Crate VC5310 power tube failure ,and fuse blown.

[gtrplayr1976]

Yes. Then check so full counter-clockwise gives max resistance of the pot. That way turning clockwise will make tubes run hotter.

What I thought. Just wanted to be certain. Thanks g-one !

[gtrplayr1976]

Yes. Then check so full counter-clockwise gives max resistance of the pot. That way turning clockwise will make tubes run hotter.

Well for some reason that didn't change anything except more noise. I may have gotten it hooked up wrong. I'll check again this evening, and I may just replace the resistor with a trim pot.

[g1]

You want to keep the series resistor. That way even if the pot gets turned to 0 ohms by accident, you still have the series resistor so you still have some bias and the tubes don't burn up.
Untill you verify it's working right, remove the power tubes. Check at pin2 of the power tube sockets that the -DC is changing when you adjust the pot. It should go from somewhere around -15VDC to more negative depending on the pot setting.

[gtrplayr1976]

You want to keep the series resistor. That way even if the pot gets turned to 0 ohms by accident, you still have the series resistor so you still have some bias and the tubes don't burn up.
Untill you verify it's working right, remove the power tubes. Check at pin2 of the power tube sockets that the -DC is changing when you adjust the pot. It should go from somewhere around -15VDC to more negative depending on the pot setting.

Ok g-one. I just can't figure why it didn't change anything. I pulled one end of R78 attached on leg of the pot to the end of the resistor ,and the wiper where the resistor had been ? It may have changed 2 ma.

[g1]

See what kind of DC variation you are getting at pin2 with power tubes out. What is the value of your pot?

[gtrplayr1976]

See what kind of DC variation you are getting at pin2 with power tubes out. What is the value of your pot?

The pot is 10K. I was reading current at JO3 ?

[g1]

Yes, you should see current variation at J03 (same conditions as before, Jumper lifted, current meter in series, good fuse).
But there must be voltage variation at pin2 of power tube sockets, this you can check with them out of amp.

[gtrplayr1976]

Yes, you should see current variation at J03 (same conditions as before, Jumper lifted, current meter in series, good fuse).
But there must be voltage variation at pin2 of power tube sockets, this you can check with them out of amp.

I'll check that this evening g-one. Thanks!

[gtrplayr1976]

Yes, you should see current variation at J03 (same conditions as before, Jumper lifted, current meter in series, good fuse).
But there must be voltage variation at pin2 of power tube sockets, this you can check with them out of amp.

Hey g-one. I never saw any change in the voltage at pin 2. I removed the bias pot ,and resistor R78 ,and read through it ,and the pot ,and it went from 0-24k ohms ,so I'm not sure what's going on.

[g1]

In case there is a schematic difference from your amp, verify that R78 is going where the schematic shows.
What was your voltage reading (that wouldn't change) on pin2 ?

Edit: Try the pot in series with a 5K instead of the 15K and see if you get voltage variation at pin2 (power tubes still removed).

[gtrplayr1976]

In case there is a schematic difference from your amp, verify that R78 is going where the schematic shows.
What was your voltage reading (that wouldn't change) on pin2 ?

Edit: Try the pot in series with a 5K instead of the 15K and see if you get voltage variation at pin2 (power tubes still removed).

It looks like the schematic is correct. I was getting 13 vdc at pin 2. I'll try the 5k ,and see what happens. It was also suggested that I take one pin off the pot ,and connect to R78 ,and the other to R77 ,and the wiper to B- ?

[g1]

I hope you mean negative 13V at pin 2 .
I'm not sure the exact alternate wiring you are speaking of without a schematic. If they meant disconnect the top of R78, put in series with the pot, other end of pot to R77, disconnect R77 from the bias filter cap which then connects to the pot wiper, then yes.

[gtrplayr1976]

I hope you mean negative 13V at pin 2 .
I'm not sure the exact alternate wiring you are speaking of without a schematic. If they meant disconnect the top of R78, put in series with the pot, other end of pot to R77, disconnect R77 from the bias filter cap which then connects to the pot wiper, then yes.

Oops .Yes sir negative 13 . There's schematics earlier in the thread,and I think that's what they meant.

[g1]

Here's the 2 alternate circuits drawn out. The #2 option may be less desirable as it effectively increases R77 at the same time it is effectively decreasing R78. Doing both at the same time may cause the pot to be more "touchy" as they both decrease the bias voltage. Method #1 does not increase the effective series resistance of R77.

[Enzo]

In chosing between those two, consider what happens if the wiper contact fails in the pot.


Hint: I never take bias off a wiper.