Fender Bassman 300 Pro Bias Question Need to know if it is safe to use

Nighttrain1127

Nighttrain1127

Nov 27, 2004
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#1
So I put in the new KT-88's and went to set the bias. Both the manual and the back panel say set the bias for 100mV and set the balance for 0mV . So I got my meter out and the bias will only go to -100mV but the balance will go to 0 mV . So is this ok or do I have a problem somewhere ? Kind of need to know before I go and use it at practice later.
 
#2
Nighttrain1127 said:
So I put in the new KT-88's and went to set the bias. Both the manual and the back panel say set the bias for 100mV and set the balance for 0mV . So I got my meter out and the bias will only go to -100mV but the balance will go to 0 mV . So is this ok or do I have a problem somewhere ? Kind of need to know before I go and use it at practice later.
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Sounds like you have the probes of the multi tester backwards (Polarity?
 
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#3
That is what I thought but the center test point is used for both measurements so I figured that would be common because the manual does not state anything on this. if the center test point is Positive then that would explain it. The inputs on the meter are clearly marked so it is not reversed there.
 
#6
Are KT88's an approved tube for the amp?

There are some unique characteristics of the KT series that raise this question. One possibility is ultrasonic oscillation causing excessive heating.

You can always call Fender's consumer relations folks and ask for technical support. They are very good in this aspect.
 
#7
Fender now recommends that you set the bias at 80mV (same as Super Bassman is set for NORMAL now). When the Bassman 300 Pro was first released, the bias was set to 100mV, but was later reduced due to tube over-bias/heating issues.
 
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#9
Happy to help! Also, while KT88s are fine for the 300 Pro, I know that Fender does not recommend them in the SB300 because the tube sockets are closer together, and with the slighter bigger diameter of most KT88s, the increased zone heating is a bit higher than what the SB300 was designed to tolerate. But again, they should be fine in the 300 Pro.
 
#11
Nighttrain1127 said:
That is what I thought but the center test point is used for both measurements so I figured that would be common because the manual does not state anything on this. if the center test point is Positive then that would explain it. The inputs on the meter are clearly marked so it is not reversed there.
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The center point should be the Negative Probe and the other two would be where the Positive probe go that should give you the + 100 or 85 .I had Old SVT and a Aguilar DB 359 and that was the way you set the bias.
 
#14
Nighttrain1127 said:
With the Negative probe in the center test point I get a -80 mV the farthest it can go + is about +58mV. I have been told by a very trusted source that bias should be a -voltage
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Again, I am suggesting that you call Fender's Consumer Relations technical support, or seek the help of somebody who REALLY knows what they are doing.

Your very trusted source is unfortunately wrong in this case. While the GRID BIAS VOLTAGE is indeed negative for quiescent conditions, that is NOT what the test points are measuring.

Setting up the quiescent conditions, what is important is the quiescent CURRENT through the plate circuit of the output tube(s). By measuring the plate current, you set the grid bias voltage to achieve a SENSE voltage that is proportional to the plate current. What you are measuring is the SENSE VOLTAGE. The polarity doesn't matter because it will always be correct. It's virtually impossible to reverse the polarity, so often it's just ignored.

In the case of this particular amp, you are measuring the PLATE CURRENT of all 3 tubes in each half of the circuit indirectly by measuring the SENSE VOLTAGE, and the two halves then need to be adjusted to match. Sometimes it takes several adjustments to narrow the gap. The adjustment that you are making is to the GRID BIAS VOLTAGE, which will end up where it needs to be for the specific tubes that you have installed. Some bias circuits do not have adequate range for KT series tubes, and some circuits may oscillate under some conditions with KT type tubes. Your amp was designed specifically for 6550's, which are a robust, well behaved tube (the reason they are used).

There are some other important details that need to be addressed as well, which is why I suggest that you get help from somebody such as an authorized Fender service tech.
 
#15
Nighttrain1127 said:
Just talked to Fender and the Bias voltage is supposed to be a Negative voltage. So once again Thank you to everyone for all the good information. It is greatly appreciated.
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This is unbelievable, the GRID BIAS VOLTAGE is negative, but that is NOT what you are measuring. You are measuring PLATE CURRENT SENSE VOLTAGE which is proportional; to plate current. The polarity of what you are measuring does not matter because it can't under any condition be reversed.

It is critical to verify that each tube is passing current as well, and that the screen circuit is operating as it's supposed to. If you have one tube that's open, that half will be over-biased by 33% and it will be "almost" impossible to balance the halves (though it might appear to until you look at the results on a scope, or look at the THD at rated power on an analyzer)
 
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