Bias question :/

[GRK]

So Bias Rite came in and I think I did everything right but there's a huge difference now with the amp. It feels like there's significantly less gain and the volume doesn't kick in until VERY loud. I can't experiment right now since it's 2:12am but I thought I should ask before I fire it up again tomorrow.

What can that mean? Biased too cold? too hot?


Is Cornford MK50h Class A or AB btw? I'm such a noob on amps technically speaking

 

[Marshall Law]

what kinda amp, basically how many watts, and what kinda tubes, what was the plate voltage??? and what did you bias it at????

 

[GRK]

what kinda amp, basically how many watts, and what kinda tubes, what was the plate voltage??? and what did you bias it at????

Good questions

Cornford MK50h 50 watts
Winged "C" (SED) 6L6GC's
Plate voltage = 466
Biased at 45mA

 

[Marshall Law]

37ma should be right were it needs to be, that is 70% dissipation, you can go a little hotter or colder either way depending on your ears and still be safe,give that a try

 

[GRK]

37ma should be right were it needs to be, that is 70% dissipation, you can go a little hotter or colder either way depending on your ears and still be safe,give that a try

How did you figure the 37mA though? When I use the calculator on weber site it says 45mA

 

[stephen sawall]

It has been a bit.
Is that for a pair or single tube (45ma) ?
37 - I belive is one tube.
65~75 for a pair sounds right to me.
But take that with a grain of salt.

 

[Marshall Law]

37ma should be right were it needs to be, that is 70% dissipation, you can go a little hotter or colder either way depending on your ears and still be safe,give that a try

How did you figure the 37mA though? When I use the calculator on weber site it says 45mA

it's a long story . it's a calculation, 50 watts = 2 tubes =25 watts each , divided by plate voltage 466 = .05364806866 and many more numbers, but that's at 100% dissipation,so.0536480 really means, 53ma at 100% dissaption, thats way to hot, so, 53ma X .7 which means 70% dissipation, equals .371 or 37ma try it, it is safe just give it a try adjust a little hotter or colder

 

[Marshall Law]

It has been a bit.
Is that for a pair or single tube (45ma) ?
37 - I belive is one tube.
65~75 for a pair sounds right to me.
But take that with a grain of salt.

way to hot, you would go thru tubes pretty fast at that bias

 

[stephen sawall]

I bias my THD Flexi 50 - (25~35) each tube ?
I helped a friend do his Einstein (50 watt) with the Bias Rite to 65ma
As recommended.
If I remember right ?

Do not go that high just on my word.

 

[Marshall Law]

I bias my THD Flexi 50 - (25~35) each tube ?
I helped a friend do his Einstein (50 watt)with the Bias Rite to 65ma
As recommended.
If I remember right.

what was the plate voltage? remember, there is no magic number, your adjusting the tubes to run with the amp, just like over time your tubes start to wear, you can bias them up to bring them back in the ball park till they can't bias up no more.

 

[stephen sawall]

I can not remember off the top of my head?
The stuff is in the bed room closet and I am not going to wake her up to get it.
I also forgot how the plate voltage changes the number/ma range.
I am half awake.
Like I said do not do it on my word.

 

[duesentrieb]

Einstein 480V DC

tube watts * coeefizient / plate voltage = idle current in mA

example:

25W (EL34) * 600 / 480V = 31.25 mA for ONE tube, cathode measuring - a bit less when you read the current off the OT

for your amp (Cornford)

30W * 600 / 466V = 38 mA

600 = 60% max. plate dissipation

if you want it "hotter":

30W * 700 / 466V = 45mA

so your range is 38-45mA per tube, cathode reading (thats what your bias rite does)

or follow this chart:

blue line: "cold" bias for 30Watt/6L6GCs
red line: "hot" bias for " " "

HTH

 

[stephen sawall]

Thanx

 

[tweed]

Great info. You know what I think would be cool? Make a DIY section here and one of the post(s) be a tutorial on biasing, that shows how to do it with a DMM(for ppl without a BR) and one with a bias rite tool. I read about this once a while ago but never actually go to doing it.

Would be cool thing to learn to do with just a DMM as I have 4 or so amps now, with different tube configs, and i'm sure they could use some serious TLC.

 

[Duolos]

I get nervous when I hear people attempting this and they have no idea what they are doing. Touch the wrong thing and you will get zapped and it could kill you. If you are unaware of how to properly bias an amp, take it to somebody who knows.

 

[duesentrieb]

The bias rite is ok, Terry - we're even advertising (suggesting would be the better word) it on the official site for "noobs"

 

[GRK]

I get nervous when I hear people attempting this and they have no idea what they are doing. Touch the wrong thing and you will get zapped and it could kill you. If you are unaware of how to properly bias an amp, take it to somebody who knows.

Well I'm not paying 50-75 bucks for a bias job. It's robbery. And why would I touch anything besides the bias pot thingy? I think it's gonna be ok. It's my first time gimme a break

 

[GRK]

Thanks a lot for the help everyone!

I went back to 42mA from 45mA and it sounds better than ever!

 

[stephen sawall]

If you play at low volumes you may want to try it a little lower.
Just to see if you like it better that way.

I also think tweed may have a good idea.....

If the proper warning is put in place.

I rather see someone try it with the right directions. Than none at all.