biasing howto for beginners question

[Baltar]

i'm going to do biasing my first time and appreciate any feedback if
my short description here is rather ok, or wrong.

some time ago in another thread i found that post from Stratdeluxer97

On the 6L6/6V6 BF amp we'll use as an example,you do the following...

On pin 8 of each output tube inside the amp and at the tube socket...You disconnect the braided ground wire and you place a 1 ohm 1% resistor rated for 2 watts or higher in series and in between the ground wire and pin 8 of each output socket...This resistor is in "series" between pin 8 and ground...Pin 8 is the cathode...

So...You now have one end of the resistor soldered to pin 8....The other end of that resistor gets soldered to the grounded braided wire...

The resistor changes milliamps to millivolts....You place the red lead on pin 8 on one end of the resistor and the black lead on the grounded end of the resistor,set you meter to read in millivolts......Set your amp to a value in millivolts normally within the range of 30 to 40 millivolts....Use the tube giving you the highest reading as the one to set to....

There is a specific formula based on tube plate dissipation and plate voltages that I have written down,but I've been biasing my own amps for many many years now and so I know what to set my amp bias to....

so all i have to do, is to follow these instructions and use the
calculator here ??...
http://studentweb.eku.edu/justin_holton/bias.html
(scroll down to the bottom of the page)

fill in values A which is 12W for my 6V6, value B which i get from following
Stratdeluxer's instruction and also C,E.
The reslult needs then to be matched with the testpoint (1 ohm resistor
again) with trimming around the small blue pot using a screwdriver?
if that is all ...... :question: ....

this sounds quite easy to me - is that really all to do?

topic crosscheck:

1)
i like this article, but my neighbours would kill me for this ...
http://studentweb.eku.edu/justin_holton/bias.html#byear

2)
i know you shouldn't rely on the unwanted glow of tube parts as bias check, ...
i just want to know:
I read some articles, saying that only thing which should be glowing in
the tube is orange shaft in the middle.
What about the upper and lower 'attachment' ? what if they start
slightly to glow red at the inner countour (very close to the shaft)
after some time?

thanks in advance for bringing verification or correction to my view


------------------------------------------------------------------
i hope i correctly described the technical part in english - sorry if not

 

[wixedmords]

Re: biasing howto for beginners question

I am of the opinion that you need to swallow a bit of the biasing information a little at a time, mull it over and then take the next bit of information.

First of all, you are on your own, no one on any forum assumes responsibility.

The voltages you will be playing with are at least 2x the normal wall voltage of an electrical outlet, and in many cases are 4x. Keep this in mind, it is important you do not mess up. Pay up the life insurance.

The first step to biasing is getting plate voltage. For that you will need a multimeter at a minimum.

1. With the amp off, pull out the chassis to get to top of tube socket pins. Find pin #3 (pin 7 on an EL84), it is marked on the tube or on the socket usually.

2. Put the Voltmeter on the highest DC Volt scale, specifically one that reads hundreds of volts. Clip the black lead of the voltmeter onto the chassis (ground). Then clip the red lead onto pin 3 of the power tube socket.

3. Checking that everything is safe, your speakers are plugged in (called pulling a load), turn the power on. Let it warm for a few minutes, then with the volumes turned down, take the amp off standby. Measure the voltage. Write the number down. Should be something like 200-600 volts. For example purposes, lets say it was 500 volts.

4. Reverse the procedure. Go back on standby, wait a minute, power down. Then unclip the leads and put the chassis back in the amp if you are done.


The next step involves getting the bias from the amp which is done so very easy using a Weber Bias Rite, a Bias King, or one of the bias tools off Ebay, which eliminate the need for adding the resistor.

Swallow that first.

If you are in a situation where this needs to be done by tomorrow. Take it to the shop. Your life is more important than a time limit.

EDIT: If this makes any admin uncomfortable, you can delete it.

 

[Baltar]

Re: biasing howto for beginners question

@admin: please don't delete this -- i like it

@wixedmords:
first. a big thank you for writing me all that infos. i do appreciate it.

Thanks also for the warning - I am aware of the risk of a) my life b) my amps life

I already dealt with 380V AC (tiny cranes, crabs) and know it's not a game.
So i care more about my Laney Top :laugh2:

Therefore i think you are right, that i should collect some more infos
regarding that topic. I agree an amp's circuits are more complex than a
delta-wye switch, but i want to get that part and want to maintain my
pretty laney myself if possible (i like to be independent).
I don't really trust the amp tech guy at my dealers (too young and hasn't
seen any Laney (like lots of my fellow-countryman) before.

