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

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.

 

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Mr. Swartz, JC, anyone?

 

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.

 

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.

 

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.

 

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.