This was a very popular P.A. amplifier with fledgling bands of the 1960’s. One of my earliest bands used the CHB50, 50 watt version of this 100 watt amp, along with a battery-operated cheap Japanese 4-channel mixer. You know, the gold ones.
This specimen was used extensively in the 1970’s with a Toronto band, and then was put into storage until a couple years ago when it was given to me to restore and put out for sale.
Other than the usual replacement of capacitors, no modifications were made, except the installation of a 3-conductor AC supply cable in place of the original 2-conductor, just for added safety.
This amplifier uses four type 7868 pentode tubes in push-pull/parallel to achieve the 100 watt RMS power output.
Upon testing of these tubes on my mutual conductance tester, it was found these were not very evenly matched. The Gm values measured had varied from 10 to 20%, but all were in the “good” range. The owner did not wish to spend the money on purchasing four new and matched 7868 tubes which are not common, and not in current production, therefore very, very, pricey. However, once the bias was set properly, this quad still put out a pretty clean 100 watts before clipping.
No other major issues cropped up during the 10 hour restoration, and the new owner is quite happy.
