Bias, Cold vs Hot vs Correct. Can you hear the difference?

[Saul Goodman]

How do you know without a meter and some knowhow? How to tell if your amp(s) need biasing?

[Dave Scepter]

Key Signs Your Amp Needs Biasing:

New Power Tubes: Always re-bias when installing new output tubes, as they won't match the previous bias settings.

Red-Plating: If you see a bright red or purple glow inside the metal plate structure of the power tubes, turn the amp off immediately; this indicates severe over-biasing.

Poor Tone: Sound becomes cold, "choked," or overly clean.

Odd Noises: Excessive hissing, sputtering, or popping sounds.

Drifting Bias: If the amp sounds different than it used to, the bias point may have drifted over time. 

[velorush]

Not too difficult to check it. I bought one of these years ago making it super easy:

https://www.eurotubes.com/product/bias-probe-head-only/

$30 for an Octal unit. The EL84 is $57, but I've never had an EL84 amp that wasn't Cathode Biased.

 

[bubs_42]

A probe and a Meter. Don’t do it by ear, tubes are too expensive to guess. The probe and a meter will cost you less in the long run. 

Too hot and your tubes wont last. Too cold, it’s going to sound steril and way too low is going to starve components and cause failure. 

I bias my own amps, because I’m cheap. You really don’t need a probe but it’s safer. 

[Jimbilly]

I've never messed with bias to try to see if the tone changes, I remember a demo that Lynn Ellsworth did when he was trying to sell his Speedster amps with the adjustable bias knob on the front, I seem to recall that you could supposedly change the tone by messing with the bias, I think it was pretty subtle, and I also don't remember being super impressed with those amps at that time.  I think I also understood that messing with that might seriously shorten tube life, with my tube collection being pretty small at that time.

  I bought one of those Allessandro dual VU bias meters ages ago, it wasn't super cheap, but I've used it a quite a few times, and I've rarely regretted buying any good quality tool. Apparently using something like above that plugs into a multi meter works well too, I have one of those for el84s that my tech made me, I seem to remember him saying something about it just being a socket with a "1 ohm resistor" wired into one of the pins or somehting like that, really simple to build iirc. The Allessandro is plug and play', but only works with Octals like 6L6s, and doesn't work with 7027s (which, in my Ampeg VT40 can be swapped for 6L6s because there's no connector in the socket to the extra pins that the 7027s have).  

[Dutchman]

The DenHer Deluxe 6V6 is biased at 60%. I find that to be easy on tubes and still have great tone. Amps that are cathode biased run hot. I've had them come in biased @ 90+%. Then you see the lettering on the tubes burn. EL 84 amp are usually running on the hot side. 

I use this for all amps!

https://www.tedweber.com/bias-calc/

This is from the Master!

I have 4 of these for the 100 watt amps. I absolutely love them!

Edited January 6 by Dutchman

[hamerhead]

The SuperChamps came out biased cold and you could definitely hear/feel an improvement when set correctly, even without a tube swap. It's the only amp I've ever tried it on.

[Jimbilly]

Ideally bias will be set within spec, AND use a variac or 'brownbox' attenuator to get the voltage consistent. The Brownbox looks really neat, but I already have a variac, it's only a bit large... - and one more thing to carry

 I was fortunate to make friends with a local amp builder hobbyist a few years ago, and had a couple of custom hand-wired builds done really affordably, using donors from amps that weren't highly collectible. This Plush, half of which is a Dual Showman, and now has bias check probe ports built in, which (as I understand it) is like having that eurotobes octal tool (pictured above) built into the tube socket in the amp.  The Plush has a 'short' amp box without much room above the power tubes, only the short 6L6s with the thin base fit in there, but I don't use it often and those tubes should be available for some time if I ever wear these out. If necessary, I can always drill holes in the top of the cab to make more headroom for different tubes.  my photo isn't sideways in my folder, sorry

 

Edited January 6 by Jimbilly

[hamerhead]

[Jimbilly]

10 minutes ago, hamerhead said:

Sorcery! Witchcraft!!

[tbonesullivan]

I don't notice too much. People have been obsessed with cooking tubes since Eric Clapton beefed up the bias on his "bluesbreaker" Marshall JTM45 combo. Vox AC30 amps also cook the EL84s over spec to get part of that sound. In these days after the military production ended, more moderate bias levels should definitely be the norm, regardless of what the guy at Eurotubes says. The move away from power amp overdrive to preamp designs pretty much makes power tube cooking unneeded. So unless you are running something vintage, I would usually keep things to around 60%.

I know some people say "well the only way you can truly tell is with a scope to look for crossover distortion" but honestly that's above my pay grade. I go with what I think sounds good, and I have noticed far more from changing brands than I ever did from cooking my power tubes.

Definitely be wary of anyone who says "you need more current" as an absolute. While most amps these days seem to have the plates at around 450V, some go higher, and some go lower (like Rivera). You need to know the current and the voltage to calculate the idle dissipation. I've never tried the NGV method of biasing, and don't intend to start now.