I don't have to haste, because my old tubes are still working somehow
and the new tubes manage a high plate voltage and first check was like plug-and-play
(sound nice warm, but not distorting to early on the one hand, on the
other hand no grey plates glowing red :burnout: ).
so everything is quite fine and i could also life with it as it is, if i
want that.

i will put your details to my checklist

one thing is still keeping me thinking:
if i take a closer look to the tube at work. is slight glowing allowed for the
carriers around the orange glowing shaft ?
Just in theory - i don't intend to blame anybody for tubes i kill.

cheers
Martin

 

[TonyBar3]

Re: biasing howto for beginners question

Sell your amp and buy a mesa/boogie. Self-biasing.

 

[Baltar]

Re: biasing howto for beginners question

Sell your amp and buy a mesa/boogie. Self-biasing.

mesa are nice amps, but too expensive for my pocket.
if i had the money, i would think of buying an orange first.

the laney is exactly my taste and also old enough to be of interest for me.

to buy a self biasing amp is some kind of sabotage to my own nature.
i have the continous drive to modify all my hobby-things.
despite my case, self biasing sounds like a great feature ...

cheers

 

[wixedmords]

Re: biasing howto for beginners question

Martin, what tubes are glowing, power or pre? A small amount of glowing is fine, but if they look like they are going to explode, that's not good.

You really can't read bias by sight of tube, it has to be measured. I am aware of the postings on sight methods, and even a "SMELL" method. Baaa-lieve it.

And..what Laney is the amp. It is shameful not to point out the model..

I have never seen a Laney before. .......

 

[wixedmords]

Re: biasing howto for beginners question

TB, you may want to check your Mesa information. It may be cathode biased, but not self-biasing. I believe Mesa's require you to purchase tubes tested to a certain specification. Someone correct me here if needed.

A lot of EL84 amps are cathode biased, which means there is a range of bias the amp will allow the tubes to operate in, but you still need to check the bias, to make sure it isn't off the map.

 

[TonyBar3]

Re: biasing howto for beginners question

TB, you may want to check your Mesa information. It may be cathode biased, but not self-biasing. I believe Mesa's require you to purchase tubes tested to a certain specification. Someone correct me here if needed.

A lot of EL84 amps are cathode biased, which means there is a range of bias the amp will allow the tubes to operate in, but you still need to check the bias, to make sure it isn't off the map.

I'll check my manual, but the jist of it was "If you buy these tubes (insert specs here) you will have to do no biasing at all."

Tha maintenance manual basically states that Mesa wants all users to be able to do simple maint. tasks like changing and troubleshooting tubes. For this purpose, the tubes are biased in some way (for lack of tech. term) so that the owner doesn't have to do anything other than stick them in there.

Doh! Manual is on-line. Here you go, straight from the book:

Most amplifiers have a "bias adjustment" which must be set right for correct operation of the power tubes. But these variable bias supplies can wander out of adjustment, and there is often confusion about the correct setting anyway. To avoid these potential problems (and save you the technician's fee) each Boogie's bias supply is built with parts selected for that exact amplifier and there is nothing to adjust. So when it comes time to replace the power tubes (after 6 to 12 months of vigorous use) just POP in a set of genuine MESA replacement tubes making sure the black spring clamps are snug against the tube base. We hear of many problems caused by using other brands, so: Preserve your amp's warranty -- use only genuine MESA replacement tubes.

There you go. Stick with Mesa and no brains are necessary. I guess that's why you pay more, they think for you. (God I sound like a moron after all that.)

 

[Gearjoneser]

Re: biasing howto for beginners question

Every tube amp owner who's the least bit interested in biasing amps should buy a volt/ohm meter and a bias tool, which is just a two sided socket with a cable that connects to your volt/ohm meter. I use the mA Bias Tool I bought on Ebay for $15

With the chassis plugged up to a guitar and speaker cab, and the bias tool in-between one of the tubes and chassis, CAREFULLY turn the mini pot till the amp sounds it's best.

6L6's should read somewhere between 30 and 33 mA and EL-34's should read somewhere between 34 and 39mA. As long as the meter is giving you numbers that are in that ballpark, and the amp sounds it's tightest and punchiest, you're fine.

