plexi 1986 vs 1987?

Monstronaut

I'm considering plexi clones at the moment and have narrowed it down to 50watt small box. So the choice is between lead and bass models. Ill be using an early 60's sg so am thinking a 1986 will provide a bit of extra warmth. Any thoughts? Also any recommendations for builders?

Ossyrocks

I much prefer the 1986 circuit, I had Ceriatone build me one a while back. Fuller, more girth, but less gain. If you want a rock monster then the 1987 will do that, but it is voiced with much less bass. 1986 will distort well if you turn it up, but it will be loud!

I had a couple of small mods incorporated into my 1986, first a .68uf cap in the second stage (like in the 1987's), for a little bit more drive and second 100k in the negative feedback on the 4 ohm tap, for more gain again. It's a great amp.

ICBM

I'd go for the 1986 every time. The Lead models are really shrill, particularly at lower volume settings. If you're intending to use it for cleans as well, I would add a 500pF bright cap on the Channel 1 volume control - it gives the sparkle without the shrillness of the 5000pF cap on the 1987. If you're cranking it it makes no difference since the cap is bypassed when the volume is up full.

Ossyrocks

I remember years ago picking up an early 70's 1987 and using it that very night on a gig. It was either on or off depending on whether it was on 1 or 2 on the volume. 1 was off, 2 was full tilt! The bass player kept trying to turn it down, but couldn't, it just went off. I clipped one leg of that cap off the following day and it was much more usable. 

jpfamps

I prefer the 1986, however if you are having the amp built for you then you can have this switchible.

Randall Aiken on the AX84 site had a diagram which he referred to as ali/plexi switching that did this.

funstuie

I probably should know more but I just got a second hand Cornell Plexi 7 and its excellent. No idea what circuit it's based on though.

gjonesy

@Monstronaut many thanks for asking this question, Ive been thinking exactly the same thing. Also many thanks to those that replied youve helped me make the decision to go for a 1986 build. Was thinking 100pf across the vol but can always swap that later. Does anyone else have a comment on the NFB resistor and which tap?

ICBM

I like 47K and 8 ohm - gives the best balance between presence control effectiveness and openness, to me.

100pF on the volume control effectively makes it an Organ model (1989), which is also nice but lacks a tiny bit of bite compared to 500pF.

These three models - Bass, Organ and PA (1985 - no bright caps at all) are all very similar and only vary in these cap values. The Lead is quite a bit different, it's not just the bright caps - it has a different V1 cathode network, a couple of different values in the tone stack, and smaller coupling caps in a couple of places, so you can't make a true switchable Lead/Bass unless you do all that.

gjonesy

Cheers @ICBM Ill post some pics later of what Ive done so far