Converting Hammond LR A (ex HR-40) to Hifi Mono Amp

Dear All,

i am planning to convert 2 Hammond LR A (from a HR-40 cabinet) into Hifi Mono Tube Amps.

I decided to use this AMP since it has an internal 200Hz (cycle) crossover which fits nearly to my Eminent LFT-8 speakers (180Hz cossover).
So I could use the L speaker out for the LFT-8 Bass and the H Speaker out for the other.

As afr as I understand, there is not such a lot to be modified in the LR A when used as Hifi Amp (in minimum)
- connect pin 3 and 4 (of the 6pin plug) to 110V AC
- connect audio out (signal) of preamp to pin 6 and 1 (of the 6pin plug) and to ground respectively

On an other forum (can't remember which one) I found the following information for the audio in for Hammond amps:
- solder 2 200ohm resistors in series
- solder them between audio in hot and cold
- additionally connect the two ends to pin 1 (signal) and pin 6 (signal)
- soldering point between the two resistors is connected to the ground of the tube amp
(see attached pdf file)

can anybody tell what this cirquit is worth for?

According to the information of Hammond, pin 1 and 6 are both signal inputs for the amp (no ground inputs). And since the amp is a mono amp, I would guess to connect them together to the hot out of the preamp...

any proposals from the professionals side?

Best regards from Switzerland, Thomas
 

Attachments

  • Strom und Audio Eingang für Hammond LRA.pdf
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Hammond intercabinet signal levels were 7 vac. Modern equipment uses 2 vac. The 7 vac was impervious to interference from the AC supply in the cable running along with it.
Hammond output transformers were upper frequency limited. HR40 is from the B/C3 days, upper limit probably 4000 hz. For high fidelity 20-20000 hz, replace transformer with one from triodeleectonics.com of IL, USA. 40 watts would be a ST70 or MKIII transformer.
I wouldn't have talked to you if you'd scrapped an organ. But an HR cabinet is just a PA device like any other.
 
Hi, thanks for answering.
So I need to replace the output transformer? There are two transformers for the output, a bass transformer (which is on the 200hz cirquit) and a treble transformer, i assume this one needs to be changed, right?
What about the two henri-transformers, do they also limit the frequency range? What are they used for?
Thanks, Thomas
 
Dear all,
first just to clarify, i havent scrapped any Hammond Organs or Cabinets; i bought these two amps on eBay.
I am planning to run them on a Parasound P FET preamp which can deliver up to 10V output. This should be enough, according to the 7V that seem to be required according to indianajo.

Question to the pro's in this forum;
Indianajo has proposed to change the output transformers of the LRA, which has a bass (T4 on schematic, ao-21106-3) and a treble (T2 on schematic, ao-21264-0) output transformer (and a internal 200hz crossover for the bass) to hifi output transformers. Does anybody know hifi output transformers with the same layout?

If I change these two output trasnformers to hifi-output transformers, do i also need to change the Henry transformers , the ones on the Hammond have 6 Henry input (L3 on schematic) respectively 18 henry output (L4 on schematic)?
I assume not, since they are Prior to the signal cirquits.

Thanks a lot
Best regards Thomas
 
The chokes (2H inductors) are between the rectifier tube and the filter caps. This is a straight 60 hz hum killing function. No replacement necessary.
This is a bi-amp. The bass output transformer should be fine. If you want frequencies above ~4000 hz you'll replace the high freq. transformer with one with a driver impedance suitable for four 6V6 push pull. I'm not familiar enough with 6v6 to know how many ohms that is. If HR40 stands for 40 watts, then one for a dynaco mark IV should work, even though the more modern tubes were only one pair. The Mark IV transformer would also do the bass, but Hammond filtered the bass off before the 4 output tubes were driven.
Talk to triodeelectronics.com that sells those transformers about four 6V6 at 295 V plate voltage. Or look at the 6V6 datasheet, triode has them available for download.
There may be other high freq filtering ahead of V2 the c6 & C5. I haven't calculated the roll off freq. Reducing those caps then making sure the circuit does not oscillate is up to the converter.
I'm a Hammond player is why I am a bit touchy about repurposing organ amps RFE. The search took me 28 years after I bought Passacaglia & Fugue Cmin folio to find a Hammond console I could afford, an H100. Not the most desirable model, but plays JS Bach & John Lord both okay. After 71 new e-caps.
Have fun.
 
