Eurotube "pro one" bias probe not measuring current

[fossilshark]

I got this pro one bias probe from eurotube and have used it to bias a couple amps now.

I am trying to bias my LA50BL clone and the current reading was at 1ma

When I turned it up a bit it then read 2ma and the amp screeched super loud so I panic shut it off

Now the amp does the screeching thing and I am unsure how to proceed

[fossilshark]

Also plate voltage reads 430v, the voltages on that schematic are not accurate

[pdf64]

1 or 2mA is rather low.
How are you reading this?
Photos of the meter when making this reading?

[AlNewman]

Aren't the bias probes just a resistor in series measuring voltage?
Is there any fuses that can be replaced in the probe?

[pdf64]

Aren't the bias probes just a resistor in series measuring voltage? ...

Not the Eurotubes probes mentioned in the first post, they use a current meter inserted into the feed to the anode.
https://www.eurotubes.com/store/pc/bias%20probes.htm

[Latole]

 The Pro one

[Latole]

I got this pro one bias probe from eurotube and have used it to bias a couple amps now.

I am trying to bias my LA50BL clone and the current reading was at 1ma

When I turned it up a bit it then read 2ma and the amp screeched super loud so I panic shut it off

Now the amp does the screeching thing and I am unsure how to proceed

Question:  ls it the first time this amp is power ON ?

[fossilshark]

No, I have been using this amp for about 7 years. When I built it I set the bias to -54v because I didnt really know how biasing works. I have not changed that and have only swapped power tubes once.

The meter reads 430v but only 1 or 2 ma when the amp is on and making sound. This is the same for both tubes.

Like I said I have used this meter on a couple amps so far with great success, I am not convinced anything is wrong with the amp or the meter (at least not catastrophically).

Also, I turned the bias back down to about where it was before and I was able to get through band practice afterwards, I am just trying to figure out how I should go about properly biasing this amp.

[Latole]

IMO you have issues with m.a. on your meter .

When you put back to -54v and the amp play fine, how many milli- amps do you read ?

[fossilshark]

Meter reads 1ma. I am just confused because I literally just used this meter on a different amp and it worked fine.

Is it possible the 5.6R resistor on the cathode is messing with the measurement?

[astronomicum]

I just bought some new meters for measuring bias. I had the exact same problem and discovered that I had the probe plugged into the wrong port. I even turned the bias up to increase current and it maxed out at 2mA just as you did. I was not seeing 2mA however. I was actually seeing 200mA!

[pdf64]

Meter reads 1ma. I am just confused because I literally just used this meter on a different amp and it worked fine.

Is it possible the 5.6R resistor on the cathode is messing with the measurement?

It's possible that the probe and meter is causing the amp to oscillate. What happens to the current reading if the ECC83 in the LTP phase splitter (V3?) is removed?
The 5R6 cathode resistor won't be affecting the bias probe's accuracy etc.
Of course if it blows, such that it's high resistance or open circuit, the anode current reading will be really low or 0.

[Latole]

I just bought some new meters for measuring bias. I had the exact same problem and discovered that I had the probe plugged into the wrong port. I even turned the bias up to increase current and it maxed out at 2mA just as you did. I was not seeing 2mA however. I was actually seeing 200mA!

IMO you may read 20 ma not 200 ma
430 volt C 20 ma = 8,6 watts a very cold bias .
That is because you put bias at -54 vdc

Best is to learn how to use you tools before making  an expensive mistake 
And how bias work , at 2 ma, a amp will not work.

Because ;
430 volts at 200ma !!!  = 86 wats for a 35 watts tube.
Tubes will blow in a second.

[astronomicum]

I just bought some new meters for measuring bias. I had the exact same problem and discovered that I had the probe plugged into the wrong port. I even turned the bias up to increase current and it maxed out at 2mA just as you did. I was not seeing 2mA however. I was actually seeing 200mA!

IMO you may read 20 ma not 200 ma
430 volt C 20 ma = 8,6 watts a very cold bias .
That is because you put bias at -54 vdc

Best is to learn how to use you tools before making  an expensive mistake 
And how bias work , at 2 ma, a amp will not work.

Because ;
430 volts at 200ma !!!  = 86 wats for a 35 watts tube.
Tubes will blow in a second.

Rookie mistake? Granted. Being most familiar with Flukes where the current ports are separate from the Volts and Resistance port, these [cheap] meters have the milliamp port common with Volts and Resistance.

But sometimes it takes a mistake to learn something new, and I for one am not ashamed to share.

I did some testing to confirm my observations. I hooked up two meters in series to see what actual current I was getting based on what I was observing. The actual current I saw was lower than the 200mA I reported (which was an estimate). But.... It was 120mA. The plate voltage was not 430V (not sure where that came from) but 600V. You can do the math. I discovered my error quickly but the tubes in this quad spent quiet a few seconds at this level until I figured out my mistake. You would have thought they would not survive. Well... they did, they are still matched, and I have a couple hours on them since. They probably survived due to the lower screen voltage (300V). These are Sovtek 5881s to boot. Lucky? Yes. Are they Damaged? Only time will tell. But they did not blow.

It is acceptable to be skeptical, but be careful of those pesky facts getting in the way of opinion.

[Latole]

Some reading must be wrong , it make no sense each tube work with 600 volt plate volts  and 120 ma .
Power ; 600 X 120 ma = 72 watts for a 34 watts tube.

You must never drive a tube in class A/B more than 70 % plate dissipation .
You drive them at more than 200 %

And the fuse don't blow ?

It is your amp and your money.

[astronomicum]

And the fuse don't blow ?

Some rough calculations:

Secondary:
600V x 0.48A = 288VA
6.3V x 6A = 38VA
Approximately 326VA Total

Primary:
117V x 3A(Fuse)
Approximately 351VA (As we know, fuses do not blow right at their ratings so consider this a minimum)

The fuse probably should not blow.

Math can be pesky too.

[astronomicum]

Apologies to the OP. Never intended for this to get off track.

[fossilshark]

Forgot the bias probe at band practice, I will have to mess around with it more after sunday.

I promise I am not misreading a decimal lol the meter reads out ma as a whole number.

Now that I have the amp in the shop I am SHOCKED that it runs reliably and sounds good right now. This was the second amp I ever built (in 2018, I was in high school) and it has kind of been a work in progress ever since.

I mean look at the state of these bias supply caps.

Now I don't know what this bias circuit is, I have to reverse engineer it (it is coming off a 28vac line off the power transformer), but am I correct in assuming those caps don't need to be 2000uF as these are?

I have alot of work to do on this amp, but I will do it because it means alot to me and I have never played through anything that sounds like it.

I will post more about the bias meter when I get it back, I have a 5150III here that needs biasing, I will post a comparison of the meter readings.

[fossilshark]

Another pic of this monstrosity.

[HotBluePlates]

The meter reads 430v but only 1 or 2 ma when the amp is on and making sound. ...

At the risk of asking the "dumb, obvious question," you weren't looking at current-readings while playing through the amp, were you?

Idle Current is a Direct Current measurement.
"Making Sound" with the Amp puts large Alternating Current where we expect Direct Current only.

Many meters get confused with their DC readings when large AC is present.