1968 JMP100 12-series...NOS?

[shakti]

Time to unveil my latest build, this time a mid-68 style JMP100. I think this one came out particularly nice, probably my cleanest build yet. I had planned to do this one with mostly new parts, but once bitten by the NOS bug it's hard to not search out those last little bits. This one has mustards, RS silver micas, nearly all Iskras, Erie filter caps (plus a TAD on top and BC/Vishays on the screens), Castelco switches, Brian H's boards, Swanson head cab, Sparky front panel, MG logo, and to top it off, some extremely tasty M e r r e n transformers, both the T2562 PT and C1998 OT.

This is my first split cathode preamp build. I've owned a '72 SL years ago, but I must say that this one has the edge. Something about the supple feel of the mid-68 specs (2.7k V1b cathode, 0.0022uF V1b coupling cap, 50uF mains, 16uF screens, 32uF PI) that's a little more appealing than the stiffer '69 and on amps. It has gobs of harmonics and that classic "chugging", woody tone with tight but round bottom and high-mid bite. It's also very quiet (after weeding out a noisy V1 tube). Currently has Mullard in V1, RFT in V2 and Amperex Bugle Boy in V3. Tesla black eagles in the output, might try some RFTs. I'm running this one at 460V currently, but this T2562 also has a 490V tap.

And now the pics:

[Roe]

very nice indeed. any reason why you use cts pots? I think you have more NOS parts than me
I went for the yellow philips .68u caps since 1968 amps seem to use those (and mustard .68 are too expensive)

[demonufo]

Tesla black eagles in the output

Whilst mine sounded good enough, all 3 sets I bought became mechanically noisy quite quickly. They seem to shake themselves apart. Especially when used in my 4104 combo (now gone). I'd NEVER experienced this with tubes before. My early 80's brown based Tesla's seem to suffer from the same phenomenon but to a lesser degree as well. Early 70's Tesla's are all good though.
Still haven't tested my RFT's yet. (Promised Neikeel I would ages ago)

[chrisom]

Thanks for the excellent pics! I didn't go quite that far but I did get all NOS Mustards except for a .68 uF chicklet on V1 (which I may change out after a/b-ing), and I used Royal '67 Sozos for the big power supply spots. Yours looks incredibly nice!

classic "chugging", woody tone with tight but round bottom and high-mid bite.

Whether it's chicks or classic amps, I guess God put it in us all to appreciate those "tight but round bottoms"...

[AustinTx]

Beautiful!

[gldtp99]

Great work !!!! Did you really build this or do you have a Time Machine hidden away ?----

[neikeel]

Really nice build Thorlief
They are very good all round 'rock' amps and they do 'brown out' with volumes over 8.
Exceptionally clean build

Tesla black eagles in the output

Whilst mine sounded good enough, all 3 sets I bought became mechanically noisy quite quickly. They seem to shake themselves apart. Still haven't tested my RFT's yet. (Promised Neikeel I would ages ago)

Only bought one set of these from tt. Sounded good well matched but destroyed themselves pretty quickly. I bought a set of RFTs anyway, but not tried them yet. They are destined for my '68SL restoration once I have wired in the PT and tested it with my 'test' mullards.

[shakti]

Thanks guys!

I used CTS pots because...I had them lying around? I think I bought them a year or two ago, and remembered that George described them as being the closest to the late-67 and on pots. Don't know if that's the case or not, but they work fine here. I also serviced a '71 recently which had CTS pots (which doesn't say much, really).

I got several quads of Black Eagles, but unfortunately many of them drifted. I also noticed one tube in this amp actually had some blue sparks very occasionally and some hum, which went away with a more moderate bias. When they work, they sound good, but I'll probably try the RFTs. I have a dual bias setup in my Hiwatt CP103 build, so that one gets the stray tubes that are hard to match up. If they withstand the Hiwatt "rumble" test I guess they should be good to go...

[demonufo]

Sounded good well matched but destroyed themselves pretty quickly.

Sounds exactly like my experience with them, but I didn't test them to the destruction stage, I simply shelved them in case of emergency.

[shakti]

As mentioned, I have 2.7k cathode resistor and 0.0022uF coupling cap for channel 1. This is more of a late '68 spec I think, and I will experiment with 820R and 22n by parallelling in another resistor and 18n cap with alligator clips.

