BF Bandmaster rebuild troubleshooting

[rocky101]

I rebuilt my BF Bandmaster a few years ago. I replaced all the caps, resistors, some pots etc. Anyway the amp worked for a few months without any problems, then it blew a screen grid resistor. I replaced the resistors and tubes. Now the amp powers up but has no sound. All my voltages are good except the phase inverter. I am getting the entire plate voltage into pin 1 of the phase inverter. Now here is where things get weird. With my multimeter negative lead attached to ground and the positive lead on pin 7 of the phase inverter the amp comes alive and has sound, but the voltages are still off ie the PI cathodes are getting less than 50 volts. I am getting a ground reference through the multimeter at pin 7. I pulled/checked the resistors in the phase inverter and they are all good. Another weird thing when the amp is left off for a few weeks the amp works, then makes a staticy noise and then sounds fades away. I also can't seem to get a bias current reading using the transformer shunt method. Any ideas or suggestions, I don't know of any good techs in my area. Thanks for any help and if more info is needed, like more measurements/readings, let me know.

[JHow]

Maybe bad coupling cap lets voltage onto grid, when your meter pulls grid down low enough, the tube turns on?

[MWJB]

List all idle voltages at pins 3, 4, 5 of the power tubes, all plates & cathodes of PI, V1, V2, V4.

Pics of the chassis, tube socket wiring & cap pan would be good.

When you say you are getting full plate voltage at pin 1 of the PI, you mean 6L6 type plate voltage? Check ground reference for tail & cathode resistors, via NFB load resistor? What are you getting at pin 6 PI?

[rocky101]

List all idle voltages at pins 3, 4, 5 of the power tubes, all plates & cathodes of PI, V1, V2, V4.

Pics of the chassis, tube socket wiring & cap pan would be good.

When you say you are getting full plate voltage at pin 1 of the PI, you mean 6L6 type plate voltage? Check ground reference for tail & cathode resistors, via NFB load resistor? What are you getting at pin 6 PI?

I will try to get pics up they are turning out blurry. anyway here are the voltages
Power tube 1 pin 3=455, pin 4=452, pin 5 -39
Power tube 2 pin 3=453,pin 4 451, pin 5 -39
V1 Plates = 254 & 269
V1 Cathodes= 2.03 & 2.25v
v2 Plates=267 & 279
V2 cathodes= 2.04 & 2.25
V3 plates= 370 & 355
V3 cathodes= 0
PI Plates = 411 & 183
PI cathodes= 61v

"Check ground reference for tail & cathode resistors, via NFB load resistor? What are you getting at pin 6 PI"--not sure what you mean here

[MWJB]

I wanted to see how mismatched your plate voltages are at the PI...the answer is very!

I'd guess that you have the wrong resistor value at pin 1 of the PI...double check this.

-39v at pin 5 sounds way low for 455v on the plate...I'd be looking for mid to late 40's as far as negative bias voltage. What plate current do you have?

Pictures would really help.

[rocky101]

I wanted to see how mismatched your plate voltages are at the PI...the answer is very!

I'd guess that you have the wrong resistor value at pin 1 of the PI...double check this.

-39v at pin 5 sounds way low for 455v on the plate...I'd be looking for mid to late 40's as far as negative bias voltage. What plate current do you have?

Pictures would really help.

Here are some pics. I can't seem to get a plate current measurement. I have my DMM set to mA with negative lead on Pin 3 of the power tube and positive on the center tap of the OT

[booj]

I can't seem to get a plate current measurement. I have my DMM set to mA with negative lead on Pin 3 of the power tube and positive on the center tap of the OT

You have the curent meter across the plate winding and are shorting it out. The current meter needs to between the plate wire that goes to pin 3 and pin 3. Or better yet (and safer yet) interrupt The cathode ground connection with the ammeter. Or better yet stick a one ohm resistor between the cathode and the ground and measure mv instead of ma. You'll get one millivolt per milliamp.

[JHow]

...Or better yet stick a one ohm resistor between the cathode and the ground and measure mv instead of ma. You'll get one millivolt per milliamp.

Can't you just get a rough idea by measuring voltage drop (VDC from cathode pin to ground) over cathode resistor and divide by the value of the resistor?

[booj]

Power tube 1 pin 3=455, pin 4=452, pin 5 -39
Power tube 2 pin 3=453,pin 4 451, pin 5 -39

Can't you just get a rough idea by measuring voltage drop (VDC from cathode pin to ground) over cathode resistor and divide by the value of the resistor?

Looks like fixed bias to me

[JHow]

Of course. Makes perfect sense.

[MWJB]

Booj wrote: "You have the curent meter across the plate winding and are shorting it out. The current meter needs to between the plate wire that goes to pin 3 and pin 3." No. He is shunting the transformer, he has the correct procedure.

Rocky check the mA fuse in your meter, in the meantime set the negative voltage at pin 5 of the 6L6 as negative as it will go (most negative volts).

I can't see an orange stripe on the plate resistor at PI pin 1, should be 82K (grey/red/orange)?

[rocky101]

Booj wrote: "You have the curent meter across the plate winding and are shorting it out. The current meter needs to between the plate wire that goes to pin 3 and pin 3." No. He is shunting the transformer, he has the correct procedure.

Rocky check the mA fuse in your meter, in the meantime set the negative voltage at pin 5 of the 6L6 as negative as it will go (most negative volts).

I can't see an orange stripe on the plate resistor at PI pin 1, should be 82K (grey/red/orange)?

Resistor is the right value. Meter fuse was blown, I replaced it. I am getting 24mA plate current. The lowest voltage I can get at pin 5 is -48v

[MWJB]

"Resistor is the right value." Try a different 12AT7 in the PI, inspect the socket, make sure you have a good connection at pin 1 to the tube socket. The fact that you are getting 411vdc at pin 1 of the PI suggests that no current is being drawn through that triode, as pins 3 & 8 share the same cathode resistor (& tail resistor) the tube/it's connection to B+ looks suspect?

[MWJB]

I don't like the way the OT secondary wires sweep around the PI tube, they should come thru the chassis & go direct to the speaker jacks (short & direct)...as this part has been changed, double check speaker connections, can't see from the photo clearly, but it looks like the black & green OT wires could go to the same tag (hot) on both main & ext jacks?

That wire to PI pin 7 could be shortened & tidied.

Confirm that there is a good mechanical connection between PI pins 3 & 8.

Measure dc voltage at the 470ohm cathode resistor where it meets the wire from pin 8&3, then measure dc at the other end of the 470ohm cathode resistor.

[rocky101]

I don't like the way the OT secondary wires sweep around the PI tube, they should come thru the chassis & go direct to the speaker jacks (short & direct)...as this part has been changed, double check speaker connections, can't see from the photo clearly, but it looks like the black & green OT wires could go to the same tag (hot) on both main & ext jacks?

That wire to PI pin 7 could be shortened & tidied.

Confirm that there is a good mechanical connection between PI pins 3 & 8.

Measure dc voltage at the 470ohm cathode resistor where it meets the wire from pin 8&3, then measure dc at the other end of the 470ohm cathode resistor.

I have already tried different PI tubes. I am getting 61 volts at the wire and 60 at the other end. It does seem the tube isn't drawing current, I just can't seem to narrow down why