Need help PT question Please.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jh45gun, Feb 23, 2008.

  1. jh45gun

    jh45gun Member

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    OK here is the deal and I am not pointing fingers as this is a learning process for me and I should have checked out the schematic better before I bought this. I was told this was a 4 6v6 amp (assumed to be quad) which should be 40 watts or there abouts, well it is not since the circuit is not a quad circuit but a double 6v6 push pull for not true stereo but a bass and treble channel amp arrangement. Now the Power Tranny is labeled on the schematic 360V 0 360V is this enough to push 6l6's if I buy a higher wattage output transformer rated say at 50 watts? I do not know the wattage of the output transformers on the chassis now but since they are for 6v6 tubes from now what I have learned know they probably cannot work with 6l6's as I would think the wattage is not enough. So can anyone help with this? I guess if not I bought an expensive chassis to strip and I will have to buy both trannies to build a 6l6 amp but I am sure hopeing that the PT will work. Help Please. Jim
     
  2. VacuumVoodoo

    VacuumVoodoo Member

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    A little more info please:

    Are both output transformers same size?
    What rectification is there: single or double tube (one per channel), or silicon?

    I understand you have the schematic - can you post a scan?
    A couple pics would be nice - especially PT with tubes showing for size reference.

    I'm asking because if the treble channel is identical to bass (on some of those systems treble is 1/4 power of bass) then the PT is probably ok to run 2x6L6 at about 430-440V for about 35-40W output.
     
  3. jh45gun

    jh45gun Member

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    Alex any help or advice you can give me I would appreciate. There is only one Rectifer a 5u4. The bass OT is bigger.

    OK here is what I got: First the Schematic:

    http://www.captain-foldback.com/Hammond_sub/schematics/JR20_early.gif

    [​IMG]

    Bass OT is on the right side first the PT then two chokes then the OT. That can on the extreme right is a cap can. The bass OT is phsically bigger than the Treble OT but then the Bass OT is a closed design while the treble ot is open design if that makes any difference?

    Treble ot on the underside of the chassis:


    [​IMG]

    Underside if the Bass OT
    [​IMG]

    Anyone else wants to add comments please do.
     
  4. VacuumVoodoo

    VacuumVoodoo Member

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    Ok. Both channels are identical but:

    1. Treble amps OT is smaller because it doesn't have to deal with low frequencies
    2. With Bass channel at 20W output rating Treble channel will never have to deliver more than 2-3W but with 20W capability you can listen to crash cymbals at deafening volume.
    3. The amps use separate secondary windings on OTs for cathode feedback - something seen in many audiophile amps of the era (Macintosh e.al), I'd like to try it in a guitar amp.

    The whole thing is probably rated at 25-30W total output, bearing in mind it's from a Hammond organ it was designed for continuous duty at high output levels which is a good thing for your intended evil deed.

    PT & rectification:

    This uses a choke input ripple filter, the advantage here is that it can deliver more current than a capacitor input filter (which what we see in 99.99% of guitar amps) but at a lower voltage. Here you're getting 285V B+ whereas with capacitor input filter it will jump to ca 400-420.
    This should be enough to run a pair of 6L6 to 30-35W with a 5A4R recto but I'd probably go silicon to save 10VA of power burned by recto tube heater.

    No need to use two chokes but it doesn't hurt to keep them as they're already there.

    You'll need to arrange a bias supply for the 6L6 in the range -40 to -55V, as there's no separate winding it will have to come off the HV. Just copy the standard Marshall o similar scheme.

    That big paper in oil cap can has in all probability dried out but with a bit of cleaning and polishing it may be a display piece.

    I'd give it a try, if you smoke the PT there are many other to replace it with.
    But first replace the cap can with electrolytics and try to run the bass amp only with some kind of preamp and give it a listen. If you can run this thing dimed with a meaty OD pedal without it giving up the blue smoke the signs are good.

    Now, why did I say "eveil deed"? Personally I'd renovate it, perhaps find a twin and bi-amp my JBL L65 stereo speakers. Maybe these are abundant in USA, over here certainly not.

    Keep us updated on the progress, don't experiment without properly fusing the PT first.
     
  5. jh45gun

    jh45gun Member

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    Thanks for the info Glad to hear that PT will work and maybe the bass OT as well. One Problem with the Bass and the treble OT is I have no idea what the impedance rateing is. I am guessing 4 ohms on the bass as it had 4 speakers hooked up in parallel which if they were 4 16's would equal 4 but I have no way of knowing for sure at this point. What I would do is buy a pre loaded turret board for a 5D8 Twin or a Bassman or Bandmaster ect. That would have the proper Filter caps and be all set to go to wire up to the tube sockets. The preamp section and PI section would be of course 12ax7s or 12ay7s depending on what I decide to build. I would take out 3 of the octal sockets and replace them with the 9 pin novel sockets. Then I would wire everything up according to the layout prints online. I do not understand what you mean by the bias remark about separate windings I am a rookie at this so are you telling me that the OT has something to do with the Bias setting? I thought that was done with resistors? Since most of the Fender Tweed amps are fixed bias or at least the ones I would be makeing I was thinking the bias setting is already on the board with the proper resistors.
     
  6. jh45gun

    jh45gun Member

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    As far as the PT goes I have to add a 3 prong power cord and a fuse like you suggested plus a Power switch. Now it is wired to a plug for a 5 prong wire set up that I suppose went to the organ for the main power.
     
  7. jh45gun

    jh45gun Member

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    Alex I am not sure that you understood I do not want to modify this amps circuit but to use the PT and OT for a complete rebuild using a pre loaded Turret board for a Tweed amp clone. What I would want to build Alex would be either a 5d8 Twin or a 5d4 super or a 5d5 pro using the infor at Fender Amp field guide link. http://www.ampwares.com/ffg/ I would clean up the tube sockets and replace 3 of the octal ones with noval ones for the 12ax7 or 12ay7 tubes. I would then wire up the tube filament wires better as these just run the old style with out being twisted and then add the board and wire to the sockets. I have been working on the chassis it is now gutted except for the PT and Bass OT and tube sockets. I will then add stuff to the chassis like jacks and pots and all the other stuff I need to make an amp out of this chassis.
     
  8. jh45gun

    jh45gun Member

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    OK Alex, I have a friend who is going to build a 5e3 so I am sending him the OT's and I am going to buy or find another
    I know will handle the 6l6's So the chassis is stripped except for the tube sockets and the PT. I will go with SS rectification to keep the V/A up like you suggested.
     

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