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Post subject: How do you like your tubes biased-Hot or Cold?
Posted: Sat Oct 19, 2013 6:25 pm
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Roadie
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Hello all-

I have a couple blackface and a few silverface amps. The silverface amps with the bias balance run around 34ma +/-. I had a 6L6 die in a 1964 Concert. When I replaced it, I experimented with bias settings. Plate voltage is 440. I found 35ma was the sweet spot. Just the right amout of clarity and creamy distortion. 33ma was too weak, 36ma to brittle. It's a matter of preference, I guess. Anyone care to share their preferences.


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sat Oct 19, 2013 7:01 pm
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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sat Oct 19, 2013 8:57 pm
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Since you didn't specify, I assume you are measuring cathode current. Plate voltage changes as the bias voltage current changes, i.e.; increase the bias voltage (make it less negative which increases the cathode current) and the plate voltage decreases, decrease the bias voltage (make it more negative which decreases the cathode current) and the plate voltage increases. So unless you measured the plate voltage at your measured cathode current of 35 mA, you have no idea what the actual plate dissipation really is. At 35 mA, if your plate voltage was actually 440 VDC, your plate dissipation would be 15 watts which is only 50% of the max plate dissipation of a 6L6GC (30 watts), and is very cold. I normally run my power tubes between 65% and 70% of max plate dissipation.

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 6:53 am
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There are many ways to measure plate voltage. Let's assume the voltage is 440. Bluesky was getting to my point-percentage dissipation. Some folks crank it up to 70% or a little higher, some like it around 50%. Biasing an old Fender amp is tricky, every MA-one way or another-affects the sound. It also depends on the tubes, but generally everyone has their sweet spot. :D What's yours?


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 7:00 am
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No, there is only one way to measure plate voltage. You still haven't addressed the question as to what your plate voltage is at your "sweet spot" of 35 mA cathode current or is that plate current? Again, you did not specify. Your post is only telling part of the story and without the other part, it is meaningless and misleading.

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 8:17 am
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I like my amps bias hot. In 6L6 amps, like my SR, and depending on what tubes I'm using, I run in the high 40s with the voltage around 440. In 6V6 amps, like my DR, I'm around 21-23 usually.


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 12:58 pm
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Figuring 35 mA at 440 VDC on the Plate, that's approx 15.4 watts, which would be just a bit low in my book. I would probably go in the range of 16.5 watts to 18 watts per tube for a 6L6. If you are measuring Cathode current (typical bias probe), the measured current will be slightly higher than the actual Plate current (3 mA to 5 mA higher), so you can adjust a little higher than your calculations indicate for idle wattage.

As pointed out by bluesky636, after adjusting the current, you have to remeasure Plate voltage and recalculate, because the Plate voltage will change.

Of course the change from 35 mA to 36 mA is nothing, and you might even have that much difference between output tubes in a well matched set. "35ma was the sweet spot...36ma [too] brittle..." Really?!?! :lol:

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 1:54 pm
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What can I say. I'm picky. :mrgreen: Going up or down 1 ma makes a difference, minimal I agree, but my point is where is the sweet spot. To many folks, it could be anything around 60% or 70%. The way an amp is biased defines the tone, all things considered. My tech, who has 40 years of experience with professional musicians, lets his customers play the amp before he signs off on it. You'd be surprised how picky they are about bias.


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 2:17 pm
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Roadie
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Guitarists are even more picky about their guitar setup-string height, pickup height,etc. Where 1/64" is a huge difference. I was courious to see how many people are that picky.


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 2:26 pm
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Krankster,

Remember, all tubes have a unique tone and bias --- including gain stage. I like to bias the output tubes to the conservative side, for push-pull Class AB1. Maybe, 65-73% of max.

However, make sure that the gain stage has healthy bypass caps on the cathode & good coupling between stages. You can't believe how much difference a proper setup, esp in input and phase inverter, makes in the amp's perfomance. Plus, don't forget a clean, potent power supply.


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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 2:30 pm
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Bias can easily drift a couple Ma. Kind of OCD to worry about 1Ma IMO. I seriously doubt anyone would be able to hear a difference. Like Bill said, without knowing the actual plate voltage, your numbers mean nothing. As Bill said, there is only one way to measure plate voltage. Sound like you should let a qualified amp tech work on your amp before you hurt yourself. :wink:

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 2:46 pm
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BMW2002Ti wrote:
I like to bias the output tubes to the conservative side, for push-pull Class AB1. Maybe, 65-73% of max.


+1

I find that 65% max is a healthy compromise between nice harmonic tones and tube longevity.

Arjay

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 3:23 pm
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This is how I bias my amps and the information that should be provided in this discussion:

Wrecking Ball (EL34):
CATHODE CURRENT 44 mA
PLATE CURRENT 42 mA
PLATE VOLTAGE 405 VDC
% MAX PLATE DISS 68%

Bluesman (6L6GC):
CATHODE CURRENT 50 mA
PLATE CURRENT 48 mA
PLATE VOLTAGE 429 VDC
% MAX PLATE DISS 68%

BDRI (6V6S):
CATHODE CURRENT 24 mA
PLATE CURRENT 23 mA
PLATE VOLTAGE 425 VDC
% MAX PLATE DISS 70%

Champion 600 Reissue (6V6S):
CATHODE CURRENT 42 mA
PLATE CURRENT 41 mA
CATHODE VOLTAGE 20.3 VDC
PLATE VOLTAGE 322 VDC
% MAX PLATE DISS 86%

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Last edited by bluesky636 on Sun Oct 20, 2013 7:21 pm, edited 1 time in total.

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 5:32 pm
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63supro wrote:
...let a qualified amp tech work on your amp...


IIUC, he did have an amp tech do the job, and he listened while the tech "adjusted" between 36 and 35. > "My tech, who has 40 years of experience with professional musicians, lets his customers play the amp before he signs off on it." If it was done by a tech, my guess, the tech did nothing at that point, saw the initial reading go from 36 to 35 (normal with new tubes), and asked; "How does it sound now?", and the OP thought he heard a difference. I have done this myself, set it where it should be set and if they think it isn't quite right, pretend to adjust and ask; "How about that?". It's not surprising how many people notice a difference when nothing at all was done. :wink:

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Post subject: Re: How do you like your tubes biased-Hot or Cold?
Posted: Sun Oct 20, 2013 7:23 pm
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Rock Star
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shimmilou wrote:
63supro wrote:
...let a qualified amp tech work on your amp...


IIUC, he did have an amp tech do the job, and he listened while the tech "adjusted" between 36 and 35. > "My tech, who has 40 years of experience with professional musicians, lets his customers play the amp before he signs off on it." If it was done by a tech, my guess, the tech did nothing at that point, saw the initial reading go from 36 to 35 (normal with new tubes), and asked; "How does it sound now?", and the OP thought he heard a difference. I have done this myself, set it where it should be set and if they think it isn't quite right, pretend to adjust and ask; "How about that?". It's not surprising how many people notice a difference when nothing at all was done. :wink:


I can tell the difference between 50% and 70% of maximum plate dissipation. I doubt I can tell the difference between 68% and 70%. :lol:

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