Which bias Meter ?? bias Probe, biasrite, biasking

glman

Member
Messages
629
Which of these 4 will give me the most reliable and accurate useage?

Bias Probe - Alessandro
bias rite - weber
Bias King
Compu-Bias

I want to be able to swop tubes and bias my Marshall Slash 2555 without taking it to a tech

the 2555 uses 4x EL34 tubes .

I've heard rumours that the bias rite can give inaccurate readings and tends to underrate plate voltage?

please help me decide.
 

fr8_trane

Silver Supporting Member
Messages
6,936
I have a weber bias rite and it works very well. Not as convenient as test points but a hell of alot easier and safer than opening the amp.
 

John Phillips

Member
Messages
13,038
But for most amps - including all pre-JCM2000 Marshalls - you have to open the chassis to get at the adjuster if the bias reading is off, anyway. It does make things safer than poking about near the power tube sockets with a multimeter though, since the adjuster is usually not near any high-voltage parts of the circuit. You can only do it with the amp fully powered up, so you must know at least the basics of safe working.

I'm not familiar with the details of any of these since I built my own, but you should avoid any which pass the actual tube current through the meter (ie require you to set the meter to milliamps) since these can both cause instability in the amp and even could even in a remotely possible case (with a shorted tube) be dangerous. The ones which use a 1-ohm resistor in the current path and require you to set the meter to millivolts and read an indication of the current are safer for both the amp and you.

I'd also avoid overcomplicated and expensive ones with multiple tube adaptors - they are more convenient, but unless the resistors are extremely precise tolerance, they will give inconsistent readings even on a perfectly matched set of tubes. The accuracy isn't that critical, but there's no sense in confusing yourself by having two or four different readings which don't appear to agree, since you won't know if it's the tubes or the metering. (This is a known problem with at least one of these units BTW.)

The other thing to avoid is blindly setting the amp to 70% of maximum dissipation for the tube type - let alone a particular current. That isn't always the most reliable or best-sounding setting, you need to know more about the amp. It's better than a total shot in the dark, but a lot of people pay more attention to getting the amp to 'exactly' 70% rather than to the correct operating point, which is usually less exact and also not always there. It's like aiming very carefully at the wrong target.

FWIW, most Marshalls need to be biased a bit below 70% - many older ones in particular sound best and are most reliable as low as 50 to 55%, with EL34s.
 

Gary Brennan

Platinum Supporting Member
Messages
1,828
Hi John,

In your opinion, in a 2 socket configuration, is the potential tolerance varience of a 2 socket type more of a problem than having to go through the turn off/cool down/switch tube/measure routine over and over again, considering how adjustments to 1 tube usually require a tweak of the other tube?

gb
 

glman

Member
Messages
629
But for most amps - including all pre-JCM2000 Marshalls - you have to open the chassis to get at the adjuster if the bias reading is off, anyway. It does make things safer than poking about near the power tube sockets with a multimeter though, since the adjuster is usually not near any high-voltage parts of the circuit. You can only do it with the amp fully powered up, so you must know at least the basics of safe working.

I'm not familiar with the details of any of these since I built my own, but you should avoid any which pass the actual tube current through the meter (ie require you to set the meter to milliamps) since these can both cause instability in the amp and even could even in a remotely possible case (with a shorted tube) be dangerous. The ones which use a 1-ohm resistor in the current path and require you to set the meter to millivolts and read an indication of the current are safer for both the amp and you.

I'd also avoid overcomplicated and expensive ones with multiple tube adaptors - they are more convenient, but unless the resistors are extremely precise tolerance, they will give inconsistent readings even on a perfectly matched set of tubes. The accuracy isn't that critical, but there's no sense in confusing yourself by having two or four different readings which don't appear to agree, since you won't know if it's the tubes or the metering. (This is a known problem with at least one of these units BTW.)

The other thing to avoid is blindly setting the amp to 70% of maximum dissipation for the tube type - let alone a particular current. That isn't always the most reliable or best-sounding setting, you need to know more about the amp. It's better than a total shot in the dark, but a lot of people pay more attention to getting the amp to 'exactly' 70% rather than to the correct operating point, which is usually less exact and also not always there. It's like aiming very carefully at the wrong target.

FWIW, most Marshalls need to be biased a bit below 70% - many older ones in particular sound best and are most reliable as low as 50 to 55%, with EL34s.
thanx for the helpful info - do you have any knowledge about the marshall signature amp and biasing it?
 

John Phillips

Member
Messages
13,038
It's a standard JCM800 output section, so you should probably aim for 60% or maybe a little lower.

It really depends on how much power they will produce when fully cranked - the older ones, up to and including the JCM800s, are way over the spec 50 or 100W, and if you bias them up to 70%, the tubes will try to operate outside the survivable range when the amp is fully cranked, even though they're well inside it at idle. You'll usually see heavy screen overload (bright orange glow inside the plate structure) which can lead to sudden failure, and maybe even red-plating which will drastically shorten the tube life even if they don't blow straight away.

