Bogen CHB10A to Guitar Amp

[jezocaster]

Recently acquired a Bogen CHB-10A Amp and want to convert it to a guitar amp. Has anyone had success with this? Any ideas or schematics of one who's done this?

Thanks for the Help.

[petemoore]

Sure

Do you have electronics experience ?
Counting the tube compliment gives you some idea as to what schematics share the tube compliment of the 10, knowing that makes it much easier to proceed with suggestions.
The first thing is to understand the dangers involved, and have someone else do it if you're in a rush and dont know about B+ dangers.
Getting a 3 pronged AC supply [separate ground bolted to the chassis] would the first order of business after unplugging the amplifier and measuring a safe B+ voltage [0.0vdc is safe].
http://geofex.com/Article_Folders/Po...s/powersup.htm
Aiken Amplification and Watkins, AX84....read the warnings and understand what is required to eliminate dangerous AC and DC voltages while you have your amp open.

[jezocaster]

Appreciate it

I've modded a Valve junior over and over, then gutted it and rebuilt it point to point, replacing xfmrs, sockets etc. keeping just the chassis and tube complement. If its got a schematic, I can build it. Theres alot out there for the valve juniors, but not for this unit.
This is another two tube amp. The existing schematic is a little unusual (to what I'm used to) with a 470K Plate Resistor and no cathode resistor on the first stage. I'm not opposed to either slightly modding what is there, or something more extreme. Mainly I'd like to keep the existing tubes & transformers to keep costs down. I forsee this as a 10 watt practice/studio type amp. Any help would be appreciated.

[petemoore]

I've modded a Valve junior over and over, then gutted it and rebuilt it point to point, replacing xfmrs, sockets etc. keeping just the chassis and tube complement. If its got a schematic, I can build it. Theres alot out there for the valve juniors, but not for this unit.
This is another two tube amp.
Getting an IEC in there can be a bastard, but that green wires eyelet bolted securely to the metal chassis, ground tested to conduct to the wallplugs offset pin.
The existing schematic is a little unusual (to what I'm used to) with a 470K Plate Resistor and no cathode resistor on the first stage.
I'm all for leaving stuff alone, but goodness that's either a cathode stage with a great bigger resistor than ever or...something else...I would think the gain would make it oscillate. @@Rate, all those Fender schematics show input stages with the 1k5 or 2k2 w/bypass on cathode, then 100k or 220k plate resistors. 470k sounds more like a grid bias R.
I'm not opposed to either slightly modding what is there, or something more extreme. Mainly I'd like to keep the existing tubes & transformers to keep costs down.
Me too, basically stick with what works...
I forsee this as a 10 watt practice/studio type amp. Any help would be appreciated.
See what's up with stage 1, measure say R of plate to it's rail, then grid to ground R, cathode to ground R...follow and draw anything else going on in there.
Then you can know what you got in there, and how far that is from what you like or think you might like.
Just setting the tube up to work in the well worn fashion...>470k grid bias resistor, ~1k5 w/<47uf bypass under cathode, 100k plate resistor, 68k input resistor for say the first stage. There's probably a similar arrangement for stage 1 in the Valve Jr. ?...that you modified...IIRC there's a 1meg R in the signal path on that one.
Some of those resistors can be pinlug rippers to get out. The ones I want to smaller value get a clean rings scraped around it's leads, parallel resistor of 'x' value tagged on to = the value I want there. much easier than trying to rip the amp apart trying to remove it, you have to watch because the pin/lugs can get to moving around if they're distorted in shape, they get hot and can break, best to not disturb them as best you can.

[Dave Curtis, dB AudioTech]

I've modded a Valve junior over and over, then gutted it and rebuilt it point to point, replacing xfmrs, sockets etc. keeping just the chassis and tube complement. If its got a schematic, I can build it. Theres alot out there for the valve juniors, but not for this unit.
This is another two tube amp. The existing schematic is a little unusual (to what I'm used to) with a 470K Plate Resistor and no cathode resistor on the first stage. I'm not opposed to either slightly modding what is there, or something more extreme. Mainly I'd like to keep the existing tubes & transformers to keep costs down. I forsee this as a 10 watt practice/studio type amp. Any help would be appreciated.

Schematic:
http://www.schmarder.com/radios/sche...n-chb10a-4.jpg

First things first; make sure it's operating properly as is. Then start changing it. I have a bunch of Bogen CHBs, but mostly 100s and 50s.

The first stage on this amp uses grid bias (aka: contact bias). As Pete suggested, set it up similar to a Fender (or your favorite guitar amp) input stage (but put the 68K grid resistor right on the grid pin). This will set up the input impedance to better match the guitar. You might want to start off with a 220K plate resistor and adjust accordingly.

Jacks?

You'll find the screw-on input jack is mounted on a disc of wafer board. Inside the amp, the case of the jack has been swaged over a ring terminal for the ground connection. If you pull the ring off (with pliers), you'll be able to pull the jack out. Very much like pulling teeth, you have to work it around in a small circle, and it will pop out. Reuse the existing wafer piece to install a Switchcraft 12A, and it will be isolated from the chassis. For the output jack, I drill out the rivets on the existing one and use reducing washers (electrical hardware) that are made to fit a switch into a 1/2" conduit knockout hole. I also use a Cliff/Rean jack as a Switchcraft is rather small for the hole. If you'd rather keep the 3-pin, I've got one of those plugs with about 5' of wire on it...

