Noob manual on Biasing

Steven_nl
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Noob manual on Biasing

« on: December 31, 2008, 10:41:07 AM »

Hi guys
can you plzz explain how to bias the power amp tubes on the OTS. I juist got a set of Sovtek 5881's. On the box it says 38mA
I do have a multimeter
http://www.soundex.nl/productenCategorieSub.asp?prodCategorieSubID=-430666581

but I think it only does Volts and Ohm. plus the pins are very thin. I tried sticking them in the the 3 points next to and in the middle of the power tubes but there was no consistent reading. maybe I need to get another model? Or stick 'm elsewhere ;-)

thanks!!


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hywelg
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Re: Noob manual on Biasing

« Reply #1 on: December 31, 2008, 11:53:38 AM »

Turn the power on, take it off standby and let it warm up for 5 mins or so. (however if this is the first time with a different set of tubes ie a change from 6L6 to El34, don't wait, get the bias into the right ball park staight away)

Turn master and volume down to 0.

Turn on your multimeter and select volts (DC if its not autodetect) (mV across the 1ohm resistor = mA)

Plug black probe into black socket and red into red. You might have to hold them to make contact if they don't fit well. Take the reading. Then move the red to the other red socket. If the valves are matched the readings should be similar.

Use the pot to get where you need to be, keep checking both sides. Play it. Try it hotter or colder within reason, find you're own preferred bias.


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Steven_nl
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Re: Noob manual on Biasing

« Reply #2 on: December 31, 2008, 12:49:14 PM »

GREAT!
I did what you said
one tube reads 34mV while the other reads 36mV. (I did order a matched pair).
I must say my first impression (after 1 minute) is that they sound A LOT nicer then the Winged C's.

Thanks!!


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hywelg
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Re: Noob manual on Biasing

« Reply #3 on: December 31, 2008, 01:02:34 PM »

34mV vs 36mV is fine I think. I also think most manufacturers quote a high bias value, I think they want you to wear them out more quickly, but I have it on good authority (my amp tech) that biasing colder will produce a better clean.


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Steven_nl
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Re: Noob manual on Biasing

« Reply #4 on: December 31, 2008, 01:09:08 PM »

Thanks a lot Hywelg. and Dannygrandpa for pointing me towards 5881's . astonishing what a difference these make


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hywelg
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Re: Noob manual on Biasing

« Reply #5 on: December 31, 2008, 07:28:25 PM »

What do you like about them in particular?


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Steven_nl
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Re: Noob manual on Biasing

« Reply #6 on: January 01, 2009, 01:26:04 PM »

Hi Bcmatt. Thanks for that. It's much to complicated for me though. The bias setting seems to be fine. The sound is good and the tubes don't run to hot I think. I didn't have to change the bias setting that much compared to the Winged C's

@ Hywelg
I have tried 3 kinds of power tubes: the JJ's, the Winged C's and the 5881's

I'm not that keen on a very direct clean sound. Some poeple seem to lke the " in your face" direct tone of a Twin reverb. It's a bit to much for me. The Winged C's (I tried them because Scott Lerner recommended them) were even more " direct" then the JJ's. Plus the OD seemed to be harsher, just not as nice, even compared to the JJ's. With nasty edge when using a bit to much gain. Plus they didn't seem to have a nice feedback. It was hard for me to dial in a good sound. Especially using 3 types of guitars (Tele, Strat and Hamer with SD '59's).
At first I thought it had something to do with the OD Trimmer. I had one installed on the back panel a the same time the Winged C's were put in.

Now I put in the 5881's and it's great. They sing more, the OD is sweeter without loosing agression. but there's no nasty edge on the sound. Even with a bit more gain they sound nice (now i can use the PAB). The sound is warm without loosing sparkle!
Well worth trying them out IMHO.

I feel these tubes go well with this perticular amp. (JJ's in V1 V2 and PI)

Funny thing is I never thought myself as being an expert. I'm no great player at all. I always ask other peoples opinion and rely on that. Turns out I do know what I like and what I don't like. Maybe I don't have Dogears (Joke) but I should trust mu own ears more.

Mayor lesson: Power Tubes are VERY important to the sound!!!


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hywelg
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Re: Noob manual on Biasing

« Reply #7 on: January 01, 2009, 01:38:16 PM »

Quote from: Steven_nl on January 01, 2009, 01:26:04 PM

Plus the OD seemed to be harsher, just not as nice, even compared to the JJ's. With nasty edge when using a bit to much gain. Plus they didn't seem to have a nice feedback. It was hard for me to dial in a good sound. Especially using 3 types of guitars (Tele, Strat and Hamer with SD '59's).
At first I thought it had something to do with the OD Trimmer. I had one installed on the back panel a the same time the Winged C's were put in.

Now I put in the 5881's and it's great. They sing more, the OD is sweeter without loosing agression. but there's no nasty edge on the sound. Even with a bit more gain they sound nice (now i can use the PAB). The sound is warm without loosing sparkle!
Well worth trying them out IMHO.

Thats usefull to know thanks. I don't like a hard edged o/d either. Just got rid of a Crowther hot cake and an Xotic BB+ because they were boths quite hard edged. I've got an Xotic BB and thats great, go figure!.


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Steven_nl
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Re: Noob manual on Biasing

« Reply #8 on: January 02, 2009, 11:56:00 AM »

Plate Voltage Huh?

Thanks for taking the time and trouble Grin


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JohnE
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Re: Noob manual on Biasing

« Reply #9 on: January 02, 2009, 05:31:15 PM »

Quote from: bcmatt on January 01, 2009, 02:11:21 AM

Quote from: hywelg on December 31, 2008, 01:02:34 PM

I'm relatively new to biasing too, but this is what I've learned:
I think that you are going to want to check your plate voltage too. That will determine whether the 35mA is within the right range.

So, to get your plate voltage, you actually need to go inside your amp the first time for each set of tubes. Carefully put the red lead of your DMM on pin #3 of a power tube socket while the black lead is on ground.
Imagine you get 425 Volts.

So, if the Plate Dissipation Figure for a 6L6 is 30 Watts --> 30 Watt / 425 Volts = 0.071Amps or 71mA
The standard is often 70% of this for idling current. I've heard about as low as 50% being suggested. So, I think I choose 60%: 71mA * 0.6 = 42mA
Because of the 1 ohm resisters installed inside, you get to read in mV on your DMM and know it is basically the mA reading.

You are right, plate voltage is part of the equation, but using the standard protocol of 70% generally runs things with a hotter bias than is necessary. My amp tech insists 30-35mA is just right and will make the tubes last longer and my experience bears him out.


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