Question re "floating" bias ...

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hunter

hunter

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I always bias my amps myself, using a Bias Rite. Obviously I don't know all the different methods to bias an amp, so this is the safe one for me.

Now on some amps I set the bias with master on "0" and the reading doesn't change if I turn the amp up with the bias rite still connected. However, on the Bogner Shiva, and as well now on my Cornford Mk50H, the reading changed, went up quite a bit, when I turned the amp up. So this could be like 35mA on master at 0 and go to like 55mA with the amp turned up.

Why does the bias float on some amps with volume, on others not?

How to adjust the bias, at master="0" or at playing volume?

Then on the Cornford forum they say to bias at 33 to 39 mA, but I measure 465V plate voltage and with my 6L6GCs it should be 44mA?

Thanks for any help.
 
The current always goes up when you apply signal, thats why it is called "idle" current - no signal into the amp = idle current.

An El34 goes up under load to 100 and more mAs.

Idle current for 6L6GC: 30 Watt * 600/ 465V = 38,7mA for your amp. Thats 60%. I always use values between 55 and 60%.

HTH
 
duesentrieb":1mjoc4y9 said:
The current always goes up when you apply signal, thats why it is called "idle" current - no signal into the amp = idle current.

An El34 goes up under load to 100 and more mAs.

Idle current for 6L6GC: 30 Watt * 600/ 465V = 38,7mA for your amp. Thats 60%. I always use values between 55 and 60%.

HTH
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Ah IC, thanks a lot Olaf, that helps! Means I could safely go to 44 but the Cornford guys like it a bit colder like you. Makes sense then. :rock:
 
465V on the plates is for many (not all) 6L6/5881-types ok, esp. with GCs you will be fine. And I wouldn't trust values, but your ears - and start with 55%, then go up. If the difference say between 60% and 65% is small, stay at 60%. Your portemonnaie will thank you :D
 
Olaf is bang on ! Anything more than 60% (in my opinion) does NOT achieve any better tone (in most amps) and just burns your tube life to crap!

Too many guys out there think that you should run these things HOTTER than hell - the tone is choked- bottom end loose and dirty - amp sounds fizzy... no clarity - no BALLS...

Olaf is the man!! :thumbsup:
 
Okido I went by ear again, now I have to say I am not playing in any band right now, so I can't really crank my amps much these days, but at home I liked it at 41,5mA, which is close to 65%. There was quite a difference between 35mA (which sounded a bit fizzy and thin), 39mA (which was better, nice and tight on lower chords, but still not the bloom I like on leads) and the 41,5, where more lower end body came in but it retained the clarity for chords.

I also went up to beyond 45mA to check out and it gave a great brown sound and notes were really super lively, but as I am far from being able to mimic a respectful EVH (and I don't have his stash of tubes) I went back to 41,5.

Thanks again for your help guys, highly appreciated!
 
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