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Thread: (Just another) Baldwin Organ Conversion Question...

  1. #1
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    (Just another) Baldwin Organ Conversion Question...

    Greetings All !

    I'm your classic long time lurker/first time poster here on the forum, and I'm compelled to break the silence as I'm venturing into some new personal amp territory with (yes another) conversion from organ parts to guitar magic.

    I've found LOTS of great info here and elsewhere about these conversions, but once you get outside of the classic Hammond organs with their wealth of info available through widely available schematics and Hammond transformers specs and modern classic amp designs based on them, info starts to be a little less specific...and in some cases downright vague. So while I'm hoping to get a little input from you all, I'm also hoping that there will be enough detail here to help the NEXT guy scouring the net for facts and ideas!

    Here's the deal: Classic estate closure, old organ no wants to move, I have a truck and love vacuum tubes and you know how it is...next thing I know it is in my shop. "It" being a 50's-era Baldwin Orgasonic (love that name) Model 41P. All 300 pounds of it's mahogany-veneered, massive steel-chassis sprouting a bajillion vacuum tubes dinosaur-like relic from a time when one could afford to pay American workers a living salary to solder wire looms by hand. It really is amazing to see.

    Or was. As it rapidly fell to pieces as I reversed the manufacturing process.

    I'm left with transformers I'm ready to build with...but wanted to check specs and math with you all. There are essentially "two issues" I will break down:

    ISSUE 1:
    The main amplifier unit consisted of a PT, choke, OT set which ran a pair of 6L6GB's, a 5u4GB, 2- 12ax7's, and 2- 12au7's into a 16ohm speaker load (a set of two 10" 8 ohm speakers wired in series).

    My current thinking is to build a simple amp like the Atlas Tweed schematic (a "6v6 version of the Matchless Spitfire" or "almost an 18-watt lite" depending on your sensibilities). Schematic attached.

    Specs for the PT: 350-0-350, 12.5v tap, 4v tap all approx as it was measured unloaded.

    Specs for the OT: Here is where my math might need double checking since the PT is an unknown and I had to determine the impedence. The only low voltage AC source I had on hand is an oddball 20VAC transformer which puts out an actual 26.1VAC. I applied that to the primary side, which gave me 1.2VAC on the secondary. So, 26.1/1.2= 21.75, or approx a 22:1 turn ratio. I square that 22*22= 484, so I have an impedance ratio of 484:1. Against an 8 ohm load that makes 484*8= 3872/4K, and against a 16 ohm load it's 484*16=7744/8K. I think?

    And all of this brings me to the (probably obvious to you) question: Sure this set will work if I stick to 6L6 (though it's a little low voltage on the primary by Fender standards I'd think)...BUT...I of course want to use it with a pair of 6V6's. I'll clearly have enough current available to power a small village, so my targeted tube layout of 2 x 6V6, 5U4 or 5Y3, and 2 x 12ax7 should be fine...but it's the OT impedance I am concerned about? After studying tube spec sheets I *think* I'll be OK using 6v6's and a 16 ohm speaker load with this output transformer...? (this is where I'm in new territory...building with spec parts to a known design, no problem...reverse engineering...problem).

    So- issue 1 is looking for a little reassurance (or ridicule as the case may be) for using this set of iron on a 6V6 Spitfire/Atlas Tweed. (I know this PT would be a little higher voltage than on the schematic but I can adjust B+ down a bit if needed).

    ISSUE 2:
    There was a second power transformer in the percussion unit of the organ. This PT is slightly smaller (will power a slightly smaller village) and fed 30 12ax7's, a 5U4, and I don't have it in my notes but a couple other tubes as well I believe. (For what it is worth I believe both PT's fed some branched feeds through some smaller filament transformers that went to power the tone generators and their total of 30 12au7's!)

    Specs for the PT: 350-0-350, 13.4 tap, 16.2 tap again all approx and measured unloaded.

    OK, same question as above- would like to use this PT for the same 6V6 Spitfire/Atlas Tweed design.

    My best guess is that I *could* use it...the main voltage is fine. I'd guess the 13.4 tap is actually the 12v tap used for the heaters in the former array of 12ax7's...so if I used that tap and wired 2 x 6V6 in series and then wired the 2 12ax7 preamp tubes of the Atlas as 12v I'd be OK...? And that would leave me with a weird tap that's too high for a rectifier, but I could simply use a Weber copper cap and leave the remaining tap unused. Right? Haha.

