive load instead of the load resistors.

A friend told me about some studio he was at in the 90's that would plug a 50W marshall into the leslie, but you couldn't turn it up very loud. I bet it was big sounding...

Sorry to hijack the thread a little bit!

 

[PRR]

> finding a decent tranny for this app that can handle the DC on the primary

You might be surprised how well a single-winding 120V+120V:120V+120V or 120V+120V:250VCT power transformer sounds here. If you are sure you hate guitar highs, a two-winding Euro-style power transformer would shave the highs.

It's only a dang geetar amp. It is literally "voice range", though full voice not telephone voice. It need not be better than, or much different than, than a classic AM radio. The standard (for 1930s) plate to P-P Grids interstage transformer is fine for prototyping. (The really shoddy ones are all long gone.) When money is available, you can try a 3-digit Lundahl. You may hear a difference. It is unlikely to be "better" in proportion to the wallet-pain, not for guitar. 50KHz response may be "worse" than 10KHz droop.

> it looks as if the input impedance of the Leslie is 1Meg, so how is this reflected back to the tranny here?

There is a 10K+100Kvar+10K gain-trim ahead of the two 1Meg grid resistors, so the transformer looks into around 100K. As low as 20K if gain is full-down, but you won't do that. The 10K or so transformer "sees" a high impedance, which is fine. Especially since you are driving it with a low but not zero source impedance (12AU7 plates).

> The nominal 12AU7 plate resistance seems to hover around the 7-8k range

Only at the show-off 10mA condition.

What do we need? In your schematic the first stage drives a 750K pot (and some other stuff). We will never get or want 75V here, but if we did, we'd only have 0.1mA in the 750K. Considered as a "power stage", we need over 0.2mA in the driver. More is not a big problem, but going much-more raises B+ filtering costs and heat in the box. In DIY, filter cap cost is not a profit-killer, but no compelling reason to be using 5mA-10mA in the tube to deliver less than (probably much less than) 0.1mA to the pot.

Find G.E. datasheet, they usually show how Rp varies. GE 12AU7 page 3. Rp is about 8K at 10mA, 11K at 5mA, 18K at 2mA. For many commercial tubes over reasonable range of current, Rp rises about as inverse square-root of current. Half the current gives Rp 1.4 times higher.

A "good" design tends to have a plate resistor value near the geometric mean of load and Rp.

Assume load is 600K, Rp is 6K. And assume triodes often work well with 300V supply and plate around 2/3rd of the way up (100V on resistor, 200V on tube).

The geometric mean of 6K and 600K is 60K. If the 60K resistor has 100V across it, it flows 1.6mA. Rp for 200V and 1.6mA is really more like 20K than 6K. Iterate. Geo Mean of 20K and 600K is near 100K. Now the tube flows 1mA. Rp is likely to be 25K. Re-iterate, 25K and 600K suggests 122K, we don't fiddle better than 20% for simple triode preamp stages, we round-off to 100K.

I think you were running 6mA or 7mA before, this plan runs 1mA. The larger plate resistor would seem to give less swing to the load, but reduced drop in B+ filtering may make it around the same. Moot since you won't get close to overloading this stage. Gain will be a hair higher. Heat will be one Watt less, which may be enuff to ponder.

> I tried to do it with as little reference as possible, save the tube sheets

Well, OK, that's one way to learn. Repeat the painful path that others have trod before you without looking at their notes. I do it too! But I also think the golden rule of audio is plagiarize-plagiarize-plagiarize (only please call it "research"). Do keep in mind that many commercial designs are penny-pinched: nobody (but Harmon and Bose) ever got rich in audio, few last a decade, and pennies are often the difference between bankruptcy next year or bankruptcy now. In DIY you don't face that pressure. Your lo-Z hi-current design is not "wrong", but extravagant for no good reason.

> Lowering the size of the cathode caps will slightly lower my gain also, due to the feedback, correct?

No. When the cap's impedance is "small, the tube gives full gain; at some low frequency where cap impedance is not-small, tube gain is less (generally around half). A large cap may give gain of 10 at 20Hz and 11 at 40Hz and higher, gain of 5 for signals 5Hz on down to DC. For any practical audio purpose this is gain of 11 -1dB at 20Hz. With a small cap like 1uFd, gain could still be tending to 11 above 160Hz but fallen to 5 at 40Hz down to DC. For a wide-band 82Hz-2KHz signal, this is nearly gain of 11 but with less bottom-octave boom.

Which is "better" depends on speaker, style, and band. A solo guitarist may want big bottom on tap to fatten up the solo sound. A guitarist working with a bassist will often leave the 41Hz-160Hz range to the bassist and "sing tenor". Some chord fingerings will use the guitar's bottom octave, but the amp's balance should favor the screaming mids, not compete with the bassist holding the foundation of the harmony.

