NOSValves Posted March 2, 2018 Share Posted March 2, 2018 On 2/28/2018 at 12:36 PM, mopardave said: 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. That not a sure thing 6L6 require less negative bias voltage than 66 too Quote Link to comment Share on other sites More sharing options...
mopardave Posted March 2, 2018 Author Share Posted March 2, 2018 12 minutes ago, NOSValves said: That not a sure thing 6L6 require less negative bias voltage than 66 too That sucks. Thought kt66 was a direct replacement for 6L6? Ok Quote Link to comment Share on other sites More sharing options...
NOSValves Posted March 2, 2018 Share Posted March 2, 2018 In many ways it is....they may work okay in that amp or they may not. In the end my advice is to investigate never assume. It all depends on how the amp is biased and how much adjustment is available. Quote Link to comment Share on other sites More sharing options...
mopardave Posted March 3, 2018 Author Share Posted March 3, 2018 I really like the reviews i'v read on the Dared VP-20's. They look to be well built for the price as well. Quote Link to comment Share on other sites More sharing options...
mopardave Posted March 3, 2018 Author Share Posted March 3, 2018 On 2/28/2018 at 11:34 AM, 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). Ok, amp is on getting warmed up and bottom cover off to get some measures. Now can anyone tell me where to measure to get the current flow measure to calculate with the 10 ohm resister which is on pin 8? The .50VDC=50ma calc. I do know with adjusting pots turned all the way down(clockwise) i get -47v on pin 5 and 500v on pin 3 . That gives 15v at the 8 pin with resister. 8 pin is where i set bias adjustment, so should i see a minus DC at 8 while setting bias? Just want to do the math to get the corrent bias settings. Thanks Quote Link to comment Share on other sites More sharing options...
NOSValves Posted March 3, 2018 Share Posted March 3, 2018 You don't need to do all that math.... I'm sure the amp is designed to be bias between .40 to .60 at the 10 ohm resistor on pin 8. THAT PIN 8 VOLTAGE IS THE BIAS CURRENT MEASUREMENT. When you adjust the bias control you are actually adjusting the negative voltage on pin 5 which causes the current to change as you measure it at the pin 8 10 ohm resistor. If you are truly getting a measurement of 15 volts on pin 8 you mostly destroying your output tube/tubes you should not be recklessly adjusting those controls up and down. Adjust them to keep the measurement on PIN 8 in a safe range (between .40 and .60) The only other thing you should watch for is red plating on the main inner structure of the output tubes. If you see any lower the bias measurement at PIN 8. Quote Link to comment Share on other sites More sharing options...
mopardave Posted March 3, 2018 Author Share Posted March 3, 2018 Yeah, i give up on it for now. The guy that sold me the tubes told me that just measuring directly without know what was needed is not the way, but could only give examples and not tell me directly how to do the math. Just wanted to know the math and how to get the numbers. I have been setting bias measuring directly with the multimeter the way you showed me some 17yrs ago. I believe that was .15 not 15v pot turned fully clockwise. I just put amp back together listening again. Biased the JJ KT77 to .38 on my meter. Thanks Craig Quote Link to comment Share on other sites More sharing options...
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