JCM800 6V6/6L6/EL34 in a Princeton Reverb Chassis

Discussion in 'Shock Brother's DIY Amps' started by robrob, May 1, 2019.

  1. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    If mine eyes doth not deceive, on RR's design the tubes that would normally go in positions V4 and V7 are "missing" (meaning, "room for more stuff"!?!!!). Just as a thought exercise, one could conceivably add a Princeton-style reverb (1 & 1/2 12AnN tubes), or perhaps trem (1/2 + 1/2 tube 12AnN) to this. Might not be appropriate sonically but there appears to be room to do either, with no chassis drilling (tube hole is already there; one could add another pot to the ground hole in the back).
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    There are extra holes and lots of room in that PR chassis for tinkering.
     
  3. NSB_Chris

    NSB_Chris Tele-Meister

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    Anybody have issues purchasing the PT and OT from Allen Amplification? I emailed David and got no reply. Will try calling tomorrow. I want to get the transformers before sourcing all the other parts.
     
  4. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    I purchased a pair of transformers from him in March. I also emailed him first and perhaps caught him while he at his computer - because he responded in ~ 1 hour. Or maybe it was because it was the weekend - who knows. I asked him if he took online orders, which he does not. He just said to call to discuss what I wanted, which I did. He takes the calls himself - nice guy - very helpful!
     
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  5. NSB_Chris

    NSB_Chris Tele-Meister

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    I called today. Had to leave a message. No call back. No “out of office” message but maybe he is traveling or something. I will try back.
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I think he's a one-man show but his stuff is of very high quality.
     
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  7. NSB_Chris

    NSB_Chris Tele-Meister

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    I think I am going to source Classic Tone transformers. I am going to stay with 6V6 tubes anyway.
    PT: 40-18094 [Bassman 5F6-A]
    OT: 40-18000 [Bassman 5F6-A]
    Choke: 40-18032 [Marshall]
    Might have to make some adjustments on the B+3 dropping resistor.
     
  8. NSB_Chris

    NSB_Chris Tele-Meister

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    Looking into this further, I would benefit from more education/advice on OT selection. If I am correct, the Bassman (40-18000 replacement) used 6L6 and 4.2k ohm impedance. For the 6V6 circuit I have, I need something between 6k - 10k ohm and should shoot for 7k-8k ohm range, correct? So if I use the 40-18000 with an 8 ohm speaker load than I should use the 4 ohm secondary tap, correct? That gives me 8.4k ohm reflected to the primary.
     
    Last edited: Jun 1, 2019
  9. FenderLover

    FenderLover Friend of Leo's

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    That OT has 2, 4, and 8 ohm taps. They all reflect 4.2k to the primary. Using Bassman transformers will work well together, but you will have an overkill situation with 6V6's. Those transformers will be quite large on a Princeton chassis. They are designed to support a 50W amplifier, and you will never get that out of 6V6's. You would also have to use a 8 ohm load with the 4 ohm tap to make it work.

    I'd have a little patience with Mr. Allen and use his transformers, it's just a better idea IMO. Or... if you want the Bassman set and can fit them on your chassis, use 6L6GC's.
     
    Last edited: Jun 1, 2019
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  10. NSB_Chris

    NSB_Chris Tele-Meister

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    I noticed I had typo mistakes on primary side impedances in post #48 so I will correct that.

    Thanks. I am making my own chassis so I can use whatever I need. If I don't use the Allen, then I will probably just use the regular Princeton Reverb OT 40-18045.

    For education purposes, if I used the 40-18000 OT, wouldn't I use an 8 ohm speaker load on the 4 ohm secondary tap to have 8k for the 6V6s on the primary side? I think RR is doing that in the other way on the JCM800 micro. The Allen OT is 7k so when he uses EL34 tubes he connects the 8 ohm speaker load to the 16 ohm secondary tap to have 3.5k for the EL34s. Do I have that right?
     
  11. FenderLover

    FenderLover Friend of Leo's

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    Yup, sorry - fixed that.

    As Rob suggests in switching output taps with different tubes, I think neglects what voltage and current the output runs on. See my post #12. It's a personal choice in what you like to hear, but with the recommended Allen set, the voltage and current (and therefore power) don't differ much at all between 6L6 and 6V6 so changing taps is not really necessary. Try it. Using the Bassman set, yes, there is a difference in voltage current and power.
     
    Last edited: Jun 1, 2019
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  12. NSB_Chris

    NSB_Chris Tele-Meister

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    My Allen transformers are ordered!
    I ordered both the PT and OT transformers although I may still use the upright Bassman PT (40-18094) as an upright might suit this chassis better. A little lower B+ and unnecessary weight increase over the smaller Allen PT. Now to order all the other components.
     
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  13. NSB_Chris

    NSB_Chris Tele-Meister

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    The Allen transformers are in, and I got right up to the point of ordering components, had an organized parts list, and I noticed a fundamental flaw in my planning logic. I will attach the schematics I cobbled together from the RR800V and the 5F6-A Bassman preamp. I usually play with two amps ABY'ed together, so this one would likely be paired with my Deluxe clone. The issue is that the way I have it, switching between the JMP800 preamp section and the 5F6-A preamp section will result in a phase mismatch which will not be good! Crap. Back to the drawing board.

