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Post subject: Bias question
Posted: Mon Oct 07, 2013 3:41 pm
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So I checked the bias on my '67 bandmaster and adjusted it to 426v plate voltage with 48ma reading on the bias probe. Prior to this it was reading about 440v plate voltage and 33ma of current, a bit on the cold side. So after adjustment this puts me at about 20w per tube. But now when measuring from the 220k bias resistors to ground I'm reading about -33v dc. Typically it's supposed to be between -48v & -52v. Should I be concerned?
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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 3:47 pm
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Was that -33VDC bias-supply measurement taken with the power tubes installed or removed?

Arjay

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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 4:18 pm
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Installed, power and stand by switch on.

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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 8:57 pm
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I decided to go over the schematic to double check my values. So when i adjust the bias to read -48v I get the 440v at the plates but the bias probe reads 33ma for the tubes. Since my voltages are correct for the specified readings on the schematic I'm beginning to suspect worn or tired tubes. Would that be a correct assessment?

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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 9:12 pm
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Why do you suspect a problem? When you set the tubes to run hotter, the bias supply is going to become less negative. Fender has a tendency to bias their amps cold. The only numbers you need to worry about are plate voltage and cathode (actually plate) current. What the bias voltage ends up to be is irrelevant as long as it can be set to provide the desired bias point as measured by your bias probe. Your values look perfectly normal to me.

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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 9:35 pm
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I think that's what I was trying to figure out. It shouldn't matter what the bias resistor reading is, so long as the plate voltage and current reading come to within 70% of max dissipated wattage for the tubes the amp will be ok.
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Post subject: Re: Bias question
Posted: Mon Oct 07, 2013 9:53 pm
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socal323 wrote:
I think that's what I was trying to figure out. It shouldn't matter what the bias resistor reading is, so long as the plate voltage and current reading come to within 70% of max dissipated wattage for the tubes the amp will be ok.
Thanks.


I don't even measure the bias supply setting unless I am troubleshooting or making a mod to get the tubes hotter. I had to mod the bias supply in my BDRI to get a less negative voltage to get my tubes closer to 70%. The stock supply would only get them to about 55%. I also had to mod the supply in my Wrecking Ball when I built it. Although I could get the desired bias settings, the adjustment pot was too close to one end of the voltage range which made it very twitchy. Changing the resistor in the supply moved the overall range less negative and made it less sensitive.

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Post subject: Re: Bias question
Posted: Tue Oct 08, 2013 1:23 am
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socal323 wrote:
Typically it's supposed to be between -48v & -52v. Should I be concerned?
Thanks.


This mean nothing ( short answer ) , good Bias prob reading is your goal = tubes working properly.


Use this calculator:

www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Bias question
Posted: Tue Oct 08, 2013 9:24 am
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Thanks guys, so I guess as long as it's sounding good and the tubes aren't red plating I won't concern myself with it.
Stratele52,
That's the calculator I was referencing when biasing the amp. It's great when the more experience repair/builders put that type of information out there for the less experienced. It's a great web site for anyone interested in biasing tubes themselves.

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Post subject: Re: Bias question
Posted: Tue Oct 08, 2013 10:12 am
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In my '67 Bandmaster I'm running a pair of Western Electric (nee GE) 6L6WGB's, with the plate voltage measured at 437VDC. I have them biased at 40mA and they've been running fine for about eight years now. In a '68 Bandmaster I have a pair of Electro-Harmonix 6L6EH's. Plate voltage for this amp is on the money at 440VDC and they work well with 42mA of bias current.

Arjay

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Post subject: Re: Bias question
Posted: Tue Oct 08, 2013 11:32 am
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Thanks to all, you've provided me with some very useful data. Time to jam away..... :D

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