1973 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. In fact, this amp already sported a mod to the Bass channel tone stack that altered the EQ and gain level to allow easier access to distortion. We decided to keep this because it sounded good, but also clean up the technique used in the installation of the mod.

This one came in with blown power tubes that also were loose/dangling in their sockets. It had previous work of mediocre quality done sometime in the past, with bad choices for screen resistors and broken wire connections present. The blown 6L6GC’S took out the undersized screen resistors and this also destroyed the resistor network in the artificial heater center tap.

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 artificial heater center tap was re-done with 1 Watt Carbon Film resistors as the originals were broken in half.

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 Balance control that Fender moved to with many of their Silverface amps only allows adjustment of how even the bias is between each pair of 6L6GC’s, it provides no way to adjust how hot or cold the bias is to the power tubes. This obviously is not a great design choice and is something I change right away anytime I see it present.

The original tubes did not test good and were all replaced: V1 = JJ 12AX7S, V2 = JJ 12AX7MG, V3 = EHX12AT7, V4-V7 = JJ 6L6GC Matched Quad. These tubes were picked for best tone, reliability, and increased headroom. The 6L6GC's Fixed Bias was set to %59 Class AB Plate Dissipation with a B+ of 420VDC. All tube sockets were cleaned with De-Oxit and re-tensioned with the power tube sockets getting all new Belton bear-trap style tube retainers - this solved the loose tube issue and noise caused by dirty connections. The pots were sprayed out with De-Oxit and the amp was cleaned inside and out. A new Fender style dial knob, a new fuse, a new Switchraft speaker jack, and a new 3 prong AC cord were installed to replace the broken originals. Amp now functions well.