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Post subject: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 8:44 am
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For you DIY guys out there. Which Bias Probe do you recommend ?? Of course, your answer will come from the fact that you actually own one and use it the way it is supposed to be used.

I'm gonna stick to Fender's Groove Tubes for my 65 SRRI and I need to make sure of bias and plate currents/voltages etc. I went through a great number of common tubes I have on hand from my Marshall amps maintenance closet and have said elsewhere, I was not impressed. The best tones for all my humbuckers and single coils turned out to be the stock tubes that come with this amp. IMHO. :cry:


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 9:42 am
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Mine is a "Quad Stage Bias Pro MQ10". I have four octal and four 9-pin probes, allowing testing of up to four tubes at once, covering most any amp. I started with the Q10 which requires a separate meter, then I upgraded to the MQ10 which has its own meter.

http://www.asharpfretworks.com/5543.html

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 10:27 am
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shimmilou wrote:
Mine is a "Quad Stage Bias Pro MQ10". I have four octal and four 9-pin probes, allowing testing of up to four tubes at once, covering most any amp. I started with the Q10 which requires a separate meter, then I upgraded to the MQ10 which has its own meter.

http://www.asharpfretworks.com/5543.html

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+1

That's the most versatile system available.

Arjay

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 10:40 am
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Ken just introduced new probes that combine current measurement and Plate voltage measurement in one probe (V1-8C and V1-9C). I might order some of those for convenience. While I am used to measuring the Plate voltage directly, the new probes would be cool.

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 11:03 am
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Thanks guys!
I just measured my pin3 plate voltage and after 15 minutes of warmup, it came in at 461. Is there no ther old school way of setting/checking bias without the investment into those gorgeous tools :?:
If I hadn't been a stinkin' player all my life, but worked a regular day job for 40 years, I'd have some money to buy that gadget y'all recommend but, I didn't - and now, can't afford it :oops:
So, crying da Blues aside; what else can me & my trusty Sears DMM accomplish on this task :roll: :?:


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 1:24 pm
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bluezguy09 wrote:
Thanks guys!
I just measured my pin3 plate voltage and after 15 minutes of warmup, it came in at 461. Is there no ther old school way of setting/checking bias without the investment into those gorgeous tools :?:

Yes .Solder 1 ohms precison resistor at each output tube cathode and read milliamp at this resistor

If I hadn't been a stinkin' player all my life, but worked a regular day job for 40 years, I'd have some money to buy that gadget y'all recommend but, I didn't - and now, can't afford it :oops:
So, crying da Blues aside; what else can me & my trusty Sears DMM accomplish on this task :roll: :?:



Cathode resistor method ;

http://www.duncanamps.com/technical/lvbias.html

Most difficult job for you ; when there is no bias pot on amp .


Last edited by stratele52 on Sun Jul 13, 2014 1:34 pm, edited 2 times in total.

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 1:30 pm
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I use amp head dual probe testers since many years .

http://www.amp-head.com/product_info.ph ... ucts_id=31

This probe use your own meter.

Disavantage : you may need your meter for other reading while checking bias. You just have to disconnect the meter from bias probe , no need to remove bias probe from the amp . Easy to do.

Advantages :
A ) not too expensive ,

B) if your meter broke you can use another meter . Bias work with any meter .


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 7:02 pm
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stratele52 wrote:
I use amp head dual probe testers since many years .



I use these also, no problems, simple, works for me.


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Sun Jul 13, 2014 7:49 pm
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bluezguy09 wrote:
...Is there no ther old school way of setting/checking bias without the investment into those gorgeous tools :?: ...


Yes, really old school. With amp off, unplugged from electricity, and caps discharged, measure the resistance of each half of the OT primary and write down the values. Then power up the amp, warm up and measure the DC voltage across the same halves of the OT primary where the resistance was measured and write down the values. You should see something in the range of roughly 3 or 4 VDC. Be careful, as the OT primary has around 460 volts applied, but you are measuring the drop across the primary halves. Ohms Law, I = E/R (current equals voltage divided by resistance). Voila!

This is the actual Plate current, which is a little more accurate than a bias probe, but at the expense of safety. Be careful!!!

