Bogen CHB20A mods

[twister526]

I have started converting to guitar, added jack, and done a few resistor changes. My goal is to have some gain and a control for the gain. I have some gain, and some breakup at high vol. I would appreciate any ideas and mods for this project. I am just getting into tube amp projects, learning as i go, very aware of safety issues, and practice good safety on any project. Thanks for any help!

[Baxtercat]

I had a Bell PA [probably pretty similar] that I turned into a 'fake' Marshall. I just wired my input jack to the microphone input. It had tons of gain already.
I made a cab for it, tolexed it dark green, etc.

[twister526]

CHB20A

Thanks for the reply BAXTERCAT, the CHB20A has one 6eu7 preamp tube and two 6gw8 or ECL86 power tubes. Im not sure how similar it is to your Bell, I have allready added the 1/4 jack for guitar input into the mic input section, i have made a few resistor changes just trial and error type thing, have had some luck, I get some overdrive type gain at high volumes, would like a little more sooner i guess you could say. I have read about project to change CHB20A to marshall type, but not sure I have enough no how to overcome unforseen problems while converting? Just getting into tube projects, have alot to learn but willing. I did build the 12au7 tube overdrive boost effect, 9V. I am currently running that in front of my CHB20A, sounds great just looking for more gain. Have any pictures of your completed Bell project? Thanks again!

[twister526]

CHB20A

Just realized you have link to pic of your project, looks great! nice job. I can only hope mine will turn out so well. Have any sound clips of your Bell/Marshall?

[olddawg]

I've done a few conversions. Right now I am converting an old Dewald mono hifi amp that came with 2 12AX7s, 2 EL84s, and an EZ81 to yet another single channel hot rodded 18 watt Marshall clone. It has been my experience that most amps (even PA amps) do not sound like a decent guitar amp when you just slap a 1/4" jack on the input no matter how pretty the box is that you build. They tend to use wierd tubes, the voicing is wrong, many have NFB circuits, etc, among other things. I just gut them and use the chassis, transformers and maybe some of the pots and tube sockets. I usually make a new turrent board or use perf board (point to point is much more difficult to pull off) and rewire the whole thing from a known good guitar amp schematic and use the tubes I want. It isn't that expensive. The trick is to have the right transformers to start with. It helps a lot if your project amp already has the tubes that you want. There are a lot of sites out there that will help you as well as this one. 18 watt.com is good and its associated links 36 watt, 45 watt and many others.

[Baxtercat]

I hear you on that. They usually have too much bass, and this one was too hot. I thought I loved it in the house but got tired of the 'grind' on every tune at gigs. I ended up pretty much rewiring into a Fenderish diagram [just two stages—like a Deluxe, no 'verb] and it's now a right-on little box. I changed filter caps, added a standby switch, old computer fan...

Twister; I got lucky w/ this salvage. The thing runs more conventional tubes. It had two blackplate 6L6s, and I'm probably getting about 35 watts.

If you want to try heating yours up you could experiment w/ the 1st preamp tube cathode[s]: a smallish cap, 5 or 10 mf, paralleled w/ a 1K resistor.

I found an EM-80 and added it....for no other reason than to make it even more goofy. The tuning eye tube glows the louder the guitar signal.

I'll stick up another pic or two hereabouts.

[antiunderscores]

im still learning myself and i have no idea how much (or how little you know) so here's an uber summarised mod list for the preamp section. By changing the preamp it allows you to access higher gain levels while keeping the volume down:

to increase gain:
-increase plate resistors
-decrease cathode resistors

to brighten amps:
-bright switch (comes in many forms but an easy example is a small value capacitor i.e. 100-300pf over the equalizer/volume control)
-caps over cathode resistors
-caps over volume control
-smaller/remove caps over plate resistors (if there are any)

to mellow amps:
-caps or larger over plate resistors
-caps to earth from cathode


Look up your preamp tube/s pin patterns and see what each one does (a good site is the valve museum) so you can change the right resistors/caps. Unless you have access to schematics with this tube type make small changes because you dont want to go overboard and ruin a tube especially if it's an unusual type.
For gain control a simple idea is a voltage divider (volume control set up found in most nearly all amps) in between gain stages (different tubes or different sections of the tube if its a dual triode like the 12ax7's). This means you can run a preamp tube loud so it distorts and then peg the volume back before the power amp. A standard value for this pot is 1M but anything from about 100k should work fine.

Sorry if that sound like i was talking down to you or anything, im just trying to cover the basics and others can expand on these for specific designs.

Edit:
It's kinda self explanatory but for decreasing gain do the opposite of the increase gain stuff.

[twister526]

CHB20A

Thanks for the info antiunderscores, I appriciate any input and help I can get. I have been doing electronics projects and repair for a long time now, but just getting into Tube amp building and mods. Learning alot from Music Electronics Forum, everyone seems really knowledgable, and willing to share the knowledge. I have done the mods you suggested, and they have all improved the amp. I am probably getting all im going to get with just the one preamp tube. I would like to add another stage if possible, and a standby switch, any help with those would be greatly appreciated. Thanks again! "Twister526"

[determined2play]

I have started converting to guitar, added jack, and done a few resistor changes. My goal is to have some gain and a control for the gain. I have some gain, and some breakup at high vol. I would appreciate any ideas and mods for this project. I am just getting into tube amp projects, learning as i go, very aware of safety issues, and practice good safety on any project. Thanks for any help!

Twister where did you hook up the new input. I have a Bogen Cha 20 that I am trying to adapt for guitar. I hooked it up to the Mic input wires, but I get a maasive amount of hum, also it sounds like the amp is being choked unless the guitar volume is wide open.

Need a lil help.

Thanks,
Chris