Bias and max current for my 7591 AB amp... - Page 2 - diyAudio
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Old 5th January 2006, 10:05 PM   #11
cerrem is offline cerrem  United States
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A couple of things to keep in mind....
Whether you are dealing with Class AB1 or Class A1, this
has nothing to do with having a 70% rule that someone made up.....You can operate the plate dissipation werever you want up to 100%....you can even go beyond with certain tubes because the data sheets were overly conservative...this is NOT generaly the rule, but the exception.....
Life expectancy is not all about where you set the DC bias point...
The plate load is crucial as well, since that sets the AC power swing...In some amps, the plate load is way too small and the load-line will spend most of it's time swinging past the MAX plate dissipation, while the DC idle current can be well below the MAX plate dissipation...this amp will have the plates glowing slightly red when it is operating at full steam...but when no signal is present, the plates cool down to the idle dissiaption....
WHen it comes to biasing for AB1 amps....it's boils down to your descresion...Technically Class AB1 has a very large biasing range..
Most Class AB1 amps will not be able to be biased into Class A range...the plate dissipation at that point would cause a melt down...
The "standard" biasing method for AB1 that our Hi-Fi forefathers used back in the Golden-era, was for both efficiency and distortion in mind... You basically start cold and work your way hot till you notice the cross-over notch on the scope just fade..
You will NEVER get the cross-over notch 100% gone in AB1 amp...
SInce you are doing this by EYE on a scope, you will have a subjective issue since 5 techs will not all agree at the EXACT point where the notch is "mostly" gone.... So this is not an exact method...but still a good method since you want the point where "most" of the cross-over is gone. ANy further adjustment is just burining up more plate dissipation and is diminishing returns and not efficient...But wait, if you don't care about efficiency and how often you replace tubes and looking for the cleanest sound, then by all means bias it hotter ... just don't get too crazy about it..know your limitations and what to expect for life expectency without melting the tubes down
Some folks prefer to use thier ears and thier eyes...meaning using the scope and then using the ears for final tweak....
especially in guitar amps, where linearity is not always the goal, but adjusting the bias for the right harmonic tone...
If you want to be extrememly anal and scientific about it, you could use a spectrum analyzer or distortion meter to adjust the bias for specific distortion limit...if you can't dial it in then you toss the tubes for better ones ....

Chris
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Old 5th January 2006, 11:29 PM   #12
Giaime is offline Giaime  Italy
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Dear Chris,

biasing by seeing the crossover distortion is a very bad thing. Aiken explained this very well, go read his article
http://www.aikenamps.com/CrossoverNotchBiasing.html
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Old 6th January 2006, 12:40 AM   #13
cerrem is offline cerrem  United States
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All due respect to Aiken... but I have my own thoughts on this...
I did look at his article and his first point of positive grid drive does not apply, since I refered to AB1 .....

His second point is valid, but is common sense, you must check your plate disipation at all times even when using a scope..

The 3rd point about plate voltage and cross-over ...depends on number of things and not soo clean and cut...

The 4th and 5th issue about biasing a Push-Pull Class A amp or Single Ended Class-A amp by way of looking at cross-over is rediculously absurd... instead with Class A, you look for symmetric clipping since there is no cross-over...it does not apply since i was refering to AB1...

If your amp can't bias up properly without over running the plate dissipation then there are some fundamental problems in the output stage design...

Chris
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Old 6th January 2006, 03:11 AM   #14
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Quote:
Originally posted by mr mojo
Jojo,


If I bias them for 40ma or even a bit more my plate voltage should actually decrease a bit-right?


The output transformers are rated for 70watts and with AC and DC balance I should in theory be able to bias the 7591s hotter without running the risk of unbalancing the output transformer-is that right?

Or, is this whole situation more complicated than I think it is?

Thanks a lot for the help!

Best,
mr mojo

Yes, your plate voltage will decrease a bit when you increase your plate current.

Yes, you can run them 7591s hotter with destroying your opt tranny.

It is not that complicated, I can relate to your situation because this is exactly what I have but a different tube. Instead of 7591 I used EL34, and I play around it a lot especially on the bias where I learn a lot. I even accidentally manage to run the EL34 with 340V plate and 100mA each when I first turned on my amp.

But we all know that tubes are more forgiving of our mistakes.
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Old 6th January 2006, 03:50 AM   #15
AJT is offline AJT  Philippines
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Quote:
Originally posted by Giaime
And for a complete treating of the argument, see here:

http://www.aikenamps.com/Why70percent.html
http://www.aikenamps.com/Biasing.html

hi giaime,

nice articles for a noob like me! i guess the whole point of the articles is that it is possible plate dissipations ratings can be exceeded that puts the output tubes at risk and that biasing can be selected such that this does not happen.

i guess biasing outputs is a balancing act and depending on our priorities one can chose between a safe amp and a good sounding one.

perhaps there is a happy compromise to be found somewhere in between.
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