Fender Bassman 50 AB165 Volume and Tone Issues

[misterc57]

Trying to get this amp right. Volume and tone act weird. Seems like the Bass and Treble pots have a big effect on the Volume pot. Or they all seem to counter-act each other.

Both channels have similar behavior. I will describe the Normal channel.

Volume at zero does not cut the volume. Signal is loud at zero and Volume pot behavior does not kick in until about the "4" indicator and above. Treble does not kick in until volume pot is at 4 or higher. Below 4 the tone is muffled and bass like regardless of treble pot setting. Once volume is at 4 and above the treble pot is responsive.

I have stepped through the wiring from input to output of V3. All seems good.

Both channels are behaving similar. Wondering if the pots need to be replaced. I did check the Bass channel volume pot with wires disconnected and it did behave properly on the meter.

Schematic that I am using is attached.

Thank you for any suggestions. MC

Fender-Bassman-50-Schematic.pdf

Normal channel V3 is in the top center of below pic.





Normal channel pots below

[Helmholtz]

I suspect a bad or missing ground connection. Measure if pot shells have zero Ohm to chassis.

[Leo_Gnardo]

It's obvious that both volume pots are replacements, they have long "sticks" intended for printed circuit board mounting instead of eyelets meant for fly lead attachment. I suggested you check with a meter how the wiper to ground resistance tracks. Either someone else was trying to fix the same problem & failed (note the major wiring goof you reported in the previous thread) and/or those replacement pots are wonky. Also, you have to expect an overall volume change as either treble or bass are turned up & down. They filter - that is remove - large parts of the sound spectrum so it's no surprise the overall volume changes especially when the EQ pots are dialed way down. Bass pot dialed down leaves a thin tone, nothing unusual about that. In Fender amps it's normal that you don't get much increase in bass as you dial it up past 4 or so. Treble pot dialed down yields a muffled tone, no "definition," no "point."

Let's find out what's up with those volume controls, and double/triple check the wiring on all the controls. Since you already found one glaring error there may be more.

[misterc57]

I suspect a bad or missing ground connection. Measure if pot shells have zero Ohm to chassis.

All under 1 ohm to ground except the Bass Channel Vol pot is 5 ohms to ground. I had to really snug up the Bass Channel Vol retaining nut to get it down to 5 ohms, earlier testing in prior thread I was reading very high resistance to ground.

[misterc57]

... I suggested you check with a meter how the wiper to ground resistance tracks. Either someone else was trying to fix the same problem & failed (note the major wiring goof you reported in the previous thread) and/or those replacement pots are wonky. ...
Let's find out what's up with those volume controls, and double/triple check the wiring on all the controls. Since you already found one glaring error there may be more.

Ah, I thought the VOL pots were original.

When I previously removed the hot and wiper wires from the Bass channel volume pot the tracking of wiper to ground looked good. I did not try the Normal channel volume pot.

I walked through the Normal channel wiring. Will check again, both channels.

I have a 500K pot I could test with. Will see if I can find a 1M. Can I remove connections to one vol pot and clip into a test pot sitting outside the amp?

Thank you!

[Helmholtz]

I have a 500K pot I could test with. Will see if I can find a 1M. Can I remove connections to one vol pot and clip into a test pot sitting outside the amp?

Yes, but don't forget the ground wire.


BTW, the ground side lugs of the vol. pots should be soldered to the shells.
Another test you could do is measure resistance between wiper and chassis with vol. at zero.

[The Dude]

Also check grounds soldered to the brass plate. I occasionally find them loose. They will look soldered, but a little tug will show them to be loose.

[pdf64]

Maybe an issue with the HT decoupling, eg a bad HT ecap or connection thereof, as they’re something common to both channels.

[misterc57]

BTW, the ground side lugs of the vol. pots should be soldered to the shells.
Another test you could do is measure resistance between wiper and chassis with vol. at zero.

