i just rebiased my two rock onyx :)

[mentoneman]

i finally found my pair of sylvania 6L6 GCs, and rft preamp tube, so i wanted to retube the onyx pronto.
went to radio shack to buy a new digital multimeter,
followed the instructions found on two rock website for the type 3 amps (chad said these would work--thank you chad!!)


and went for it--first time i ever rebiased my own amp after 20 years of tube amp playage!

and it almost went badly----stupid meter was faulty out of the box!

i set it for millivolts, DC, neg test probe to open speaker jack for ground, pos probe to test point provided on chassis, and had screwdriver ready to tweak bias adjustment screw

started ok but then i got some strange readings and the meter then kept increasing and then decreasing without any adjusting of the screw, which i kept trying to compensate for.

shut off the amp.

checked online and found out the meter was notorious for such nonsense.

users suggested replacing the meter's internal fuse with the provided spare.

after doing so the meter began working normally but the bias was way off--was supposed to be set at 64mv and it was in the 100s!!!

tweaked it back asap into the proper range and crossed my fingers i didn't do any damage to the tubes.

perfection!

thank goodness for robust NOS USA tubes.

now i'm running:

tonegrinder GX100A solid state 12ax7 in V1 (clean channel)
NOS mullard ecc83 in V2 (lead channel)
phillips 12at7 in V3 (reverb driver-this tube was found in V4/wrong spot by prior owner so i swapped it into proper place)
GT in V4 (reverb/effects)
RTF in V5 (phase inverter)
pair of sylvania 6L6 GCs in power section

i am really impressed by that tone grinder "tube" and the sylvanias are so smooth and sweet!

[ked]

curious, what is a Tonegrinder tube?

Ken

[mentoneman]

http://tonegrinder.com/

i discovered them at NAMM this year and gave one a spin after hearing the demo. Pretty darn good so far. I had my mullard in V1 and actually prefer the sound of the tonegrinder in that spot.

[mentoneman]

the two rock website suggested that sturdier tubes can run higher bias settings

are there any biasing tricks i should know about these particular tubes--sylvania 6L6GC??

[Structo]

The bias current is just an arbitrary number unless you know what the plate voltage is.
But for the inexperienced, measuring plate voltage is just too dangerous.
We're talking 400-500 DC volts here.

But what Two Rock and other amp sellers do is give you a number in their user manual that will most likely work OK and not be too hot which will shorten power tube life.

You see, power tube has a specification called maximum dissipation.

It is the max wattage the tube can dissipate in heat.

The 6L6GC is a 30 watt tube.
It is the generally accepted rule on fixed bias amps not to exceed 70% of maximum dissipation.

The bias formula is:

(P * .7) / Vp = Ip

P = Tube Max Diss in watts
.7 = 70%
Vp = Plate voltage
Ip = Plate current

So lets assume you have 450v on the plates of the 6L6GC tubes.
(30 * .7) / 450 = .046 or 46 milliamps or 46ma

The max you should set each 6L6GC is therefore 46ma at that plate voltage.
But you can see how important knowing the plate voltlage is due to the formula.

Now this does not take into consideration what the screen current is.
This will usually be a small number 3-5ma which can serve as a safety buffer when setting the bias to keep you from setting it too hot.

Now Two Rock probably knows where the approximate plate voltage will be within a certain range due to different wall voltages and different tubes.

So they give you a number to go by.

Dumble amps tend to like a bias that is on the cooler side, like 55-60% but you can set it just about anywhere below that 70% number and be OK, although I probably wouldn't go below 50% to keep the tubes happy.

[mentoneman]

thanks for that info!

yeah i'm not sure how they arrive at the value they gave on the manual but it said 60mv, and chad gave me a value of 64mv in an email specific to the onyx, but the manual also says some sturdier tubes can handle up to 80 mv readings...does the higher bias make the tone break up sooner, make it more touch sensitive...etc?

my friend once mentioned he likes setting his bias cold on his fender to break up later which makes me confused....:D