Introduction & Thomas Organ Amp Question

[ThomasTube74]

Hello everyone,  I'm Gary, longtime lurker, first time poster.
I recently re-aquired an amp from my past.  Back in the early 2000's, I had an old Thomas organ in my apartment/my band's rehearsal space.  It functioned perfectly & I tracked with it now & then. In 2003, my main guitar amp (a 74 Super Reverb 4x10) went down & took weeks to repair.  In the meantime,  I plugged into the Thomas amp (via a 1/4" to RCA adaptor) and used a tube screamer as a preamp & it sounded glorious!  After I got my main amp back and moved, the Thomas went into a band mates storage unit.  I ran into him the other day & apparently he just pulled the amp & tossed the rest.
Luckily, he still had the amp & returned it.  I have a 2x12 cabinet I'm dying to power with this.  I rewired with a 3 prong power cable years ago.  I only plan to replace the RCA input with a 1/4" instrument jack & add an 8 ohm output.  Problem is I have no idea what each of the holes in the 10/12 pin socket on the end correspond to.  Haven't been able to find a schematic.   Any help would be greatly appreciated!  Everyone pointed me in the direction of this board, said if any knows, it's a member here!
Thanks in advance!

[bmccowan]

I believe that rectangular socket is a connector for the control panel. Check out this thread https://el34world.com/Forum/index.php?topic=25376.0 you might want to ring up 1blueheron. Its not the same amp, but I bet with the exception of the power tubes, its pretty similar.
You will likely need to do more than you suggest to get a satisfactory guitar amp.

[ThomasTube74]

Yep, back when it was complete, that socket had wires that fed to the upper part with more tube's (I guess a tone generator) a volume pedal, and a speaker.
I wouldn't be confident about using it without modification if I hadn't used it in the past.  It sounded very solid as is, and underneath everything seems to have held up well.  I just wish I had the opportunity to trace the leads from that connector before my buddy tossed the organ!

[ThomasTube74]

Oh, and I did send a dm to 1blueheron.  When you search Google for anything concerning a thomas organ to guitar amp mod, his posts & replies are right at the top!

[ThomasTube74]

Here's some pics of the business end...

[ThomasTube74]

It's 2x 6V6GT, 1x 12AX7, 1x 12AT7, 1x 12AU7, 1x OA2, 1x 5Y3GT

[SoundmasterG]

It's 2x 6V6GT, 1x 12AX7, 1x 12AT7, 1x 12AU7, 1x OA2, 1x 5Y3GT

I think that anything that was connected to that rectangular connector is going to or coming from some other chassis that was in the organ. I also think that this amp will need all caps replaced, and likely many of the resistors also since you would want to tailor the values to whatever circuit you want to build. So due to that, I think that you can probably just build it the way you want and disconnect all of the wires from that connector. You have a power transformer, and output transformer, sockets (should replace these too as those don't look that great) ....basically you have all you need to be able to gut it and start over. I would not include the OA2 if it was mine. That is a regulator tube and works to keep the power supply voltages stable, but you generally want the power supply to sag in a guitar amp under heavy load, so having that in there is counterintuitive. You could go with 2x6V6 for output, 2x12AX7 for preamp, 1x12AT7 for a phase inverter, and 1x5y3 for rectifier and have a decent 12-15W amp depending on the voltages.


I have a similar Thomas organ chassis that used a pair of 6973 power tubes. I haven't got around to rebuilding it yet but likely I would gut it and start over.


Greg

[ThomasTube74]

I hear ya.  I was thinking that would probably be a safer bet for a good outcome myself.  Thanks for the recommendations.  I'd love to build something like you suggested.   I also have a spring reverb unit I'd love to incorporate.

[ThomasTube74]

Soundmaster G, do you have one of the Fender layouts, or otherwise, that you'd recommend?

[AlNewman]

If you can't find the original organ schematic, you'll have to draw your own based on what you have.  Then fill in the dots.

[SoundmasterG]

Soundmaster G, do you have one of the Fender layouts, or otherwise, that you'd recommend?

