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Post subject: Odd Bias voltage
Posted: Wed Apr 19, 2017 5:59 am
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A couple weeks ago, the 12AX7 in my SCX2 went out. I replaced it and all is fine. I figured it was a good time to replace the 6V6's as well. I ordered some JJ's, and they finally got here. I opened the amp and checked the bias with the original tubes, the factory bias. It measured 60mV, not the expected 40 I swapped in the JJ tubes and checked the bias - still 60 mV. The schematic says to set it to 40 mV, so that's where I put it. It sounds fine. I'm going to check it again tonight with a different meter, maybe mine is not accurate. Has anyone seen a bias on this amp set at 60mV?


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Post subject: Re: Odd Bias voltage
Posted: Wed Apr 19, 2017 7:24 am
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Shematic show 375 volts plate voltages on 6V6GT tubes. What is yours with news tubes ?

With 375 volts, a bias of 26 ma / mv for each tube is ok = 52 mv / 2 tubes at the test point.
Many amps sound good with colder bias so 40 mv/ ma may be good. Test with your ears too.

I won't go to 60 mv/ma

Without knowing your plate voltage, your bias reading are not accurate

Use this calculator

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


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Post subject: Re: Odd Bias voltage
Posted: Wed Apr 19, 2017 7:41 am
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Thanks. I'll measure the voltage tonight while it's open. With the bias set to 40 mV, the amp had a bit more grit to it. Not sure if the bias had anything to do with that, or if it was the JJ tubes. I had the volume set quite low, so the tube shouldn't have been anywhere near distorting. Anyway, from what you say, I could safely bias upwards of 50 mV?

With the measured bias voltage high or low, do I have this right? If it's set lower, the tubes will likely last longer but not sound as good. If it's set higher, the amp will breakup sooner and the tubes like won't last as long. Correct?


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Post subject: Re: Odd Bias voltage
Posted: Wed Apr 19, 2017 7:55 am
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pdawg wrote:
Thanks. I'll measure the voltage tonight while it's open. With the bias set to 40 mV, the amp had a bit more grit to it. Not sure if the bias had anything to do with that, or if it was the JJ tubes. I had the volume set quite low, so the tube shouldn't have been anywhere near distorting. Anyway, from what you say, I could safely bias upwards of 50 mV?

50 mv is safe and sound good at 400 volts plate voltage, if plate is 365 volt, tube will be little hot, safe too.

With the measured bias voltage high or low, do I have this right?
Yes
If it's set lower, the tubes will likely last longer but not sound as good. If it's set higher, the amp will breakup sooner and the tubes like won't last as long. Correct?


A correctly biased amp will run efficiently and maximizes the life of your tubes.

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


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Post subject: Re: Odd Bias voltage
Posted: Wed Apr 19, 2017 3:58 pm
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I borrowed a meter from work that had just come back from calibration. The bias read the same with it as with my meter. So, the bias from the factory (GT tubes) was 60 mV, a bit high. The plate voltage read 384 volts. I set the bias at 44 mV. The amp sounds great. The individual notes seem more clear and defined. It seems to have a stronger bottom without muddying the highs. High volume has a lot of punch and at 6 or 7 I wasn't hearing any hint of breakup. I didn't feel like going louder than that. I wasn't expecting new tubes and proper bias to have such a noticeable effect. I've always liked this amp, and this is definitely a step up.

Thanks for the help stratele52!


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Post subject: Re: Odd Bias voltage
Posted: Thu Apr 20, 2017 12:56 am
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You are welcome.
You are now a pro for bias adjustment

Enjoy your amp :)


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Post subject: Re: Odd Bias voltage
Posted: Mon May 01, 2017 10:11 am
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What are using to read plate voltage? I looked at Weber Bias-Rite, but even they describe their own probes as not being robust.

Any recommendations?


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Post subject: Re: Odd Bias voltage
Posted: Mon May 01, 2017 10:36 am
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I used a Fluke voltmeter and measured the plate voltage at the pin of the tube socket. I had removed the chassis from the cabinet and had it powered on for measuring the plate voltage and test point voltage for the bias setting.


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Post subject: Re: Odd Bias voltage
Posted: Mon May 01, 2017 11:44 am
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Since many years I have two Amp-Head Dual Bias Testers.
One for 6L6 /6V6,EL34 and one for EL84

Amp-head.com look out of business :(


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Post subject: Re: Odd Bias voltage
Posted: Tue May 09, 2017 2:51 pm
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Thanks for the info, guys!


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Post subject: Re: Odd Bias voltage
Posted: Tue May 09, 2017 2:57 pm
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pdawg wrote:
A couple weeks ago, the 12AX7 in my SCX2 went out. I replaced it and all is fine.


What brand 12AX7 did you install?

I replaced mine with a Tungsol 12AX7.


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Post subject: Re: Odd Bias voltage
Posted: Tue May 09, 2017 3:26 pm
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Tungsol for the 12AX7 and JJ for the 6V6. I'm very happy with the sound.


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Post subject: Re: Odd Bias voltage
Posted: Tue May 09, 2017 3:44 pm
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Thanks again, pdawg! I'm thinking of trying TAD 6V6s after reading a review of them at the TubeStore.

John


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Post subject: Re: Odd Bias voltage
Posted: Wed May 10, 2017 5:43 am
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I picked the JJ's because they were among the lowest cost. Then I read that they (supposedly) put up with more abuse and sound better than a lot of the more expensive 6V6 tubes.


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Post subject: Re: Odd Bias voltage
Posted: Wed May 10, 2017 8:12 am
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pdawg wrote:
I picked the JJ's because they were among the lowest cost. Then I read that they (supposedly) put up with more abuse and sound better than a lot of the more expensive 6V6 tubes.



Very good tube... and the price is low


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