Fender Bassman 5B6 amplifier

I wanted a nice clean amp to play rockabilly tunes on. I'd heard good things about the Fender Bassman line and found the 5B6 model over at the Fender Amp Field Guide. I've never heard it but it's a bit simpler than the more popular 5F6-A model and (I think) lower power. It may sound like crap, but if so I'll have to tear it apart and build it into something else.

• schematic
• layout

Below are some high-level thoughts on how this circuit works. I'm not an amplifier expert so there will probably be some mistakes.

Preamp

This is a pretty simple cathode-biased triode preamp stage. Looking at the load line the gain appears to be around 60. One different thing about the 6SC7 preamp tube is it has a single cathode that is shared between both triodes. This works for this input stage because both the inputs are similar signals. If different frequency responses were needed we would want something with separate cathodes (like a 12AX7, for instance).

The two plates are tied together for a single output to the tone/volume circuit. This prevents separate tone/volume controls for the two inputs.

Tone/volume control

The tone control here is a simple lowpass RC filter (aka a "treble rolloff"). Not much else to say here.

Phase Inverter

The signal arrives at the V2a grid (from the volume pot) and is amplified at the V2a plate. The gain here is ???. A small fraction of this output is voltage divided out between R9 and R14 and fed into the V2b grid. An amplified version of this appears at the V2b plate. Because a tube inverts its output signal the signals at C6/R9 and C7/R10 will be 180 degrees out of phase with each other.

Poweramp

I don't yet understand this well enough to say much. It's a push-pull amplifier.

Amplifiers are expensive to build. The transformers alone for this project would run a good $150. For much less than that I got an old Conn organ amplifier off of ebay with all the transformers and tube sockets I need plus a chassis that will probably work fine for my project. And I get some nice old tubes that I can reuse.

(click for full size)

(click for full size)

I checked the B+ line with all the tubes except the rectifier pulled, it's reading 480 VDC. Strangely, the filter caps seem to be holding a voltage fine, it took about 5 minutes to drain down to 20 V. Hopefully under load this will drop down to the 400 VDC that I need.

There's a few things in this amp I won't be using. There's a 3rd transformer that appears to be some sort of line-out transformer for an external amplifier. I have no use for it. There's also a small choke in the power supply circuit that I don't think I'll need.

The 5B6 uses 5881 power tubes, which are essentially identical to the 6L6's in the organ amp. So the output transformer ought to work for my amp. But there's 3 secondary taps and I didn't have a datasheet for this ancient transformer. Someone from a forum showed me a trick to measure the impedance ratios of a transformer.

I took a function generator and set it to output a 1 Vrms sine wave at various frequencies. This 1 V signal is connected to a secondary tap and the voltage at the primary is measured. The impedance ratio is (Vp/Vs)^2. If you multiply this by your speaker impedance you get the impedance seen by the output stage.

(click for full size)

The output stage needs to see an impedance of about 4 kOhm. So it looks like my output transformer will provide 8 and 16 ohm outputs (perfect for guitar amps) and a 32 ohm output that won't be useful for me.

For my reference and anyone who happens to find an amp with these transformers, here's what I figured out for the wiring.