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Hi inidanajo,

remember, the highest note (F#''''') of a B-3's 91-note TWG puts out a frequency of 5925 c/s, the highest one (B''''') of your H-100's 96-note TWG means 7899 c/s. I suppose the OT's might have been designed for reproducing these frequencies.

Not enough for high fidelity purposes, though.

Best regards!
 
I don't have a picture of the amp, but David A who runs a repair service in NC, USA, finds the sardine can type 12 & 4 uf caps used in late 1940's and 50's hammond amps never go bad. He's used an ESR meter on multiple samples. The achilles heel of most electrolytic caps is the "rubber" seal that deteriorates running or sitting on the shelf. The hammond sardine can caps have a crimp seal like a tin can of food.
David says those don't deteriorate.
This long life doesn't apply to tall alumimum can electrolytic caps, from late fifties to early 80's, those do go bad pretty frequently after 20 years. I replaced 5 tall cans in my 1968 build Hammond, plus 66 other electrolytic caps.
 
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Dual capacitors are made for convenience, unless you are doing a full restoration using NOS parts, there is no reason why you can't replace them with modern individual capacitors, as you clearly have a lot of space available. The exact value of the capacitor isn't that important as long as the B+ voltages are close to the original spec. 47uF is a common value, so should be easy to get.
 

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> the sardine can type 12 & 4 uf caps used in late 1940's and 50's hammond amps never go bad.

They are probably oil caps. Well-sealed as they are, they never go bad. Do NOT tamper with them. More uFd is not needed, and will not be better.
 
Hammond intercabinet signal levels were 7 vac. Modern equipment uses 2 vac. The 7 vac was impervious to interference from the AC supply in the cable running along with it.
Hammond output transformers were upper frequency limited. HR40 is from the B/C3 days, upper limit probably 4000 hz. For high fidelity 20-20000 hz, replace transformer with one from triodeleectonics.com of IL, USA. 40 watts would be a ST70 or MKIII transformer.
I wouldn't have talked to you if you'd scrapped an organ. But an HR cabinet is just a PA device like any other.
Good afternoon. Per the schematic at "captain-foldback.com", this looks like a pretty straightforward "bi-amp": I have a line on a pair of these (actually not to far from you in So.IN); and already-scavenged, so the organ murder is not on my conscience... Do you have any specific familiarity with this LR-A chassis type (beyond the usual re-capping and resistor value drift)?

Thanks in advance for any guidance!
 
All - 'quick question to those familiar with this H40/LR-A's inputs: The schematic (e.g. input socket) appears to imply almost a "balanced" XLR-like driving of the input-stage tube grids...<?>. Unusually lacking a phase-splitter, it seems "pre-split" throughout the signal chain - suggesting a separate pre-amp may have created the +/- w.r.t.-gnd? Being ignorant of typical Hammond overall systems, is my presumption that (quite sensibly by the mf'r) they sent a noise resistant balanced (higher-than-line-voltage) signal out of the organ itself to this external cabinet?

Thanks in advance for the education!
 
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Yes, that's exactly the way Hammond did it. From the very first model A on, all consoles without built-in power amplifiers and speakers, and even some of those with, featured balanced ('G-G') preamplifier outputs, fed via an output transformer with center tapped secondary winding.

Best regards!
 
Supplement: I think the main reason for Hammond to do it this way may be that both mains leads are run in parallel with the signal leads within the same 5 or six conductor cable. Anyway, in more recent Leslie cabinets with 9 or 11 pin connectors Hammond returned to unbalanced inputs, even though the mains supply via the same cable remained.

Best regards!