The Jubilees don't seem to push the tubes quite as hard as the 2203/2204/2205/2210, but 70% is still too much for good reliability.
 

Jube2550

Member
Messages
3,087
It's a standard JCM800 output section, so you should probably aim for 60% or maybe a little lower.

The Jubilees don't seem to push the tubes quite as hard as the 2203/2204/2205/2210, but 70% is still too much for good reliability.
+1 and John knows.

I have the Biasrite which works well, I cannot comment on the others. The initial results on the stock Jubilee indicated some drift or it was set to 70% at the factory based off using the Biasrite A = Vp 462 Ik 38.1 & B = Vp 460 Ik 37.4 readings. This is a bit high and unnecessary, especially for tube life. I will set external bias trimmers like on a red knob twin for instance but I am a bit unnerved poking around at the PCB mounted bias pot on the Marshall so I took it to a tech.

The tech set it around 60% or lower to 31-33ma range which he also recommended.
 

allynmey

Member
Messages
328
The Compu-Bias from Mission amps is the best one I've used! With most others, you still have to open the amp to get your plate voltage. Compu-Bias gives you plate voltage, Cathode Current, and dissipation in watts all at once on the screen for two tubes t a time!

Allyn
 

donnyjaguar

Member
Messages
4,199
Just educate yourself on how an amplifier operates and set the bias with the unit apart. I've never used a bias probe so I can hardly recommend any of them.
 

fr8_trane

Silver Supporting Member
Messages
6,936
The other thing to avoid is blindly setting the amp to 70% of maximum dissipation for the tube type - let alone a particular current. That isn't always the most reliable or best-sounding setting, you need to know more about the amp. It's better than a total shot in the dark, but a lot of people pay more attention to getting the amp to 'exactly' 70% rather than to the correct operating point, which is usually less exact and also not always there. It's like aiming very carefully at the wrong target.
I just put a SS recto in my Deluxe with JJ's. Without rebiasing I notived the amp had less clean headroom. That's the only tonal difference. When I checked the bias with the bias rite I had 415 V on the plates (Yes, the Bias rite give you plate voltage also) up from 400 and the current read 22/23 ma respectively (up from 20-21 at 400v). I rebiased for 18/19ma and now the amp has exactly as much head room as before. No real tonal differences.

So I guess what I want to know is what am I supposed to hear when the amp is in or out of the sweetspot because I didn't hear much besides lack of headroom when biased too hot.
 

JackButler

Silver Supporting Member
Messages
3,823
I have a Bias Rite and it works fine. I know zero about electronics, can't even solder a cable. The Bias Rite is easy to use and has been a great investment for me. Thoguh now I feel I can get in the safe and good range with just my ears,
 

John Phillips

Member
Messages
13,038
I just put a SS recto in my Deluxe with JJ's. Without rebiasing I notived the amp had less clean headroom. That's the only tonal difference. When I checked the bias with the bias rite I had 415 V on the plates (Yes, the Bias rite give you plate voltage also) up from 400 and the current read 22/23 ma respectively (up from 20-21 at 400v). I rebiased for 18/19ma and now the amp has exactly as much head room as before. No real tonal differences.

So I guess what I want to know is what am I supposed to hear when the amp is in or out of the sweetspot because I didn't hear much besides lack of headroom when biased too hot.
Allegedly it's supposed to give a fatter, richer tone when you bias hotter.

I find that it doesn't make all that much difference - it does increase gain very slightly, which can give the impression of fatter and richer, but if anything it tends to make the sound harsh and overly compressed if you turn it up too far. Too cold is bad as well (too thin and 'cold' sounding), but the best point tonally is not necessarily as hot as you can go.
 

squeally dan

Member
Messages
5,711
I have a Bias Rite and it works fine. I know zero about electronics, can't even solder a cable. The Bias Rite is easy to use and has been a great investment for me. Thoguh now I feel I can get in the safe and good range with just my ears,
Cool. Do most folks get where they can find the good/safe range by ear?
 

Lonely Raven

Member
Messages
1,003
Cool. Do most folks get where they can find the good/safe range by ear?
When I'm familiar with an amp, I can bias pretty close by ear. I built a Trainwreck type amp and I had loaned my Fluke to a friend, and lost my Bias Rite around the apartment somewhere (packed up probably since I don't use it much). So I dialed the amp in by ear...starting cold and working my way up till I found a sound I liked...went past it a bit and it started getting a bit hairy and backed it down to where I thought it sounded best.

When I got my Fluke back and found my Bias Rite, I checked the bias to see where I had put it by ear. I was a few mV shy of what most TW style amps (with the correct plate voltage) should be dialed in at, but perfectly within range. Dialing it up and down a bit I found that I kept going back to where I had done it by ear. :BEER


A side note; my Bias Rite (an early model mind you) was accurate within 1.5mV compared to my $300 Fluke. I'd have to say that's good enough for me.
 

Lex Luthier

Member
Messages
1,765
I've used a Hunt Dabney BiasProbe on a ton of amps since the early 90's, and it still works great to this day. Back then I don't recall anyone else making one.
 




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