Using old Hammond iron

[PeteMcK]

I have a pair of OP transformers, Hammond code 28185 – no, I don’t think they’ll be found an old catalogue because they are OEM, I pulled them out of an old church organ ~ 40 years ago, can’t remember the brand, possibly Gulbransen. Comparing physical measurements with those in an old Hammond cat. show the OPTs to be capable of 20 or 30W.

The amps used 6L6’s, measuring the trannie’s voltage ratio, I get 10:1, so a Z ratio of 100:1. The secondary has a C.T., so the only sensible way to use these with 6L6’s seems to be a 16 ohm speaker across one half of the secondary (to get a 400:1 Z ratio) for a primary a-a impedance of 6400 ohms (assuming my maths is correct).

However, I want to use these with 8 or 4 ohm drivers, and I’d like to not use 6L6’s either. Attracted by the elegance of the circuitry, I wanted to build an RLD, but these trannies don’t seem to be suitable. Perhaps I could use the same topology with a pair of EL34’s (3200:8) ? (This would probably suit the power transformer better too, it gives 350-0-350v, (the Hammond plate rates it at 125VA, 125V, 40Hz? What the?...)

[wakibaki]

I never heard of an OPT with a centre-tapped secondary.

Why would you have 2 halves to a secondary and only use one? In an OEM transformer?

It just doesn't make sense.

Then there's RLD.

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?????????????????

Rotating Loudspeaker Doofer.

Is that what it was used for? 2 back-to-back speakers on a rotating thingie? For an organ. As in Hammond...

40Hz?

This post is like a cryptic crossword. Or a secret wartime code to the French underground.

w

In the beginners mind there are many possibilities. You're not Al Qaeda?

[wakibaki]

Rotating Loudspeaker Doppler.

[Lingwendil]

Use them to build a homebrew Rotating Leslie.

[tubelab.com]

I wanted to build an RLD, but these trannies don’t seem to be suitable.

Most readers of this forum are quite familiar with the RLD since the thread is now nearly 700 posts long. It now has offspring, the WLD, the OLD, and the NLD.

I pulled them out of an old church organ ~ 40 years ago

It the organ was parted out 40 years ago, it must have been old enough then to destroy. A lot of old organs used multiple speakers. 16 and 32 ohms were fairly common. Assuming the tap is actually center (same voltage on both sides) I see two possibilities. It was somewhat common for one set of taps to drive an internal speaker and another set to drive the "extension speaker". It was also common for one amplifier to drive different sized speakers. I took apart some Hammond organ extension cabinets in the 60's that had 4 32 ohm speakers in each one.

This would probably suit the power transformer better too, it gives 350-0-350v, (the Hammond plate rates it at 125VA, 125V, 40Hz? What the?...

This stuff must be really old. Today most power in North America is 60Hz while much of the world is 50Hz. Not too long ago there were isolated power companies scattered across the US that did whatever they wanted. Odd voltages and frequencies did occur. The 40Hz rating was the minimum and the transformer would work just fine on 60Hz. The military used (and still uses) 400Hz power in aircraft to cut down on transformer weight. Never plug a 400 Hz transformer into an ordinary wall outlet unless you like some really stinky smoke!

If the transformers are in good shape (doubtful) you can build a RLD using 6CW5 tubes. These want a 3200 ohm load but use a lower B+ voltage (about 250 volts). EL34's could be used, but I doubt that your power transformer could power two channels, and the 12AT7 may not drive them to full power.

[dcgillespie]

New member here, and hopefully I can shed some light on your transformer question.

It is very possible your transformers came out of a Hammond Organ product as they manufactured many tone cabinets in the early days of electronic organs, and even wound their own transformers for a period of time as well.

Going down that road then, in the earliest Hammond models, the power amplifier proper (usually located in the tone cabinet) consisted of nothing more than a push-pull output stage, preceded by a push-pull driver stage. The amplifier was driven by a balanced to ground transformer originated signal from within the organ, so no electronic phase inverter in the power amplifier was necessary. As for the output transformer and it's center tapped secondary, it was very common for Hammond to apply push-pull feedback from the secondary of the output transformer to the cathodes of the output tubes (PPP in the largest designs), as that was the easiest and cheapest way to do it. Hence, the requirement of the secondary being center tapped. The speakers were always connected across the full secondary. This was often 16 ohms, but hardly always. Sometimes it was 32 ohms. When it was 16 ohms of course the CT would represent 4 ohms, and an 8 ohm CT if it was a 32 ohm secondary -- although the transformer was typically not wound for that purpose or service. If it were, the low ohm winding portion would have been wound with wire gauge that was more appropriate for the current flow that impedance required.

In those days, two 6L6s were often used in 20 watt tone cabinets, while four were used in the 40 watt models. Is it possible that your transformer came from a 40 watt unit? If so, it would have been appropriate for use with PPP 6L6s and a 32 ohm load, based on the turns ratio your unit offers.

In any event, the old organ transformers were hardly a high-fidelity grade product, often with a very limited frequency response and power bandwidth. That may or may not be a problem depending on your application. I hope this helps.