    My thought now is to do one of two things (either way I have to add another tube to the circuit):
    1. Just put both preamp sections in with separate input jacks and mix them in with 270k resistors before the power section. This would have no channel switching in the amp and I would just put a dedicated ABY box with phase inversion capability in front of it.
    Pro: This has the benefit of being able to blend the two channels together and having a much more simple layout and construction.
    Con: The master volume would have to move back into the preamp section and there is no way of having a separate presence setting for each.
    2. Add an extra gain stage on the 5F6-A preamp side. This is a departure from the 5F6-A circuit and might require some fine tuning of the voltage drop between the first and second stage.
    Pro: The master volume can stay after the phase inverter section and have separate presence settings.
    Con: This would be a departure from the 5F6-A preamp and would take some dialing in. With the channel switching it is a pretty complicated build.

    Thoughts?


    Existing circuit to be changed!
    RR2204 & 5F6A Preamp V03.jpg RR2204 & 5F6A Power Amp V01.jpg
     
  14. FenderLover

    FenderLover Friend of Leo's

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    I guess I'd favor option 1, and even simplify the Beast a little more. Channel switching outside the amp automatically gives you separate preamps, EQ and gain with the A-B before two short FX chains feeding the two inputs. That simplifies some of the circuit baggage inside the amp. It really depends how you like to play, and I wouldn't discourage you from blazing your own trail. It's just that I have tried the 'kitchen sink' approach myself, and it seems to take away from the guttural essence of a amplifier. And I like simple construction.

    If you already A-B with two amps, you might review how you use them separately and/or together to determine how much you need here. Maybe you've done that already. I often use two amp too. I'd be curious to know how you use yours. Dual mono, stereo, Lead- Rhythm?
     
  15. symbiotic

    symbiotic TDPRI Member

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    Please pardon this question, it will show my ignorance. That is a crossline master volume in there isnt it, post phase inverter? Won't the DC grid bias voltage make the control scratchy? Maybe I'm wrong, or maybe that is something you're prepared to live with? Asking for my own learning.
     
  16. NSB_Chris

    NSB_Chris Tele-Meister

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    The master volume in my circuit is intended to honor Rob's circuit in the original post. I have never done the post PI Trainwreck Type-3 MV so I can't comment. Others surely will.

    Lately I have been using an ABY going into amps:
    1. a Mesa Mark Five:25 head with a closed back 1X12 (vintage 30)
    2. home grown Deluxe clone head with an open back 1X12 (Weber 12F150)

    I set the Deluxe to the edge of breakup
    The Mesa top "crunch" channel is set to just a tad more breakup than the Deluxe. The second channel of the Mesa I have set for a solo channel with a healthy level of distortion.

    I have a pedal board feeding the ABY but honestly mostly I only use an OCD overdirive with just a little grit. For a cleaner sound I just leave the OCD off and roll back the volume a tad. This setup works really well for most everyting, but falls short when a Marshall distortion sound is needed. Using pedals for that sound just gets lost in the mix. The distortion pedals sound great outside of the mix, but just disappear in the mix or sound "fizzy". There is no substitute for a tube amp with asymmetric clipping. I built a Marshall JMP2204 clone and it was magical, but I just can't bring three amps into my friends studio.

    My thought was to replace the Deluxe with this new amp and the Mesa would be able to be switched in with the Bassman-ish amp side or the Marshall-ish amp side depending on the song. However, if I like it enough, this amp might replace the Mesa and Just have this amp as the rhythm/lead switching amp and keep the Deluxe as the second amp to round out the tone.

    If I go with option 1 on this build I could just get an ABC switcher (with 180 phase and ground lift capability). Pairing two amps, i have to keep phasing in view. Honestly I have no idea how many gain stages the Mesa is using on the clean and dirty channels that I am using. They don't give you much information on that. One benefit of using the amps that I build is I know how many gain stages everything has.
     
  17. NSB_Chris

    NSB_Chris Tele-Meister

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    I went back to the drawing board and am redoing it. I think I will go with option 1 to remove the complexity of the channel switching and to have flexibility on what goes in front of the preamps. The first input stays pretty true to Rob’s circuit except for the master volume and the mixing resistor. I am calling this the RR800V_Plus. I cant say that the second input is a Bassman. It is more of a tamed down and re-voiced Marshall preamp. Interested in advice for component values. In this draft I took out most of the bright caps and am considering putting a 12AY7 to tame the distortion. Thoughts?

    rr800v plus preamp v01.jpg


    rr800v_plus power amp v01.jpg
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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  19. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  20. Wally

    Wally Telefied Ad Free Member

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    Wise words? That link is full of sage explanations of the considerations. My opinion??? If you are going to run this type of circuit hard for extended periods, use speakers that can handle significantly more than the power @5% distortion that the amp produces. I have no idea what power out would be for this amp at that 10% distortion point. Say maybe it is 40 watts. 2x20-25 watt speakers for a 40-50 watt total is borderline imho if a person is going to be pushing the amp hard all of the time. You might never have a problem, or the speakers might live a shortened life. If you don’t push the amp hard, maybe the speakers live forever.
    Another consideration outside of the amp is the use of overdrive/distortion effects. Speakers like to ‘rest’ in between excursions....really small pieces of time are important rest periods, so to speak. When a speaker is asked to reproduce distorted signal, those rest periods between excursions is decreased greatly as I understand it because the distortion keeps the cone at or very near full excursion continuously. Even if the speaker arrangement is rated for much more capability than the amp produces, the w3ar and tear is greater with distorted signals than clean signal...even if the clean signal is at a higher power.
     
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