What circuit you decide to build needs to be based on your transformer set and voltages, and tube complement. I don't think you know what the voltages in your amp are yet correct? If it will power up at all under a current limiter then you can get an approximation of what the voltages are, and that will help to inform what circuits might be good to use. Any of the 2x6V6 and 5Y3 circuits might be a good guess, and if your voltages were higher you could always use the voltages you have and change the dropping resistors between stages to get the phase inverter and preamp down to voltages closer to an pre-existing Fender circuit. If I was in your place I would find out what the voltages are in your amp first, and then use other examples as a starting point, but design my own circuit to use based on those examples. A 5E3 Tweed Deluxe is a good starting point, and uses two 6V6, a 5Y3, and similar preamp tube complement. The preamp and phase inverter tubes you can change up as most of the common types interchange and have similar filament current draw and pinouts, so I wouldn't worry too much about that part of it. You could change those later or now. The 5E3 is a GREAT sounding amp and many people have built clones so there is lots of help available. They aren't too hard to build either assuming you have a basic skill level. I happen to have a real 1956 Tweed Deluxe so I'm partial to those amps, but the tubes used and voltages are probably similar to what your Thomas Organ amp has now. You do need to find out your existing voltages though to get started.

Greg

[mresistor]

If your organ was similar to one from 59-60 that used 6973 9pin pentodes then this hand drawn schematic may be similar
Also here is the thread where I saw this  it also has pictures of some of the organ parts. https://audiokarma.org/forums/index.php?threads/thomas-organ-6973-amp-schematic-needed.336049/

[dogburn]

It might help if you knew the model number of the organ or amp - anything printed on the chassis? Also, any codes on the transformers? Sometimes the specs can be tracked down from those.

[ThomasTube74]

It might help if you knew the model number of the organ or amp - anything printed on the chassis? Also, any codes on the transformers? Sometimes the specs can be tracked down from those.

Thanks so much to everyone for the tips.  I'm trying to track down some old pics of the organ from around 2001, when it was all in one piece.  I'll keep y'all posted.
As far as the voltages, I'll see if it powers on & try to get some readings.
Thanks again guys, everyone recommended this board & said y'all would be helpful & not judgy if I wasn't a seasoned pro, and I see why!

[ThomasTube74]

It might help if you knew the model number of the organ or amp - anything printed on the chassis? Also, any codes on the transformers? Sometimes the specs can be tracked down from those.

The code on the main transformer is: 
B-50068 606641
Output transformer:
B-50069 606647

[mresistor]

It might help if you knew the model number of the organ or amp - anything printed on the chassis? Also, any codes on the transformers? Sometimes the specs can be tracked down from those.

The code on the main transformer is: 
B-50068 606641    Schumacher 41st week of either 66 or 76
Output transformer:
B-50069 606647    Schumacher 47th week of wither 66 or 76

[mresistor]

Look at the photos of organs on this page   do any look familiar ?


http://theatreorgans.com/hammond/keng/kenhtml/Thomas%20Organs%20Page.htm

[ThomasTube74]

Look at the photos of organs on this page   do any look familiar ?


http://theatreorgans.com/hammond/keng/kenhtml/Thomas%20Organs%20Page.htm

Unfortunately no.  The closest it would be is the Silvertone.  It had no drawbars or flip switches, just a row of a few knobs left of the bottom keyboard.  I've looked far & wide and can't find a photo that corresponds.
On another note, I went ahead & cleaned out the chassis, now I just have the parts.  Thinking of going with a 5E3, something relatively easy for my first full build.

[mresistor]

The 12AT7 is no good. I'd toss it as it's lost vacuum.

[ThomasTube74]

The 12AT7 is no good. I'd toss it as it's lost vacuum.

Yep, I was wondering what the white around the top was about.  No problem,  I have plenty of backups I got recently with an old Eico amp.

[SoundmasterG]

Look at the photos of organs on this page   do any look familiar ?


http://theatreorgans.com/hammond/keng/kenhtml/Thomas%20Organs%20Page.htm

Unfortunately no.  The closest it would be is the Silvertone.  It had no drawbars or flip switches, just a row of a few knobs left of the bottom keyboard.  I've looked far & wide and can't find a photo that corresponds.
On another note, I went ahead & cleaned out the chassis, now I just have the parts.  Thinking of going with a 5E3, something relatively easy for my first full build.

