Fender Bassman 5B6 amplifier
- Circuit analysis
- Parts from the graveyard
- Output transformer characterization
- Transformer pinouts
- Chassis finishing
- Wiring it up
- Hum hunting
- Too much bass
- Hum hunting II
- Phase inverter distortion
- Hum hunting III (power supply ripple)
- Bass control
- Master Volume
- Bumping up the power supply voltage
- Experimenting with 6V6 power tubes
- Sound clips
At the first filter cap (B+1) I was measuring 16 V of ripple. That's about 4.6% of 350 V, pretty decent especially for a push-pull amplifier. In my layout I used a single 33 uF cap instead of two 16 uF caps. Since I've been having problems with 120 Hz hum I tried adding another 47 uF (80 uF total) cap here. This brought the ripple down to 2.3%. There was a noticeable reduction in hum but it's still there. My theory is there is that ripple is somehow leaking into the preamp or phase inverter stages.
Mucking with the preamp cathode capacitor helped quite a bit but was never quite enough. I decided to try changing the treble rolloff to a bass rolloff control. Someone suggested using the bass control from a Fender 5C8 "Twin" model amp. This worked very well. If I hadn't finished the chassis already I might have both treble and bass controls but I don't want to redo the chassis and usually don't find much use for treble knobs anyways (most guitars have them).
I tried a couple master volume circuits to see if I could get a little grit out of this amp at reasonable volumes. The first was a "cross-line" master volume (CLMV) where part of each side of the phase inverter is bled to the other side. The bled signal is 180 degrees out of phase with the main signal so it cancels out a portion of the signal going to the power tubes, resulting in a lower output volume.
The second was a post phase inverter (PPIMV) design by Bruce Collins. This uses a dual-ganged pot to simply attenuate the signal going to the power tubes.
Both of these methods were functional but I wasn't able to detect any distortion even with the (preamp) volume cranked all the way. I'm not sure yet why there isn't some distortion at max volume.
I had been playing around with smaller preamp plate resistors to bias the preamp tube hotter. Someone suggested decreasing R15 to give a higher voltage at the preamp plate. Earlier I had left the B+ voltage at 350 VDC, much lower than the spec'd 400 VDC. Swapping in a different Zener diode brought this up:
|preamp plate, V1a/V1b||132|
|phase inverter plate, V2a||198|
|phase inverter plate, V2b||200|
|phase inverter cathode, V2a||2.43|
|phase inverter cathode, V2b||2.48|
|power amp plate, V3||392|
|power amp plate, V4||392|
|power amp cathode, V3/V4||26.7|
|power amp screen, V3/V4||305|
The higher voltage resulted in a slightly clearer sound. The preamp tube may still sound better if biased a hotter, but this was an improvement.
One odd thing I noticed was the bass control will bring the preamp plate voltage from 132 V to around 40 V. It still sounds fine but I thought this was suspicious. I added a .1 uF coupling cap between the preamp tube and the tone circuit and now it's steady.
6V6 tubes have the same pinout and similar characteristics as 6L6 tubes but have less output and tend to distort earlier. I had some old 6V6's from an amp I retubed so I plugged them in. They were signficantly quieter. I think there's two reasons for this. First, 6V6's expect a plate load of about 8 kOhm vs the 4 kOhm for 6L6's, so the impedance wasn't matched very well. Second, I don't think the biasing is correct. I drew the loadlines for each:
I think the 6V6's would be operating entirely in class A based on the load lines, but I don't have a good understanding of that yet so may be wrong. I need to look at this under the scope to be sure.
In any case, the output transformer isn't matched well for 6V6's unless I switch to a 16 ohm speaker. I didn't care for the sound of the 6V6's as-is, but maybe properly biased and the right output impedance would make them sound good.