The attached pic shows the new jacks and how the reducing washers had to be trimmed to fit.

[jezocaster]

Thanks!

All voltages are within 5-10% of the schematic. Amp works, overdrives very nice. Loudness about what you'd expect for a 5-10 watt amp. Using a strat and with amp Treble control maxed and , still not enough high end. Probably extra muddy with a humbucker.
Attached is what I plan to do with the first stage (copied largely from Gibson GA5) and using the 200K you recommend. Should I do the same with the second half of the 12AX7? That jumpered point between R8, R9 and R11 is different than what I saw with the junior. Is that whats called a negative feedback resistor there?

By the way THANKS to Everyone for the help!!! I'm very green at the engineering side of this.

[Dave Curtis, dB AudioTech]

All voltages are within 5-10% of the schematic. Amp works, overdrives very nice. Loudness about what you'd expect for a 5-10 watt amp. Using a strat and with amp Treble control maxed and , still not enough high end. Probably extra muddy with a humbucker.
Attached is what I plan to do with the first stage (copied largely from Gibson GA5) and using the 200K you recommend. Should I do the same with the second half of the 12AX7?

Just do the first stage for now and give it a listen. That might be all you need to do. While the 68K grid resistor is OK on the jack side of the 1M (like Fender did), it's better to have it right on the tube pin.

That jumpered point between R8, R9 and R11 is different than what I saw with the junior. Is that whats called a negative feedback resistor there?

R11 is the NFB resistor; you could put a switch somewhere in that loop, or replace R11 with a 100K pot for adjustable NFB.

[RogerWilco99]

I have a Bogen CHB14A that I'm going to rebuild. I would think they would be somewhat similar. Here's the schematic for it. Hope that helps. You can buy copies of the owner's manual for many of the Bogen amps on eBay. They include the schematic for that model. That's where I got mine.

[gldtp99]

Thanks for the CHB14A schematic--- i've got a CHB14 here (w/tube rec) that's similar--- haven't worked on this one yet.
I've also got a similar Bogen MU-10 head that i have converted for guitar use--- this also uses ECL86 x 2 (cathode biased power pentode sections---- as opposed to the CHB20A that uses ECL86 x2 in fixed bias--- and larger iron) but has a 6eu7 pre tube---i've replaced the filter caps using two F&T 16/16uf/450V cans---- i've re-voiced the preamp, moved the guitar input to the front panel, removed the Aux inputs, removed the Mic channel Vol pot (Master Vol pot is now the "vol")---- i've kept the Treb and Bass cut slide switches.
With no tone stack this amp has a very aggressive response---it's on "Boost" at all times but cleans up pretty well with the guitar vol pot--- i've re-voiced to limit the bass response that would get muddy at higher vol levels---- when cranked up thru a 4x12 cab it's pretty impressive if you're in the mood to rock out. People seem suprised that a 10 watt rated amp can put out that much sound---- but a loud drummer and bass player would still bury this amp in a live, un-miked situation-- some players have liked the tone of this Bogen MU-10 better than my real Marshall JMP 2204, and several modded 2204 clones i've built (50 watters, 25 watters, and a switchable 14/7 watter) ----i keep it around my workshop to plug into if i feel like rocking out without blasting ears out---i get enough of that from testing bigger amp builds----i also keep a BF Princeton-type head i built (w/1964 Schumacher 4/8/16 ohm OT pulled from a Bell 6V6 x 2 PA amp) for lower vol clean/pedals use--- very nice sounding general purpose lower wattage head.
The CHB14A isn't built much like his CHB10A----- the CHB14A is a push-pull amp using two ECL86's as the PI triodes and output pentodes--- the CHB10A is a single ended design using one 7868 output tube and is built on a smaller chassis than the larger sized CHB14A----- the CHB14A- sized chassis was also used for CHB20, CHB35A, CHB50, CHB100 heads---i have or have had several of all these different sized Bogens.
I also had a CHB10A that i converted for guitar use---i agree with the posts about converting the first gain stage and then deciding to go farther if needed------ It also might benefit from new power supply filter caps---- mine was sold on ebay several yrs ago and the owner liked it very much.........................gldtp99

[jezocaster]

First stage

Got the First stage rebuilt....finally.
Used a 47K input grid resistor, 220k Plate resistor,
2.2k Cathode resistor paralleled with a 22uf cap
Initial Sound was MUDDDDDD. Replaced 22uf with a 1uf and the brightness returned. Overdrive is good, but begins way too early, about 1/4 volume. It allows a variety of Distortion from subtle to Crunch, but I'd like more clean headroom before getting here.
My plate voltage is still only 88 volts, up from original 73 with 470k Plate resistor. (original Bogen setup had No Cathode resistor or capacitor).

AJ

[jezocaster]

Changes and questions

Ended up changing 1st stage Plate resistor to 110k, Cathode resitor to 1.1k.
Sounds better. First stage Plate voltage about 150 Volts.

Changed second stage cathode resistor from 470 Ohms to 1k and added a 22uf Cathode reistor. Sounds Better, but plate Voltage on pin 6 is almost 160 volts,
Is this too high?

Also Don't understand what's going on at the original junction point of R9, R11 and the second stage cathode resistor. Have I negatively affected this circuit by replacing the original 470 Ohm Cathode resistor with the 1kOhm and 22uf pair? I see that R11 is the NFB from the output, but I don't understand why you'd want the voltage tied here through the 150k Ohm R9.