When in doubt, start flat. The classic variations are:

Flat -- cheap but unexciting and nasal

Top-boost -- adds "zing". If done with a cheap speaker, is cheap.

Bass and treble boost -- masks the nasal midrange, can give the impression of orchestra range instead of a box of strings.

Notch near 600Hz -- takes out the nasal, brings out bass and treble, reduces inharmonic distortions.

The classic Fender tonestack can do all of these. It is usually used with bass and treble boost and mid-notch. Alternatively, rhythm guitar may play with treble cut to sit behind the lead guitar.

> I don't actually know what kind of output the Leslie is expecting

"Input"? i.e., the output of "your box"?

6550 are shown biased at 25V G-K. In audio, this is usually the Peak Grid Signal: we can't force the grid positive of cathode, but we must get near that point to reach Max Output. So we need 25V peak.

The driver is 12AU7 and probably gives gain near 15. So 25V/15= 1.66V needed at each 12AU7 grid to force each 6550 to all the power we paid for. There's two of these grids, so we need 3.3V peak or like 2.5Vrms grid-grid. That's what you must deliver from your "console" (gitar preamp).

Your pickup may be able to deliver 0.5V, but that is hard work. Older (early 1950s) amps needed 0.05V for full output; modern guitarists feel it is hard to overdrive such amps. (Older styles used overdrive sparingly, and also those guys started by whacking acoustic dreadnoughts behind big brass bands.) Mid-1960s pro amps could be driven to full output with 20mV, and suit many styles. Some "high gain" amps have sensitivities below 1mV for full output; these can not be used with all knobs on "10", or if they are, they hiss bad but nobody notices because any string motion is making gigantic fuzz.

So ballparking and rounding: you want 25mV from pickup to make 2.5V at 122's input, you want gain of 100. Single triodes don't go that high; anyway gain of 100 without a turn-down wants to overload on hard plucking. Two stages with gain of 10 is spot-on. Or since the second stage may be pulling a transformer, you might go 14 and 7. And since you have a volume/gain pot and don't really know how it will all come together, you might go for a bit more gain.... it is easier to waste some than to find more after you already built it.

 

[Nishmaster]

An update -

I finally put this thing together. Thanks to all that chipped in, and especially PRR.

Some changes to the schemo:

TR1 is gone, as is S1 and C1.
R3 is 1Meg
C2 + C3 are 10uf
C4 is 270pf
The wiper of R8 is tied to the volume pot
R1, R2 is 22K
R4, R5 is 1.2K
I'm now putting the leslie switching voltage straight into the output transformer center tap, removing C6, C7 and R11. R10 is 470K into TR2 CT. I removed D1.

I am, however, getting rather massive buzz and hum. The heaters are AC powered from a Hammond 12.6V tranny with the center tap going to the cathode of the first stage to raise it above ground. With both the Leslie and Hammond cranked, I can't hear hardly any buzz. With my pre and the Leslie cranked, it's pretty ugly.

Everything is connected to one big star ground which leads to a single chassis point. The IEC power connector also connects at this point. The switching on the input jack is tied to the star ground, as is the ring. 47K Grid stoppers have no lead on them, leads to pots are twisted. Heater tranny wires are tightly twisted. The input lead going to the first stage isn't shielded, but is only about 4-5 inches long, if that. Pin 2 from the Leslie ground is going to the star as well.

There were some things I tried. I had assumed that the B+ coming from the Leslie was going to be regulated somewhat better, but I had about 1.5v of ripple on that. I corrected that by putting a 300R in series and 220uf to ground, which brought it into the 25mV range. There is also more like 280V coming from the Leslie as opposed to 200V, but I'm not bothered by that as I'm not really near max dissipation or overloading either of the stages, and more gain never hurt anyone.

I'm thinking I'm getting some RF into the box. I've got the tube mounted to the outside of the chassis, hi-fi style. If I put my hand near the tube, the buzz gets worse, but if I touch the tube, it goes back down to where it was. Before I corrected the ripple, I could get the amp to oscillate by turning our light dimmer down to about halfway (I know, I know, a studio should have dimmers, we're changing them out). The odd part in all of this, though, is that if I pull the tube and shut off the heater tranny, the buzz doesn't really get much, if any, quieter. Having this thing single ended probably isn't doing me any favors, as the only things that I can think of are either that mains AC carrying wires inside the box are radiating into the OPT, or that ripple is still coupling into the output (it doesn't seem like 25mv would really sound like much here).

Any suggestions?