Bogen CHB-35A - rebuilt for the millionth time (it's staying this way)

[thehoj]

So over the past year or so I've tried quite a few things with this Bogen CHB-35A that I had kicking around. It's a bit of an oddity because it runs on 7868's in the power section. I think they sound really good, and so I designed a new amp around that original bogen power section.

I have pics of the original gut shots in this other thread, as well as my first attempt at doing a rebuild on it, and then retrofitting it into a vox ac15 combo cab.
http://music-electronics-forum.com/t14750/

I've since gutted it again and built it with a completely different preamp (which I happen to really like). The preamp is pretty much a dead on marshall 2204 layout right up until the PI. I only have a single triode for the PI, so I have to use a different type of PI than the 2204, and so from that point on I'm using fairly close to the original bogen layout.
I also built the whole thing on a turret board to keep things tidier. I think it's turned out really well. It's still a beast of an amp, not pretty at all, but I really like it and it's much more solid than it was before. I wouldn't hesitate to take it out gigging now.

Here's the schematic I'm running with currently (red lines are shielded cable):
http://home.westman.wave.ca/~vwtweak...uly10%20jp.jpg

The Presence control is a bit of an oddity also.. It's wired into the tone stack, and it almost seems to be a combination presence/depth control. I like what it does, so I'm leaving it.
I also wired in a switch to have low or high gain on v1b. It gives a nice contrast.

Here are some pics of the newest build:













I've learned so much from this amp, It's really been an awesome tool to have around, and I'll be leaving it as it is now.. Might change out the speaker but that's it.

Out of interest, does anyone know what I'm actually doing with that "presence" control connected right into the tonestack?

[bob p]

just wondering ... is that turret board from AES?

[thehoj]

Yup.
More Info for item P-HTUR-60

[RWood]

Really nice work. You're persistence and willingness to rework the circuit (and the schematics!) is admirable. It's good to see a turretboard fit so nicely; I'd like to try that, too.

How do you feel about 500V on the plates of those 7868s? Do you have a good range of negative voltage to supply to the grids?

RWood

[thehoj]

Thanks for the kind words. I never could have gotten to where I am with this amp without your help, so I really appreciate that.

I was a bit off on my voltages. I just took some new readings, and updated the schematic with readings at a bunch of locations and the chart.
I also decided to add a large grid stopper (470K) to the PI based on the info here (http://www.freewebs.com/valvewizard/cathodyne.html) to reduce frequency doubling. Maybe it's in my head, but it seems to have improved the fullness of the sound.

I'm actually 490V on the plates, and yes I can get more than enough negative voltage for the grids.
I'm biased at about 70% max plate dissipation right now. Max plate diss is 19w, so 13.3w/490v = 27.1ma. So the highest one is reading 26.8ma and the other one is at 24ma (the tubes aren't perfectly matched I guess). I can get it down as low as 20ma if I wanted to.

I've just got to get the back closed up in some sort of clean manner now.

Really nice work. You're persistence and willingness to rework the circuit (and the schematics!) is admirable. It's good to see a turretboard fit so nicely; I'd like to try that, too.

How do you feel about 500V on the plates of those 7868s? Do you have a good range of negative voltage to supply to the grids?

RWood