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Please need help! TUBE BIAS PROBE
Please need help! TUBE BIAS PROBE
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Old 11th April 2007, 01:36 AM   #1
fiendish is offline fiendish  United States
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Join Date: Apr 2007
Default Please need help! TUBE BIAS PROBE

Ok so i would like to see if anyone can help me. Im very new to building vacuum tube stuff. I came across this shematic for building a tube pias probe.I wanted to build it for my 1964 fender bassman amp so i can bias it myself. It uses 2 6L6's and 4 12ax7's. So i went ahead and started to build it and now im having a few problems with the tubes not heating up al the way on the probe. When i put the 2 6L6's back on my amp them heat up fine. I drew some picture of what i did. Can someone please tell me what i am doing wrong? Here the pictures


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Old 11th April 2007, 02:47 AM   #2
kevinkr is offline kevinkr  United States
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Location: Boston, Massachusetts
Please need help! TUBE BIAS PROBE
From your photograph it would appear that you have neglected to make most of the connections.

You need to make connections to from pins 2 to 2, 3 to 3, 4 to 4, 5 to 5, and 7 to 7. Insert the 10 ohm resistor between pin 1 and 1 to make the bias measurement.

Connect the switch connections across the 10 ohm resistors as shown in the diagram to make the connections for measuring the bias.

Verify very carefully that you have made all connections properly otherwise you may destroy output tubes and damage the amplifier.

Make sure everything is well insulated and that no contact is possible with any high voltage point. Read and become familiar with the safety and high voltage threads here if you have not already done so. A mistake could damage you or your amplifier or far worse be fatal to you. Be careful!!

It would probably be a good idea to mount everything in a metal chassis and have a couple of banana jacks for the meter measurements. Provide a connection with a wire lead with clip to ground said chassis to your guitar amplifier chassis for safety.

"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 11th April 2007, 03:25 AM   #3
gingertube is offline gingertube  Australia
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Join Date: Oct 2004
Location: Adelaide South Oz
I've never put a lot of faith in the tube bias probes.

Its a MUCH better idea to install those 10 Ohm resistors into the amp permanently.
Not only does it give you a place to quickly measure the individual output tube currents, BUT if you use 1/2 watt resistors they will also act as fuses in the event of a tube short or a bias failure, possibly saving you from cooking the output transformer.

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Old 11th April 2007, 06:44 AM   #4
fiendish is offline fiendish  United States
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Join Date: Apr 2007
Default bias probe

Thanks for the help. I didnt expect help so soon.First off i am taking the danger very seriously and taking precaution.
What i used to make the connection was 18 stranded wire insulated with 600vdc insulator. i will take pictues of my work and post tomorrow so that you two can check it out and then i will test it. Oh and i have a plastic enclouser.

I have heard maybe i should not bias it this way but i really just want to finish this since it is my first project.
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