My clone works, well I might add. Thanks to all, especially Dana for the boards. I am currently running some JJ KT77's with excellent results. They compare favorably with my Mullard (used) EL34's. My question concerns the change in tone with a change in bias. With the idle current at 35ma (391 vdc B+) the it's good, but somewhat crunchy and rough, but with it running hot (45ma), I get better compression at cleaner settings and smoother overdrive tones. The volume difference between clean and overdriven is smaller with the higher bias current. Is this the way it's supposed to be, 'cause I like it! I was hoping others would share their experiences.
I am extremely pleased with the KT77's, btw. I also like the JJ 6V6's and the ECC803s.
D
How does bias affect tone?
Moderators: pompeiisneaks, Colossal
Re: How does bias affect tone?
Assuming KT77s have 25W max dissipation like El34s, 391V at 35ma is a very cold bias. Where'd you get that number?
I prefer to shoot for 70% of max dissipation and do the math to get me in that range.
25W/391V = .06393A
or 63.93mA
70% of that is .04475 or 45mA
In other words, your 45mA is dead nuts perfect.
You can work the math backwards, too:
.045 A * 391 V = 17.595 W
17.595 / 25 = .7038 or 70%
So, your 35mA was:
.035 * 391 = 13.685W
13.685/25 = .5474
So you weren't even at 55% with 35mA. Very cold.
(Note -- folks who can do math better than me, please check my number so I'm not talking out my ass, here)
For tone -- that sounds about like my experiences with bias. Too hot can lead to problems, but running 'em cold can make a harsher breakup. On amps with poweramp breakup, I tend to prefer a slightly hotter bias and I almost never go much below 70% at idle, though I prefer not to get much higher if I can avoid it.
I've read a good deal about crossover distortion and reasons for biasing this way or that, but it all comes down to using your ears to fine tune it in a range you determine to be safe.
I prefer to shoot for 70% of max dissipation and do the math to get me in that range.
25W/391V = .06393A
or 63.93mA
70% of that is .04475 or 45mA
In other words, your 45mA is dead nuts perfect.
You can work the math backwards, too:
.045 A * 391 V = 17.595 W
17.595 / 25 = .7038 or 70%
So, your 35mA was:
.035 * 391 = 13.685W
13.685/25 = .5474
So you weren't even at 55% with 35mA. Very cold.
(Note -- folks who can do math better than me, please check my number so I'm not talking out my ass, here)
For tone -- that sounds about like my experiences with bias. Too hot can lead to problems, but running 'em cold can make a harsher breakup. On amps with poweramp breakup, I tend to prefer a slightly hotter bias and I almost never go much below 70% at idle, though I prefer not to get much higher if I can avoid it.
I've read a good deal about crossover distortion and reasons for biasing this way or that, but it all comes down to using your ears to fine tune it in a range you determine to be safe.
Re: How does bias affect tone?
Thanks, Moose. I really would like to hear others' opinions about optimum bias settings with regards to tone. I was being very conservative with my new KT77's, but I agree with you-HEAT 'EM UP!!!. Also, when using the "transformer shunt" method, I got much lower readings (10% lower) than using the 1 ohm cathode resistor method. I forgot about the screens. 
I have a much better grip on "correct" now and have put away my fear of burning up my new tubes. Now if I can get my family out of the house long enough to record some clips.......
Thanks again, Moose. Anybody else have experience to share?
D
I have a much better grip on "correct" now and have put away my fear of burning up my new tubes. Now if I can get my family out of the house long enough to record some clips.......Thanks again, Moose. Anybody else have experience to share?
D