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Thomas organ AR-1 to 18watt conversion help

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  • Thomas organ AR-1 to 18watt conversion help

    I picked up an AR-1 the other day and plan on gutting it and putting in an 18watt type circuit, as there is a post from a member here, Rwood, stating that has been a successful conversion. As long as I'm looking at the current circuit, before I rip it out, i have a few questions, most of which will be helpful when I convert.
    I can't find a schematic for it anywhere. compliment is 12at7,12ax7, 2x 6gk6, 6ca4

    1. The only out jack is one of those 5 pins found on old PAs as the speaker outlet, but this one is labeled to go to the "tone cabinet". It's odd to me as one of the OT taps is connected to the jack, but so is the other side of the fuse! This is just bugging me, even though I'll just put in a modern speaker jack when I convert. What's is a tone cabinet? Is it an enirly other amp, thus the VAC feeding the jack? This has prevented me from plugging the amp into the wall, bc I don't want to do it without a load, as I'm unsure about the fuse/jack situation
    2 . All this is to say I haven't been able to read voltages. Does anyone have any idea about the voltages for this PT? I'm also concerned with the current draw
    3. Besides the pinout difference, what do I need to do with the 6gk6's to make them work like el84?
    4. I've identified most PT and OT taps, but I'm not sure what the OT tap is that is connected(or will be) to speaker jack. There is only 1 that could be 4 or 8 ohms I'm assuming. Any idea?
    5. For PT, there are two blue wires that go to diodes, then across a huge cap, then to a single yellow PT tap, as well as to the grids of power tubes. Any idea what these are? This amp obviously has a tube rectifier

    Any help is appreciated.

  • #2
    Go to the Ceriatone (CeriaTone.Com - DIY Guitar Tube Amp) website. There are lots of board schematic and layout diagrams there. If it were me I would completely gut the thing and start over. If the amp is functional now, measure the B+ first. If it seems low for an 18 watt, you can always use a full wave bridge rectifier to bring it up a bit. IMHO, there isn't a noticeable difference between an EZ81 and an SS rectifier in these amps. I would replace all of your tube sockets with fresh new ceramics. Be careful of your lead dress and make your filament runs tightly twisted pairs along the edge of the chassis if possible. Use different colored wire following a normal convention for the different circuits. I would probably make an 18 watt Lite with the PT you have unless it powered other peripheral tube circuits. Also using an LED pilot lamp instead of a filament lamp will free up some heater current. Copy the ground scheme on the layout diagram. You don't need to reinvent the wheel. Use fresh new or NOS 12ax7 and EL84 tubes. I find JJs to work well in these amps with a NOS tube in V1.

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    • #3
      Originally posted by olddawg View Post
      Go to the Ceriatone (CeriaTone.Com - DIY Guitar Tube Amp) website. There are lots of board schematic and layout diagrams there. If it were me I would completely gut the thing and start over. If the amp is functional now, measure the B+ first. If it seems low for an 18 watt, you can always use a full wave bridge rectifier to bring it up a bit. IMHO, there isn't a noticeable difference between an EZ81 and an SS rectifier in these amps. I would replace all of your tube sockets with fresh new ceramics. Be careful of your lead dress and make your filament runs tightly twisted pairs along the edge of the chassis if possible. Use different colored wire following a normal convention for the different circuits. I would probably make an 18 watt Lite with the PT you have unless it powered other peripheral tube circuits. Also using an LED pilot lamp instead of a filament lamp will free up some heater current. Copy the ground scheme on the layout diagram. You don't need to reinvent the wheel. Use fresh new or NOS 12ax7 and EL84 tubes. I find JJs to work well in these amps with a NOS tube in V1.
      Thanks olddawg. I am totally planning on gutting it, and doing the 18watt lite iib. I'm a comparatively inexperienced builder, so I wanted to take the opportunity to just kinda get to know this stock organ circuit, if for no other reason than for learning. Kinda a 2 amps for 1 deal with this build. Anyways on with the 18watt.....
      I obtained a schematic for the organ amp: http://www.whiteghostshivers.com/ima...0ar1%20001.jpg
      This cleared up my confusion about the PT. It is 250-0-250. I don't feel comfortable turning the amp on, as the only out is the tone cabinet jack, and there is AC wired to it as well. Nonetheless, I see most folks using 290-0-290 for 18watt builds. However, the PT looks to put out B+ of 340VDC at the rectifier, which is very close to others' builds that I can find on the net. Also, it seems the two PT blue wires and yellow wire is part of a 25-0-25 bias network for fixed biased power tubes, so those will get capped for the 18watt build in favor of traditional cathode bias.
      The OT singular speaker tap is 8ohms, too. For the record, PT is 80-5018-6, OT is 80-5031-1, Schumachers

