Hammond Tone Cabinet Reverb Drive Transformers

[David Anderson]

I don't get hired that often to restore/rebuild Hammond Tone Cabinet power amps from the 1940s and 50s. This year, I've done four of them. Among those four, in three of them, the 6SN7 reverb drive transformer has been bad, and it's the same failure in each of them: open primary winding. There's no apparent reason for the damage like a tube problem.

The good one was actually in a 1946 Type RA reverb preamp paired with a Type G power amp, and it looks different from the others. The bad ones have been in HR-1 and Type LR power amps in DR-20s and HR-40s made between 1948 and 1956.

Has anyone else noticed a trend like this, or is this a statistical fluke?

Just in case anyone ever needs a point of reference, DC resistance measured across each half of the primary winding should be ~740 Ohms or 1.5k across the 6SN7 plates.

[Wes]

Have you been curious enough to unwind a transformer and see why it opened up?

[David Anderson]

Have you been curious enough to unwind a transformer and see why it opened up?

I've got two bad ones in a box, but I haven't done an autopsy yet. For you, Wes, I may get out my razor knife and start cutting open the paper wrap :-)

[Guest]

The bad sounding reverb unit (in naturally good sounding churches) was unplugged by the skilled organist or by the minister himself thus leaving the reverb OT with no load (by design!?!) operation for MANY decades powerful celebration of Glorious Acts of Him which eventually put to much stress on isolations, primaries being switched between two mighty plates of almost 300 VDC each ...
Isn't that a matching transformer more of alike?

[Hamman]

Why Boyan, are you suggesting a Pentecostal church?? LOL!

The bad sounding reverb unit (in naturally good sounding churches) was unplugged by the skilled organist or by the minister himself thus leaving the reverb OT with no load (by design!?!) operation for MANY decades powerful celebration of Glorious Acts of Him which eventually put to much stress on isolations, primaries being switched between two mighty plates of almost 300 VDC each ...
Isn't that a matching transformer more of alike?

[Guest]

Purely unintentional!
It is that I couldn't have imagined it in a... ....mosque.
Cheers,

[2Tracker]

Repurposing Hammond Reverb Pan?

Hi folks,A friend gave me one of the giant 4' reverb spring assemblies from a PR 40 (or so I'm led to believe).The idea is to make a standalone reverb unit from it. Anyone have experience attempting such a thing? The driver shows a nominal impedance of about 5 Ohms (which makes sense if this is essentially an 8 Ohm speaker driver); the pickup on the other end measures about 1 Meg.My question is: As I've yet to locate a correct schematic, can someone point me in the right direction in terms of circuitry, and what would make appropriate driver and recovery stages? I'm not a tech, but have built or modded several utility and guitar amps, and other studio gadgets.Much obliged!

[David Anderson]

As I've yet to locate a correct schematic, can someone point me in the right direction in terms of circuitry, and what would make appropriate driver and recovery stages?

Hammond Type RA Reverb Preamp
Hammond Type RB Reverb Preamp
Hammond HR-1 power amp
Hammond Type JR power amp
Hammond Type LR power amp
Hammond AO-15 power amp

All of these are in the main Hammond console organ service manual (B3, C3, RT3, and their ancestors).

The reason you don't get a low DC reading from the pickup is that it's a piezoelectric crystal pickup, not a magnetic coil pickup. Due to age and environmental effects, these tend to fail, so you have maybe a 50/50 chance that it still works. I would verify that it works before putting a lot of time into trying to build a driver stage.

It did not come from a PR-40, that's for certain. It may have been an HR-40. PR-40s use the necklace reverb.

[2Tracker]

Thanks much, David! I appreciate your pointing me towards some circuit options; my issue was that I only have the spring unit, and I don't know what part # is associated with it. So, thanks again!

Yup, I figured that's about what I'd see on a crystal unit. I will for sure test it before proceeding; another source suggests an element from an Astatic-type mic can be substituted....?

[David Anderson]

There are a number of relatively inexpensive piezoelectric transducers available these days, though I'm not sure how they would work as substitutes.

[2Tracker]

Well fortunately, the crystal element is intact! It was a bit fussier ascertaining this than I hoped, but treating the 2-pin output as "hot, neutral, ground" did the trick (for whatever reason I assumed otherwise). So, good news!

Now I have to figure out a simple circuit around the tank. I haven't dug into the circuits you suggested yet, David, and while I'm assuming they service other parts of the organ (or are otherwise overly complex for my needs), I'm open to learning otherwise.

I've looked over this document a bit too: http://theatreorgans.com/hammond/faq...TSBs/TSB49.pdf

which details replacement of the original oil-damped reverb with a smaller unit, but I'm thinking this isn't really suited to my needs.

This unit will be used in a recording studio, so unbalancing and balancing transformers on the input and output should do the trick.

Thanks, again, for the truly helpful tips and advice!

[David Anderson]

As you've already ascertained, the spring drive transducer is basically an 8 Ohm speaker without a cone. Hammond drove it with a 6SN7 running push-pull into an output transformer.

Since Hammond was a proponent of balanced audio, the crystal pickup unit plugs into a balanced gain stage for recovery. The earliest recovery stages used a pair of 6J7 or 6SJ7 pentodes, but for the rest of the oil-tube reverb run, they used a single 6SC7 dual triode, somewhat similar to a 5751 miniature dual triode. The circuit is very simple.

So, the output of the crystal pickup would be Signal +/Signal -/Ground, like you'd see on an XLR connector.

[2Tracker]

Thanks David, this is most helpful! So I'm inferring that the 6SN7 receives more or less "line level" signal from a preamp elsewhere. A Google search doesn't immediately turn up specs for the transformer driving the reverb but I suppose I can extrapolate from the fact that it's being driven by triodes in push-pull.

For that matter, I suppose I could also build a generic Fender (or Ampeg, or whomever) style circuit around it too. In your experience, is there anything unusual about the Hammond setup that would require specific treatment?

[David Anderson]

This is Hammond elementary level information, but you are new to this.

To keep the signal/noise ratio low when driving a tone cabinet via a 40 foot long cable that also supplied AC power, Hammond's original 1935 design involves the preamp in the organ sending a balanced-line signal to the power amplifier that's considerably hotter than what is now standard line-level, so the input to the reverb driver is fairly high.

I can test one of the reverb drive transformers for you, but you are correct that there's nothing particularly unusual about the circuit. That the output of the crystal pickup was intended for a balanced recovery stage may or may not end up being significant.

What you may need to do some experimenting with is what kind of damping oil to use in the tubes. I've read a number of suggestions, and I've tried one type of mineral oil, but wasn't thrilled with the results. I tend to think that a very light oil might yield better results. That's where I'd personally like to know more. I know about these things, but I don't get to work with them directly very often. I've never owned one.

[Wes]

Where did you source the mineral oil, David? Do you know the viscosity? Did it seem thicker or thinner than Hammond generator oil? Typical drug store mineral oil comes in three viscosities, but usually they only have "medium" on the shelf.