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Post subject: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Wed Jan 07, 2015 11:02 pm
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Hey All,
Forgive me as I'd rather be redundant and annoying than have pedestrian tone or be dead.
I just replaced my power tubes and I am using a bias probe to see if I need to adjust.
So...with a bias probe that sits between the socket and the tube...
My meter on it's lowest DC range...I get .16mV and .17mV.
1- Am I correct in my understanding the each tube should be reading .2mV (.4 total or 40mV)?
2- Are these readings close enough to not require adjusting up 3-4mV? 8% off target seems a bit...if I'm understanding the process...and the readings.
I have not done this before and I have read as many other posts as I can find, but...like I said...I don't want to be lazy or hasty about this.
Grateful to those willing to share their expertise...and tolerate my deficit in this area.


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 3:15 am
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SCXD ??? Ahhhhh it a Super Champ XD , please no sms

You write 0.16 and 0.17 mv are you sure ? With this current, tubes are dead.
Do you mean 16 and 17 mv ? This is good

Schematic here ;

http://support.fender.com/schematics/gu ... ematic.pdf

6V6 cathode current you read 16 and 17 mv should match with plate voltage wich is 375 volts as schematic show . Best is you own reading , voltage change with bias value.


MV without plate voltage is not usefull , both work together

With Weber bias calculator 6V6GT with 375 plate voltage at 70 % plate dissipation = 26mv
6V6 at 70% = 22 MV
Most fender amp work well at colder bais . 17 and 16 MV are low for 6V6GT . Good for 6V6 . Wich tubes do you have ?
Few MV between tubes are better than 0 MV as Gerald Weber wrote on his books .

http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 8:26 am
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Thanks for the response.
6V6GT tubes...matched Soviet
I guess my question is more specific to the probe I am using, which goes directly into the socket rather than reading from the test point.
I am not sure if:
a) I'm reading the meter correctly. The lowest setting is 2 in the DCV section of my meter (the directions say set to 200mV...I can't set that low) and the tubes read .16 and .17. I am taking that to mean...16 and 17 mV or mA.
b) If I am reading it right...should I adjust closer to 20 per tube (.20 on my meter setting)?
I hope my description makes sense.
Thanks again!


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 9:36 am
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jbear wrote:
Thanks for the response.
6V6GT tubes...matched Soviet
OK 6v6GT
I guess my question is more specific to the probe I am using, which goes directly into the socket rather than reading from the test point.
I am not sure if:
a) I'm reading the meter correctly. The lowest setting is 2 in the DCV section of my meter (the directions say set to 200mV...I can't set that low) and the tubes read .16 and .17. I am taking that to mean...16 and 17 mV or mA.
You are right with 16 and 17 ma
b) If I am reading it right...should I adjust closer to 20 per tube (.20 on my meter setting)?
I hope my description makes sense.
Yes it make sense . 6V6 ask 22 mv , 6V6GT 26 mv . Both are maximum
Thanks again!


If you can't read safely plate voltage ( 375 is lethal ) leave it at 20 mv each tube , the lower tube.

You can do some hear test.
Do not go over 26 MV . If sound is good at lower MV leave it like that .
_______________
A correctly biased amp will run efficiently and maximizes the life of your tubes.

Under - biased ( too many MV ) amps will lack of punch and tubes will run hotter , shorter tube's life .
Overbiased ( low MV ) amps will sound thin and brittle. with tubes running too cood for proper performance .
A correctly biased amps will sound clean and thight a moderate volumes., than at higher volumes breakup and distort musically .


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 10:46 am
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Thank you for working through this with me!
Very grateful!
I may see if I can get them a little higher. Amp sounds good, but volume is a little low I think.


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 12:26 pm
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jbear wrote:
. Amp sounds good, but volume is a little low I think.



If youre reading are true ( 16 , 17MV ) , issue is not output tubes or bias .

Check 12XX7 tubes or power supply voltages


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 3:06 pm
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Interestingly it is plenty loud with my Teles, and an LP with mini's, but I have a Goldtop with a 490/498 combo (recent acquisition) that seems quiet. I have another LP with the same PUP's and I am going to A/B it so see if it could be the wiring in the guitar.
Thank you again!


