6V6 2204...

Discussion in 'Shock Brother's DIY Amps' started by Che_Guitarra, Dec 5, 2017.

  1. Che_Guitarra

    Che_Guitarra Tele-Holic

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    ... Has begun. I've moved away from my original idea of a 5F6A front end grafted onto a Deluxe power section, as my previous thread suggested. The new plan is a JCM800/2204 preamp, EQ and PI, grafted onto some kind solid state rectified 6V6 power section, specs of which are yet to be set in stone.

    [​IMG]

    Knowns at this stage:
    - 2204 front end.
    - JJ 6V6 power section.
    - Classictone Deluxe Reverb trafo set: 40-18017 (PT), 40-18087 (OT), 40-18040 (choke).
    - 16" long chassis.

    Unknowns at this stage:
    - everything else!
    - i'm 90% set on SS rectification, but I haven't shut the door totally on going the GZ34 route.

    Lots of concepts to master that i'm yet to attempt in an amp build:
    - zener voltage dropping.
    - ss rectification of 6V6s.
    - modifying a 50W SS B+ rail to suit operation @ 20W (filter capacitance changes, optimization for 6V6s, etc).


    Anyway, i'll update as I make progress. Hopefully the good folks of TPDRI can help me as I plod along. I expect i'll have many, many questions.
     
  2. Mart the Hat

    Mart the Hat Tele-Holic

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    Mark Huss' 6V6 Plexi schematic could be a useful reference in adapting a Marshall circuit to 6v6. The first two preamp stages are arranged differently, but from V2 onwards it's not far different from a 2204.
    http://mhuss.com/Plexi6V6/Plexi6V6c.gif

    I built an amp along similar lines this year, using a transformer set made for an 18 watt. My power amp ended up very close to Huss', though I used 47u filter caps in the first two positions, no dropping resistor before the choke, and a little more negative feedback.
    http://www.tdpri.com/threads/6v6-plexi.664113/
    My amp has the plates at around 410 volts with SS rectification without using anything to drop the B+, but I can imagine you might need to do that with a Deluxe Reverb transformer with SS diodes.
     
    Last edited: Dec 5, 2017
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  3. Phrygian77

    Phrygian77 Tele-Afflicted

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    With that PT and a SS rectifier, I'm betting your B+ is going to be above 450VDC. I got 422VDC out of my Classictone PR PT (630 @ 100mA) with a GZ34 and a pair of JJ 6V6S biased at 64%.
     
    Last edited: Dec 5, 2017
  4. Che_Guitarra

    Che_Guitarra Tele-Holic

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    I guess the ideal (B+) for this project would be an 18W Marshall style PT. But by the time it lands on my door in Australia, it will have cost me $250.

    If I opt for GZ34 implementation, and throw a 5751 in V1, I suspect i'll end up with something akin to a Marshall Astoria. That's not a bad thing. But not entirely my goal either.

    My preference is to stick with my SS rectification, because I don't like sag at all... well, I don't like the type of sag and wooly bass incumbent within vintage Fender circuits.

    But if tube rectification is a better option than any type of SS B+ voltage dropping alternative (and associated resistor heat radiation, zener noise issues, faux sag due to dropping resistors, or $250 outlay, etc), then a GZ34 it might as well be.
     
  5. Mart the Hat

    Mart the Hat Tele-Holic

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    I suspect the low-end character of a Deluxe Reverb may be more down to the preamp voicing than the rectifier. The 2204 does several things to trim the amount of low bass, so it would have a good chance of avoiding wooliness.
     
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  6. Wyatt

    Wyatt Tele-Afflicted

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    Looks like you already had the transformer, the GZ34 may be the best bet. Classictone did make a few more versatile Deluxe PT's with multiple HT taps.

    Ironically, there used to be quite a few JMP PT's available with low-voltage taps to get a plate voltage around 370-375V, like real late '70's Marshalls and the Germino LV. I think Marstan, Metro and a few others used to offer them. I guess there are some stand-up options.
     
    Last edited: Dec 5, 2017
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  7. Fiat_cc

    Fiat_cc Tele-Holic

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    Hammond transformers are available on au.mouser.com

    There are 18 watt style PT’s amongst pretty much all of their guitar amp line. Look in the Hammond website, then search Mouser by model number. Most orders over $60 get free freight unless they’re heavy, but I ordered an AC30 PT and OT in one order and hit free shipping.
     
  8. Che_Guitarra

    Che_Guitarra Tele-Holic

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    I'm going to go the GZ34 route. Not a huge fan of rectifier sag, but the reality is I don't anticipate i'll be running this amp dimed @ max gain, except for kicks on rare occasions, so sag levels should be reasonably in check.

    So that will mean, for ease of splicing the circuit: 2204 preamp and EQ, Deluxe PI and power section. Pretty straight forward - i'm open to better suggestions.

    The biggest challenge will be correcting the B+ feed. I'll need to recalculate the dropping resistor scheme (green), insert a choke (blue). Filter capacitance (red)... now this is something I need assistance with. Massive filter capacitance levels on a 2204, minimum filter capacitance levels on a deluxe... i'm not sure how to resolve these differences to best outcome... i'm very much hoping for suggestions. 40/20/20/20 indicative values only.

