Bogen CHB-20A Rebuild Project




So I picked up an old Bogan CHB20A. It had a bad output transformer (of which I was not aware of when I bought it) so I changed that. One thing led to another, and I ended up rebuilding it stage by stage. What follows is what I did, a few notes, and documentation regarding the rebuild.

NOTE:At the time of this writing, it is not complete. ALL info is considered 'suspect' until the amp is done and I'm happy. I will remove this warning when that time comes.

And here are some pictures of the old monster in the condition I recieved it in.





For a preface, here is the schematic. (p.s. - You can click on most of the pictures for a larger view.)



First thing, was the preamp. I made the preamp exactly how it was, but removed all the inputs except for a 1/4" mono jack.

Here is a picture of the pre amp before:



Here is a picture of the pre amp after:



Here is my hand drawn layout of the preamp:



So.. the pre amp might be done.



Next, the power tubes.
Here is a picture of the power amp before: Notice the charred resistors and excess of bare wire where the shielding has been scorched off.



Here is a picture of the power amp after:



Here is my hand drawn layout for the power tubes: This is now rev. 2 of this drawing. You will notice R14 I have 75K and 82K. The schematic calls for the 82K, I used a 75K/2W since that is what I had on hand. Also, on this layout I added the 220K resistor I used for the NFB.



Here is a redraw of the layout for the power amp stage - provided by Nigel - THANKS Nigel!!!



At this time, I also chose to redo the heater wiring.
On the finished picture of the power stage, you will see on the left side there is a 3 lug terminal strip I used for grounding. I ran a bare wire between the holes to allow all 3 lugs to be grounded as opposed to just the middle lug.



NFB - Negative feedback:
Again, I had to replace the output transformer on this amp. In the schematic, you'll notice a second secondary winding coming off of the output transformer for the negative feedback. I tried, but could not find an output transformer that had that odd secondary widning, so I just got a regular OT. I used a 220K 2W resistor, just as specified in the schematic.



The Tone Stack:
NOTE: I have NOT yet modifed the tone stack. Also, I want to add, this is a James/Baxandall tone stack. I also have contemplated putting in a standard bass/trebel Fender based tone stack - as it might do a bit better for guitar.... Again, I'll post my final results/decision once I build it.
Coming soon





Here is completed/modified pre-amp & power amp shared by Nigel - friend of the site - Thanks!!!!



We're going to skip ahead and post this just so the info is available. Nigel, also sent a layout for the power supply. He added an extra diode in the layout ... as he put it "with one extra series diode for safety can be installed at builders discretion ( personally I like the extra protection )". Thanks again Nigel.