It seems there are a couple of labeling discrepancies between my chassis and the schematic diagram on p.1 of this thread, itself taken from the Hammond L-100A Service Manual. In any case, I was errant in using the point corresponding to pin 7 on V6 as my input - which you will notice sends the signal on a winding R-C path which effectively blows off all of the gain from V6. The signal eventually ends up about where it began (gain-wise) at pin 7 of V8, which is the "proper" input to the power amplifier section.
From that point forward it's a standard console hi-fi type circuit, with 1/2 of V8 acting as an AF amp and the other 1/2 of V8 being a balanced phase splitter. So now input runs directly into pin 7 of V8; V6 can also come out of the socket, as it's no longer needed. As you can imagine, the signal is a lot 'cleaner' sounding with the new input. Now I need to look at things like C307 and the circuitous path for the pin 7 grid resistor, which seems to be an aggregate ~100K(?) of about 4 or 5 parts in that network. A lone 470K or 1M to gnd would probably be just fine.
But what is the purpose of the C314 & R335 (10N / 4K7) network, across the primary of the opt? Recall this is the one that smoked out when I was making full power measurement at ~20KHz.. is it there to snub out ringing in the opt xfmr? The amp does come off fairly bright, a bit glassy in the highs - the cheap CD player that feeds it is sounding much more like a cheap CD player now. Things were much more tubey and euphonic prior to the input change and removal of the network.
From that point forward it's a standard console hi-fi type circuit, with 1/2 of V8 acting as an AF amp and the other 1/2 of V8 being a balanced phase splitter. So now input runs directly into pin 7 of V8; V6 can also come out of the socket, as it's no longer needed. As you can imagine, the signal is a lot 'cleaner' sounding with the new input. Now I need to look at things like C307 and the circuitous path for the pin 7 grid resistor, which seems to be an aggregate ~100K(?) of about 4 or 5 parts in that network. A lone 470K or 1M to gnd would probably be just fine.
But what is the purpose of the C314 & R335 (10N / 4K7) network, across the primary of the opt? Recall this is the one that smoked out when I was making full power measurement at ~20KHz.. is it there to snub out ringing in the opt xfmr? The amp does come off fairly bright, a bit glassy in the highs - the cheap CD player that feeds it is sounding much more like a cheap CD player now. Things were much more tubey and euphonic prior to the input change and removal of the network.
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It never ends, does it? 
What is the purpose of C307, that 100pF (0.0001) part from pin 7 to 6 (G to K) on V8? Is it shunting HF off of the input line (see R323 47K in series) or something else?
Also note that for some reason, the factory gain-adjust is in fact a rheostat in the global NFB loop. At max volume, the effective resistance is 13K9 - at minimum, it's 3K9. Both of these are much lower than the 22-27K you typically see in this spot. So I assume it's "correct" to run it at max volume, which is the least NFB, but is there any reason the value has to be so low - why not 22-27K - and why is there no compensation cap?
Questions, questions.. this is how we learn.
What is the purpose of C307, that 100pF (0.0001) part from pin 7 to 6 (G to K) on V8? Is it shunting HF off of the input line (see R323 47K in series) or something else?
Also note that for some reason, the factory gain-adjust is in fact a rheostat in the global NFB loop. At max volume, the effective resistance is 13K9 - at minimum, it's 3K9. Both of these are much lower than the 22-27K you typically see in this spot. So I assume it's "correct" to run it at max volume, which is the least NFB, but is there any reason the value has to be so low - why not 22-27K - and why is there no compensation cap?
Questions, questions.. this is how we learn.
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In other words: Your amp now reproduces the input signal more correct than before, more HiFi'ish. Remember that it originally has been designed as a musical instrument's amp, or as an effects box, if you like.
Do you own a function generator? With that, I'd do the following:
Connect a dummy load to the amp's output with your scope across it. Disconnect the NFB path completely. Apply a 1 kHz sine signal to the input and determine the input level when clipping occurs. This gives you the open loop gain. Then close the NFB and adjust R336/R337 to your needs, i.e. maximum signal input right before clipping. The needed total value can be calculated as well and be replaced by a single resistor. Then, as yet suggested by me, apply a square wave signal and jiggle and wiggle with the lead and lag compensation, the Zobel etc., 'til the output signal comes close to optimum. It's just try and error. The amp must not overdrive under this square wave test, otherwise it doesn't tell you anything, so reduce the input signal quite a bit.
