EuroTubes Bias Probe faulty - Amp issues

G

GuitarGuy08

Well-known member
This is an odd one. I have a EuroTubes Bias Probe, the cheap one for around $20. I have used it a number of times successfully in the past.

Well, I pulled it out of my little gear bag, which is admittedly a bit cramped at times, and plugged it up to bias a set of RCA NOS outputs.

Not working. Not only is it not working, it seems to almost surge or spike the power to the tube(s), making me shut off the amp.

I tried a few times and with different sets. The tubes and amp work completely fine and do not do this if the probe bias isn't connected.


What could have happened? A short in one of the leads from being rolled up? I've never seen this.

The meter will keep the 1 at the front and then 'blink' in other numbers. When removed from the probe and amp, the ohm readings are totally what you would expect.


I would think it's a problem with the amp, but it is fine if the bias probe is not in it, making me think the probe has some sort of issue.

Going to try to get to a friend's house so I can bias this up so I have my amp back. Any ideas are appreciated!
 
midnightlaundry said:
What is the Battery status in the meter??
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The meter itself seems completely normal when not using the bias probe. Battery made me jerk back the 9v is so fresh. IDK why the battery being out in the meter would cause the tubes to runaway bias anyhow, but it was the first thing I checked.
 
glpg80 said:
The probe is shorted internally. There’s a 1 ohm resistor shorting out another pin. Throw it away and get another.
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Welp, just went to a buddies house and tried to bias my new set of tubes with his tester. My amp blew the HT fuse. I'm guessing something is up with the new set of tubes possibly? Will replace the fuse and Will try and older set that was working fine and see how it goes.

If it blows another fuse or has issues, I wonder what it could be? Was working fine on the old output set.
 
GuitarGuy08 said:
Welp, just went to a buddies house and tried to bias my new set of tubes with his tester. My amp blew the HT fuse. I'm guessing something is up with the new set of tubes possibly? Will replace the fuse and Will try and older set that was working fine and see how it goes.

If it blows another fuse or has issues, I wonder what it could be? Was working fine on the old output set.
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Probe probably damaged the tubes you tried to bias with the defunct probe.

Since you blew a fuse it probably means you need to have screen grid resistors checked to make sure they haven’t been damaged.
 
glpg80 said:
Probe probably damaged the tubes you tried to bias with the defunct probe.

Since you blew a fuse it probably means you need to have screen grid resistors checked to make sure they haven’t been damaged
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Not sure how to do this. Will have to learn it up while the fuses are on the way. Wish I had a local amp tech I could trust. You would think Houston area would have plenty but the ones I have dealt with have been no good.
 
GuitarGuy08 said:
Not sure how to do this. Will have to learn it up while the fuses are on the way. Wish I had a local amp tech I could trust. You would think Houston area would have plenty but the ones I have dealt with have been no good.
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Take it to a tech. Not something you should attempt and it’s way beyond what is considered safe especially since your damaged probes made the amp/tubes unpredictable. The voltages inside an amp can be lethal even when the amp is turned off. There are extra steps that must be taken and I won’t mention them since you don’t know what you’re doing.
 
glpg80 said:
Take it to a tech. Not something you should attempt and it’s way beyond what is considered safe especially since your damaged probes made the amp/tubes unpredictable. The voltages inside an amp can be lethal even when the amp is turned off. There are extra steps that must be taken and I won’t mention them since you don’t know what you’re doing.
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Yep, don't have a tech around here. Wonderful.
 
griff10672 said:
https://el34world.com/charts/Schematics/files/Rivera/Clubster_25_45.pdf
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Thanks! This may be the chart I found when I had to solder a leg and restore a circuit previously, cannot recall. I'm thinking the analysis above is correct.

Faulty probe led to faulty tube

Tube reinserted into amp

Left in too long checking with faulty tube and blew fuse

I would check the grid resistors but unable to do that in a manner I believe to be safe enough at this time.

Need a tech I can trust but there are none around here apparently.

Big oof.

Will take time to study and think. Super shitty it took out a very rare tube as part of a matched set, possibly from two sets....Ugh.....

The pedal win has my spirits up in these low times lol still kinda depressed about it all.
 
glpg80 said:
Take it to a tech. Not something you should attempt and it’s way beyond what is considered safe especially since your damaged probes made the amp/tubes unpredictable. The voltages inside an amp can be lethal even when the amp is turned off. There are extra steps that must be taken and I won’t mention them since you don’t know what you’re doing.
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Hey, my buddy who built a few amps, has a sweet setup. He has a modified bias probe so that we get the plate voltage and mA and all of that. If measuring the grid resistors can be done by measuring certain things like pins, while the amp is on, may not be that difficult to make a modified bias base.

It would be nice if I had a tech I could take/send it to for a good once over and all of that as well. Time will tell.
 
GuitarGuy08 said:
Hey, my buddy who built a few amps, has a sweet setup. He has a modified bias probe so that we get the plate voltage and mA and all of that. If measuring the grid resistors can be done by measuring certain things like pins, while the amp is on, may not be that difficult to make a modified bias base.

It would be nice if I had a tech I could take/send it to for a good once over and all of that as well. Time will tell.
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His probe won’t do any good unfortunately. You can search for TV repairmen as well who could take a look. I reiterate it’s not a working amp so it does need to be checked out overall by a professional.
 
glpg80 said:
His probe won’t do any good unfortunately. You can search for TV repairmen as well who could take a look. I reiterate it’s not a working amp so it does need to be checked out overall by a professional.
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Well, my old buddy IS an old TV repairman lol just sayin
 
Hello all, as an update:

We replaced the fuse and tested all of the tubes one at a time. One of the recently received output tubes (NOS JAN RCA), took quite a while to heat up and get going, but then has been good to go since. I think the events went as follows:

Root cause: Faulty tube bias used for too long on amp outout tube(s), which caused fuse to blow.


Evidence:

Replaced fuse and used old output tubes - Good to go.

Put new tubes in after warming up the set properly - Good to go.

Biased and running as normal with no other faults or issues at this time.

Will monitor results and report back any odd findings or issues.

Will also perform a full board diagnosis and ground trace for gfood measure, but all is well currently and sounding great!

Currently, I have RCA 12ax7 blackplates in V1, V2, & V3. RCA Smoked glass JAN 6V6GTs in the output section and this amp sounds absolutely massive and tight for cleans and lead alike.
 
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