Hammond hot-rodding; mystery cap Q and tube tweakers please

Well the Hammond X-5 hod-rodding is underway. Adding "leakage" to the sand-state tone generator now that the tube overdrive is pretty much successfully prototyped. Only one thing bothers me about the latter...

My clone of the Leslie 122 preamp section works pretty well, except only if one part is omitted--i'll get to that later. I built the circuit verbatim from the original schematic; from the console pins 1 and 6 on the the input side to the .1uF output caps and 220K load resistors on the output. I sandwiched the whole thing between a "textbook" pair of DRV134 and INA134 chips bookended with pots for drive and output level. (Scroll down in this thread for a couple images)

Here's the original Leslie 122 schematic: http://www.captain-foldback.com/Leslie_sub/Leslie_schematics/122.GIF

When I first fired it up, the clean sound was great--nice and warm. My grin vanished when I drove it harder. Instead of the expected distortion, I got loud, crackling farting noises. (While that sound is not atypical of some really neglected Leslie speakers I've encountered over the years, it's not the effect I was going for. :wink

Putting a scope on the circuit confirmed my guess that it was oscillating at about 75Khz. Removing the .001uF mica cap between the grids stopped the oscillation. It sounds pretty good now, but I still want to get more distortion out of it.

All the component values I used are identical to the schematic above except I used a 12AX7A because that's what I had on hand and I figured it would give more punch. (I tried all the makes of tubes I had on hand, with pretty similar results although the Sovtek sounds best to me) Also the B+ is running hotter--300V instead of the 260V on the schematic.

Any idea what that .001uF cap was supposed to do and what I might be losing by omitting it? Any suggestions on specific improvements I can make on this? I'm sure the different tube and B+ calls for some bias tweaks, but I'm not really that up on how to best make tubes sound musically interesting. I'm planning a trip to the local NOS emporium later this week--looking for suggestions for my shopping list. :green:

Input from fellow tonewheel-heads (Jim?) is especially welcome! :thumb:

--Bob

The .001 mfd is for RF roll-off, limiting the high frequency gain of the triodes. Try lifting the ground on the secondary of the output transformer and see if the oscillation stops.

[quote author="BYacey"]The .001 mfd is for RF roll-off, limiting the high frequency gain of the triodes. Try lifting the ground on the secondary of the output transformer and see if the oscillation stops.[/quote]

I thought the cap was for RF suppression, disconnecting it was actually an accidental "fix". There is no output transformer on my circuit -- I didn't duplicate the PP output section, although I did give some consideration to doing some sort of balanced line out and still might consider it if there is "color" value to doing so in a low-power version. The circuit I am dealing with is described below:

I don't have anything to add except:

A) I want to hear this.

B) Where did you find an X5?

[quote author="b3groover"]I don't have anything to add except:

A) I want to hear this.

B) Where did you find an X5?[/quote]

A. No problem! I'll let you know when it's put back together again. My chops are so rusty that I'll have to insist that only you play it--in exchange I'll promise not to make you try to play the Chapman Stick. :wink: It'd be nice to get the opinion of an "expert ear".

B. I bought it new in 1976. I got it from some piano/organ store in Muskegon (that I'm sure doesn't exist anymore) because the Grinnell's store in Woodland Mall (or was it North Kent Mall - now a Lowe's) refused to special order one for me. I think Grinnell's went toes-up pretty soon thereafter.

I'm still trying to wrangle hearing you guys. It seems there's always something going on, somebody sick, no sitter, or some other reason we can't make it whenever you're in town.

--Bob

Because the front end circuit with the cross-coupling doesn't have any HF roll-off feedback, the stage is going to be unstable driving what amount to a dead short at RF frequencies. I would remove the .001mfd cap, and install one in the feedback loop of the first opamp, around a 25 to 50pf or so would be a good starting point. You can also get rid of the 10mfd electrolytics if you still have the .015 mfd caps in the circuit.

[quote author="BYacey"]Because the front end circuit with the cross-coupling doesn't have any HF roll-off feedback, the stage is going to be unstable driving what amount to a dead short at RF frequencies. I would remove the .001mfd cap, and install one in the feedback loop of the first opamp, around a 25 to 50pf or so would be a good starting point. You can also get rid of the 10mfd electrolytics if you still have the .015 mfd caps in the circuit.[/quote]

Unfotunately the op-amps in that diagram are internal to the DRV134 chip. Perhaps the single-chip driver isn't the best solution?

Your schematics disappeared. Anyways, why do you need the preceeding solid state input stage? It's sort of redundant if I recall. I think by the resistor values it was a unity gain buffer with balanced out.

[quote author="BYacey"]Your schematics disappeared. Anyways, why do you need the preceeding solid state input stage? It's sort of redundant if I recall. I think by the resistor values it was a unity gain buffer with balanced out.[/quote]

(what browser are you using BTW? -- I had the images disappearing intermittently on IE7 but not on Firefox) anywayy...

The objective is to convert an unbalanced signal to balanced. Leslie 122's are typically driven by a floating balanced (actually transformer) output. I had the chip and +/- 7.5V analog PSU rails handy, so I went with that. Here's the datasheet on the chip.

Well, at any rate dump the 10Mfd caps and the .001Mfd. , and if all is well use it that way.

With the cap question answered, now on to getting the gain/overdrive dialed in....

So considering I have a 12AX7A differential pair originally designed as:

B+ = 260v
Rp = 56K (each leg)
Rk = 1.2K (without a Ck cap)
Rg = 1M (each leg)
RL = 220K (each leg) AC coupled with .1uF

Running that circuit off the hotter B+ at hand (305v) gives some of the desired overdrive tone, but I'd still like more grit given the same input level. Would I be better off dropping the B+, changing some of the resistors, or both?

My tube learning, although rusty, has been on attaining fidelity. I'm not really up on the black art of "Tone."

It also occurred to me that since the original preamp design fed a pair of 6550's, should I include some small capacitive loading where the tube feeds into the solid state output buffer?

Thanks
--Bob

maybe you should add a 12au7 or 12bh7 after that 12ax7. set it up like a push pull power amp with output trasformer and all. i don't think you'll get much drive out of that single 12ax7 as you have it set up.

kevin

Try adding the 10K input resistors as in the 122 input stage.
They will keep the driver chip from seeing the .001uF cap.
The volume control shouldn't be necessary.