Best regards!
Where I would have said it sounds more 'Hi-End-ish' - in the sense of more revealing, transparent and neutral. 'Hi-Fi' evokes that 'warm', tubey sound of the classic / vintage gear, with its older parts, circuits and design sensibilities. If anything, it's more of the former now. Having spent quite a while messing with hi-end gear, I'm not sure that's necessarily all for the good, either.
Well yes and no, I suppose. There are definitely blocks of the design, several without the chassis, some within, that are dedicated to effects, but there's really nothing unusual about the power amp block(s), as far as that goes. It's the same circuit with similar values as many vintage console amps. And for our good fortune, it seems that they decided it was ok to use a proper hi-fi output xfmr. And you know that in this old gear, the magnetics are +everything+.. if you're good there, you're good.
Yes, I have a pair of Wavetek 111 ;-) Not exactly state-of-the-art, but they are a good instrument. Sometime I should replace them, but I don't know what I'd want! The 111 does have a fast- rise 5uS square wave output, but I don't think I have a correct terminator for it. Suppose I can just solder in a 51R?
We'll see about that, maybe later tonight. Thanks again.
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On the first go, I went with a 1KHz square into the 8R load @ 5W or so. With the FB resistance at max (13K9) adding any parallel cap only increases the rounding of the leading edge (mostly at the upper left corner). So if anything, it's as if it's already 'over' compensated?
Is this where I should go after the lead compensation - C310 in the network (220P / 47K) parallel with the 470K Rp of V8?
There are still L100s in good use, and they still crop up for refurb 'cos they are small enough to fit in homes from the 60s and 70s.
I refurbed an L100 about 18 months ago, along with a Leslie 145 it sounds just fantastic. It is a classic organ, used in many and various recordings such as Booker T, John Mayall, Focus, Keith Emerson etc etc etc.
Although there are emulations, once one hears its authentic sound including the valve Leslie 145, there's no going back.
The end result is that this L100 and Leslie 145 are back in regular use by a band and on the road again. All 35 stone of them ! They sound fantastic, really !
Although it's a minority interest sport, restoring valve Hammond organs is the 'correct' thing to do IMO. Conserving spare parts like the amp/control/psu chassis, rather than converting to what would be an unremarkable guitar amp IMO, is the right thing.
The internal amp and speakers for the L100 sound pretty mediocre IME, but the amp module can't be removed alone: it contains PSU and control functions necessary for the organ. The organ needs to be paired with a Leslie before hearing its true potential, and perhaps that's why L100's currently don't have much reputation. Back in the day, they knew this, and one always sees the Leslie in vintage photos/films.
Where's the love for the L100s ? !
LD
On an electronic organ forum, or other musician's site?
I still find it hard to believe that there are many (if +any+) L100 out there that can't be put to use for lack of a chassis. Most of the organ parts I've come across were salvaged from consoles found rotting in alleyways, and at dump sites. I still have a complete B3 tone generator in the attic, just sitting there.. with no place to go. Can't throw it out, can't put it to use.
I’ve just “eviscerated” a similar Hammond...
I just acquired an AO-43-1, and have more modest intentions for it; guitar amp and re-use of the reverb tank. With your investigations/input-observations, might you have sketched an updated (modified) schematic? I won’t be “line-driving” so need to rework input impedance and insure sufficient gain, but your results sounded very positive!
Thanks in advance for any “as-built” that resulted from your efforts!
I just acquired an AO-43-1, and have more modest intentions for it; guitar amp and re-use of the reverb tank. With your investigations/input-observations, might you have sketched an updated (modified) schematic? I won’t be “line-driving” so need to rework input impedance and insure sufficient gain, but your results sounded very positive!
Thanks in advance for any “as-built” that resulted from your efforts!