1972 Fender Bassman 100
The Fender Bassman has gone through so many different iterations that they, in actuality, constitute a plethora of distinct and in many cases unrelated circuit designs that just happen to share the same name. The Bassman 100 is the perfect example of this. Unlike it’s far more famous 1950’s Tweed cousin (The amp that inspired the Marshall JTM-45 and defined early overdrive sounds) this amp is one of the cleanest and most Hi-Fi sounding of any Fender amp produced. It reaches that 100+ Watt RMS rating easily, in fact it’s enormous Output Transformer is actually larger than the Power Transformer. The OT on this amp really defines it’s tone and is hard/impossible to saturate keeping things very dynamic, punchy, and sparkling clean. That, along with it’s unique two channel setup, one designed for the frequency range of Bass Guitar - named Bass, and the other designed as a standard Silver-Panel Guitar preamp - named Normal, set this amp apart from the rest of the 70’s lineup. It is also dead simple inside and out sporting no Reverb or Tremolo - which also makes it a great amp to mod. This amp came in the shop in original condition - it had blown power tubes, burned open screen resistors, burned grid stoppers, and leaking filter caps. It needed a full restoration and that is what we did.
All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated MOD/CE brand electrolytic’s. New 2 Watt Metal Oxide/5 Watt Cement power dropping resistors were installed for better reliability and lower noise floor. The cathode resistors for V3 were replaced with 2 Watt Metal Oxide/3 Watt Cement type units due to heat damage. All plate resistors were replaced with 2 Watt Metal Oxide type units for preventative maintenance and reliability. All screen resistors were replaced with 3 Watt Cement 470 ohm type units for same reasoning. All power tube grid stoppers were replaced with 1/2 Watt Carbon film type units due to heat damage.
The Bias Supply was given my standard modification to be an Adjustable Fixed Bias control rather then the original Balance control - this allows the power tubes to be set to idle at the correct current/voltage.
The original tubes did not test good and were all replaced: V1 = JJ 12AX7MG, V2 = JJ 12AX7S, V3 = EHX12AT7, V4-V7 = JJ 5881 Matched Quad. These tubes were picked for best tone, reliability, and to lower the overall volume of the amp and bring on earlier power tube distortion. The switch to 5881’s (approx. 75% of the output power of a 6L6GC) was desired by the customer so that it would be easier too use in lower volume situations and to not have it be so pristinely clean in tonality. The 5881's Fixed Bias was set to %57 Class AB Plate Dissipation with a B+ of 430VDC. All tube sockets were cleaned with De-Oxit. All new screws/hardware were installed to keep the front grill in place, the original wood screws were all gone and the holes stripped out. Larger stainless steel screws and washers were used to provide good grip to secure for the future. The pots were sprayed out with De-Oxit and the amp was cleaned inside and out. A new 3 prong AC cord was installed to replace the frayed original. Amp now functions well.