    [​IMG]
     
  9. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Scratch that last schematic - it was a rushed cut and paste, an ugly layout, and missing a few elements.


    This one will be my running schematic - the same 2204 front end grafted onto a true DR power section (as opposed to a 6G3 power section).

    Pink dots = components knowingly needing value correction... i'm all ears if you have suggestions. If you have advice in regards to other elements around the circuit, i'm very much open to those suggestions too.

    [​IMG]
     
  10. radiocaster

    radiocaster Poster Extraordinaire

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    To calculate the voltage drop across the unlabeled resistor, I think you have to measure the current before it.

    I seriously doubt you're going to be able to get 415V/420V for the power section out of that 330V transformer without a solid-state rectifier though. But I'm not experienced so I may not know what I'm talking about.
     
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  11. tubeswell

    tubeswell Friend of Leo's

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    Each 6V6 screen draws around 2mA at idle

    Each 12AX7 triode draws around 1mA at idle (so 2mA per bottle)

    Knowing this, you can work out the required DC choke rating and the voltage drop across any given resistance in the power supply rail.

    For an amp with 2 x 6V6s and 3 x 12AX7s, a 50mA rated choke is ample for filtering the screen supply node.

    FWIW, if using a 12AX7 in the LTP position, you will want an 820R to 1k bias resistor (not 470R), and somewhere between 6k8 and 10k of tail resistance will give maximum gain.

    Check out this link http://www.valvewizard.co.uk/smoothing.html
     
    Last edited: Dec 12, 2017
  12. Old Tele man

    Old Tele man Friend of Leo's

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    Know thy "power supply voltage ladder"
     
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  13. Che_Guitarra

    Che_Guitarra Tele-Holic

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    It's funny you should say that. I've also been told the PT will be far, far too hot for my plans. I'm just a novice tinkerer - Classictone's spec sheet suggest i'll be getting 415V with a GZ34... i'll build and hope they're right.

    Won't need much dropping if that is the case, considering i'm chasing 334V at the preamp.


    I'd say it's 99% likely i'll be using a 12AX7 in V3.

    By 'maximum gain' do you mean overdrive, or output level? Probably a moot question: being a 2204, it's probably one and the same.
     
    Last edited: Dec 13, 2017
  14. radiocaster

    radiocaster Poster Extraordinaire

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    Supposedly it's VACx1.1=VDC for valve rectifiers, at least that's what I read on this forum.
     
  15. clintj

    clintj Friend of Leo's

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    It's a fairly good rule of thumb, but if you know the HT winding voltage and anticipated current draw you can get a more accurate number off the datasheet graphs. 50ma is on the lighter side of loading for a GZ34, and it'll run closer to 1.25 to 1.3 * HT, for example.
     
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  16. tubeswell

    tubeswell Friend of Leo's

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    Output level. The smaller the tail resistance, the more plate-to-cathode voltage there will be (for any given HT voltage).
     
  17. Phrygian77

    Phrygian77 Tele-Afflicted

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    If you're set on building the PI and the output stage like an AB763 Deluxe Reverb, and you want a presence control like you have it drawn in the schematic, you're going to want to make the first resistor in the NFB voltage divider an 82k, not 820 ohms (which would be WAY too much voltage/NFB).
     
  18. Phrygian77

    Phrygian77 Tele-Afflicted

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    I'm not sure I understand why you wouldn't just build the whole thing like a 2204, except maybe using 470 ohm screen resistors and 220k grid leaks. I'd stick with a 12AX7 PI with a 10k tail and 470 ohm bias resistor. Ken Fisher used the same values on the Trainwreck Express. I'd also use a 100k to the 4 ohm tap, or 200k to the 8 ohm tap, which would give you the same NFB level as a real 2204 (also the same as any early-mid 70s JMP 1987). Maybe 32+32uf cans instead of 50+50uf cans.
     
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  19. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Another forum suggested SS rectification would make the resulting B+ a 6V6 killer... but on reflection, I think they got the impression I wanted to plug 6V6's into a real 2204, not a DIY 20W 2204 with DR trafos. That's why I was suggesting zener strings, etc in earlier replies... I thought I may be having to deal with B+ of over 450V... thus I shifted my plans to the cooler B+ option of GZ34 rectification.

    I'm very open to the idea of going back to my original idea - but - further Youtube investigation, it does seem the best sounding 20W plexis and JCMs all use the GZ34/6V6 combo. Hmmm.

    I might resolve this the most scientific way possible - flip a coin o_O
     
    Last edited: Dec 14, 2017
  20. Phrygian77

    Phrygian77 Tele-Afflicted

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    You could still build the rest of the circuit like a 2204, but using a GZ34. You could do a couple of 100uf caps in series for the first filter, or a single 47uf. You could also use the JJ 40+20+20+20 can like Hoffman and add a cap on the board for the last node.
     
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