Hammond Tone Cabinet Amplifier Restoration/Conversion

[elarose333]

Hello all,

I recently acquired a Hammond AO-40 chassis (from a P40/Q40 tone cabinet.  Schematic is attached.
It appears to take a balanced input.  My question is twofold:

1) Can anyone explain how the 2 phase inverters work?  They look similar to long-tailed pairs, but I don't think they are.

2) Would it be possible (or feasible) to convert the input to an unbalanced line in or guitar signal?

It would be easy to just completely gut this and build a single guitar amplifier with 4 6V6's, but I am interested in the challenge of restoring and modifying the circuit as it exists.

Thanks in advance for any help!

[TimFox]

V3 and V10 are not literally phase inverters.  They seem to have a push-pull drive from whatever is connected to the preceding sockets.
V3 is close to a long-tailed pair:  the three resistors at the cathodes can be replaced by a single resistor to ground with two degeneration resistors to the two cathodes (delta-wye equivalent).
The voltage gains of these two stages are noted on the schematic.  True phase inverters would have similar (probably smaller) gains:  is that voltage gain sufficient for your new application?

[elarose333]

The voltage gains of these two stages are noted on the schematic.  True phase inverters would have similar (probably smaller) gains:  is that voltage gain sufficient for your new application?

I'm not sure...I think perhaps I will restore this thing to full functionality first and then do some further testing with a signal generator to determine what amount of gain I need.

If V3 and V10 are not phase inverters, how is the push-pull output accomplished?  Or does the whole thing run in Class A?

Thanks for your help!

[TimFox]

The input voltages at the two grids have already become push-pull by the box before the one in the circuit drawing.
Note the symmetry of the R-C networks before V3's grids.
The V3 plates, in turn, drive the grids of the 6V6s with the appropriate push-pull waveforms.

[Benta]

It's basically two separate push-pull amplifiers, one for treble and one for bass. Both symmetrically built.
There's no phase inverters anywhere, they need differential inputs to work, which must be generated at the source.

Class A or B? Needs to be analyzed, but Class A operation is not unusual in tube amps. Who cares about a bit of extra heat?

Nice Hammond quality, no expense saved anywhere.

[elarose333]

Ok, so I think I understand now.  The balanced signal is already phase-inverted from the output of the organ, and the triode stages in this amp are just for further amplification?  I guess the cathodes being tied together confused me at first, because that is a common feature of phase inverters.  If this is the case, then I'll have to have a separate preamp with a balanced out if I want to use this as-is.

Or, I can always build one guitar amp and one bass amp using the massive transformers!  All I need is another power transformer, lol!

[gbaddeley]

Consider restoration. This is an historically significant amplifier.

[elarose333]

Well, restoration was my original plan, but as I am primarily a guitar player, I don't know how useful to me the amp will be in stock form.  I suppose I could restore it anyway, but then it's likely to just sit on the shelf.  I really want to be able to use it.  For now, I guess I'll just do some more research into how to drive this thing and see if I can make it work if I do restore it.  Thanks!

[TimFox]

A simple way to drive it from another source would be to use a coupling transformer.

[eugene]

The two amplifiers have similar circuits, but were clearly intended to handle separate high and low frequencies with the frequencies being split before they got to these power amps. The output transformer for the bass amp (T4) is much larger than that for the treble amp (T2.) I can see that from the diagram in the lower left corner of the schematic.

Since these are power amps only, the input signal required to drive them to full output is probably more than 1 volt. For guitar use, probably two additional gain stages is required, maybe more depending on the sound you're looking for. All of this could be built on a separate chassis along with volume and tone controls, and the phase splitter(s) of course. B+ for preamp stages is available on pin 5 of the main connector. In fact 120VAC power comes in through that connector so the power cord and power switch would also be on the preamp chassis and they would be connected with a single umbilical.

Certain Fender guitar amps had a very special tremolo circuit that separated the high and low frequencies then modulated them separately. (See, for example, the concert 6G12.) In the simplest form, something like this would modulate the two frequencies 180 degrees apart. In other words, when the bass was made loud the treble would be soft and vice versa. But I've thought for a long time that it might be good to have one lead or lag the other by and arbitrary phase. All of that could be built into the preamp chassis.

In any case, I would get what you have working as intended by replacing the old electrolytic caps and anything else that has failed. That should be fairly easy and educational. Then cobble together a simple preamp with a couple 12AX7 just get sound out of it. Then you can decide where to go from there. Start out with the attitude that it will be a continuous process of experimentation and change. It's a mistake to try to design the ultimate finished product on paper then build it. Know from the start that there will always be something else to try.

[elarose333]

Ok, thanks Eugene!  I will go with your suggestion.  The restoration itself should be pretty straightforward, and then I'll experiment further.

[Wolfram]

The Hammond tone cabinets are designed to be driven by a Hammond preamplifier, for example the AO-10 or the AO-28, with a transformer-coupled symmetrical drive signal of over ten volts peak, for sure more than a guitar can deliver. I would consider keeping the cabinet stock, and build a preamplifier to drive it. A balanced output stage built around a pair of op-amps should do the job for example, or you could build something based on the original output stage of the Hammond preamplifiers if you want to have it all-tube.