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Thu Jan 08, 2015 3:22 pm
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Yes check the Goldtop.

Pickups height first.

Should be louder than a Tele IMO


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Fri Jan 09, 2015 6:50 pm
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jbear wrote:
...I am not sure if:
a) I'm reading the meter correctly. The lowest setting is 2 in the DCV section of my meter (the directions say set to 200mV...I can't set that low) and the tubes read .16 and .17. I am taking that to mean...16 and 17 mV or mA...


If your meter is set to 2 VDC scale, and if your readings are .16 and .17, that is 160 mV and 170 mV respectively. Something isn't quite right.

For 16 mV and 17 mV, you should read .016 and .017 on a 2 VDC scale.

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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Fri Jan 09, 2015 9:25 pm
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The XD already has a 1 ohm resistor(#R20) installed on the board as a bias test point. It reads both tubes so the bias should be set to about 40 mVs(20mA/tube) per the schematic. I would use the test point and not the probe-seems like using both would give you two 1 ohm resistors in the circuit which could affect the reading if they are in series.

You should be reading .016 and .017Vdc to equal 16/17 mAs on your meter if set to the 2 Vdc scale. Or maybe your meter is only accurate to .01 volts? I'd look for another meter.


Last edited by t-luxe on Fri Jan 09, 2015 10:01 pm, edited 1 time in total.

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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Fri Jan 09, 2015 9:57 pm
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Yup...sorry...the readings are .016 and .017.
I suppose I'm not articulating my question...
Since I have the probe, I haven't pulled the chassis...and the numbers being what they are...if they're close enough...I won't re-bias. It sounds fine as it is now, but if it could sound even better...sure...I'll re-bias.
Just to add info., this is an amp I keep at school to play for my students and to practice when time permits. It gets fairly light duty. I wouldn't attempt to mess with my "duty" amps (Ceriatone OTS, Winfield Plexi, Yamaha t50c, HRDX, and a Blues Jr.), but I figured that I could perhaps get my feet wet with this SCXD without causing too much damage...or electrocuting myself.
As of this post...I am still alive. Thanks for the guidance to date.


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Fri Jan 09, 2015 10:07 pm
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The schem calls for 40mV at the bias point. That's where I'd set it, using the built-in bias test point(R20), not the probe.
http://www.agileguitarforum.com/threads/how-to-bias-the-scxd.51442/

But if you like it where it is, no harm. However, you might like it better biased a little hotter. Not a big deal to slide the chassis part way out to where you can reach the Blue trim pot and test point at R20. Be sure the speaker is hooked up!
Good luck.


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Sat Jan 10, 2015 2:03 am
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shimmilou wrote:
[

If your meter is set to 2 VDC scale, and if your readings are .16 and .17, that is 160 mV and 170 mV respectively.

.


Tubes was red plating and destroyed since loooooong time :lol:


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Sat Jan 10, 2015 2:06 am
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t-luxe wrote:
The schem calls for 40mV at the bias point. It is 20 MV each tube .


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Post subject: Re: Biasing SCXD w/ Bias Probe...Due Diligence
Posted: Sat Jan 10, 2015 10:14 am
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stratele52 wrote:
shimmilou wrote:
[

If your meter is set to 2 VDC scale, and if your readings are .16 and .17, that is 160 mV and 170 mV respectively.

.


Tubes was red plating and destroyed since loooooong time :lol:


:lol: No doubt! :lol:

Glad that it was .016 and .017. :)

I agree that if you are perfectly happy with the tone, no need to change the bias. Typically, Fenders bias recommendation is cold enough (schematic 40 mV at TP), so 20 mV per tube using the probes should be a minimum setting, and might sound even better than 17 mV.

There is no problem having the TP resistor and using bias probes, the probes likely being more accurate than the TP. Plus, with the probes you can check each tube to check matching, and you can't do that using the TP since the TP is for both tubes.

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