I have a 94 Blues Deluxe and want to check the bias. After searches, I've read the early non reissues had no bias trim pot, but the reissues do, and a bias test point to test it. I'm confused as my 94 does have a blue bias adjusting pot but no bias test point in the area seen in some of my searches. I have a bias tester that can read mv's with my digital meter. If I put a tube in my bias tester, I can see the mv's going up or down as I adjust the trim pot. I put it at 38. Am I OK? Also the range I see on the trim pot is from 17 mv's to 92 mv's . Does that sound right?
Thanks
1994 Blues Deluxe biasing question?
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Gibsonman63
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Re: 1994 Blues Deluxe biasing question?
You have to know the plate voltage of the power tubes before you can correctly set the bias.
But, there may be a range that is considered safe for that amplifier.
In other words, it may be safe to set it between 32 and 35ma.
I = P / E
Where I = Current in Amperes
P = Power in Watts
E = Voltage in Volts (This is plate voltage in a tube amp)
Generally we don't want to bias any higher than 70% of the maximum rating.
In fact, many amps sound their best set a bit colder.
That is good for the tone and good for tube longevity.
The Blues Deluxe uses 6L6GC tubes which are rated at 30 watts each.
So if we want to limit that to 70% of max it would be 21 watts.
The unknown at this point is the plate voltage.
The schematic for the Blues Deluxe states it as 427v.
For this purpose we will assume that is correct but it may be higher or lower in the actual amp.
This does not take into consideration the screen current.
When we measure current at the cathode we are measuring Plate +Screen current.
But that can serve as a safety buffer as it is generally small, like 5ma or less.
So our formula looks like this.
(30 *.70) / 427 = .049ma or 49ma (the bias probe converts this to millivolts or mv)
So the maximum we would set the bias is 49mv.
It depends on the amp but many sound better down around 55-60%.
So at 60% it would be
(30 *.60) / 427 = 42ma or 42mv that is your 60% setting.
(30 * .5) / 427 = 35ma or 35mv is your 50% setting.
Also, the plate voltage can change with the bias setting but for your purposes, if you set your bias between 35 and 42mv you should be OK.
But, there may be a range that is considered safe for that amplifier.
In other words, it may be safe to set it between 32 and 35ma.
I = P / E
Where I = Current in Amperes
P = Power in Watts
E = Voltage in Volts (This is plate voltage in a tube amp)
Generally we don't want to bias any higher than 70% of the maximum rating.
In fact, many amps sound their best set a bit colder.
That is good for the tone and good for tube longevity.
The Blues Deluxe uses 6L6GC tubes which are rated at 30 watts each.
So if we want to limit that to 70% of max it would be 21 watts.
The unknown at this point is the plate voltage.
The schematic for the Blues Deluxe states it as 427v.
For this purpose we will assume that is correct but it may be higher or lower in the actual amp.
This does not take into consideration the screen current.
When we measure current at the cathode we are measuring Plate +Screen current.
But that can serve as a safety buffer as it is generally small, like 5ma or less.
So our formula looks like this.
(30 *.70) / 427 = .049ma or 49ma (the bias probe converts this to millivolts or mv)
So the maximum we would set the bias is 49mv.
It depends on the amp but many sound better down around 55-60%.
So at 60% it would be
(30 *.60) / 427 = 42ma or 42mv that is your 60% setting.
(30 * .5) / 427 = 35ma or 35mv is your 50% setting.
Also, the plate voltage can change with the bias setting but for your purposes, if you set your bias between 35 and 42mv you should be OK.
Tom
Don't let that smoke out!
Don't let that smoke out!