1968 JMP100 12-series...NOS?

[shakti]

I hadn't really considered the PI filtering no...when you say the 12-series had 32uF or 100uF, do you mean the dual 50uF used on the later "all caps on top" chassis? That one is 50 mains, 50 screens, 100(?) PI and 50+50 preamp, right? I have a feeling that my preferred specs are somewhere in between these extremes.

I'm using one of the skinny TAD 32uF cans for the PI, and the small cap clamp, so I don't know if I have many choices for the PI actually.

[Roe]

I actually mean the early steel chassis with only one cap on top. the early amps seems to have had 32 or 32+16 on PI, but the later amps had 40+40 or even 50+50 (cf tazin's findings): http://forum.metroamp.com/viewtopic.php ... 24#p262524

[shakti]

Wow, hadn't seen that thread...lots of very useful info! It seems like I actually have very late 10-series specs for my amp right now, like the very earliest split cathode SLs. That makes a lot of sense, as the actual feel of the amp is very much like late-period Cream, but the sound is more in-your-face.

I think I should try to up the PI filtering first of all. 50uF is a good start, but I could probably go even higher.
Would you happen to know how long they kept the low 16uF screens filtering? George's amp had 4 x 100uF on the filter board, so 50uF screens, but I haven't seen many with that layout.

[shakti]

I've been checking out photos at the Amp Archives. Looks like they kept the 100+100 for mains and 32+32 for screens throughout the 12-series range (or more precisely, for all the amps using that particular chassis and power supply layout, from late '67 and probably through most of '68). There's the odd amp using 100+100 for screens as well (like George's amp), but that seems like an anomaly if it's original.

The PI filtering is harder to make out - the very earliest ones have only a single 32 like you said Roe, then there are examples of 32+16, 40+40 (typically a grey Marconi cap) and 50+50 (blue Daly?). It's very hard to see if they're using both sections of that dual section cap.

That does open up the possibility of "early" and "late" '68 specs within this amp with the flick of a switch though. If I use a dual section cap for the PI, I can easily switch both voltage and PI filtering with the same switch. Maybe try a 32+32 and use only half of the cap for early '68 specs and maximum grease, then the additional 32 for a little more punch and cleaner sound.

[neikeel]

I've been checking out photos at the Amp Archives. Looks like they kept the 100+100 for mains and 32+32 for screens throughout the 12-series range (or more precisely, for all the amps using that particular chassis and power supply layout, from late '67 and probably through most of '68). There's the odd amp using 100+100 for screens as well (like George's amp), but that seems like an anomaly if it's original.

The PI filtering is harder to make out - the very earliest ones have only a single 32 like you said Roe, then there are examples of 32+16, 40+40 (typically a grey Marconi cap) and 50+50 (blue Daly?). It's very hard to see if they're using both sections of that dual section cap.

That does open up the possibility of "early" and "late" '68 specs within this amp with the flick of a switch though. If I use a dual section cap for the PI, I can easily switch both voltage and PI filtering with the same switch. Maybe try a 32+32 and use only half of the cap for early '68 specs and maximum grease, then the additional 32 for a little more punch and cleaner sound.

Both my 10 series 68 Superleads came with 32+16 PI caps (Hunts silver). The one up and running at the moment has 50uF mains and 16uF screens and 32/32 in preamp - no ghosting. I reformed the caps before use. That amp is very smooth and fluid or as you say 'greasy' once you get the volumes up to 7 or 8 although it is pretty loud from the outset as it has a 5000pF bright cap. Certainly not lacking in punch.
The second one is not ready yet although board is fully rebuilt with Iskras and Hunts 32/32. had to buy new screens an mains caps and will try to reform the lot before start up. Thankfully have a couple of spare 16/32 if it does not work.
FWIW 32/32=64 sounds/feels very similar to 16/32 = 48. The switch idea will work but not sure that I would recommend switching it over with the caps full of juice very often.

[stef]

I like 5000pf (lemco 5k) for the bright cap (32uF sreens, 32+32 preamp, 100 Pi)

[shakti]

Both my 10 series 68 Superleads came with 32+16 PI caps (Hunts silver). The one up and running at the moment has 50uF mains and 16uF screens and 32/32 in preamp - no ghosting. I reformed the caps before use. That amp is very smooth and fluid or as you say 'greasy' once you get the volumes up to 7 or 8 although it is pretty loud from the outset as it has a 5000pF bright cap. Certainly not lacking in punch.
The second one is not ready yet although board is fully rebuilt with Iskras and Hunts 32/32. had to buy new screens an mains caps and will try to reform the lot before start up. Thankfully have a couple of spare 16/32 if it does not work.
FWIW 32/32 sounds/feels very similar to 16/48. The switch idea will work but not sure that I would recommend switching it over with the caps full of juice very often.

