Biasing amps - recommend meter/probe

lpaul626

Gold Supporting Member
Messages
848
I'd like to start biasing my own amps. I'm very handy with tools, electrical, etc. I understand I'll need a bias probe and voltage meter? Can someone recommend an easy to use bias probe? Anything else I'll need? What are the better resources (vid, books, etc.).

Thanks in advance-
 

Robal

Silver Supporting Member
Messages
1,274
I use the Compu-Bias and the BiasMaster. There are less expensive bias meters available, but these are reliable and well designed. I particularly like the Compu-Bias, because it will display for two tubes their respective plate voltages, cathode currents, and calculated wattage. Could not be easier to use, and the company will make custom probes if you are checking less common power tubes with different pin outs, e.g., 7591A for old Ampegs. http://www.compu-bias.com/ http://www.mojotone.com/amp-parts/TubeAccessories_5/TAD-Tube-Bias-Master

FWIW, I think matching power tubes in the amp in which they will be used by using the Compu-Bias, or comparable meter and probes, is often more effective than buying tubes that a seller claims were "matched" on his tube tester. I have a decent Hickcok tube tester I use to screen power tubes for issues before I put them in an amp, to avoid failures.
 
Last edited:

tele_player

Member
Messages
732
I use the Compu-Bias and the BiasMaster. There are less expensive bias meters available, but these are reliable and well designed. I particularly like the Compu-Bias, because it will display for two tubes their respective plate voltages, cathode currents, and calculated wattage. Could not be easier to use, and the company will make custom probes if you are checking less common power tubes with different pin outs, e.g., 7591A for old Ampegs. http://www.compu-bias.com/ http://www.mojotone.com/amp-parts/TubeAccessories_5/TAD-Tube-Bias-Master

FWIW, I think matching power tubes in the amp in which they will be used by using the Compu-Bias, or comparable meter and probes, is often more effective than buying tubes that a seller claims were "matched" on his tube tester. I have a decent Hickcok tube tester I use to screen power tubes for issues before I put them in an amp, to avoid failures.
Matching tubes only on idle current isn't enough. Two tubes can idle at the same current, and differ in transconductance. Effect is that their currents will only match at one bias setting.
 

TimmyP

Member
Messages
2,485
If Compu Bias had a quad we'd consider getting it. Currently, we use the Weber four-head unit.
 

EADGBE

Member
Messages
12,342
The Bias King may be a good one too.



I biased a Marshall 100 watt head with a Eurotubes Pro one.

 

Vanyu

Member
Messages
738
You don't necessarily need a probe to bias your own amps. As long as you know where your OPT center tap is, and you're able to get an alligator clip on pin 3 of your output tubes, you could bias using the transformer shunt method. You could also solder a 1 ohm resistor onto pin 1 of your tube sockets and read your bias that way. Plate voltage is taken off of pin 3 and ground, no need for a probe for that. Let's say I wanted to bias a 100w Marshall using the first method I mentioned, like a 2203. I would attach a lead to the HT fuse (centertap connection), and pin 3 on V4, and then V7 (V4/V5 and V6/V7 are seen as one tube each by the OPT, since they are being run parallel, so only one tube per side needs to be measured). Then, I would bias to 70mA per side (again, because they're seen as one tube on each side, so current is doubled here. Assuming the tubes are matched, they will be running at 35mA). That's all it takes, no probe required. The centertap is the wire on the primary side of the OPT that isn't running to a tube, it's usually pretty easy to find. To help you bias this way, it helps to know how a push pull circuit works. Matched tubes are also a must this way, since there is no biasing induvidual tubes. Look up 'transformer shunt biasing' for some more info. Using a 1 ohm resistor is more accurate, but it does require some soldering. I've never actually biased this way, so I'm not 100% certain on the process, but it's talked about frequently in other threads.

Where a probe comes in handy is when you have an external bias control, or have PCB mounted tube sockets. Other than that, there's ways to bias with just a meter and a little bit of care. With that in mind, you must also consider that pin 3 and pin 4 carry anywhere between 400v-500v, so bias at your own risk.
 

dazco

Member
Messages
14,629
You don't necessarily need a probe to bias your own amps. As long as you know where your OPT center tap is, and you're able to get an alligator clip on pin 3 of your output tubes, you could bias using the transformer shunt method. You could also solder a 1 ohm resistor onto pin 1 of your tube sockets and read your bias that way. Plate voltage is taken off of pin 3 and ground, no need for a probe for that. Let's say I wanted to bias a 100w Marshall using the first method I mentioned, like a 2203. I would attach a lead to the HT fuse (centertap connection), and pin 3 on V4, and then V7 (V4/V5 and V6/V7 are seen as one tube each by the OPT, since they are being run parallel, so only one tube per side needs to be measured). Then, I would bias to 70mA per side (again, because they're seen as one tube on each side, so current is doubled here. Assuming the tubes are matched, they will be running at 35mA). That's all it takes, no probe required. The centertap is the wire on the primary side of the OPT that isn't running to a tube, it's usually pretty easy to find. To help you bias this way, it helps to know how a push pull circuit works. Matched tubes are also a must this way, since there is no biasing induvidual tubes. Look up 'transformer shunt biasing' for some more info. Using a 1 ohm resistor is more accurate, but it does require some soldering. I've never actually biased this way, so I'm not 100% certain on the process, but it's talked about frequently in other threads.

Where a probe comes in handy is when you have an external bias control, or have PCB mounted tube sockets. Other than that, there's ways to bias with just a meter and a little bit of care. With that in mind, you must also consider that pin 3 and pin 4 carry anywhere between 400v-500v, so bias at your own risk.
Thats what i do. dead simple. 1 ohm resistor from each cathode to ground and read millivolts across it. with a 1 ohm resistor the number translates exactly....30 millivolts=30 milliamps. Simple, safe, easy, only a multimeter needed. Why anyone would go to more trouble than that i dunno because from my experience it's as accurate as you need and biasing is not the super critical thing it's sometimes made out to be. Just use one of the many online bias calculators and bias reasonably ans the amp will sound as good as it;s going to w/o eating tubes. I'm not amp tech but it;'s worked for me for years and i hear no difference till the bias is way out of spec. Just get some 1 ohm 5% tolerance resistors and be done with it.
 

cristo

Member
Messages
367
+1 on the 1 ohm resistor from cathode to ground.
That's what I did on my Pro Reverb and Pignose.
I compared the results with the transformer shunt method on
my Pro reverb and got the same results, but once you put in
the resistor, it's much more convenient to check across the
resistor.
 
Last edited:

Kurzman

Silver Supporting Member
Messages
1,235
Weber Bias Rite for me. Great support from Weber, too, if you ever need it. They're good people.
 

EADGBE

Member
Messages
12,342
...With that in mind, you must also consider that pin 3 and pin 4 carry anywhere between 400v-500v, so bias at your own risk.
That's why I use a bias probe for certain amps. With my two Marshall TSL60s however all I need is a multimeter set to DC mV.
 




Trending Topics

Top