great 800

[Smokebreak]

I had this 100W '89/'90 JCM800 come over for repair. When I opened it I saw some modifications on the board, and didn't seem like anything out of the ordinary. Turns out the mods weren't, and after I traced everything out, they rung a bell...Tone Lizard mods!!...at least some of them. I mean no disrespect, but I never tried them in the past, as maybe I thought they were "old mods" or too simple or something and had read bad things about them in passing. That's the psychology of the situation!

So once I got the amp fired up, it almost sounded really, really good.
Basically the mods were : 470K//470p to the pot became 68K, and the divider after the 2nd stage became 220K/470K. 1n across a plate. Without any peakers it lacked that sizzle, so I put a 560p on a flying switch across the 220K for the customer, and we agreed that's what it needed. Even across the 220K, it added just a bit of sizzle, that was a good balance between "nothing" on a 470K, and a 470p across a 470K. More subtle than 1n+ across 470K, as far as the upper mid/high dance goes.

Then there's the power supply. It took me a while to trace because it seemed so odd, but it is drawn correctly. Consequently the preamp voltages were relatively high, but this no doubt contributed to the beastly nature of this amp. You had to get on top of this thing and go for a ride. I couldn't get the master past 8 oclock and not feel like the world was coming to an end. Amazing amp.

[Reeltarded]

Killed a node like a 45.

Does that amp like to feedback under your fingers?

[Smokebreak]

Yea it blasted off pretty good, and I started to go further with it, as it retained an unbypassed 10K, but my dude was like "too much". He's more of a plexi guy

[Smokebreak]

My fave recipe, today, is 100k/1M resistor feeding the grid, then a 500K pot feeding 470K//470p-2n2/470k, with 10K Rk, or the same but 100K/56K with 2K7//.68 or 4K7//.47. Pot after 2nd. The latter will not shutup.

[John_P_WI]

Thanks for the scheme Smoke, that's a lot of strangling on at the first stage with the 1n and 100p.

[Smokebreak]

For sure. That 1n was the previous mod and the 100p was stock, and I looked at it as 1n1. I prefer to kill it a bit later, and maybe not so much, but I figured it kept that 68K on good behaviour.

edit : is that correct in that the sum of the plate to B+ cap and the plate to cathode cap is the same as the additive value from plate to either? I can't hear a difference, but the cathode isn't exactly ground, and neither is B+

[jelle]

Thanks for sharing that! Got any gut shots of this amp?

[Smokebreak]

I didn't take any pics of this one. Just traced it out and sent it on it's way. Owner said the guy that sold it to him has been begging to buy it back!

[pdf64]

is that correct in that the sum of the plate to B+ cap and the plate to cathode cap is the same as the additive value from plate to either? I can't hear a difference, but the cathode isn't exactly ground, and neither is B+

Assuming that the B+ caps are good, for ac purposes the B+ line can be assumed to be equivalent to ground.

Regarding the 100pF 'plate to cathode' cap, although (for full range audio purposes) the cathode is only partially bypassed, for anything that the 100pF cap can pass, it seems reasonable to assume that the cathode too is an ac ground equivalent .

[Bob S]

Yup - Thanks for sharing.

[jelle]

Ok, thanks! I was curious to see what caps were used.

jelle

[Smokebreak]

Well, I definitely remember the massive .68 sozo, and the rest were the stock grey box caps.