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Post by edoetsch on Jul 23, 2007 13:01:10 GMT -7
I was tube tweaking in my Route 66 the other day. I was using a Weber Bias Rite for reading the mA on the tubes. I tried 2 different sets of tubes first 1 then the next then back to the first set.
One set of tubes had these readings:
I was able to best bias them at 33.8 mA and 38.2 mA. My thought was the tubes are not matched. The volts were very near 410 for both though. Not good since 34 to 38 is the recommendation.
When I put the same tubes back in 30-40 minutes later, I was able to bias:
36.8 mA and 37.3 mA. My only thought was I reversed the tubes haphazardly and this time they seemed matched. Again the volts near 410 for both. This was within 34-38 so I've left the amp like this.
The tubes are brand new from the tube store KT66 rated 48.
Is this an indication of a problem in the amp? Is something else in the circuit is sending different values to the sockets?
It sounds fine though I have not had a chance to crank it up with this new set of tubes.
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Post by woody on Jul 23, 2007 17:09:25 GMT -7
I wonder if it has anything to do with the tubes warming up properly?
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Post by dongiesen on Jul 23, 2007 18:13:24 GMT -7
That's what I was kind of thinking from hearing that sort of thing before on other posts such as Myles but I'm the last guy to ask for electronic advise
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Post by whitestrat on Jul 23, 2007 18:49:00 GMT -7
I was tube tweaking in my Route 66 the other day. I was using a Weber Bias Rite for reading the mA on the tubes. I tried 2 different sets of tubes first 1 then the next then back to the first set. One set of tubes had these readings: I was able to best bias them at 33.8 mA and 38.2 mA. My thought was the tubes are not matched. The volts were very near 410 for both though. Not good since 34 to 38 is the recommendation. When I put the same tubes back in 30-40 minutes later, I was able to bias: 36.8 mA and 37.3 mA. My only thought was I reversed the tubes haphazardly and this time they seemed matched. Again the volts near 410 for both. This was within 34-38 so I've left the amp like this. The tubes are brand new from the tube store KT66 rated 48. Is this an indication of a problem in the amp? Is something else in the circuit is sending different values to the sockets? It sounds fine though I have not had a chance to crank it up with this new set of tubes. Only 410 plate volts measured with a Bias Rite? That seems very low. I consistantly get around 460 plate volts in my Route 66 w/ a GZ-34 rectifier. What kind of rectifier are you using in the amp?
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Post by edoetsch on Jul 23, 2007 18:55:21 GMT -7
I am using a 5u4
Thanks!
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Post by nitehawk55 on Jul 23, 2007 19:25:53 GMT -7
A good GZ34/5AR4 will up your voltage probably close to 450 . 5U4 is OK , it will give you lower output and more sag .
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Post by zdogma on Jul 24, 2007 4:46:18 GMT -7
If it sounds okay, I'd leave it. 410 plate volts sounds about right for a 5U4.
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Post by myles on Jul 24, 2007 9:31:29 GMT -7
As a side note, a match within 20% is considered an industrial match. Matches within 10%, or about 3mA in this case is a good static match. You will not hear hum from a 10% difference. Tubes like current more than voltage so if you are running at 410 B+ then anywhere in this range is fine: 50% 60% 70% 30 37 43 I would go for about 35-37mA at your voltage with KT66s. This is even conservative as I calculated this as a 25 watt tube and they dissapate 30 watts easily. For the GEC KT66 data sheet - www.guitaramplifierblueprinting.com/Tube/KT66.pdf
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Post by edoetsch on Jul 25, 2007 6:24:45 GMT -7
As a side note, a match within 20% is considered an industrial match. Matches within 10%, or about 3mA in this case is a good static match. You will not hear hum from a 10% difference. Tubes like current more than voltage so if you are running at 410 B+ then anywhere in this range is fine: 50% 60% 70% 30 37 43 I would go for about 35-37mA at your voltage with KT66s. This is even conservative as I calculated this as a 25 watt tube and they dissapate 30 watts easily. These are actually the tubes I mentioned in post drzamplifiers.proboards41.com/index.cgi?board=tubes&action=display&thread=1183132372 and I found that with the standard rectifier (JJ GZ34), I could not get the bias lower than 41-42 mA. Would that be safe considering the plate voltage is probably near the 450-460 with that rectifier? i.e. are these tubes really acceptable with this amplifier? (Sorry to ask so many questions, but with this being my only amp, I cannot afford to blow via tweaking...)
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Post by myles on Jul 25, 2007 10:14:41 GMT -7
As a side note, a match within 20% is considered an industrial match. Matches within 10%, or about 3mA in this case is a good static match. You will not hear hum from a 10% difference. Tubes like current more than voltage so if you are running at 410 B+ then anywhere in this range is fine: 50% 60% 70% 30 37 43 I would go for about 35-37mA at your voltage with KT66s. This is even conservative as I calculated this as a 25 watt tube and they dissapate 30 watts easily. These are actually the tubes I mentioned in post drzamplifiers.proboards41.com/index.cgi?board=tubes&action=display&thread=1183132372 and I found that with the standard rectifier (JJ GZ34), I could not get the bias lower than 41-42 mA. Would that be safe considering the plate voltage is probably near the 450-460 with that rectifier? i.e. are these tubes really acceptable with this amplifier? (Sorry to ask so many questions, but with this being my only amp, I cannot afford to blow via tweaking...) That is too hot. Tube life would be short, amp would run quite hot and the other components would not be happy with the heat and the amp would sound harsh.
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