Dave

[ChrisA]

Originally Posted by dcgillespie

...
In any event, the old organ transformers were hardly a high-fidelity grade product, often with a very limited frequency response and power bandwidth. That may or may not be a problem depending on your application. I hope this helps.

Dave

Yes they WERE "high-fidelity", very much so. but they did it different then we do today. Hammond would place multiple amplifiers in the tone cabinet. One would be for bass, another amp would handle treble and a third would be for the output from a spring reverb unit. Each amp would then drive it's own set of speakers.

Also, and this explains the center tap, the amps were "balanced". basically this meant the schematic looed like two single ended amps on of the speaker. No phase splitters as the source itself was balanced.

What you likely have is a "bass output transformer" that is good for up to (guessing) about 800Hz or 1KHz. The typical setup from Hammond would have also two treble amps in the same box each with it's own "treble output transformer".

Today we would call this a "bi-amp'd active PA speaker".

I think the only reason Hammond's system was not used with home HiFi was cost. Their bi-amp balanced system hade about four time more parts. Thse old tome cabinets sold for more money than a new car cost. I have some old adds that $1,800 prices. That was a lot of money in 1950. These were not built cheap

What you need to do is learn to play bass guitar. This would make a really great pratice amp for a bass. I've been wanting a low poer 6L6 tube bass amp for a long time. (BTW, bass guitars players call 200W "small" and 40W would be a living room sized practice amp. Most would haul a minimum of 400W to a gig.


I think you should be able to test your transformers using a large solid state amp connected to the secondaries. Play test tones into a 5K ohm dummy load Turn the amp's power up and see how much bandwidth you get at 40W

[dcgillespie]

Chris -- We'll have to just disagree on the what the term "high-fidelity" means then. I realize that the use of multiple amplifiers and reproducers can make for wonderful sound, and while an organ amplification system of the type we are discussing here does have to reproduce copious amounts of low frequency energy, it has to produce little to nothing over say 8 kHz. True high fidelity in the hey day of vacuum tubes came to be defined as a single piece of amplification equipment that could produce full rated power output over a 20 Hz to 20 kHz bandwidth, at typically less than a percent or so of THD and IM distortion within 1 db of rated power. None of the Hammond tone cabinet amplifiers or their output transformers were capable of that kind of performance. No doubt that if the OP wanted to use the transformer for a precise bandwidth application, it would work fine -- which I as much noted at the end of my post.

Also,when I referred to the "earliest" models, I was referring to models much earlier than you describe. I have an example of one of Hammond's earliest amplifiers (model A-20), employing PPP 2A3s. From the period it was manufactured up until the later 40s, there were no such things as "bass" amplifiers, "treble" amplifiers, "reverb" amplifiers or the like. There was just a power amplifier in a tone cabinet, that during that period typically employed electromagnetic speakers. Type "G" amplifiers were the first to employ push-pull feedback from a center tapped secondary to the output tube cathodes, and employed PPP 6V6s. This was done to maintain a low output impedance since these were among the first amplifiers to employ pentode output tubes. The type HR tone cabinet was the first to include reverberation -- but again it was only one amplifier producing all the organ output.

Against the backdrop of not knowing the age of his transformer, or the intended purpose of his build, I was simply offering a caution to the OP that it's extremely doubtful his transformer would produce high fidelity results as it generally became known to mean....the same performance level as that of say an Acrosound or Dynaco transformer would produce. It is general application information that would be worth knowing about an otherwise unknown transformer in the course of a project under consideration.

Dave

[PeteMcK]

Tubelab, re:' you can build a RLD using 6CW5 (=EL86) tubes. These want a 3200 ohm load but use a lower B+ voltage (about 250 volts). EL34's could be used, but I doubt that your power transformer could power two channels, and the 12AT7 may not drive them to full power" - thanks, that's the sort of info I was looking for, I'll search for EL86 schematics. I'm not looking for heaps of watts, 15-20 would be ok, so perhaps EL34s would do.

Power Trannies - actually, there are two, the one mentioned above was in the extension cab, another rated at 180va with massive wire to power all the 12ax7 heaters was in the organ itself. (& both actually have dual primaries wired in series for NZ 230v)

DCG & Chris - I definitely wasn't a Hammond organ, and there were on 2x 12" drivers and amps with one 6L6 pair each, one in the organ and in and extension cab - no fancy Rotating Loudspeaker Doofers (although that doesn't mean that the parts weren't the same as used in a fancier version of the organ). There is also what appears to be a very low impedance winding on the OPTs (for feedback?), or perhaps a screen. I'm going to take the covers off and investigate further this weekend.

Perhaps it's possible to split the secondary & parallel the windings, or is this not a good idea?
Thanks for the responses.
(P.S. my bass playing days are long over, I'l be happy never to see another road case again....)

[PeteMcK]

I see Tubelab has heaps of info on the 6CW5 on his site, my concern about this tube is availability & low Va;
EL34s won't do either, the driver stage of the RLD puts out ~8V which according to the Mullard Data sheet p9 should give me a whole 1.5W :-(