I just finished converting a Fender 59' Bassman Vintage to a Hoffman version 5F6-A and once again I had to change the main bias dropping resistor to get the bias supply to a range that worked for the amp. The Hoffman layout shows a 1k ohm resistor at the front of the bias circuit and in order to get the range where middle was 70% for several sets of test tubes I had to go with a 2k2 ohm value. I also noticed that the reissue board had a 1k8 ohm in that position. The reissue transformer was producing 79 volts AC out of the PT bias tap. So, that is probably why I had to change the resistor value. This prompted me to check the original schematic against the current one and I see that the original fixed non-adjustable bias voltage was -48 compared to the reissue value of -52. That's a pretty big difference and the reissue obviously runs most tubes pretty cold compared to the original. Of course the more recent "Vintage" version added a bias pot to adjust the voltage so one can get a hotter idle current. I ended up with -50 on the Winged C set to acheive 70% idle current and the 2k2 ohm resistor put that right in the middle of the pot's range.
So, I'm thinking the fixed bias of -52 volts, the sand rectifier, the cheap components and the bundling of wires all together are what contributed to the anemic sound of the orginal reissue. The Vintage model put back the tube rectifier and added the bias pot but kept the pcb and bundled wire so it too pales in comparison to a real 5F6-A or a Hoffman hand wired unit. Everytime I A/B a reissue with a hand wired unit the difference is night and day even to my old ears so it amazes me that Fender keeps doing things this way.
Topic: Finished another Hoffman Bassman (Read 4374 times)