Hammond M2 and M3 organ

[olddawg]

In the Hammonator, he discusses the the fact that the transformers were designed for 110-115V and at 120V the filaments will run hotter and he offers up a way to fix this by putting the 5V filament winding in series with the primary. Are there any safety issues with doing that? Could you not just create a voltage divider to take away the bit of excess? I might want to keep the 5U4.

Or just a couple of rectifier diodes back to back in series with the filament string? Will drop .6-.7 volts per pair of diodes?

[g1]

Putting the "bucker" coil in series with the primary is a fairly common, safe, method. Of course it could be wired in an unsafe manner, but so could a fuse-holder. The bucker must be able to handle the full power current, so different units will have different requirement.
The reason this is a good solution is because it is not just for the benefit of the filaments. All the running voltages will be higher with modern line voltages. So tubes and capacitors may also be stressed by the higher voltages, which may reduce tube lifespans, and could even blow capacitors if they were running close to rated voltage at old line voltage levels.

But you can just drop the filament voltage with diodes like olddawg said, or even with low value power resistors.

[olddawg]

Putting the "bucker" coil in series with the primary is a fairly common, safe, method. Of course it could be wired in an unsafe manner, but so could a fuse-holder. The bucker must be able to handle the full power current, so different units will have different requirement.
The reason this is a good solution is because it is not just for the benefit of the filaments. All the running voltages will be higher with modern line voltages. So tubes and capacitors may also be stressed by the higher voltages, which may reduce tube lifespans, and could even blow capacitors if they were running close to rated voltage at old line voltage levels.

But you can just drop the filament voltage with diodes like olddawg said, or even with low value power resistors.

If the OP intends to gut the chassis and just use the iron, lowering the filament supply with diodes will give you the benefit of a higher B+ if you want it. A couple of more watts maybe depending on what one builds?

[Pdavis68]

I thought about that. I don't know that a higher B+ will do me too much good. I don't know what the OT is rated for, but I imagine it's not too much higher than what it's designed to run at.

What's odd to me is they've got 290V going to the transformer, 280V to the plates, and 260V going to the screen grids, but the transformer is 340-0-340. Someone mentioned it's a field coil speaker. It looks like they have -85V going to the speaker field. I guess I'll need to do that if I reuse the speaker? That may make the speaker less useful for me as I'd like to be able to make this a head instead of a combo.

But the point is, I think there's going to be more voltage than I'm going to need as it is. The 6V6s are maxed at 315V on the plate.

[DRH1958]

6V6s are routinely run at 350 v or higher on the plate. The Fender Deluxe Reverb has I believe 420 v, but I think that's too much. Just a personal preference. But it must be remembered increasing only the plate voltage in a pentode does little good by itself. You also need to increase the screen grid as well, so take care of the screens with proper resistors no matter what you do. But I'm with you on the lower voltage as this will lengthen tube life. Not much of a difference in squeezing out a couple more watts vs. shortening tube life. If you assume 15 watts output and increase it to 18 watts, that's only an increase of .8dB, hardly enough to hear. It might add some clean headroom before breakup but that's all.
Yes, you will need to use that voltage on the field coil to reuse the speaker. I'm not sure about which brand speaker is used here, but I have built a few amps out of Hammond organs and have had some speakers that came with the organ sound absolutely horrible for guitar, so you might want to have a backup plan.

[Enzo]

yeah, Fender ran over 400v on the 6V6 plates, and yet those Deluxe amps have been running that way for 50 years or more, and do not have a reputation as a tube eater. Dissipation kills tubes, not voltage.

Transformer ratings? If the amp has a pair of 6V6s, then assume the tranny is more or less like any other tranny designed for a pair of 6V6s.

The missing B+ voltage level? Remember your tube rectifier soaks up a considerable voltage, plus if you have that field coil speaker, as shown in the schematic posted, that drops about 100v too.

[Pdavis68]

Yeah, it's the field coil that's eating up the voltage. It's taking 85-100V (depending on the serial#). Duncan's PSU designer estimates about 385 after rectification with the 5U4. So yeah, if you knock off 100V, that gives you 285 and that's awfully close to the 290V they're putting into the OT.

I'm not sure what to do with all that voltage. Assuming 385V (and that's probably conservative since it wasn't designed for 120V primary), I'm going to have to bleed off at least 70V. Seems so wasteful. I'm trying to think of something useful to do with it.

I have some 5881s and 6L6s I could throw at it, but then the OT becomes an issue. My guess is, since the power transformer is already designed to handle handle 7 tubes (not including the rectifier) and quite a lot of other junk, that it could probably handle the extra power those tubes require...

Personally, I don't have need for a really high power amp. That's why the amps I'm designing are low wattage SE amps.

[Enzo]

Well, even at 385, 6V6s won't be hurt by that, and the fender Deluxe that would use such a scenario is hardly a high power amp.

And if you are desperate for 290v B+, you could easily replace the field coil with a 700 ohm power resistor, and use the organ circuit as drawn.

