Crate BV120H , 500 plate volts, no current ?

[gtrplayr1976]

Ok, so I'm working on a friends amp. It had blown a fuse (f3) which goes to the cathode return on V6. I replaced the fuse , powered up without tubes, fuse ok, installed preamp tubes , powered up , fuse ok, installed power tubes, powered up , fuse ok, hooked up my bias probe to V5 ,and V8, 500 plate volts ,but no current flow ...any thoughts , or suggestions? I actually installed the old tubes because I didn't want to risk blowing the new ones.

Thanks, Tom

[Gtr_tech]

And the screens? If you don't have voltage on the screens, you won't get but just a little current thru the tube.

[gtrplayr1976]

And the screens? If you don't have voltage on the screens, you won't get but just a little current thru the tube.

The voltage I'm getting on the power tubes is consistent as: power on = pins 3&4 have 513VDC ,pin 5 is -54VDC all others at zero except on V7 which also has 505VDC on pin 6.

[Jazz P Bass]

And the screens? If you don't have voltage on the screens, you won't get but just a little current thru the tube.

That is a real good thought!
Now onto the puzzle.
The B+, screen & bias voltages are there.
What measurement do you get from the cathode (pin 8, output tubes) to chassis ground?
(amplifier off, measurement in ohms)
If that is open circuit, no current will flow.
I would install the new tubes if everything checks out.

[g1]

schematic: Crate Bluevoodoo 120W Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

Pin 6 on a 6L6 is not connected to anything so they are probably using that pin on V7 socket as a tie point, no concern.
Have you verified your bias probe? Any chance it needs mA mode rather than mV?
Otherwise look at the cathode ground scheme as JazzP suggested, maybe a fried trace or something like bad solder on connectors?

[gtrplayr1976]

That is a real good thought!
Now onto the puzzle.
The B+, screen & bias voltages are there.
What measurement do you get from the cathode (pin 8, output tubes) to chassis ground?
(amplifier off, measurement in ohms)
If that is open circuit, no current will flow.
I would install the new tubes if everything checks out.

Pin 8 to ground,and I get 0 ohms , so just replace the tubes ,and go from there ?

[gtrplayr1976]

schematic: Crate Bluevoodoo 120W Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

Pin 6 on a 6L6 is not connected to anything so they are probably using that pin on V7 socket as a tie point, no concern.
Have you verified your bias probe? Any chance it needs mA mode rather than mV?
Otherwise look at the cathode ground scheme as JazzP suggested, maybe a fried trace or something like bad solder on connectors?

My probe only reads Voltage ,and Current. I checked the cathode grounds ,and they seem to be fine. Thanks for the input. Much appreciated.

[g1]

My probe only reads Voltage ,and Current.

The bias probe (depending on brand/type) will usually require a meter set to read current OR voltage. If it requires a current meter then you will get no reading if your meter is set to read volts. If the bias probe has it's own meter, then this is not a concern. Sorry for the confusion, just a double check. You could also remove the cathode fuse and take a current reading there.

[gtrplayr1976]

The bias probe (depending on brand/type) will usually require a meter set to read current OR voltage. If it requires a current meter then you will get no reading if your meter is set to read volts. If the bias probe has it's own meter, then this is not a concern. Sorry for the confusion, just a double check. You could also remove the cathode fuse and take a current reading there.

No worries. Mine reads both ...I just flip a switch. I think I got it from Weber.

[g1]

Do you get sound? From the schematic I get the impression they are biased pretty cold. Do you get any change by adjusting the bias pot?

[gtrplayr1976]

Do you get sound? From the schematic I get the impression they are biased pretty cold. Do you get any change by adjusting the bias pot?

Originally there was no sound because it blew a fuse. I have replaced the fuse ,and just getting around to adding new tubes. I need to figure a good bias setting to see if it improves sound.

[Jazz P Bass]

Keep in mind that these amps are set "on the warm side of cold" from the factory.

[gtrplayr1976]

Keep in mind that these amps are set "on the warm side of cold" from the factory.

Thanks Jazz. I looked at this,but I'm not sure what exactly this means. I have a bias probe,and I can read volts ,and current. So what would be a good MA number for bias ?.. Plate volts at 500, and tube dissipation at 30 watts. I get 60 milliamps divided by .7 would be 42...is that too cold ?

[Jazz P Bass]

Try it.
If you have a meter that can read ac current, hook it up to one wire if the mains lead (in series).
That will tell you the current that the whole amp is drawing at idle.
Obviously, Crate does not consider "bias" settings critical.

[g1]

Originally there was no sound because it blew a fuse.

The fuse F3 would have taken 1 power tube out of commission. It should not have caused a no sound condition. Not sure what is going on there. Maybe the old tubes are really weak and the one connected to F3 was the only one doing any work?
JazzPB: I saw that bias note at the bottom on page 1 of schematic, then I saw the same note on page 2 but with 1.3A draw spec'd rather than .85A so I wonder if they thought it was too cold and increased it.