Bad plate resistors

[daveyajd]

I am unable to get sound out of the V1 channel of my Fender. Checked the usuals: good tubes, continuity from input jack to pin 2. I haven't started measuring for good connections elsewhere yet but I did notice that the measurement on the plate resistor from the first half counts up toward infinite. Obviously a bad CC but would this cause the no output issue?

[HotBluePlates]

What specific amp?

You did measure with the amp off and caps discharged, right? Measuring resistance on a live circuit can damage your meter.

For the suspected bad plate resistor:
Switch your meter over to volts, turn the amp on, and measure the plate voltage for the stage with the suspected bad plate resistor. Do you get B+ voltage, 0vdc or something reasonably close to the schematic plate voltage for that stage?

If 0vdc: Plate resistor is open, confirming your resistance measurement.

If B+: plate resistor is not open, but no current is flowing through that triode; suspect cathode resistor is open or has a poor ground connection or poor solder joint.

If ~schematic plate voltage: Plate resistor is likely fine, and tube is passing current. You might have not had good contact between meter probe and resistor during your resistance measurement.

[daveyajd]

The amp is a Fender Bantam Bass.



Yes, I took measurements with no power. I can't recall what the exact plate voltages were when I measured them earlier. I know they seemed off. Something like 335 on pin 1, 160 on pin 6. V2 plate voltages hold steady at 260.

[HotBluePlates]

I can't recall what the exact plate voltages were when I measured them earlier. I know they seemed off. Something like 335 on pin 1, 160 on pin 6. V2 plate voltages hold steady at 260.

That suggests the plate resistor is okay (otherwise, no voltage would be seen on the plate side of it).

Unsolder one leg of each of V1's cathode bypass caps (25uF 25v, across the 1.5k resistors). Measure the resistance of each of the 1.5k cathode resistors. Is either one near-infinite (or a 15k instead of a 1.5k, if replaced)? If so, replace the bad resistor. If not, turn the amp on and measure V1 voltages again.

If they are still wrong, you might need to verify the grounding of each cathode resistor and/or reflow the solder joints. If they are good, you might need to replace the bypass caps (the stage with the low voltage, but with good plate and cathode resistors, suggests a leaking bypass cap).

[daveyajd]

I lifted the cathode bypass caps. Those resistors measured ok. I went ahead and replaced the plate resistors. There was also a bypass cap on the plate resistor of pin 1 so I removed that and fired it up. V1: 257/271, V2: 270/268, and channel 1 is working. Over all plate voltage increased to 427 also. Running smooth now. I am curious about the tone controls on the normal channel though. I like how on Vibrato channel you can turn the sound off by turning all tone knobs down. On the normal channel there is a fixed amount of mids always present. How can I change this? Thanks for all the help HotBluePlates.

[sluckey]

I like how on Vibrato channel you can turn the sound off by turning all tone knobs down. On the normal channel there is a fixed amount of mids always present. How can I change this?

You mean "Normal" channel and "Bass" channel, right? It is a stock Bantam???

The Fender Treble, Middle, Bass tone stack operates as you say. I don't know of any way to make the Bass tone stack operate that way, except to strip it out and replace with the TMB stack. I'd rather have the Bass channel than two identical channels.

[daveyajd]

Yes I did mean the Normal channel. Most people think of the normal channel on Fenders as the one with the bass tone stack that goes unused. I don't want to identical channels either but I was hoping to eliminate the fixed mids so that when "jumping" I could still have a flexible eq. I may try to revoice that channel a little to give me something different. Any ideas?