hyper Posted July 19, 2013 Posted July 19, 2013 Hi guys Got a question I've got a DYI kt120 amp running triode/UL 9/18w the output transformer is 3.5k and choke is 200ma, plate voltage at 400v, now my current bias is at 70mv. My question is isn't this running it abit cool? Shouldn't it really be running at 90-100mv to get a decent amount of class A? Also if I wanted to go above 90mv then I would need to also upgrade choke as well?
Sierra Posted July 19, 2013 Posted July 19, 2013 The optimal biasing current would be determined by the design of the amplifier. Can you supply a bit more detail about the amp, circuit diagram etc so that others more technical can advise
hyper Posted July 20, 2013 Author Posted July 20, 2013 The optimal biasing current would be determined by the design of the amplifier. Can you supply a bit more detail about the amp, circuit diagram etc so that others more technical can advise I suppose my question is providing all parts are capable of a higher bias, is it generally speaking better to be running the kt120 at higher bias setting. I suppose there is no absoulute but asking as a general guide. I know it is fine to run at 70mv and I may reduce heat and increase tube life but is 70mv abit on the cool side of range?
VanArn Posted July 20, 2013 Posted July 20, 2013 I believe you mean that the quiescent cathode current is biased at 70mA which would indicate a dissipation(anode + screen) of about 28 W for each o/p valve.You can safely run the valves a little harder,but expect more heat and a lower life span.The overall performance of a valve amplifier running in class AB mode is dependant on the quality of the o/p transformer and is tied up in how well their design reduces leakage between the primary winding halves and where the H.F resonances occur.
lovetube Posted July 20, 2013 Posted July 20, 2013 Hi guys Got a question I've got a DYI kt120 amp running triode/UL 9/18w the output transformer is 3.5k and choke is 200ma, plate voltage at 400v, now my current bias is at 70mv. My question is isn't this running it abit cool? Shouldn't it really be running at 90-100mv to get a decent amount of class A? Also if I wanted to go above 90mv then I would need to also upgrade choke as well? with the KT120 you can run them much higher than 100ma and in your case you can run your bais to 100ma but you won't get class A operation . reason is when you increase your current draw you decrease the B+ voltages therefore it little help in term of overall performance of the amp , unless you can keep the same B+ at 400v and have the tube bias at 100ma or more then you will be more in class A bias than AB . i currently run my mono block in similar condition and they sound superb . cheers Lovetube 1
hyper Posted July 20, 2013 Author Posted July 20, 2013 Hi guys Got a question I've got a DYI kt120 amp running triode/UL 9/18w the output transformer is 3.5k and choke is 200ma, plate voltage at 400v, now my current bias is at 70mv. My question is isn't this running it abit cool? Shouldn't it really be running at 90-100mv to get a decent amount of class A? Also if I wanted to go above 90mv then I would need to also upgrade choke as well? with the KT120 you can run them much higher than 100ma and in your case you can run your bais to 100ma but you won't get class A operation . reason is when you increase your current draw you decrease the B+ voltages therefore it little help in term of overall performance of the amp , unless you can keep the same B+ at 400v and have the tube bias at 100ma or more then you will be more in class A bias than AB . i currently run my mono block in similar condition and they sound superb . cheers Lovetube Thanks for your feedback tube love your setup!!! Are they hashimoto irons you got there??? Yeh thats more or less what I was looking for more class A operation. I've changed over to a 320ma choke this arvo everything seems nice at 100ma haven't done a full test comparo yet though... Out of interest what are you running bias are you running yours at? Also what's te watt output of your setup looks nice!
lovetube Posted July 20, 2013 Posted July 20, 2013 Thanks for your feedback tube love your setup!!! Are they hashimoto irons you got there??? Yeh thats more or less what I was looking for more class A operation. I've changed over to a 320ma choke this arvo everything seems nice at 100ma haven't done a full test comparo yet though... Out of interest what are you running bias are you running yours at? Also what's te watt output of your setup looks nice! good day. the transformers used in these amp are new old stock Tamura top of the line . chokes are Lundahl . i running these in UL but zero Global feedback . in memory i run these at 100ma at 400v B+ or so with fix bias . similar to your amp . lot of friend reckon this amp sound more like SE than PP . Cheers Lovetube
hyper Posted July 20, 2013 Author Posted July 20, 2013 Nice mate great setup looks good to! Mines switchable between SE and UL
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