1987/1986 power supply filtering - how low is too low?

Discussion in 'Shock Brother's DIY Amps' started by 11 Gauge, Feb 19, 2009.

  1. 11 Gauge

    11 Gauge Doctor of Teleocity

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    My 2204 is about to go under the knife again, and basically get transformed into a mix of 1987 & 1986.

    I'm mainly playing amps with really light filtering these days, though - your typical 22uFX5 doghouse array in BF/SF Fender amps.

    With that in mind, I was wondering how much below the 50uFX2 cans I could get away with. The JTM45 is probably excessively low, but on a lark, has anyone tried it in these amps?

    My best guess would be an initial 32uFX2 followed by a 32uF for the screens, and probably some on board 22uF's for everything from the PI on down.

    The CE cap cans only make it harder to decide, since they offer a 30/20/20/20 can in the 1 3/8" diameter. That would be cool, cuz I'd just have a pair of cans (no pun intended) with nothing to mount elsewhere (again, no pun intended).

    Thoughts? BTW - any ghost notes that I'm getting from the Fenders is not an issue.
     
  2. 11 Gauge

    11 Gauge Doctor of Teleocity

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    --bump--

    bump...

    Mr. Swartz, JC, anyone?
     
  3. Tim Swartz

    Tim Swartz Friend of Leo's

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    Try the 32uFX2 followed by a 32uF for the screens and then 22uF's for everything else. I too don't like an overly filtered amp, please report your findings here.
     
  4. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Well, she's a mixed bag at this stage, due to components I had on hand, as well as time constraints.

    I've got 22uF for both stages at the board, and it's a good start. The PI will be next, obviously. I also added the add'l 10K B+ drop to the screens that the 2204's don't have.

    I yanked the MV, backdated the NFB loop to 1987 specs, bumped up the slope resistor a tad (47K), and made a switchable option for 330uF on the cathode of V1A (in conjunction with the .68uF). I also dropped V1A's cathode resistor down to 1.2K, a "supposed" magic spot in some old 1987's.

    I did forget to connect the NFB loop to the 8 ohm tap instead of the 4 ohm. Probably not a deal breaker, but just something to do on the next round of surgery.

    The 470K grid stopper is bypassed with a 270pF as opposed to a 470pF.

    I want to make the amp pre jumpered and use the 330uF on one channel and the .68uF on the other. That big cathode cap is really magical, but only at lower volumes. I'm also going to bump up the slope resistor to 56K, and probably drop the treble cap to 270pF.

    ...This leaves a question of the coupling cap for the bright channel, should I choose to go down that path. .0022uF seems a bit squeaky to me - not sure if I might try .0047uF or even .01uF. I think I really like the .022uF like the JTM and 1986...

    The amp now sounds big and full and bright at lower volumes, and really mean as you turn up the wick. Switching on the 330uF cathode bypass cap at lower volumes is really gorgeous. But the remaining 50uF caps are still a bit too punchy. Maybe dropping the PI to 22uF and the screens to 32uF would be the best way to go.

    She's much more usable now, and I'm about to build a dedicated booster pedal around the amp. That's my findings, thus far.
     
    Last edited: Feb 20, 2009
  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I've done it a few times... I prefer gigantic filtering though, plus I'm a huge ACDC, GnR, and The Cult fan - so my taste in music AND amp may be different.

    As far as the Bright channel coupler - I'd stick with the stock small value. I like my two channel amps to have channels that are really different from each other. I've found some magic in that. If you're going to be jumpering often, you may have to tweak it while jumpered by ear, then go back and fine tune each channel... some jumpered channels didn't seem to work as well until I fine tuned em - though individual channels may have sounded good.

    As far as the cathode resistor, you could try a 820 ohm for a touch of extra preamp gain (if we're talking about the Bright channel, which is normally 2.7k). The 2.7k is not too often my cup of tea either.

    Are you going to be using a OD pedal all the time, or are you planning on a dual ganged PPIMV?

    I have a dual channel JCM800 I built, but may view it as a prototype and go your 1987 route as well soon (I want to use a different scenario with my next dual channel'ed JCM800)... weird coincidence. I think I'll leave it stock to schem and only pop in a PPIMV for now (though I may tweak each channel's preamp a little).
     
  6. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Yeah - I think I'll go with the .0022uF and work my way up if needed - I just don't like anything switchable with caps coming off of plates.

    Bright channel cathode resistor is currently ~1.2K. I've read about a bunch of accounts where amps had CC 820's in this stage and they drifted up to around this value.

    I played the amp again today, and now the 330uF on the (quasi) normal channel seems a bit too loose, even at low volume. I'll have to play with that, as well. I see a bunch of references to 2.2uF caps for this stage, but none are for an 820 ohm. I'll have to play with both the cap and resistor. I guess it's time to install my rotary switch. I know I can use the calculator, but it never seems to gel when I audition it.

    No PPIMV - I want the amp clean at lower volumes and mean at higher volumes.

    Since the amp was originally a 4210 and has the dual footswitch, I was thinking of wiring up the channels pre jumpered, and then using the footswitches to short out the channels' inputs. That would be optimal - normal channel for some uses, bright channel for others, both for a third, and then a booster for a fourth. A cool K.I.S.S. setup as compared to a Boogie or Bogner, without any relays, optoisolators, or a ton of preamp bottles.

    I'm also STILL really digging Adam Jones' tones, especially on Undertow, where all he's using is a '76 Bass head.
     
  7. Tim Swartz

    Tim Swartz Friend of Leo's

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    Keep in mind that if you are using EL34s, 6V6s may be an option.
     
  8. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Yeah - I float between the two with this amp. The JJ 6V6S makes rebiasing a breeze, since they are flexible enough to fudge it a bit.

    But nothing beats the thump of those EL34's, especially thru my 2X12 cab.
     
  9. GUITARmole

    GUITARmole Tele-Afflicted

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    I don't think you'll hear much difference with a.0047. Everyone I know prefers .022 over the .0022.

    I personally wouldn't hardwire jumper the channels but instead would put a switch to select between shared and split cathode inputs to optimize the amp for single coils or humbuckers.

    FWIW, I have a PPIMV and I can get clean at low volumes and dirty at high volumes too. When it's 'dimed' it's effectively out of the circuit and the amp is bone stock. When turned down it really helps to tame the beast so that your ears don't get destroyed.

    I like AC/DC, the Cult, and GnR too but I prefer underfiltering...or filtering similar to '67 Marshall specs.
     
  10. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Which PPIMV do you run?

    I vacillate between being for/against one.

    I like the Fischer model best, but still am reluctant to the 2.2M resistors saving the power section if the pot fails. I know that Ken knew his stuff though. I guess I just need to get over it.

    The original Marshall 2205 ("James" model?) seems the most failsafe, but is extra wiring and the add'l pair of coupling caps. The taper is also a little hinky, IMO.

    The Rich mod is a good compromise with a low parts count, but leaves D.C. on the pot minus the extra pair of couplers.

    I really like the Matchless blender, since it only requires a mono pot and a cap. But I've heard that it doesn't work well with Marshalls, so I haven't played with it too much.
     
  11. GUITARmole

    GUITARmole Tele-Afflicted

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    I use the L/M PPIMV off of the Metroamp forum which I believe is basically a variation of the Fischer. I'm not sure what your reluctance to trust the 2.2M resistors is based on??

    I've tried several other master volumes and the L/M is by FAR the best; FWIW, I think it alters your tone less than an attenuator and for $5 in parts the price isn't bad either!
    I wouldn't use anything else on a 50 or a 100W Marshall.
     
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