I finally finished my power scaled Express. It's a Ceriatone Express kit with a London Power Power Scaling kit (as well as Bias Mod kit) incorporated...I used the Ceriatone preamp circuit board and did the three other circuit boards myself. I got the trannies separate from Heyboer. I didn't like having the power cord attached so I used one of those connectors with the built-in fuse...then I used the 'stock' fuseholder for the B+ fuse. I'll post some pix of the final amp when it's done but here's the chassis. My camera sux so the pix aren't very good.
I've experimented with the Express circuit before and sort of expected my amp to be a bit too bright for my tastes. I also usually run the Presence control on zero, they're bright and gainy enough for me with it on zero. In addition, the power scaling circuit requires a post phase inverter master volume which when turned down low renders the Presence control useless anyway (since there's virtually no NFB to 'adjust'). So I yanked out the Presence control and put in a master tone control. The power scaling kit calls for a cross-line post-PI MV but since I never seem to like those I went with a dual-pot post-PI MV (with caps to isolate from the bias supply). I considered using a Vox-style 'Cut' control but since that's just a cross-line MV with a cap I decided to try a dual-pot post PI 'Tone' control (just two separate 'treble bleeds' to ground). I used a dual 1M-A pot and two .0022uF caps. Since the PPIMV is right next to it the wiring is super easy (and short). This control is infinitely more useful for me than a Presence control. I tried to pre-plan this amp as much as possible but still ended up with a bit of a kluge over there by the PPIMV. Oh well. No oscillation or anything yet but I haven't really even opened it up. But it certainly can get gainy at low volumes...I keep wanting to play old ZZ Top with it. I'll post my review on power scaling after I've played it for a while.
Oh yea, the power scaling mucked with the preamp voltages a bit. Instead of the power dropping resistors going 18.2K - 9.1K - 9.1K; I went with 10K - 10K - 8.2K and got these voltages in the amp:
plates = 394vdc
screens = 382vdc
PI node = 299vdc
V2 node = 280vdc
V1 node = 268vdc
Do those voltages seem right? I also 'lifted' the center tap of the heaters to 60vdc (that funny business on the preamp filter cap board).
I have heard some people way prefered Pre phase inverter MV instead of either version of the the post with the PS.
Looking good! But is that the PS mosfet hanging in the air like that?
It is going to burn up, you need to bolt that to the chassis or a big heatsink
Paul, I plan to post more pix and clips once it's completely complete.
drz400, that's one of the trannies for the bias supply and it's in no danger of burning up. The MOSFET for the PS is bigger than that one and is mounted to the sidewall of the chassis (using an insulator) tucked down by the pilot light. The chassis does get warm around there but not hot. I drilled some extra holes in the chassis just in case I'll need a fan (and that unused grommet in the middle of the chassis is for the wires) but since the amp is only 35W and ~400vdc I was hoping to get away with just mounting it to the chassis without a fan.
I'm probably going to do some mods to the preamp. I can see how a pre-PI MV would help, there's a lot of gain in that preamp and it just slams into the PI completely unimpeded. The thing is just so gainy and bright that most of the travel of the preamp volume is unuseable and the distortion has a little bit of 'fuzztone' to it unless the bass control is on zero. My first thing will likely be to add a grid stopper to the input wire and then try V2a without a bypass cap. But I'm also going to buy my first O-scope, learn to see what's-happening-where in the amp and go from there.
Faceplates are almost done, then I'll post some final pics. I haven't even touched the preamp yet regarding mods. I'm waiting until I get an o-scope and sig gen and I'm hopefully going to 'learn' about them on this amp...so I want to scope it as stock before I start modding. Will probably try some Komet-type mods first.
I'll post my review on power scaling after I've played it for a while.