Are you saying that you gutted the chassis already? If so you can't power it on now. :)


You can check the transformer windings though and find out the specs of the transformer. See this video I linked below. There are others on youtube that tell you how to do this also. Once you know the voltages the transformer supplies, then that will inform you on a suitable circuit to build if you are still wanting to use a Fender as a starting point.


Greg




https://www.youtube.com/watch?v=o_NAVysrJn4&t=7s

[ThomasTube74]

Look at the photos of organs on this page   do any look familiar ?


http://theatreorgans.com/hammond/keng/kenhtml/Thomas%20Organs%20Page.htm

Unfortunately no.  The closest it would be is the Silvertone.  It had no drawbars or flip switches, just a row of a few knobs left of the bottom keyboard.  I've looked far & wide and can't find a photo that corresponds.
On another note, I went ahead & cleaned out the chassis, now I just have the parts.  Thinking of going with a 5E3, something relatively easy for my first full build.

Are you saying that you gutted the chassis already? If so you can't power it on now. :)


You can check the transformer windings though and find out the specs of the transformer. See this video I linked below. There are others on youtube that tell you how to do this also. Once you know the voltages the transformer supplies, then that will inform you on a suitable circuit to build if you are still wanting to use a Fender as a starting point.


Greg




https://www.youtube.com/watch?v=o_NAVysrJn4&t=7s

Thanks for that link!  I'll be testing the leads on both later today, for now, it looks like this is what I'm working with on the main transformer: 2 black leads, 2 red/yellow, 1 yellow/red, 2 green, 1 green/yellow & 2 brown.
On the output transformer I've got 2 black, 1 brown. 1 yellow, 1 green.
I'll update after I've tested.
Thanks again for all the assistance.

[ThomasTube74]

Greg (SoundmasterG)
I tested the main. Here's what I got:
Red/yel: 735v
Red/yel to yel/red: 366v
Grn: 7.35v
Grn to grn/yel: 3.66v
Brn: 5.89v

Update soon with output transformer
EDIT:  Not really sure how to go about getting readings from the Output transformer,  any help would be appreciated.

[SoundmasterG]

Greg (SoundmasterG)
I tested the main. Here's what I got:
Red/yel: 735v
Red/yel to yel/red: 366v
Grn: 7.35v
Grn to grn/yel: 3.66v
Brn: 5.89v

Update soon with output transformer
EDIT:  Not really sure how to go about getting readings from the Output transformer,  any help would be appreciated.

The green and green/yellow are likely 6.3V filament and filament center tap. Brown is likely the 5V filament. Red/yellow and yellow/red are likely the primary windings and center tap, and the transformer was probably a 350-0-350 back in the day with the lower mains voltages. With higher mains today then it goes to the 366V AC. You didn't measure your wall AC voltage but I am guessing it is around 125V or so when you made those readings. The multiplier for a 5Y3 is around 1.2 for an unloaded amp. So for 366V AC, times 1.2, you get 439.2V DC, and that will likely drop to around 400V for when it is under a load. So you can assume around 400V DC will be on your power tubes. That is a little high for 6V6 unless you are using JJ 6V6, but it is doable as long as you bias appropriately. That's a bit higher than the 5E3 Fender, but you could make it work like a 5E3 by using higher value dropping resistors in the amp to drop the voltages to the phase inverter, the preamp, etc., The power section would have higher voltages so you would have a bit more power than a stock 5E3 and that is fine.


To get readings from the output transformer, you are more concerned with what the impedance is from the transformer. If you have a known lead on the transformer such as an 8 ohm tap on the secondary, then you can feed a small voltage in and figure out what the primary leads are and approximately what the primary impedance is. But you already know that the transformer is suitable for a pair of 6V6 so there isn't really a need to figure that out. All you need to know is which leads are for the primary, and which ones are for the secondary, and on the secondary, what the impedances are. There are videos and pages out there for how to figure out primary impedance for an output transformer. Do a search and see what you can find.


Greg