DIY 1624 from Hammond M3 Parts

[dawsonaudio]

I am putting together a diy supro 1624 amp build. I am trying to utilize some of the components from a non working hammond m3 that I have in my garage. I have two other's that work great. I'd like to use the power supply and 12 inch field coil speaker, as well as the output transformer from the broken organ. All the other parts, tubes, etc...I will purchase separately. Below is the m3 power supply schematic:

*Wrong Schematics* Below are correct schematics.

I'm semi-savvy with electronic assembly, but looking for some tips/help here. I will be building my own cabinet to house everything. Probably out of pine. Any thoughts? My initial concern is the dc voltage coming off the hammond m3 and making sure it matches the supro 1624 circuit specs.

Thanks,
Nate

[Phil_S]

I don't comprehend how a field coil speaker works. Let's assume that will be OK. <shrug>
The Hammond was spec'd to run on 110V. Today's line voltage is generally 117 to 129VAC depending on where you are and the time of day. Let's assume 121V for the sake of discussion. That means all voltages on the Hammond schematic should be increased by 10%. So, the preamp supply is likely to be a little on the high side, but not by that much. You can probably manage this by modifying the resistor values in the B+ supply ladder.

I'd be more concerned with filament voltage, which could easily be 7+V and that's high. Similar concern for the 5V rectifier.

Wire it up, plug it in, and see what you get! Observe all usual and customary safety precautions.

[dawsonaudio]

The field coil speaker needs power to create the magnetism. It isn't a permanent magnet like most current speakers being made.

As far as the voltage coming off the transformer, without any load on the secondary, the voltage is around 380-0-380ish, 6.6ish filament, and 5.5ish on the rectifier side. The incoming voltage is a little higher, but not sure if that will affect the circuit too much. I've run the other m3's without any problem. My a/c voltage here seems to run around 117-119 most of the time.

[Smokebreak]

I built the 1624T into a Bogen PA, and liked it so much did a proper scratch build a couple weeks ago.
Concerning the circuit, and it's history, I believe the one you posted is one of the later versions of the circuit. Nothing wrong with that, but it seems the first couple versions are more highly regarded, may be the Page thing. The tremolo is simpler, and there is 1 tone control, and the channels are mixed in a different spot.

There is some great reading here : http://www.dirtygirlamplification.com/A ... 1624T.html
I did a board layout and a half-assed chassis layout. let me know if you are interested in that. It's for the scheme I've posted here, but could easily be modified.
Seems voltages ran about 350V plates, so it looks like you may be a bit high with that PT and today's voltages. I used SS rectification(just b/c I always do) and used a 275-0-275 PT and ended up right at 350V. Even if you end up 400V+, I've had good luck running 6V6s up there. Are you gonna use 6937's?
It's a great amp, and the vibrato is very nice.

Edit: just saw your post, looks like your voltages will be OK under load for this circuit. Looks like the FC is dropping a lot

[martin manning]

I think the voltages will be fine just the way it is if you keep the field coil speaker. The organ schematic says 300V on the plates, so like Phil says you'll be 10% high with today's line voltage so about 330.

[dawsonaudio]

Appreciate the feedback here. I would definitely be interested in your layout/s. I am essentially going for the 'Page' version. I've got a thunderbolt that I really like, but really have to crank it to get it to shine when recording. I'm hoping the 1624 will get me there with lower volumes.

[Smokebreak]

No problem. I'll get my notes together and start a thread with everything I've got, hopefully tomorrow.

[dawsonaudio]

Ok, so I'm getting back to building my 1624 based amp. I've got the cabinet together and have all the parts ready to order. I would like to make sure that the Jensen field coil that I'm using will fit into the circuit. Below is the Hammond M3 power supply schematic that the speaker came from. Also, is the schematic from www.dirtygirlamps.com that I'm using to build my amp. I already have the speaker, the transformer and the rectifier tube, but not sure which other components I will need to incorporate into the 1624 circuit.

Thanks again for the help here,
Nate

[img:785:332]http://www.dawsonaudio.com/M3.png[/img]
[img2100]http://www.dawsonaudio.com/1624T.jpg[/img]

[Smokebreak]

I already have the speaker, the transformer and the rectifier tube, but not sure which other components I will need to incorporate into the 1624 ]

Those are the only components I would use for the 1624. Sure, some of the reistors in the power supply may be capable, but I'd start with new 3watts. I would not use the electrolytic caps either. Start fresh.

As far as the field coil goes, it looks like it was tapped for negative voltage, in a back-biasing arrangement, probably for the output tubes' biasing. You won't need that arrangement for the 1624, as the output tubes are cathode biased.

Field coils also did duty as chokes. Personally, I would look at it that way. Chokes usually go between the first 2 nodes in the power supply, where the 2K2 is. Someone will correct me if this isn't the best solution, but since your FC has a resistance of 700ohms, I would put it in series with a 1K5 between the 20u and 10u.

Also, when ordering, get some other values around the 15K mark in case you need to tweak your preamp voltages.

[JazzGuitarGimp]

Personally, I would leave the field coil in the cold side of the power supply. It will do just as much to filter the B+ in the cold side as it would in the hot side, plus, you won't have potentially lethal voltage in the (perhaps open back) speaker compartment.

[Smokebreak]

Personally, I would leave the field coil in the cold side of the power supply. It will do just as much to filter the B+ in the cold side as it would in the hot side, plus, you won't have potentially lethal voltage in the (perhaps open back) speaker compartment.

That's a better solution.

Lou, concerning keeping B+ around 350V w/ modern wall voltage, would it just be best to just keep the PS structure as-is(replacing all elytics, and adjusting droppers for proper B+1,2,3)?

That PT is 380-0-380, and B+1 is 310V. Is this mostly due to the parallel dropping string or the field coil, or a combination? Now I'm worried that if he uses that PT with the 1624 hot side string exclusively(albeit w/ cold side left as-is), voltage would be way above the ~350V B+ mark, especially if he's not getting the loading that maybe the original amp had?

[JazzGuitarGimp]

Is there a schematic available for the rest of the organ's amplifier circuitry?

[dawsonaudio]

Here is some more of the schematic I'm referencing.

[img1806]http://www.dawsonaudio.com/M3a.gif[/img]
[img1806]http://www.dawsonaudio.com/M3b.gif[/img]

[Smokebreak]

Lots of tubes pulling on that power supply, and some voltage dividers. Check R45 A/B on OT primary

[dawsonaudio]

The power supply that I have has been disassembled years ago. No way to test those two resistors in the circuit.