Machine Gun Amps
Notes on the tweed Bassman.

If you plan on working on, or building a tweed Bassman, there are a few things that you might want to know. But, there is a disclaimer: If you have an original tweed Bassman, don't touch it unless it's broken! Don't mod it! These are all time classic amps and are destined to become highly priced museum pieces. Here are a few notes on this amp:

1. The 47mmf cap on the plates of the phase inverter is a 47pf. The local shop may not know what you are talking about if you ask for a 47mmf! This cap is in there to help control oscillation. It's a good idea.

2. Ken Fischer and Gerald Weber talk about the tone stack on the Bassman, and how it deviates from the schematic in practice. Ken Fischer says that the .02 cap in the "middle" (actually the bass cap) is usually a .1. Gerald Weber also says he's seen a lot of amps this way, but he personally prefers the .02. Also, you may notice that the reissue Bassman has a totally different tone stack than this schematic I've got up. Gerald Weber also says he's seen original amps that have a tone stack similar to the reissue (the .1 cap, plus a 100k slope resistor instead of 56k). Fender probably directly copied a particularly good sounding original and it was this way. For what it's worth, I built my project 5F6-A with the schematic here and with the reissue schematic, and I like the old one better (with 2 .02 caps and 56k slope resistor).

3. The previous version of the Bassman (5F6) had a 25uf-25v bypass cap on the second tube (12AX7) across the 820 ohm cathode resistor. This added a little gain. You might like this if you add it to the 5F6-A circuit. Also, you'll notice that just about every other Fender amp has "grid stopper" resistors on the grids of the power tubes. These are usually 1.5k's. They help control oscillation. It's not a bad idea to install these. It's unlikely that you are going to notice a difference in tone.

4. A lot of people run a 12AX7 in the first socket instead of the 12AY7. The 12AY7 sounds nice, but the 12AX7 will give more gain. A 5751 also sounds nice in this slot! I sell mil spec versions of all these tubes. Go to my tubes page to check them out.

5. Original power tubes in these Bassmans were Tung Sol 5881's. Tung Sols are still available, but are very rare and expensive! The Philips 6L6WGB is nice for these and will give tone that is in the ballpark of the Tung Sols for less than 1/3 the price. I usually have these in stock, go to the tubes page to check them out. Regular 6L6GC's work fine as well. If your power transformer can take it, EL34's sound nice in these too. They give that early bluesbreaker tone.

6. The first Marshalls were virtually identical to this schematic. One exception is the feedback loop was connected to the 16 ohm tap on the power transformer on the JTM45, compared to the 2 ohm on a Bassman.

7. If you are building this amp from scratch, it's a good idea to install a blackface or later Marshall style bias supply with an adjustment pot. This will not change tone in any way, but the amp will be VERY much easier to bias. I use the standard blackface method in my Bassman project, but there are certain advantages to using the Marshall method. Some of the resistor values will need to be adjusted to get the proper range out of the pot. You want to have something like -35 to -55 volts to be the range.

8. Anoter area of concern with this schematic is the filter supply. Fender put the whole filter supply after the standby switch. The inrush as the filter caps charge up can cause the rectifier tube to arc and possibly short. This has been particularly a problem with the new Chinese GZ34/5AR4. The best solution is to put the first two caps before the switch or just use a blackface style power supply. If you decide to use a solid state rectifier permanently, then this is not a concern. The standard filter supply that you see on the schematic will be fine.

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Last updated 7/11/99