Thanks for that info, Neil. Your two '68s must be very early split cathode amps since they still have 10-series serial numbers. Makes me think that the single 32uF for the PI must have been very short-lived.

My amp is sounding a little better and more focused after more burn-in. I just don't have enough time...too many amps to play! I didn't bother to form the caps since I used NOS Erie 100uFs and 32+32 for the preamp which had supposedly been reformed already, but maybe I should have. Ghosting seems to have improved slightly but is still noticeable. It has that greasy sound in spades, I just have to open it up to install a bright cap. Have a ceramic Lemco 5000pF which should be very appropriate.

I'm not sure if I understand you correctly though...are you saying 32 screens/32 PI sounds similar to 16 screens/48 PI?

I can't see why the extra cap at the switch should be a problem - it simply a make/break switch on the jumper between the two sections of a PI cap. Just have to decide if I should use a dual 32 or dual 50...

[shakti]

I like 5000pf (lemco 5k) for the bright cap (16uF sreens, 32+32 preamp, 100 Pi)

Thanks for the tip. Will install a 5000pF (ceramic Lemco). Will have to see what I end up with on the PI, it's either switchable 32/64 or 50/100.

[shakti]

A little update: I've been struggling a little getting this amp to sound good. Maybe I'm just too used to the JTM preamp? However, yesterday things finally started sounding right. I tried a 5000pf ceramic bright cap, but took it out again. I remember now why I took it out of the '72 SL I used to have...harsh, hissy, no usable clean sounds. "Parkhead" mod with a 470k resistor does sound great though - a bit beefier at lower volumes. I'll experiment with a smaller value bright cap as well, inspired by Pete Thorn's killer video of his Suhr SL68.

The biggest difference though is probably burn-in. The difference from first fire-up is pretty dramatic. It seems to slowly come into focus, where it was just blurry and smeared sounding in the beginning, particularly the clean and semi-clean tones. Seems to become quieter too for some reason.

I still have to get the switch to select B+ voltage and PI filtering, but in its current state it does a mean Trower tone in "early '68" mode, with 460V and 50uF PI filtering. Very juicy and "greasy" sounding like Trower's mid 70s sound. Still hoping that "late '68" specs with 490V and 100uF PI filtering will give a little cleaner and punchier sound a la late period Hendrix though.

[shakti]

The 490V setting has bern duly tested. I ended up with about 500V om the plates with the current wall voltage. 100uF PI filtering along with the higher voltage. It does exactly what I hoped it would do - bigger, cleaner and tighter and better suited for late period Hendrix. With those specs it's more or less like a '69 type circuit but less screens filtering. It sounds a little more strident with humbuckers and cranked than the previous setting. I like both settings enough that I'd love to be able to switch them.

The big question now is: will it be safe to use a 3PDT standby switch and do the switching? George's '68 spec amp does this, but it uses two poles to switch the AC secondaries before the diodes. The third pole is used to drop the bias voltage a little in high voltage mode (presumably). Since I'm only switching from 490 to 460V - i.e. roughly a 6% change, I figured I'd leave the bias alone and live with the compromise. Set it for maybe 72% in high voltage mode and 66% in low voltage mode. Then I'd use the third pole to switch in the extra 50uF on the PI in high voltage mode.

BUT: is it safe to switch both HV AC and B+ on different poles of the same switch?

[Roe]

I'd do it like george does it personally. perhaps with a dual 32uf PI cap (or w/100uf). the last 120000 series reportedly had 100uf on the PI

[Roe]

you could add a polarity switch like jimi had to switch PI filtering

[shakti]

Yeah, but in that case I'd probably rather put the switch on the rear. Don't want to drill any holes on the front.

Is it that much of an issue though? For one, is it safe to do it like I propose? The bias compromise I can live with...to be honest I don't typically hear a huge difference between 65 and 70% dissipation.

BTW, I just made a cool discovery. Old news to many I guess, but I was fooling around with some settings and decided to try the old daisy-chained Plexi trick. I'd done this before, but never with a SL/split cathode amp. With JTM type amps I felt it muddied things up too much, but here it sounded very cool! Fatter, warmer, but still tight and cutting. I even think a large value bright cap might actually work with that kind of setu. And the best thing was that the other amp wasn't even on...just a "dummy" receiving amp.

[shakti]

Bump for opinions.

[shakti]

Sunday bump. How would you set up the switching?