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Tube amp mods


mopardave

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No absolutely not..... if you have a set of tubes that will not bias most likely you have a set that bias way hot. Many modern production tubes bias excessively hot.  

 

 When you adjust the bias you are actually adjusting a negative voltage that enters the tube via the control grid which on your amp is pin 5. The measurement you are taking on pin 8 is the current the tube is drawing through that 10 ohm resistor from ground. We use a 10 ohm resistor so the voltage translates into how much current it equals. .50 VDC = 50 mA and .75 VDC = 75 mA and so on... to make a set of tubes run cooler you have to be able to apply more negative voltage to pin 5...  

 

 

 Rather than modify the amp I'd return the tubes and request a cooler biasing set of tubes.  With some tubes it might be impossible to get a cool enough running set. 

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1 hour ago, NOSValves said:

No absolutely not..... if you have a set of tubes that will not bias most likely you have a set that bias way hot. Many modern production tubes bias excessively hot.  

 

 When you adjust the bias you are actually adjusting a negative voltage that enters the tube via the control grid which on your amp is pin 5. The measurement you are taking on pin 8 is the current the tube is drawing through that 10 ohm resistor from ground. We use a 10 ohm resistor so the voltage translates into how much current it equals. .50 VDC = 50 mA and .75 VDC = 75 mA and so on... to make a set of tubes run cooler you have to be able to apply more negative voltage to pin 5...  

 

 

 Rather than modify the amp I'd return the tubes and request a cooler biasing set of tubes.  With some tubes it might be impossible to get a cool enough running set. 

Ok, thanks Craig.   Stupid me, I have been setting bias all this time off pin 8.   Thanks for clearing that up.     I will try the KT66's again using pin 5 for bias adjustment.    

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Craig, you saved the day. The KT66's bias, but i'm not able to bias higher than 44ma. Pots are all the way up.   Must be very cold tubes.    What would a good bias setting be  for these KT66 gold lion?     I read some where they take 200hrs to burn in, so i'm gettin started.     Thanks again

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4 hours ago, mopardave said:

Ok, thanks Craig.   Stupid me, I have been setting bias all this time off pin 8.   Thanks for clearing that up.     I will try the KT66's again using pin 5 for bias adjustment.    

 

Craig is telling you a negative voltage is being applied to pin5 ( the control grid of the tube). Reading the voltage drop across the 10 resistor going to pin 8 (cathode of the tube) is still what you should be measuring and is the equivalent to the current being drawn through the tube that he gave an example of the conversion.

 

What amp are you using these tubes in?

Do you have a schematic of the amp?

 

To actually know how much wattage you are asking the Tube to dissapate at idle you need to know the B+ voltage applied to the plate and multiply that times the current to see if you are in a safe range of that tube type/ brand.

 

mike tn

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6 minutes ago, mikebse2a3 said:

 

Craig is telling you a negative voltage is being applied to pin5 ( the control grid of the tube). Reading the voltage drop across the 10 resistor going to pin 8 (cathode of the tube) is still what you should be measuring and is the equivalent to the current being drawn through the tube that he gave an example of the conversion.

 

What amp are you using these tubes in?

Do you have a schematic of the amp?

 

To actually know how much wattage you are asking the Tube to dissapate at idle you need to know the B+ voltage applied to the plate and multiply that times the current to see if you are in a safe range of that tube type/ brand.

 

mike tn

Ok, i'm confused.   I thought he said to measure bias at pin 5?  You are saying that pin 8 with the 10 ohm resister is where i should measure from?    Plate voltage measured at pin 3 is 480v. 25w/ch amp.  How do i compute this?       

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3 minutes ago, mikebse2a3 said:

 

To be clear the bias current you will need to use with the tube will vary based on any particular amps Plate to Cathode voltage drop across the tube.  

 

miketn

My amp is a Audcom 900 chinese amp that Craig modded years ago with new caps and bias pots.

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If the bias measure is taken from pin 8 these kt66's will not bias.    They are at 80ma with pots turned all the way down.    Off pin 5 ma is set to 35.   Have them playing now and those plates sure are glowing.  Something i'm sure is not good.  This is an EL34 amp, sure it was never intended for KT66. Just thought i would try them.  Thanks

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1 hour ago, mopardave said:

My amp is a Audcom 900 chinese amp that Craig modded years ago with new caps and bias pots.

 

To minimize confusion and since Craig is familiar with your amp and the modifications he performed it would probably be best to give him time to respond to your situation.

 

But definitely don’t continue to operate the amp this way. 

 

miketn

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3 minutes ago, mikebse2a3 said:

 

To minimize confusion and since Craig is familiar with your amp and the modifications he performed it would probably be best to give him time to respond to your situation.

 

But definitely don’t continue to operate the amp this way. 

 

miketn

Absolutely.   Looks like the kt77 are going back in and ill keep these kt66 for a future amp.  Thanks

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15 hours ago, NOSValves said:

No absolutely not..... if you have a set of tubes that will not bias most likely you have a set that bias way hot. Many modern production tubes bias excessively hot.  

 

 When you adjust the bias you are actually adjusting a negative voltage that enters the tube via the control grid which on your amp is pin 5. The measurement you are taking on pin 8 is the current the tube is drawing through that 10 ohm resistor from ground. We use a 10 ohm resistor so the voltage translates into how much current it equals. .50 VDC = 50 mA and .75 VDC = 75 mA and so on... to make a set of tubes run cooler you have to be able to apply more negative voltage to pin 5...  

 

 

 Rather than modify the amp I'd return the tubes and request a cooler biasing set of tubes.  With some tubes it might be impossible to get a cool enough running set. 

Totally misunderstood you Craig.   Ok, i get it.  Set bias off pin 8.    This set of tubes was the coldest he had.   I'll just keep them for future use.     Thanks

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Well, removed the KT66 and reinstalled the KT77 set bias to .40ma using resistered pin 8.    Those KT66's were cooking at 100+ma.    Not good.    After all the reading how to select correct bias for tube using plate current (485),watts(19.2) and dissipation (86.4) i just gave up and set to .40ma.     listening now and sounds good to me.    Those 66's sounded good at mid volume but higher and they started to sound vailed, but they were cooking, so......

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Dave, 

   It's not totally unusual for KT66's to not bias in an amplifier designed for EL34's. EL34's bias with -34 to -45 volts applied to pin 5 control grid while KT66's usually take well over -50 volts. Oh and realize the more negative (higher number) you adjust the pin 5 control grid voltage the cooler the tube is biased (pin 8 will go down). 

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1 hour ago, NOSValves said:

Dave slow down and read more carefully. I never said measure at pin 5. I was explaining why changing then 10 ohm resistor would not work....I did tell you to return the tubes they bias to hot in your amp. 

Yes i do understand what you were saying now. I can not return them, that was part of the deal because the KT66 runs hotter anyway.  Just trying to figure the math and measuring to fully understand what bias is really needed.

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56 minutes ago, NOSValves said:

Dave, 

   It's not totally unusual for KT66's to not bias in an amplifier designed for EL34's. EL34's bias with -34 to -45 volts applied to pin 5 control grid while KT66's usually take well over -50 volts. Oh and realize the more negative (higher number) you adjust the pin 5 control grid voltage the cooler the tube is biased (pin 8 will go down). 

Yes i noticed that will measuring things. Im now looking at the Dared vp20 i think is the model.   Takes 6L6 and should be able to use the kt66 in them.

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