That Didn’t Go As Planned

[Wayne Patton]

Purchased a set of replacement tubes for my Plexi clone and a bias probe to use with my multimeter. I biased my DSL40C so had a good idea what I was doing.

I installed all the new tubes and turned on the amp and to get an idea of how it sounded with the new tubes even before i attempted to adjust the bias. Well, the amp roared like never before! It sounded unreal! But i looked and noticed one of the power tubes was glowing almost white hot. So i turned it all off let everything cool and switched the tubes around and the issue followed the tube. Looks like I received a bad power tube. I put the old tubes back in and hooked up the bias probe to see what they read and followed the directions that came with it but my Fluke 116 meter displayed an “OL” meaning out of limits. I tried both sockets, both tubes. Not sure if the bias probe is inoperable or I’m not using the meter correctly. In any event. The amp sounds fine with new preamp tubes and the original power amp tubes. Gonna exchange them for a new set. Just trying to understand what I did wrong g to not get a reading for milliamperes. I was able to read voltage at 487v so…

 

[fitz]

What are the old tubes and what are the new tubes?
And what is this bias probe that you mentioned?
How is the bias adjusted on this plexi clone?
Did you try adjusting the bias all the way down to see if you get a "low as it goes" reading?

 

[Wayne Patton]

Both sets are JJ.

Got the probe from eurotubes.

I didn’t mess with it at all. It’s a Rockitt Retro 50w clone. It has a small black adjustment pot inside.

Not sure if I used the probe wrong or the meter. No harm done. The amp runs fine. I had a bad preamp tube and wanted to retube it completely but for now it’s fine.

 

[FleshOnGear]

Purchased a set of replacement tubes for my Plexi clone and a bias probe to use with my multimeter. I biased my DSL40C so had a good idea what I was doing.

I installed all the new tubes and turned on the amp and to get an idea of how it sounded with the new tubes even before i attempted to adjust the bias. Well, the amp roared like never before! It sounded unreal! But i looked and noticed one of the power tubes was glowing almost white hot. So i turned it all off let everything cool and switched the tubes around and the issue followed the tube. Looks like I received a bad power tube. I put the old tubes back in and hooked up the bias probe to see what they read and followed the directions that came with it but my Fluke 116 meter displayed an “OL” meaning out of limits. I tried both sockets, both tubes. Not sure if the bias probe is inoperable or I’m not using the meter correctly. In any event. The amp sounds fine with new preamp tubes and the original power amp tubes. Gonna exchange them for a new set. Just trying to understand what I did wrong g to not get a reading for milliamperes. I was able to read voltage at 487v so…

Are you sure the multimeter is supposed to be set to read mA rather than mV? I was under the impression that bias probes like this measure idle current indirectly by registering a voltage across a 1 ohm resistor.

 

[Wayne Patton]

Well it read voltage fine when I selected the voltage setting on the meter. So…. But that tube is crap so I’ll have to exchange the pair out.

 

[RLW59]

Purchased a set of replacement tubes for my Plexi clone and a bias probe to use with my multimeter.
Just trying to understand what I did wrong g to not get a reading for milliamperes.

Is that a typo, or did you really have your meter set to read milliamperes?

Bias probes use 1 ohm resistors as test points just like the DSL40C, so you set your meter to read voltage instead of current.

The plate voltage is measured directly so for that you'll get correct readings.

 

[fitz]

I have a Eurotubes probe for a DMM.
The instructions clearly state to set the meter to mA for bias readings.

 

[dtier]

Looks like your Fluke 116 does not measure milliamps. It only measures microamps.

 

[Dogs of Doom]

Looks like your Fluke 116 does not measure milliamps. It only measures microamps.

 

[mickeydg5]

I have a Eurotubes probe for a DMM.
The instructions clearly state to set the meter to mA for bias readings.

But only if it designed to work that way.

He did not state anywhere what probe is used.
My guess is that it is the other type, maybe.

 

[mickeydg5]

Both sets are JJ.

Got the probe from eurotubes.

I didn’t mess with it at all. It’s a Rockitt Retro 50w clone. It has a small black adjustment pot inside.

Not sure if I used the probe wrong or the meter. No harm done. The amp runs fine. I had a bad preamp tube and wanted to retube it completely but for now it’s fine.

Which bias probe did you get?
Link?

 

[RLW59]

I have a Eurotubes probe for a DMM.
The instructions clearly state to set the meter to mA for bias readings.

I didn't know that.

Makes me wonder why just about everyone else uses 1 ohm resistors to read the plate/screen current as an equivalent voltage -- I always assumed there was some problem with reading current directly. Seems a lot simpler to measure current directly and eliminates any tolerance or drift in a 1 ohm resistor?

 

[mickeydg5]

I didn't know that.

Makes me wonder why just about everyone else uses 1 ohm resistors to read the plate/screen current as an equivalent voltage -- I always assumed there was some problem with reading current directly. Seems a lot simpler to measure current directly and eliminates any tolerance or drift in a 1 ohm resistor?

Eurotubes sells more than one type.
Be sure to use the probe correctly.

 

[Pete Farrington]

my Fluke 116

probe from eurotubes.

Your meter and probe are not compatible.
Your meter will show its overrange indication above 0.6mA (600uA) https://www.fluke.com/en-us/product/electrical-testing/digital-multimeters/fluke-116
You need a different probe or a different meter.

Eurotubes probes and meters have a USP of directly measuring anode current

Bias Probes

Eurotubes Bias Probe Page

www.eurotubes.com

With their unshrouded banana plugs, they seem horribly dangerous for unskilled users.

 

[2L man]

Measure if there is 1 ohm resistor between probe banana plugs! Multimeter resistance reading might come 1.2 or so ohms. If there is a resistor the correct multimeter setting is mV when measuring bias current.

 

[Ronquest]

I biased my DSL40C so had a good idea what I was doing.

So, did you use the same meter and probe on the DSL ?

 

[fitz]

Which bias probe did you get?
Link?

Bias Probes

Eurotubes Bias Probe Page

www.eurotubes.com

Eurotubes Bias Probe Instructions

Eurotubes Bias Probe Instructions

www.eurotubes.com

They sell the DMM probe and the Pro One probe with plate voltage and current displays.
I have one of each - makes biasing a pair easy-peasy.

 

[Pete Farrington]

I installed all the new tubes and turned on the amp and to get an idea of how it sounded with the new tubes even before i attempted to adjust the bias. Well, the amp roared like never before! It sounded unreal!

That’s completely the wrong method. The new valves may well be put into unacceptable conditions.

My guess is that the bias was far too hot for the new valves, so their anode current (and hence gain) was much too high. That’s why the amp sounded unreal.

Really, the fact one has been wrecked is on you.

The correct procedure is to set the bias as cold as possible with the old valves.
Then install the new valves and check their current draw ASAP.

Maybe 95% of the time people get away without setting the bias cold before powering up with replacement valves. But occasionally that’s not going to work out well for them.

It may be that the bias is fine and you just got a bad valve, eg got knocked in shipping or something, but it’s hardly that much more hassle to play it safe and start with the bias cold.

 

[dtier]

View attachment 135442

So the OP won't be confused, you can convert microamps (uA) to milliamps (mA) but the meter will only read up to .6 milliamps this way. He needs to read around 40 mA.

 

[Ken Underwood]

I rest my bloody case OMG