Fender Bassman 5B6 amplifier - Projects - Audio Artillery

Fender Bassman 5B6 amplifier

  1. Intro
  2. Circuit analysis
  3. Parts from the graveyard
  4. Output transformer characterization
  5. Transformer pinouts
  6. Layout
  7. Chassis finishing
  8. Labels
  9. Wiring it up
  10. Testing
  11. Debug
  12. Hum hunting
  13. Too much bass
  14. Hum hunting II
  15. Phase inverter distortion
  16. Hum hunting III (power supply ripple)
  17. Bass control
  18. Master Volume
  19. Bumping up the power supply voltage
  20. Experimenting with 6V6 power tubes
  21. Cabinet
  22. Sound clips

11: Debug

The 120 V Zener diode dropped B+ down to 402 VDC - perfect. But it's still quiet and distorted. The layout suggests the 3 filter cap voltages should be 400, 320, and 300 VDC but I'm seeing 402, 387, and 375 VDC. The cathod of the preamp tube ought to be 2.1 V but instead it's at 1.6 V. It turns out this is because I didn't connect the 6L6 screens to the power supply!.

voltages with 120 V Zener installed but power amp tube screens floating
NodeVoltage (V)
preamp plate, V1a/V1b113
preamp cathode1.64
phase inverter plate, V2a60.6
phase inverter plate, V2b61.4
phase inverter cathode, V2a1.09
phase inverter cathode, V2b1.04
power amp plate, V3416
power amp plate, V4413
power amp cathode, V3/V40.502
power amp screen, V3/V4-0.994

With that corrected things look and sound much more normal:

voltages with 120 V Zener installed
NodeVoltage (V)
preamp plate, V1a/V1b81
preamp cathode1.16
phase inverter plate, V2a38
phase inverter plate, V2b39.2
phase inverter cathode, V2a0.85
phase inverter cathode, V2b0.82
power amp plate, V3324
power amp plate, V4312
power amp cathode, V3/V423.3
power amp screen, V3/V4283

Now it sounds like a real amp. It gets loud and stays clean. There's some hum and the volume never shuts off completely, but the main concern is B+ being 320 V instead of 400 V.

voltages with 100 V Zener installed
NodeVoltage (V)
preamp plate, V1a/V1b89
preamp cathode1.25
phase inverter plate, V2a43.8
phase inverter plate, V2b45.2
phase inverter cathode, V2a0.905
phase inverter cathode, V2b0.87
power amp plate, V3346
power amp plate, V4333
power amp cathode, V3/V425
power amp screen, V3/V4305

It looks like a 50 V Zener would be ideal. These are only available rated for 10 W. Is this good enough? RC-30 says two push-pull class AB 6L6 tubes can sink 140 mA. The smaller tubes will only draw 1.5 mA between them. 50 V * 140 mA = 7 W, so a 10 W Zener should work.


12: Hum hunting

Things I've tried so far to eliminate the background hum:

  • Wired the power tube heaters in phase.
  • Rewired all the heaters with a really tight twist and careful layout.
  • DC biased the heater center tap at 25 V (tied to power tube cathode).
And... no change so far.

With some help from a couple guys on some forums I found out I had the cathode connections swapped on the phase inverter. How did this work at all?

voltages with 100 V Zener and correct phase inverter cathode wiring
NodeVoltage (V)
preamp plate, V1a/V1b84.6
preamp cathode1.25
phase inverter plate, V2a41.7
phase inverter plate, V2b43
phase inverter cathode, V2a0.87
phase inverter cathode, V2b0.82
power amp plate, V3339
power amp plate, V4339
power amp cathode, V3/V421.9
power amp screen, V3/V4305

More help from a forum guy and I found out I had wired R4 in series with R7 and R8 instead of in parallel. This caused really low PI plate voltages.

voltages with 100 V Zener and correct phase inverter plate wiring
NodeVoltage (V)
preamp plate, V1a/V1b119
preamp cathode1.67
phase inverter plate, V2a164
phase inverter plate, V2b164
phase inverter cathode, V2a2.44
phase inverter cathode, V2b2.44
power amp plate, V3347
power amp plate, V4347
power amp cathode, V3/V422.5
power amp screen, V3/V4305


13: Too much bass

This will sound unsurprising but there's way too much bottom end in this Bassman. It sounds fine with a telecaster bridge pickup but with the neck pickup (which as a lot more bass response) all you hear is the bass strings shaking the windows. According to the Valve Wizard, the half-boosted frequency is:

f1/2 ~= 1/(2*pi*Rk*Ck)
where Rk and Ck are the cathode resistor and capacitor. For the 2500 ohm/250 uF combination this comes out to about 0.25 Hz. Great for a bass amp (as far as I know). Replacing the 250 uF with 1 uF the cutoff is around 63 Hz. Since the low E on a guitar is 82 Hz this is just about right.

Initial testing showed it sounds much better. But there was still way too much bass. I put another 1 uF cap in series to bump the cutoff up to 127 Hz. This did the trick. It's still got a lot of bass response but it sounds bad-ass instead of just boomy.


14: Hum hunting II

Armed with a cheap oscilloscope I now have more visibility into the signal chain. Here is what I see with inputs grounded.

At the output of the preamp what looks like a 100 Hz 10 mVpp signal with some jaggedness to it. Grounding the preamp plate doesn't make the audible noise change.

(click for full size)

At the V2b phase inverter plate a very complicated signal that measures 5 mVpp and I think repeated at 200 Hz. The image was taken with the preamp plate grounded.

(click for full size)

At the power amp plates there is this 1.5 Vpp, 200 Hz sawtooth signal. Is that my hum? I'm not sure.

(click for full size)

V2b plate (upper) and V2a plate (lower). V2a plate shows 20 mV @ 120 Hz signal that looks really bad.

(click for full size)

An attempt at plotting the load line for V2a based on voltages measured above. Looks like we're in cutoff?

(click for full size)

I'm not sure what to make of any of this. The noise level at the phase inverter seems too small make a big difference. I think the sawtooth at the power tube plates just indicates the power tubes aren't perfectly matched.


15: Phase inverter distortion

No phase inverter is perfect, especially not the "paraphase" type used in this circuit. The below oscilloscope shots illustrate the mismatch of the PI outputs. The first shows the two outputs and the 2nd shows the two outputs summed.

(click for full size)

(click for full size)

You can see in some cases one side of the wave is a little larger than the other. This generates some distortion since the positive half of one output and the negative half of the other are amplified by the power stage to construct the entire waveform. Ideally the two PI outputs would sum to zero. The two levels can be fine-tuned by adjusting R14 (or so I'm told) but I may just leave it be, it sounds good.

Here are some the videos thatt these were taken from:


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