[DRH1958]

Yeah, it's the field coil that's eating up the voltage. It's taking 85-100V (depending on the serial#). Duncan's PSU designer estimates about 385 after rectification with the 5U4. So yeah, if you knock off 100V, that gives you 285 and that's awfully close to the 290V they're putting into the OT.

I'm not sure what to do with all that voltage. Assuming 385V (and that's probably conservative since it wasn't designed for 120V primary), I'm going to have to bleed off at least 70V. Seems so wasteful. I'm trying to think of something useful to do with it.

I have some 5881s and 6L6s I could throw at it, but then the OT becomes an issue. My guess is, since the power transformer is already designed to handle handle 7 tubes (not including the rectifier) and quite a lot of other junk, that it could probably handle the extra power those tubes require...

Personally, I don't have need for a really high power amp. That's why the amps I'm designing are low wattage SE amps.

Remember, at lower dissipation, the 6L6s have around 6000 ohms for pri impedance on your OT depending on where you operate them at. So this might not be an issue. Do some research on this and try to settle in at an impedance that is closer to 6V6 if you choose to go this route. For example the 6L6 operated at 360v plate and 270v screen specs out at 6.6k ohm pri. impedance for 26.5 watts out. Also to consider is bias voltage when switching to 6L6s but for the above scenario, it is only -22.5v. So pretty close to 6V6 range.
Also remember about heater current if you do this.

[Pdavis68]

I guess my father-in-law has rubbed off on me a bit. He's always big on safety and staying within the limits. 385 seems like a huge jump over the 315V in the datasheets, not to mention the 420 of the Deluxe Reverb... But you guys and Fender know better than me. If you say it's safe, no doubt it is.

I'm going to hold off on the design a bit. I've focused most of my attention on SE amps, so I'm going to have to read up on Push Pull. I understand the basics, but I just haven't focused on it nearly as much.

Kind of ironic that I just designed a 6SJ7 preamp and now I'm about to get an amp with a couple of them in it.

[Enzo]

The data sheets were published by RCA or whomever as a guide to engineers for design. But back then everything used tubes, and the designs in mind were for table radios, TV sets, and so on. The published specs were for long reliable tube life. Mom and dad did not want to replace the tubes in their table radio once a year. Those numbers are for nice polite designs of consumer electronics. Think of them like speed limits on a highway. If you drive faster than the speed limit, it won't hurt the car. it may place you in some danger from other traffic, but the car doesn't know the speed limit is 70 or 45. MAybe a better analogy is cars. GM or Ford makes a car intended to be a nice family sedan for dad to drive to work or mom to the store. It should live a long reliable life in stat service. Now junior starts to race that car on weekends. The tires will wear out sooner, maybe the engine is more stressed, but the car will work. In guitar amps we expect to wear out tubes in a year. It is the price we pay for performance.

[Pdavis68]

Well, I just got off the phone with the guy. He's not so anxious to have his old organ chopped into bits and he has someone else who wants to keep it intact who might pick it up tomorrow. But then he went on about all these other amps (like an amp from an L100 series which has EL84s) he has (apparently he spent a lot of his youth picking this stuff up at estate sales and such and has quite a collection). He actually did a conversion himself about a decade ago, so he understands what I'm after. Anyway, he's going to make a list of what he has and e-mail me tonight. So looks like I'm going to be taking a slightly different tack on this. Glad to not have to deal with all the wood, though :-) That was turning into a headache. Kind of wish I had more money to buy a few of them from him. Sounds like he could keep me in the amp conversion business for some time...

[Enzo]

I used to work in a large music store, and they sold pianos and organs. people always way overestimated the value of their old organs, but the store always gave a good trade in for them, there was plenty of profit built into the new ones. People would leave us their old organ assuming it would go to a new home. But almost all of them were immediately scrapped in the back room.

When talking about them, they always referred to the beautiful wood, etc. About the only use for the wood I ever had was I'd save the music stand, and use it for a shelf somewhere. The big wide top of some old ones could be used as a larger shelf.

[Pdavis68]

Well, he's only selling it for $50, so I don't think he's really overestimating the value. In fact, I suspect he could ebay the parts for quite a bit more than $50. But he clearly has some sentimental attachment to it. And really, from looking at the photos, I don't think it's all that far away from a complete enough restoration to make it worth $300 or so.

But it sounds like he's got more amps than I can afford to buy anyway, so I'll see what he has and go from there. He seemed to be fishing for whether or not I'd take them all off his hands. If I had the spare cash, I would. I'm waiting on one final part of finish fixing my old Ace Tone amp, at which point I'll probably sell it. If I do that, maybe I'll use the proceeds to buy a few more amps from this guy.

[DRH1958]

Grab that L100 amp. This is my favorite Hammond amp to rebuild because it has a built in reverb circuit. Make sure he is right about it being from an L100 and that it is an AO-43. See if he has the reverb tank. It should have a DCR of around 170 ohms for both in and out. Usually it has an L stamped somewhere on the under side. This amp sounds as good as any 18 watt amp I have built. If you decide to get it, I can send you the schemo. The AO-43 is easily more mod/rebuild friendly than that AO-29 in the M2 organ.