If you have two output tubes, each calculation will be for each tube (one tube for each half of the OT). If you have four output tubes, each of your calculations will be the current for two tubes, so divide each calculation by two to reveal the current for one tube.

If something isn't clear, just ask. :)

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Mon Jul 14, 2014 5:48 pm
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shimmilou wrote:
bluezguy09 wrote:
...Is there no ther old school way of setting/checking bias without the investment into those gorgeous tools :?: ...


Yes, really old school. With amp off, unplugged from electricity, and caps discharged, measure the resistance of each half of the OT primary and write down the values. Then power up the amp, warm up and measure the DC voltage across the same halves of the OT primary where the resistance was measured and write down the values. You should see something in the range of roughly 3 or 4 VDC. Be careful, as the OT primary has around 460 volts applied, but you are measuring the drop across the primary halves. Ohms Law, I = E/R (current equals voltage divided by resistance). Voila!

This is the actual Plate current, which is a little more accurate than a bias probe, but at the expense of safety. Be careful!!!

If you have two output tubes, each calculation will be for each tube (one tube for each half of the OT). If you have four output tubes, each of your calculations will be the current for two tubes, so divide each calculation by two to reveal the current for one tube.

If something isn't clear, just ask. :)


I don't understand "each half" of OT primary. One side has 3 wires, other side has two. What do I apply the 461 vdc measurement I got on pin 3 of each socket? You mention 460 volts approximately for the half of OT measurement ... any relationship?


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Mon Jul 14, 2014 6:26 pm
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Your amp operates via a push-pull output stage, class AB. The output tubes each have a duty cycle of 50% -- that is, each tube only conducts one half of a sine-wave signal. Thus for every second that the phase inverter is feeding a signal to the power tubes, only one of them is amplifying that signal for 500 milliseconds. When one tube is at rest, the other tube is conducting. During that time, the resting tube requires no plate voltage while the other needs the full B+ available from the power supply. And you'll note that your output tranny's primary winding is center-tapped for that very reason. And so the magic occurs.

Arjay

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Mon Jul 14, 2014 6:33 pm
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I use one of these. Cheap and effective.
https://www.tubedepot.com/products/tubedepot-bias-scout-kit

You'll also need a DMM that will measure DC millivolts(a cheap one works fine). You can buy two probes, but one will do the job with a bit of switching tubes.


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Mon Jul 14, 2014 6:53 pm
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OT = output transformer. The primary is the input side of the transformer, connected to the tubes, the secondary is the output side of the transformer, connected to the speaker(s).

OT primary has three wires, red, blue, brown. From red to blue is one half of the primary. From red to brown is the other half of the primary. The red wire is the B+ at approxc 460 volts, fed through the OT primary to the tubes' Plates. The "voltage drop" across each half of the OT primary is approx 4 VDC (at idle).

The other wires on the OT are the secondary, or the output part of the OT.

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Post subject: Re: Opinion on Best Bias Probe?
Posted: Tue Jul 15, 2014 2:18 am
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t-luxe wrote:
I use one of these. Cheap and effective.
https://www.tubedepot.com/products/tubedepot-bias-scout-kit

You'll also need a DMM that will measure DC millivolts(a cheap one works fine). You can buy two probes, but one will do the job with a bit of switching tubes.



This probe did not the same job as Amp Head probe . It is very basic .

You can read only tube cathode current ,the most important .

You can't read Plate voltage . You must disconnect your meter from the probe to read plate voltage.


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Post subject: Re: Opinion on Best Bias Probe?
Posted: Tue Jul 15, 2014 12:23 pm
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stratele52 wrote:
t-luxe wrote:
I use one of these. Cheap and effective.
https://www.tubedepot.com/products/tubedepot-bias-scout-kit

You'll also need a DMM that will measure DC millivolts(a cheap one works fine). You can buy two probes, but one will do the job with a bit of switching tubes.



This probe did not the same job as Amp Head probe . It is very basic .

You can read only tube cathode current ,the most important .

You can't read Plate voltage . You must disconnect your meter from the probe to read plate voltage.


Nope.
It does read plate voltage, you just have to un-plug one of the 3 probe leads and plug a different probe lead into your meter. Takes 5 seconds. Works great. For $20. The Amp head probe is $85.
I've biased many amps with it with no problems.

YMMV, I guess.


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