My disclaimer: DO NOT STAND IN A TUB OF WATER WHILE WEARING LOOSE HANGING JEWELRY WHILE WORKING INSIDE OF A LIVE AMP!!! HAHAHAHAHA

 

[Xeromus]

Re: biasing howto for beginners question

I still don't trust myself enough to bias my own amps, I always take it to someone. I know some people don't like the fact that you can't bias Mesa amps, but that's really attractive to me since I don't have a lot of time or money to spend. It's great to be able to pop in new tubes and go.

 

[wixedmords]

Re: biasing howto for beginners question

just POP in a set of genuine MESA replacement tubes

You did fine Tony. What happens if you want to go with different tubes than Mesa?

They are tying you to the specifications of their tube. Which is fine, as long as you see what I am talking about.

 

[Baltar]

Re: biasing howto for beginners question

Martin, what tubes are glowing, power or pre? A small amount of glowing is fine, but if they look like they are going to explode, that's not good.

You really can't read bias by sight of tube, it has to be measured. I am aware of the postings on sight methods, and even a "SMELL" method. Baaa-lieve it.

And..what Laney is the amp. It is shameful not to point out the model..

I have never seen a Laney before. .......

it's about my power tubes (6v6 type)

bias by smell sounds like a great invention

... my Laney is a PT30-MV. If i can believe the sticker, it's from 1990.
It has just one preamp-section, but those push-pull tone pots.

it uses 2 6V6GT for the PowerAmp and 3x ECC83, 1x ECC81 for the premamp

cheers

 

[TonyBar3]

Re: biasing howto for beginners question

What happens if you want to go with different tubes than Mesa?

I guess I'll never know. Actually, "Mesa's" tubes, are actually Sovtek tubes. Oh sweet irony. And I think if you match up the specs with another brand tube, then screw it. Plus, I've got a Mark III, my warranty has long since been over, so I have no worries about losing it. Also, it's just a manual. If you call Mesa and ask them, hey can I put this brand tubes in, it's blah blah blah, they'll compare and tell you if it's a safe bet. They are really customer friendly.

I think they should pay me as a sales rep though.

 

[Baltar]

Re: biasing howto for beginners question

and..what Laney is the amp. It is shameful not to point out the model..

... my Laney is a PT30-MV. If i can believe the sticker, it's from 1990.
It has just one preamp-section, but those push-pull tone pots.

do you think, it's possible to get the blueprints of my amp?
Laney said, it's one of the models of which they lost the manual :blackeye:

cheers

 

[Zerberus]

Re: biasing howto for beginners question

You did fine Tony. What happens if you want to go with different tubes than Mesa?

Usually nothing, but I´ve seen cases where the ias resistors had to be replaced becaiuse the newer tubes were soo off spec (Compared to the Mesas)... most competent techs will make the bias adjustable by installing a pot in this situation... I mean, if you already have to void the warranty, then you may as well spend 5 cents more and save the hassle for the next guy

 

[wixedmords]

Re: biasing howto for beginners question

do you think, it's possible to get the blueprints of my amp?
Laney said, it's one of the models of which they lost the manual :blackeye:

cheers

I haven't seen one for it in my travels, but will keep you in mind.

 

[wixedmords]

Re: biasing howto for beginners question

Usually nothing, but I´ve seen cases where the ias resistors had to be replaced becaiuse the newer tubes were soo off spec (Compared to the Mesas)... most competent techs will make the bias adjustable by installing a pot in this situation... I mean, if you already have to void the warranty, then you may as well spend 5 cents more and save the hassle for the next guy

Yep, it will need to have a bias pot added so it would be then adjustable cathode biased.

That is kinda like "it is a dry heat", or "That ice burned my mouth". :laugh2:

 

[Baltar]

Re: biasing howto for beginners question

I haven't seen one for it in my travels, but will keep you in mind.

thank you.
the complete name on the front imprint is
'Laney PT30MV - PRO TUBE Series II'

all the stickers on the back are faded. just the signs of the people who
did the wiring and qa are there with date 1990

cheers
Martin

 

[Rich_S]

Re: biasing howto for beginners question

I think I already told you this, but download "Dave Sorlien's Excellent P1 Theory Document" from The P1 page at AX84.com. Print it out. Read it over and over until you understand it.

Then, you need to understand the differences betwen your push-pull amp and the P1's single-ended stage. Then you'll be ready.