For the record, what are you guys running in your 12-series builds, and why? And when did these specs change? It's very hard to tell from photos, so far I'm not sure if I've found a single photo showing a 22n for channel 1. The cathode resistor is usually hidden, so it's very hard to see if it's 820R or 2.7k.

Neil - what's in your very early split cathode SL with the grey RS 0.68uF caps?

[shakti]

Bump. Still curious about people's preferred specs for these amps.

I find this amp sounds much better with a B cab and with bass cone speakers. A slant cab with G12M lead cones sounds a little screechy to me, even though the amp itself actually sounds softer and darker than I expected (feared?). The B cabs and bass cone speakers add a bit more depth and grunt to the amp, which by itself does compress a fair bit. It sounds killer with both a '70 B cab with G12M 55Hz (super fat EVH style...not that I can play that stuff) and a '69 B cab with G12H 55Hz (late period Jimi, though perhaps a touch more compression and dirt than he usually had, probably due to the lower filtering in this amp vs a '69).

[neikeel]

As mentioned, I have 2.7k cathode resistor and 0.0022uF coupling cap for channel 1. This is more of a late '68 spec I think, and I will experiment with 820R and 22n by parallelling in another resistor and 18n cap with alligator clips.

For the record, what are you guys running in your 12-series builds, and why? And when did these specs change? It's very hard to tell from photos, so far I'm not sure if I've found a single photo showing a 22n for channel 1. The cathode resistor is usually hidden, so it's very hard to see if it's 820R or 2.7k.

Neil - what's in your very early split cathode SL with the grey RS 0.68uF caps?

I use 2k7/0.68 but I am pretty sure it came with 820R (my slightly later 68 50watter with yellow block 0.68s with intact dye has 820R there).
I prefer 2k7 as it is a little cleaner when you dial back on the guitar volume, on the 100watter that was a bit grainy. When I asked about it Larry said that was why Marshall went to 2k7 for late 68/69 on. Funnily enough never bugged me with the 1987
Both run 0.0022uF on V1b - thats what gives it the Superlead cut - oh and I also use a 5000pF bright cap (RS on the 100 and Lemco on the 50). Neither amp is harsh or screechy. Both sound best through my 68/69 B cab with prerola G12H T1217s (75%hz). My b/w A cab is a 1935 with M55 prerolas which needs a bit more pushing. I am scared to run either hard through my T1221 20w loaded pinstripe cab unattenuated but every amp I run through it sounds sublime
Old amps well run in just do not sound as harsh and bright as new builds even with identical specs.

[shakti]

Thanks Neil!

I didn't mean to imply the amp sounds harsh...there's not a trace of it! But the match with that particular A cab wasn't the best to my ears. It sounds phenomenal with the B cabs. It needs burn-in of course, but those M e r r e n transformers are really something...

[shakti]

It's a little flubbier and not as focused as expected, but needs a lot more burn-in and a few tweaks. It's also ghosting quite a bit...here's a few thoughts, but I'd love the opinions of seasoned 12-series builders and users!

- I have no bright cap on volume I. Will try 5000pF and 1500pF. Any favorite bright cap values out there? I tend to like 100pF for the earlier amps.

- currently running it at 460V. I typically prefer EL34s with a more moderate plate voltage in the 460-80V region with 100-watters, but maybe 490V works better for the late period Hendrix sounds I'm after.

- screens filtering is low at 16uF per early '68 specs. I'm considering upping the screens filtering for less ghosting, more punch and slightly cleaner sound.

I had an interesting idea to use a 3PDT standby switch like George's 12-series clone, to choose either early or late '68 specs:
UP: 460V, 16uF screens = greasy, Van Halen style tone
DOWN: 490V, an extra 32uF added on the screens for a total of 48uF = tighter, cleaner, late period Hendrix style

How does all that sound to you guys?

[Roe]

switching voltages sounds like a good idea. I have 473v/395v on my 1969 1959 and only use the higher voltage setting (with the exception of using a strat). I have experimented a bit with voltages and prefer voltages from 460-80v on a typical 100w el34 plexi.

Have you considered the PI filtering? 12000 series went from 32uf to 100uf. Also, bright cap is a must imho