Help me figure this bias issue out, please.

[V-man]

A ways back I got a dual bias probe and a meter and started biasing some of my amps. Readings were disturbingly low on my 2550, which I chalked up to an old bias pot component, but then the same lowballs were coming on my 4100.

Fast forward to today, I brought the 4100 into the office (with a modern outlet, something I figured may be contributing to the problem) and I am getting some bizarre readings all over again. As such, I was hoping you could help me isolate what the hell is the problem with this biasing - whether the amp(s), the tool(s), or the user.



1. 1993 4100 Dual Reverb with 4 Winged-C EL34 Power Tubes, through a MF 400 cab @ 8ohms with the HIGH power "Pentode" mode engaged.

2. Tools used: Dual bias probe and multimeter in the mA/A position.

3. Inserted bias probe sockets on 1 and 4 (the outer tubes), then installed all 4 power tubes.

4. Measured plate voltage by inserting probe at pin #3 on tube(s), registering 466V.

5. Applying Bias Formula: B = Max Watt Dissipation/Plate voltage X 70%, I arrive at 25W/466 x .70 = 37.5 (or 34.8 for 65%).

6. If my math AND interpretation is correct, EACH tube should register @ 30-something mv and I will fine-tune them within 1 milliamp of each other, somewhere in the range of 35 to 38 milliamps.





What I am getting is about 15 milliamps, and I have about 1.5 milliamps worth of play with the bias trim pot? What is the deal here? Even if I am wrong with my understanding (e.g. it should be 37 milliamps per PAIR, not per individual tube) the readings are too cold.

Here is a shot of the multimeter to demonstrating the readings and setup, to indicate what an idiot I am/am not:

The only other thing that seems off is the pins in the bias meter do not seat 100% thanks to ill-placed wiring (which interferes with the tube retaining spring) I don't think this really is an/the issue, but I figured I'd throw it in to give you as much info as possible to help diagnose.

Any idea what's going on here?

 

[V-man]

Update: It gets even better...

Tubes 2 & 3 register as 17.88 milliamps and ZERO!


So, with that craziness I realize this becomes something bigger than a bias issue (especially since a tube blew in this amp). As such, I will have someone check the bias supply components and the screen grid resistors. Maybe that will fix the issue, but there is still the matter of 2 other amps with very cold readings (dates of manufacture ranging from 1984-1993). I will check those as well and see what is going on here.

 

[Marshallhead]

Hang on, your meter says you are reading in milliamps, however if your bias probe is the typical 1ohm resistor to cathode deal, you should be reading the voltage drop in millivolts (which, thanks to ohms law, where V= I*R, can be taken as a proxy for milliamps).

 

[Bart Eeltink]

Is this one of those 5881 JCM 900s, that were accidentally equipped with EL34s?

5881: screen resistors: 470 Ohm
EL34: screen resistors: 1KOhm
The screen resistors are the white ceramic blocks connected to pin 4 of each power tube.

 

[V-man]

Is this one of those 5881 JCM 900s, that were accidentally equipped with EL34s?

5881: screen resistors: 470 Ohm
EL34: screen resistors: 1KOhm
The screen resistors are the white ceramic blocks connected to pin 4 of each power tube.

Negative. I am the original owner and it came with el34s (still have 1 or 2 of them around somewhere). Nevertheless, I will be checking all the components before going any further, so I'll keep that in mind!

 

[hbach]

What Marshallhead said: Most probes work with mV. Do you have the manual of your probe so you can check?

 

[V-man]

What Marshallhead said: Most probes work with mV. Do you have the manual of your probe so you can check?

thanks for the suggestion. I will check when I get home whether I still have it.

 

[scat7s]

heres a dumb question, as im not familiar with using bias probes, but, im just wondering, should they be monitoring on one tube of your outside pair, and one tube on your inside pair?

or does it not matter?

nevermind, i think i get it...but yeah, sounds l,ike you should have your meter set to mV.

 

[hbach]

heres a dumb question, as im not familiar with using bias probes, but, im just wondering, should they be monitoring on one tube of your outside pair, and one tube on your inside pair?

or does it not matter?

nevermind, i think i get it...but yeah, sounds l,ike you should have your meter set to mV.

If I had a double probe and a four tube amp I would use the probe on both tubes on one side to bias that side first checking both tubes are OK. I guess if you are positive you have very well matched tubes you can do both sides at once and be a bit quicker.

 

[clel miller]

Hang on, your meter says you are reading in milliamps, however if your bias probe is the typical 1ohm resistor to cathode deal, you should be reading the voltage drop in millivolts (which, thanks to ohms law, where V= I*R, can be taken as a proxy for milliamps).

I had the same question.
Have you biased with this tool before.?
Are the batteries good in your meter.?
Some of these tools use a 1 Ohm cat resistor and require your meter to be set to VDC.
Other bias tools break the connection at Pin 8 and send it through your meter. This would require you to have the meter set to read DC Amps.

 

[aspguitars]

By no means an expert, but as far as I'm aware, you have to connect your test meter differently for measuring voltage and current. Voltage is in parallel and Current in series. I don't think you can connect your meter up the same way to measure both, so one of your readings is probably wrong. I've always measured bias using DC mV (parallel) but please double check with someone like Lane.

 

[Bart Eeltink]

By no means an expert, but as far as I'm aware, you have to connect your test meter differently for measuring voltage and current. Voltage is in parallel and Current in series. I don't think you can connect your meter up the same way to measure both, so one of your readings is probably wrong. I've always measured bias using DC mV (parallel) but please double check with someone like Lane.

You're right, but if you measure the voltage drop over a 1 Ohm precision resistor, you're actually measuring current, that's why a probe has a 1 Ohm resistor between pin 1+8 & ground.
V/R=I
45 mV / 1 Ohm = 45 mA