Conversion: 1987 to Ceriatone Yeti

[MrHaroldA]

I have too many amps. JMP 2203, JMP 2204, 20th Anniversary 4104, 1987x, and two Sovtek MIG50's. Many of them work and sound alike, so there's only one option at hand: make them more diverse (and maybe sell one or two? )

One of the Sovteks is made somewhat into a Sunn Model T, but with the standard Marshall power amp. Cool but not great. A friend of mine has a big interest in my white tolexed 4104 combo, and the '80 JMP 2204 could eventually be up for sale ... that leaves the 1987x.

The '93 1987x was already converted to a turretboard-style plexi. but the Sovteks are way better and cheaper amps for the raw Plexi-tones. Since the 1987x is turretboard, it's also way easier to mod them into something more brutal: a "Yeti". So I stripped the preamp, removed the useless PostPIMV, transformed the Ceriatone layout into something that will fit on my limited space (only 9 rows available) board and started building the yeti.

After one big blunderous mistake (switched PI wires after removing the PostPIMV) it was up and running, without any oscillation or excessive noise! It was also very dull sounding (treble knob didn't have a noticable effect), but after changing the presence pot to Yeti-specs, all was fine again!

Except for too much sag at higher gain settings.

The original Yeti has a separate regulated heater power supply, could that be the reason for the sag?

[img:775:369]http://img.sait.nl/schematics/Ceriatone ... 0V1-V2.png[/img]

I still have the 1987 voltage droppers present, and the Yeti has a slightly different configuration in the power filtering. I'll mod that into Yeti territory next.

Preamp: http://img.sait.nl/schematics/Ceriatone ... acabra.png
Power amp: http://img.sait.nl/schematics/Ceriatone ... 0Power.png
Yeti50 (layout): http://www.ceriatone.com/images/layoutP ... Yeti50.jpg
100W Chupacabra: http://img.sait.nl/schematics/Chupacabr ... b03611.png


And of course: some random pictures

[img:800:600]https://lh3.googleusercontent.com/-MlOo ... 220258.jpg[/img]
[img:480:640]https://lh4.googleusercontent.com/-setX ... 205742.jpg[/img]
[img:800:600]https://lh4.googleusercontent.com/-dW2t ... 205722.jpg[/img]
[img:800:600]https://lh5.googleusercontent.com/-yD6g ... 210448.jpg[/img]
[img:800:600]https://lh3.googleusercontent.com/-RmMB ... 230427.jpg[/img]

[MrHaroldA]

Except for too much sag at higher gain settings.

The original Yeti has a separate regulated heater power supply, could that be the reason for the sag?

Ehm, no. Blunder #2.

You see that 2 red wires in the bottom/middle of the board? I moved the right one (CF anode) to the other side of the 10k dropper, and forgot to solder it. It was just a mechanical connection, which probably couldn't handle higher currents.

[img:800:600]https://lh3.googleusercontent.com/-MlOo ... 220258.jpg[/img]

I discovered this error when I swapped the resistor for a wire, since the Yeti doesn't have that 10k in the anode power line. I do believe the sag is gone, but I'm a bit biassed listening to it now, so we'll know for sure when I fire it up again next week. Maybe even for a full volume test!

[xtian]

Have any amp with Fender tone stack and PI?

[Reeltarded]

By sag, I think you mean it's blocking on the second stage? It looks cool!

[MrHaroldA]

Have any amp with Fender tone stack and PI?

Nope, I'm more of a Marshall guy, and yes I know the plexi was a Bassman copy

Friend of mine has a Blues Deluxe, which I like, but don't want to have.

By sag, I think you mean it's blocking on the second stage? It looks cool!

With "sag" I meant "the open note after a palm mute has no attack". The amp had to recover from a spike in the input volume.


I played the amp again after letting it sit for a couple of hours, and the "loss of attack" seems to be completely gone! I did cheat a bit this time: I put my trusty old Coron Distortion 10 in front of it. Man, this chewes nicely! Brutal and crunchy, just the way I like it!

[Reeltarded]

That either says the filtering can't keep up, or more likely it's blocking after that distortion unit of a first stage.

[Smokebreak]

Hey Miles, I've having a similar issue. It's not bad at all, but noticeable enough to bug me. It's a 4 stage +CF, and it only happens when running 3 stages. When I introduce the 4th stage up front, it's gone. Notes compress slightly, volume drops a little, all for a split second, then everything blooms a little back to normal. If I back off the gain it goes away too. Filtering is 33u so stiff enough, I suppose. I've got 33K stoppers on all stages up to the 820r stage. Any suggestions?