      Should I just go for it(18watt lite iib) with the current 250-0-250 PT, and adjust voltages at that point, if that's gonna be possible, or even necessary? I'd like to stick with the current tube compliment. I'd just like to make work what I have.

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      • #4
        As I said before. Using a diode rectifier instead of a tube, which a lot of the 18 Watt Lite projects do, will raise the B+. You can put it all on the same turret board along with the filter caps with modern electrolytic caps. Especially for the Lite. I don't know if 18watt.com is taking new members, but there is a lot of info there and some conversions as well. If the B+ is a little low you will get 15 or so watts instead of 18-20. Still a nice little amp. You could also consider a Vox AC15. You have the expensive parts. Don't recycle any components or tube sockets. You are just asking for headaches.

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        • #5
          Ok thanks again. i was actually thinking about doing a spitfire too....maybe another build.

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          • #6
            Originally posted by Smokebreak View Post
            I obtained a schematic for the organ amp: http://www.whiteghostshivers.com/ima...0ar1%20001.jpg
            I don't feel comfortable turning the amp on, as the only out is the tone cabinet jack, and there is AC wired to it as well.
            That 'Tone Cabinet' socket is an extension of the circuit.
            It does not need to be hooked up to anything.
            Follow the wiring for the socket.
            All of the pins are simply duplicates.
            Vac, ground & speaker.
            Cool that you found the schematic.

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            • #7
              I have this same amp and I would like to keep the tremolo circuit but I do not have the tremolo controls. I can see where they were connected on either side of R711 but I do not know what the organ had for tremolo control. Can anyone tell from the schematic what I would need to do with these two leads to operate the tremolo?

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              • #8
                Originally posted by scrimpus View Post
                I have this same amp and I would like to keep the tremolo circuit but I do not have the tremolo controls. I can see where they were connected on either side of R711 but I do not know what the organ had for tremolo control. Can anyone tell from the schematic what I would need to do with these two leads to operate the tremolo?
                Yea scrimpus, I dug up the entire schem : http://www.whiteghostshivers.com/ima...kebreak/thomas amp.pdf
                You can see on one of the pages that the ends of R711 goto terminals 1 and 4, which go to two switches...the on/off which effectively places a short across R711(removing Rk and any voltage drop across it, disabling electron flow in the tube?), and then a "full vibrato" switch that shorts R641. That looks like a set value for "intensity", as R705 looks like factory set "speed". Looks like you could play around with that, or stick in a pot as a variable resistor to control that. That said, I am far from an expert on vibrato circuits..
                I've rebuilt my 18w IIb 3 times now inside this chassis and finally settled on the cascade/parallel mod...never quite could get the last of the fizz out. It's a pretty loud amp despite the puny OT. Good luck!

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                • #9
                  That link doesn't seem work, could you post or send me the complete schematic please? I can I PM you my email?
                  Last edited by scrimpus; 05-24-2013, 12:26 AM.

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                  • #10
                    Originally posted by scrimpus View Post
                    That link doesn't seem work, could you post or send me the complete schematic please? I can I PM you my email?
                    Sure, but here it is : my bad..
                    http://www.whiteghostshivers.com/ima...omas%20amp.pdf

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                    • #11
                      Thanks, that's just what I needed.

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