Baldwin Model 40 conversion

[spsimmons]

I've been collecting organ parts for a while and think I might have a chassis that might make for a great conversion.

I'm trying to get a schematic for it, but no luck yet. I have most of it drawn out, but for an amp having so few parts, it gets really confusing. I've sent an e-mail to a guy that I know either has or had the schematic, hopefully he'll come through.

In the meantime, I need to start thinking about a preamp to use. That's where you guys come in. Does it look like there is enough room on the chassis to add a preamp? (I don't really understand the concepts behind chassis layout and lead dress yet. Seems a little like voodoo.) If adding it to this chassis will cause problems, I suppose I could build the preamp on a separate chassis and eventually build it into the same cabinet (but for all I know this might pose problems as well).

I'm also open to suggestions as to which preamp designs might be a good match. I have a bunch of 12ax7s, 12au7s, and 6sn7s laying around. (The 6sn7 might be cool 'cause of the octal base.) I don't need a bunch of controls, volume, treble and bass would be nice. The amp already has a pot on it labeled "gain". Would this act as a Master volume?

Any other tips or suggestions would be welcome as well.

The tubes are: 2x6L6GC, 6SL7, 5U4GB




Thanks, Shannon

[petemoore]

Kinda crusty...test the transformers yet ?
Use the existing tube compliment, probably the same stages, then dig-up or work up a preamp..often for something like this 18v of solid state boost can be wired up to drive it pretty good, or look for the next preamp to try on it.
As far as fitting it in there...tight-packing/edge-ing a preamp right next to power amp might be troublesome and ugly.

[JHow]

Looking at the first picture, I thought: "Wow! That's pretty clean!"

Then I saw picture two.

Maybe, as they say in the used-auto trade, it's just surface rust.

[petemoore]

I should have said the state of the transformers shows oxidation, hopefully the end caps are still keeping the coils free of excessive state-change.
And there's a boost circuit available or you could possibly attach something to the input jack, outside the chassis is generally easier if an internal preamp introduces noise.

[spsimmons]

I still haven't got a good schematic for this thing yet. I'm trying to recreate one from an out of focus photo that's posted on a telecaster forum.

The top side is a little on the crusty side, but it's mainly on the chassis. There is some rust on the transformers, but it does seem to be fairly superficial. It doesn't appear to be from water, but from the rodents that were using the organ as a home/peanut storage facility.

I hadn't tried to power the amp up before today. I spliced a cord and fuse to it, pulled all the tubes and plugged it in. I measured 450v at pins 4 and 6 of the rectifier socket and 2.5v at pins 2 and 8. Unfortunately, when I try to power it up with the rectifier tube in, the 5A fuse blows immediately. Problem.

Well, at least the main transformer seems to work. The choke made a slight hum and the speaker made a little noise when I was probing the rectifier socket.

[unparalleled]

Try powering up with the 6L6's and 6N7 out and check DC volts at the big can capacitors,(if the fuse doesn't blow) then put the 6L6 valves back in and see if fuse blows.
You need to really get it working to prove which bits are good.

If I was doing a conversion I would check these things first to see which parts are bad, then strip the whole chassis and clean/paint, lose the can capacitors & shift stuff to one end/ side, making room for preamp.
Strip the cans off the transformers, wire brush rust off ends and laminations - paint.
Should make a useful amp.

[spsimmons]

So I replaced the fuse (used a 3A slo-blo) and powered the amp up with only the 5u4 installed. The tube flashed brightly a couple of times, settled down and then the plate started to go red. There was a loud hum coming from the speaker. I unplugged it after about 60 seconds. Didn't take any measurements, the amp seemed too unstable to approach. Any ideas on taming this so I can move forward.

Thanks

[roughcut studios]

tube gows red, loud hum

Sounds like new caps in the power supply would be a good place to start.

[olddawg]

This may not have enough heater current spec to add a preamp stage. It looks like a slave amp to me. I guess you could add a 12v filament transformer to heat some preamp tubes. And yeah, this crust old amp has bad electrolytics for sure. I would be worried about the PT as well.

[spsimmons]

Thanks, roughcut. I figured those three can caps would have to go eventually. I'll go ahead and replace all of those. Should I have concerns about the Micamold tropicaps or the three Mallory caps (at least I think that's what they are, they don't have markings on them that I understand)?

Olddawg, this was just the power amp. There are very large transformers on both the tone generator chassis and the percussion unit chassis. There is a 4th chassis that has the tone and "drawbar" controls on it. It has three tubes on it that look suspiciously like a preamp. It also has a small transformer, maybe for the preamp heaters? I don't know yet.

If I can confirm that the transformers are good, I do intend to strip, clean, paint and probably re-arrange the whole chassis. Adding a small 12v transformer wouldn't be a big deal since the thing already weighs a ton. I definitely should consider ditching that monster of a choke.

[spsimmons]

Replacement cap values

Two of the can caps on this thing are 4uF 600V. I've only found one source for these so far (justradios.com). Is this an oddball value? Is 600V overkill? They only cost $4 each, so I don't mind ordering from there. It just seemed strange to me that they aren't more common (for example, Mouser doesn't seem to have them).

[RWood]

Are you sure they're not 40uF? That would be about right for a 5U4GB reservoir cap.

I've been using two 80uF 450V caps in series to produce 40uF for 5u4s. F&T makes them, AES sells them.

RWood

[spsimmons]

The existing can caps are clearly marked as 4MFD 600VDC and 4.0MFD 600VDC for the two largest, while the smaller can is a multi-cap with 20MFD 450V, 20MFD 450V, 20MFD 450V and 100MFD 50V values.

[imaradiostar]

I took one of those apart many many moons ago. Assuming it's the same critter, the output tf is 6600:16/8 ohms, other large tf is a choke, probably a swinging choke.

I still have the chassis, OT and choke and an assortment of resistors and sockets from it. You may find it's more rewarding to build in a new fresh chassis with a known power transformer that's a good match for the output. Save the choke until you need a big swinging choke. I think it's around 13 Hy and 100 ohms DC resistance (from memory).

jamie