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Biasing KT66 power tubes in Hotrod DeVille

Messages
2
Hey people question about biasing KT66 tubes in my HRD 4x10.
I got a new set of Genelax Gold Lion Kt66 tubes to replace the 6L6s(was advised this would be okay by the place I bought them) the problem is with my bias adjustment pot all the way down, I'm getting a reading of 104mv at my bias test point. with a plate voltage of 470v-480v on the hotrod and the tube rating at 25w, I'm positive that these are running too hot and i cannot adjust it any lower. BUT I played through it regardless for about 30 seconds and I loved the tone of the tubes. Is there a way to Mod the bias circuit in order to accommodate the new tubes, or maybe I'm missing something. Thanks all!
 

cap47

Member
Messages
2,272
KT66 bloom at about 40-44 ma each depending on PV. My VK112 was about 460 PV at 44 ma and sound nice!
 

fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
11,003
Hey people question about biasing KT66 tubes in my HRD 4x10.
I got a new set of Genelax Gold Lion Kt66 tubes to replace the 6L6s(was advised this would be okay by the place I bought them) the problem is with my bias adjustment pot all the way down, I'm getting a reading of 104mv at my bias test point. with a plate voltage of 470v-480v on the hotrod and the tube rating at 25w, I'm positive that these are running too hot and i cannot adjust it any lower. BUT I played through it regardless for about 30 seconds and I loved the tone of the tubes. Is there a way to Mod the bias circuit in order to accommodate the new tubes, or maybe I'm missing something. Thanks all!

You need to change the bias feed resistor. A KT66 is still a 25 watt tube, you want to keep it at 70% max dissipation. So 25 * .7 = 17.5. Then, 17.5 / 480V = 36.4 mA. the test point sums both tubes, so that adds up to 73mV at the bias test point. You have to take the board out to do this properly. I recommend you take it to a tech.
 

Ben S.

Member
Messages
2,181
KT66 bloom at about 40-44 ma each depending on PV. My VK112 was about 460 PV at 44 ma and sound nice!
So true in my experience. 42 is usually my minimum. Too bad the Pv is that high compared to the Deluxe.
 

fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
11,003
So true in my experience. 42 is usually my minimum. Too bad the Pv is that high compared to the Deluxe.
IMO, its not the mA setting that is important, its the dissipation....
 
Messages
2
So true in my experience. 42 is usually my minimum. Too bad the Pv is that high compared to the Deluxe.
So getting my tubes to say 40ea..puts me at 76%. How much shorter of a life will my tubes have? And do you think it would be worth it with the tone bettering?
 

fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
11,003
Once again, bias myths abound. The actual dissipation is what you should be aiming for, not the mA. It will sound like mush going that high imo...
 

Ben S.

Member
Messages
2,181
Of course it is dissipation, what myths exactly? And with a lower plate voltage you can run the bias at a higher quiescent current. With that said, for some reason ( due to all kind of different scenarios for tubes being pushed to cutoff) KT66's have always sounded better to me biased in this range. In lots of different amps also.

It could be total phooey, probably not, but it has been my experience. I could say it is due to a preferred range of plate resistance, available voltage swing at the grids, or how it affects the opposit waveforms, but I'm sure you can find a more profound answer reading Randall Aiken's site or having someone else here with more experience offer their opinion (oh John Phillips, where art thou :cry: )

But with all my myths abounding, I'm sure you can find a way to discount my personal opinion about my own personal experience.
 

fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
11,003
Of course it is dissipation, what myths exactly? And with a lower plate voltage you can run the bias at a higher quiescent current. With that said, for some reason ( due to all kind of different scenarios for tubes being pushed to cutoff) KT66's have always sounded better to me biased in this range. In lots of different amps also.

It could be total phooey, probably not, but it has been my experience. I could say it is due to a preferred range of plate resistance, available voltage swing at the grids, or how it affects the opposite waveforms, but I'm sure you can find a more profound answer reading Randall Aiken's site or having someone else here with more experience offer their opinion (oh John Phillips, where art thou :cry: )

But with all my myths abounding, I'm sure you can find a way to discount my personal opinion about my own personal experience.

You misunderstood me. Its not what you posted that is a myth, its the idea that a certain "magic" value of current regardless of plate voltage, well, is "magic".

What you are saying is correct. If your plate voltage is lower, of course your mA reading will need to be higher to get the tube between 65-70% dissipation. I had no intention of directing my comment to you.... Please forgive me if that is how you took it... I will endeavor to be more clear in the future...
 

cap47

Member
Messages
2,272
I have used KT66 and as with any tube you have to find what suits your ears and still not burn anything up. I took information from thetubestore reviews and suggestions. http://www.thetubestore.com/Resources/Product-Reviews/KT66-Tube-Review
The larger bottles also dissipate heat at a faster rate. Shugang was recommended at 75% dissipation for one and I tried it with the Tung-Sol and liked it at about 40 to 44 ma.. That is my memory since I gave the amp to my grandson and put regular 6L6GC back in it. I kept the KT66 for future use. Fusionbear knows a lot of what he is stating, you can see and hear some of his Videos about some of his builds so don't discount his advice! I have the building bug too! Presently doing a 2nd 5E3 and already planning another yet undecided build.
 




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