Adapting Hammond Tone Cabinets

[Rhodesplyr]

I'm both a Hammond/Leslie and guitar amp tech, and I had an interesting project come in last week. A guy has two Hammond tone cabinets (DR-20s) from 1948 that he wants restored and adapted for guitar use. He's a jazz player and wants a darker, full-bodied tone, so he wants to use the full cabinets. Both these amps use a pair of Jensen A12/F12 speakers, and via a gerry-rigged test, I can already tell that it's going to sound very nice for guitar. All the iron is still good.

These are balanced-line input power amps that require a fairly hot signal to drive them to full output, and the amps are pretty inaccessible when inside the cabinets. Thus, my idea is either to design an interface that allows him to drive the tone cabinets from the output of another amp (his idea) or design a guitar-appropriate preamp on a separate chassis that would hook up to the cabinet via the traditional 6-pin Hammond/Leslie cable.

The pre-1950 Hammonds supplied B+ from the power amps to the preamps in the organs via the cable, so it would theoretically be possible to build a tube preamp running on B+ from the power amp. (Hammond tone cabinets through 1975 supplied B+ in order to be backwards-compatible.) Hammond preamps only needed their own filament supplies. B+ would be limited to about 250-275V, and you'd need balanced output and a low enough output impedance not the be adversely affected by the length of the cable. Hammond used output transformers on their preamps for this purpose.

Any ideas on guitar amp preamp circuits that could easily be adapted for this purpose?

FWIW, Leslie speakers in the 122 family also supply B+ in order to be compatible with early Hammond Organs, so, with appropriate switching, such a preamp would also work with this type of Leslie.

[olddawg]

He will probably want some kind of tone control. If everything is operational you might just consider using a $30 Art tube preamp. (They have both balanced and unbalance output and switchable line / instrument level input) and maybe an equalizer pedal in front of it (although many have a voicing rotary switch) then an A/B/Y switch to select that chain or the guitar amp.

[loudthud]

I would be concerned about the reliability of the speakers when used for guitar. As far as interfacing with the cable, that shouldn't be too hard. The first thing to do would be measure the sensitivity of the input, perhaps with new and old tubes just to get a range. Next, find a suitable transformer to drive the input if you want to go that way or commit to a solid state line driver. You should look at new and vintage sound reinforcement equipment or high end audiophile gear. Hammond has several types of audio transformers that should work, the price could be high for "Broadcast Quality" types.

Just about any tube preamp could be adapted to work. A lowish B+ can be worked around. The voltage gain will be slightly lower than a normal design, but you don't need high gain in this case. Probably two dual triodes would get the job done. Adapting to use the speaker out of a Champ or any guitar amp is a good alternative.

A related but off topic question I have is, Don't the 122 and 147 Leslie cabinets require a different cable to interface with a B3?

[Rhodesplyr]

Interesting idea, however, Hammond Tone Cabinets require a good bit hotter signal than what has become standard line level to drive them to full output. They wanted as little noise as possible introduced in the cables, so they designed it for a high output from the organ.

Tone controls are a given. That's one reason I'm thinking about this topic. There are a few different ways to drive a Hammond Tone Cabinet or Leslie with a line-level signal, but none of those I know of incorporates a preamp specifically designed for guitar with the appropriate input impedance, volume, and tone controls.

[Rhodesplyr]

Speaker reliability would be an issue if I didn't recone speakers as part of my business. Jensen field-coil speakers are fairly easy to recone, and these may be reconed as part of this project since the cones are stiff with age and could crack around the edges. The cones have also sagged a bit as a result of their downward horizontal mounting. These speakers are far superior to the speakers typically found in a Champ. Also, the speakers' field-coils are integral parts of the amplifiers, so using permanent magnet speakers would require amp modifications.

(I've been running Leslies for 23 years with 1950s Jensen field-coil speakers in them. Some are reconed, but some are not.)

Leslie hookup is a complicated topic, but the 122 hookup is very similar to the Hammond Tone Cabinet hookup with the addition of a isolation transformer used to supply DC motor control voltage to the Leslie. The standard 147 family hookup is different because the 147 amp uses an unbalanced input and a different motor control system. Also, the 147 does not supply B+ because, unlike the 122 family, they were originally designed for Wurlitzer Organs that didn't need it. All can use the same 6 pin cable, which is why I always label hookups with a label maker. I usually do at least one Leslie hookup a week--or sort out a hack job I run across in the field.

[loudthud]

IIRC the C3 organ only has 5 tubes, none bigger than a 6V6. I don't think the actual power needed to drive the power amp is more than 1 Watt, it's just a question of impedance. Even if it's 100V, the impedance is high enough that the power isn't very high. Something as unassuming as the Blackface Reverb driver will work with the right transformer. Checkout the Hammond 560, 800 or 850 series.

But without hard data, everything is just a guess. We need to know the input impedance and sensitivity of the amps.

[Rhodesplyr]

Here's the schematic of one of the amps and a Hammond C-2 preamp. Note that since it was originally driven from a center-tapped output transformer in the organ, the power amp gets its ground-reference for the input grids via the transformer that drives it.

The C-3 has 8 tubes and uses half a 12BH7 to drive the OPT; the C-2 used half a 6SN7.



I tried to upload the C-2 preamp schematic, but the File Upload Manager is just churning--had that problem before recently--so here's a link:

http://captain-foldback.com/Hammond_...atics/AO10.gif