Getting the most out of a Bogen CHB-35A conversion

[Rockn]

I recently acquired this Bogen amp and have done sme mods to it and it actually sounds pretty good if you want a really clean sounding amp with lots of chime. I am looking to get a little more crunch or distortion out of it and was wondering how I would get more out of the preamp section by cascading the output of the first half of the 6EU7 into the second half.

Looking at the schematic, stock with no mods the voltage on the plated of the 6EU7 is supposed to measure 43V. Looking at the datasheet it doesn't give you much headroom to play with. I had changed the plate resistor to 220K and cathode resistor to 2.2K and I am getting around 55V now and it is a touch louder. Would bumping the plate voltage up to a steeper curve and putting the operating point lower give some more compression at higher volumes?

Any insight or help would be greatly appreciated.

Also this thing is not terribly loud for a "35W" amp, but I think the 7868 tubes are only rated for around 19W each so in reality it would only be around 30W max.

[RWood]

Lots of good threads here on the CHB-35a, and even more possibilities exist. Check for a recent post from TheHoj, who has done extensive work and rework with this amp.

If you want to cascade the first two triodes, try this circuit which comes from the good folks at 18watt.com:

http://i95.photobucket.com/albums/l1...ilZModRev3.gif



I have built this preamp as drawn, using the switch to go from parallel mismatched triodes to cascaded triodes. There's a level control for the cascaded triode so you can dial in the amount of boost you get. You can get fancy and put in a relay to make that footswitchable, which is what the Hoffman 18 watt Stout (el34.com) does.

My experience with the CHB-35a is that it is difficult to go in and just tweak a few things. It's more of a one-thing-leads-to-another situation. Look at the tone control circuit; ever seen a tube biased like that? Then there's the voltage-doubler power supply which will leave you with more voltage (490 +/-) than you really want......so a VVR, or a bridge rectifier, becomes appealing.

One thing that should be a given is to remove the high voltage taps that go to the terminals on the back panel. Those things are like Death Waiting To Happen, or at least a nasty bite. UL Labs must have been out back getting high when that amp rolled through for inspection.

Good luck; lots of fun possibilities. Oh, and this amp can be plenty loud. Last one I worked on seemed like it was going to levitate itself offa the bench, once I cranked it up. You could simply try a 100k Ra, 1.5K Rk, 25uF bypass on the first triode and check out your results.

RWood

[Rockn]

I did look at TheHoj work on here and have gotten some ideas out of that. I would like to keep the 6EU7 section just to see how it works out instead of rewiring it for a 12AX7.

I have been looking at a few circuits online as far as preamp sections and there is on that looks faily simple that is on a SE plexi build. It gives the option for building it with parallel or cascaded preamp section. The only problem I am having is getting the plate voltage to something useable. The schematic shows 43v on the plates which seems pretty low even for that tube. I did change out the plate and cathode resistors and got a plate voltage of around 55V and it did make it a bit louder and cleaner.

I was looking at the scheatic as well and it has 240v on the plates and on the cathode...is that hte weird biasing scheme you are referring to...LOL. It looks like that would keep the 12AX7 form being driven at all and keep it at a constant amplification state.

I have taken most of the high voltage taps off of the back of the amp so far, but did find that there is also a 70V tap back there that needs to come off. BTW, I see no UL stickers anywhere on this thing....:O Kind of scary that these were sold to the public back then. I did just look and there is a UL screened on top of the chassis....must be like home inspectors "yea....looks good"

EDIT: I just tried the amp out again and I do like the way the amp is now with the exception of the bass muddiness, it is like the bass and some mids are getting squashed and really do not sound too good. THe current bass control really doesn't od anything and maybe a tonestack rebuild would be in order. And maybe just make a hi - low input and cascade the preamp section for a little more drive.

I do like the way it sounds otherwise and when it is turned up past 5 it really barks. I wonder how a telecaster would quck on this thing?