    That's the best solution my knowledge base could muster. How'd I do?

    I know these kinds of projects and questions likely annoy many of you and please know I understand that and appreciate it. The fun of something like this is absolutely the randomness and upcycling and adventure...so it's being creative with things that appeared, and not a situation where I was wanting a couple of new amps and so am trying to cut corners. I know I can go buy perfect spec transformers for the Atlas Tweed and wouldn't then have to challenge my knowledge level...but really, where is the fun in that? (ok, ok that would be fun too)

    Thanks so much to any and all who are willing to pitch in.

    -Joe W
    Attached Thumbnails Attached Thumbnails atlastweed.jpg  

  2. #2
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    You can change your B+ voltage by which rectifier you use. The unloaded voltage for a 350-0-350 transformer when using a GZ34/5AR4 is 350 * 1.3, or 455V. Solid state diodes is 350 * 1.4 or 490V. If I remember right the 5U4 is 350 * 1.2 or 420V and the 5Y3 is 350 * 0.9 or 315V. So change which rectifier you use and the amp can change radically for sound and power. If I was you, I would stick with 6L6GC's and put in power scaling to get the volume down. The bigger amp will have more headroom and sound bigger yet with power scaling you can still get it to a useable volume range easily. Adding power scaling requires a master volume and the power scale control too. I gutted and rebuilt an old Bogen CHB-100 with my own circuit and it is about 50 watts using four 7868's and 480V B+. The power supply in that uses a voltage doubler. With power scaling the B+ dropped to about 460V and I put in a switch so I can disable power scaling if I want to. It works great and I can get great tones appropriate for a blues jam at stage volume, or I can get those same tones playing super quiet at a Starbucks or something.

    You could make it work with 6V6's but you should use the next output tap up....so if 6L6's were meant for the 8 ohm tap, use the 16 ohm tap with 6V6's for example. Organ amps tend to have large and heavy PT's so I like to have more power with an amp built from them rather than less so I can justify the weight! About 4k is the ideal load for a 6L6GC pair running around 425-450V and the 6V6 is twice that so your approach should work. There is a range of impedances that will work with those tubes though so don't worry about getting exact a whole lot. The tone and distortion and power out will change a bit depending on the load.

    Greg

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    Hey Greg, thanks that's helpful.

    I was actually just earlier today thinking along similar lines...not necessarily sticking with 6l6 but splitting the difference and maybe using 5881's or JJ6v6s since I have a set of each hanging around. This OT only has one tap, which makes sense I guess since it was dedicated use and I'm sure made for Baldwin and cheaper to just make what you need...but it's struck me as really weird they were going into a 16ohm speaker load and giving the 6l6's twice the spec in reflected impedance...but I guess to your point a range will work.

    Truth be told I have kept to small bottle amps for a few years now just based on where my ears are now...just can't take the volume so I don't tempt myself...but I didn't think about the power scaling option. I look into that, and if I stick with 6v6 I'll just use a 16 ohm speaker so they are getting closer to what they want to see from the OT.

  4. #4
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    Quote Originally Posted by Denali View Post
    Hey Greg, thanks that's helpful.

    I was actually just earlier today thinking along similar lines...not necessarily sticking with 6l6 but splitting the difference and maybe using 5881's or JJ6v6s since I have a set of each hanging around. This OT only has one tap, which makes sense I guess since it was dedicated use and I'm sure made for Baldwin and cheaper to just make what you need...but it's struck me as really weird they were going into a 16ohm speaker load and giving the 6l6's twice the spec in reflected impedance...but I guess to your point a range will work.

    Truth be told I have kept to small bottle amps for a few years now just based on where my ears are now...just can't take the volume so I don't tempt myself...but I didn't think about the power scaling option. I look into that, and if I stick with 6v6 I'll just use a 16 ohm speaker so they are getting closer to what they want to see from the OT.
    That amp was designed to work as it was, so you can bet that it was working in its correct range for 6L6's with the 16 ohm tap. Was it cathode biased? If an amp is cathode biased the proper match for primary impedance usually goes up a bit and an 8k load isn't too far off from ideal depending on the voltages. There really is a fairly wide range that will work as far as primary impedance matches and different tube types, meaning that the different impedances will work with power out, distortion/overdrive and the tonal spectrum changing. I used to run a Silvertone 1484 into an 8 ohm speaker. The OT tap being used was a 4 ohm tap and on that amp with that super small transformer it compressed a lot and the top and bottom frequencies kind of got muted and you were left with a perfect lead tone, though it didn't clean up well. Go the other way with a 2.6 ohm load on the 4 ohm tap and it got a lot cleaner and harder to distort. In both cases the power was lower than being at 4 ohms on tap and speaker.