To reduce confusion on my part, I am going to focus on the Bass Channel only for now, since both channels behave similar, and this channel it is easier to access the wiring,

Bass Channel

Ground side lug of VOL pot has a wire soldered to the brass plate (it is not soldered to the pot shell).
All solders on the brass plate are solid.
Resistance from ground side of VOL pot to chassis is under 1 ohm. Resistance from VOL pot shell to chassis ground is 5 ohms.

Resistance between VOL pot wiper and chassis with vol. at zero. = 21 ohms

Schematic shows a connection from the 047 cap to the leg of the bass and treble pots. This does not exist. I ran a jumper wire to make that connection and there was no change in behavior.

The bass pot does not seem to do anything to the tone.

Guitar is connected to Bass Channel input only. The Normal Channel (nothing is connected there) VOL pot, when dialed above 6 increases the overall volume of the amp significantly.

When putting the DEEP switch in the UP position, the VOL, TREB, & BASS pots are completely non responsive.

[Helmholtz]

Bass Channel

Ground side lug of VOL pot has a wire soldered to the brass plate (it is not soldered to the pot shell).
All solders on the brass plate are solid.
Resistance from ground side of VOL pot to chassis is under 1 ohm. Resistance from VOL pot shell to chassis ground is 5 ohms.

Resistance between VOL pot wiper and chassis with vol. at zero. = 21 ohms

That looks ok.

Please measure plate (pin 6) and cathode (pin 8) voltages of the second triode to see if it's working properly. Do tube voltages change when you turn the vol. pot?

[misterc57]

Please measure plate (pin 6) and cathode (pin 8) voltages of the second triode to see if it's working properly. Do tube voltages change when you turn the vol. pot?

Amp is connected to an isolation variac. VAC = 117

V1 pin 6, volume from zero to 7 stays steady at 250 VDC. Volume from 7 to 10, VDC starts dropping and lands at 232 VDC with volume at 10.

V1 pin 8, volume from zero to 7 stays steady at 1.94 VDC. Volume from 7 to 10, VDC starts dropping and lands at 1.8 VDC with volume at 10.

Overall amp current draw, volume from zero to 7 stays steady at 0.7 A. Volume from 7 to 10, current starts rising and lands at 1.1 A with volume at 10.

Not sure this matters but. Resistance between VOL pot wiper and chassis with vol. at zero. = 21 ohms, rises up to 300K with VOL at 7.5, when dialing VOL above 7.5 resistance to ground starts dropping

[pdf64]

Amp is connected to an isolation variac....

Are you sure it's isolated?It would be best not to be under a misapprehension.

[misterc57]

Are you sure it's isolated?It would be best not to be under a misapprehension.

Sencore PR-57

[Helmholtz]

V1 pin 6, volume from zero to 7 stays steady at 250 VDC. Volume from 7 to 10, VDC starts dropping and lands at 232 VDC with volume at 10.

V1 pin 8, volume from zero to 7 stays steady at 1.94 VDC. Volume from 7 to 10, VDC starts dropping and lands at 1.8 VDC with volume at 10.

Voltages look good. No signs of leaky caps.

Check the V1b cathode cap (pin 8) to ground for capacitance.

As you mentioned some interaction between channel vol.pots, what happens if you pull the normal channel input tube?

Otherwise you need to verify the complete tonestack wiring(s). Finding the original Fender layout should help. Verify all connections using your Ohmmeter.

[Leo_Gnardo]

Resistance between VOL pot wiper and chassis with vol. at zero. = 21 ohms, rises up to 300K with VOL at 7.5, when dialing VOL above 7.5 resistance to ground starts dropping

Ah this is what we're looking for. That the resistance doesn't go to zero tells us there will be some signal passing through although that volume pot is dialed down all the way.

At the other end, with the EQ pots still connected to the volume control, you're reading the composite resistance of the volume pot with treble & bass shunting that volume control resistance, that way the readings you observe are to be expected. Disconnect the jumper between treble wiper and input terminal of the volume control and you will likely see its resistance climb to 1 Meg ohm as you dial it up.

21 ohms doesn't seem like much but it's enough to fox you if you're trying to dial volume completely off.