1974 Benson 300-H

This is an extremely rare hybrid tube/solid-state guitar amplifier made by the Benson company - which is not related in any way to the current company from the pacific NW also called Benson. Co-designed by studio session musician Howard Roberts and businessman Ronnie Benson this was intended to be a cutting edge recording tool made especially for session guitarists. Howard Roberts was a Jazz session guy who also played on Tom and Jerry cartoon soundtracks. Benson as a company only lasted for a few years due to manufacturing and marketing difficulties, so there are only ten examples of this amp still around at this point - that we know of. It has one of the oddest schematic designs I have ever seen and the internal construction is just as idiosyncratic. The two preamps are each designed around a 12AX7 preamp tube, but each channel has a very different tone stack to set them apart. Channel two actually takes part of the tone stack and puts it into a replaceable plug-in module. Apparently the company said in its promotional literature that they were going to manufacture 6 different modules that specialize in different genres and frequency ranges. However it seems like they only ever got around to making two of these odd 9-pin yet octal sized socket modules. The module has a resistor and two capacitors that change the values of key tone stack components through a multiple parallel wiring scheme. This is a really cool idea that I think could be used in new designs - except this weird module base on the Benson is octal sized yet having 9 evenly spaced pins instead of 8 - it is almost impossible to find, but some Leslie speaker/amp units used this socket and plug attachment. The power amp is a pair of 6CA7/EL34 in fixed bias (50 Watts RMS)and came from the factory biased extremely hot, which likely lends to its unique tone. The power amp is fed by a Cathodyne phase inverter (also a 12AX7) that is normally only found in amplifiers with less than 20 watts of power. The effects are all Solid-State and designed around uncommon transistors that are impossible to replace exactly with modern units. It has a brutal Fuzz Face/Octavia type fuzz built into the amp, a very wet Spring Reverb that sounds darker than a Fender, and a Tremolo that is nearly a square wave in its oscillation. This is all rounded out by a very high-end and expensive Altec 418B 15” speaker that is more often seen in old Hi-Fi systems from the 50’/60’s. Nothing about the amp is normal or related to any other design I have seen. It was very cool to get to work on one and play through this beast.

This amp came in to my shop in completely original condition. This one needed a full restoration including a cap job, new power resistors, cleaning, a 3 prong AC cord, biasing, and some hardware.

Amp was completely restored. All axial-leaded filter/bias/bypass caps were replaced with upgraded voltage and temperature rated MOD/Vishay brand electrolytics. The electrolytic cap can was replaced with a new USA made CE brand (40/20/20/20uf x @ 525v) unit for the filter section. The high voltage filter section was re-designed to use just one 4 section cap can. Originally there were two 2-section cans present (these had lower voltage ratings), attached to these were more electrolytics in a balanced series topology. This entire circuit was made much simpler thanks to a modern capacitor can with a 525 volt rating, negating the need for the multiple 450 volt cans in series to add up to a +500 volt rating. The hole left by the unused can capacitor was covered with a friction fit nickel plated unit for safety. New 2 Watt Metal Oxide power dropping resistors were installed for better reliability and lower noise floor. The "ice-cube" style electro-mechanical relay in the Fuzz circuit was taken out and its contacts were cleaned with de-oxit so it would switch in/out better. A new 3 Prong AC cord was installed to replace the worn out original. There were a number of broken wires and cold solder joints that were making Channel 1 have some terrible humming/buzzing noises and stopping the solid-state Tremolo from working intermittently. All of the circuit boards were completely taken out of the amp and these connections were re-soldered and fixed.

The original tubes all tested good, except for the V3 12AX7. This was replaced. Final tube lineup: V1-V2 = GT 12AX7, V3 = JJ 12AX7S, V4-V5 = GT EL34 Matched Pair. The EL34’s Fixed Bias was set to a hot %85 Class AB Plate Dissipation with a B+ of 482VDC. The factory spec for biasing this amp sets the plate dissipation far beyond what is safe for 6CA7/EL34 power tubes - 70% is the standard max PD, 132% PD is where the tubes were at when using the factory spec guidelines. So a compromise was used placing the tubes at the absolute safe limit listed anywhere which is %85 Class AB PD, this was done so that the B+ was also not unsafely high - B+ rises as PD% goes down. I'm not sure what the designers of this amp intended with their guidelines, but either they are a misprint, or they were depending on NOS military stock tubes which could often be used far above their ratings suggestions/limits.

The tube sockets were re-tensioned to stop the tubes from jiggling or falling out while they hang upside down and they were all treated with De-Oxit to remove/prevent corrosion. The pots were sprayed out and the amp was cleaned inside and out. New hardware for the input jacks were installed as the original isolation washers and nuts were gone. Amp now functions perfectly.