    I'd seriously consider the power scaling, from London Power, not VVT or something. Talk to Kevin O 'Connor about it and he can give you more details since he came up with the idea, but since I put it into an amp I plan to use it on all higher power amps I make....it is really that useful. It does take a little bit of dialing in on each amp you put it into but once you have that squared away it is great. There is no substitute to the big amp sound either....your cleans are nicer and the amp just sounds bigger...and when you can get that at a lower volume too....it works out nicely.

    Greg

  5. #5
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    Exclamation Baldwin Power Amp Consolidation

    This post links to the few Baldwin organ power amp threads that I've found on this forum. In no particular order.
    Baldwin organ tube amp
    Baldwin Vintage Amp conversion
    1958 Baldwin Orgasonic amp project

    But don't tell this guy I've killed another of his pets!
    The electric guitars nearest kin requests you stop killing off our family members:)

    I recently picked up a power amp from a 51P (?) Baldwin, and took some measurements to confirm what others have said.
    Power transformer (line voltage 120vac at time of reading)
    2ndy HV 360-0-360 (wire color was very dingy but might have been red at one time) center tap wire striped red/yellow (maybe). I expect this to be nominally 350-0-350?
    5v heater on dingy (originally yellow) pair. voltage read 5.8vac
    6.3 heaters on brown pair. voltage read 6.6vac
    12.6 heaters on black (possibly VERY DARK green?) pair. voltage read 13.2vac

    OT with Black/Red/Green primary, Yellow/Black secondary. Using a 12.6vac filament PT (reading 14.02vac under load from the OT) I got 0.716vac on the secondary.
    Using E squared over R power law -> (14.02)^2/Z1 = (.716)^2/Z2 gives me Z1 = Z2 * (14.02/7.16)^2 or ~ Z2 * 383.4, so at 16 Ohm load the OT is about 6.135k (call it 6k?)

    I went so far as to trace out the power tube and driver circuit. I recognized the PI as a paraphase, having stumbled across a few designs that use it; only a couple of weeks ago! I think I'll leave the sockets and tag strips in place, only converting the PI to LTP for a more grind-worthy power section. The filter cans will go, but anything else riveted in - even the weird proprietary jacks - will stay in place. I'm lazy like that. I'll rebuild the PSU filter and bias circuitry, may re-use some of the extremely heavy-duty-looking Mallory wirewound resistors in the process. This will be my first grounded cathode power amp. Definitely modeled after something simple enough to use the tag strips as they are. The multitude of accessory circuitry is being yanked, and without an idea of what it feeds or feeds it (via the weird proprietary jacks) I didn't bother to trace it out before snipping the leads. This leaves more than enough room for a guitar amp pre.

    I'd like to figure out how to guesstimate the inductance of the choke, although it's a moot point. It must work OK for this amp. right? The other two wound-core devices are interesting too, but probably have no place in a guitar amp. One is a tranny-looking thing that has three wires. Mine must be broken, because two wires show a path (less than infinite resistance), and the third shows open to either wire and also open to the case. This may have been why the amp was sold on eek!bay? The other transformer is a little 2:1 gizmo who's secondary ran from the plate of one preamp tube to its own grid. Something to learn about another day.

    I hope some of this info can help others who have run across the Baldwin amps.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  6. #6
    Supporting Member eschertron's Avatar
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    I tested the choke from the 51P today to measure XL and calculate inductance. It came out around 7.6H, reasonable I guess. As noted by others, the choke fed the screens of the 6L6s and all downstream tubes, potentially 60 or so 12A_7 types.
    There was also an autotransformer in the chassis. This had one lead open to the other connections. It is now sitting in a jar with some denatured alcohol to dissolve the shellac on the case and laminations. I checked that alcohol did indeed have an effect on the lac before committing a jar-full! Once I have the transformer apart I can check to see where exactly the winding opened. I really have no plans to try to fix/reuse the transformer lams or bobbin, but you never know... maybe after a few more projects, including making a pickup winder from the old sewing machine also stored in the basement!
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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