need help with BIASin' a la Ken Fischer...

[antosimoni]

hi,
when Ken Fischer states :

"...the positive lead of the current meter will connect to the point of the center tap of the output transformer..."

wheres this center tap of the OT ???

PS : the positive lead is the red one? :oops:

thanks a lot guys and sorry if I'm a zero in this... :oops: :wink: :wink: :wink:

http://img30.picoodle.com/img/img30/4/4/13/f_Dscn3846m_a257324.jpg

[VikingAmps]

Standby switch? Is that some sort of Marshall reissue?

[antosimoni]

yes
its a 2203 reissue

[Nolatone Ampworks]

I can't say which one is the center tap from that photo, but be careful poking around in that chassis. Make sure you understand how to do that safely.

[Blue Strat]

Can't say for sure in your amp, but it's often the one connected to the HT fuse and is a red wire.

[VaughnC]

I can't over stress this enough but, if you don't know your way around inside an amp, this biasing method is very dangerous for the novice as there's typically around 500 lethal volts at that location. It might be best for you to purchase a bias meter so you're not subjected to the high voltages inside any amp (a bias meter usually has adapters that install between the output tubes and their sockets). Biasing a guitar amp is best done by ear anyway...only using a meter to tell you that you're not exceeding a tubes design limits.

[Blue Strat]

What VaughnC said!

[antosimoni]

so how can I bias my amp in a better way ???

[jbltwin1]

I pesonally would wire in a 1 ohm resistor to ground from the cathode, ASSUMING it's not cathode biased. Then you can read direct VOLTAGE to ground which translates into current by ohms law. Then set accordingly. You don't say what kind of amp this is so this is the generic explanation but pretty much applies everywhere. I would suggest that you get a little more understanding of what you are doing before poking around in there. At the LEAST, you have(depending on the amp, tubes, etc) 350-500 volts bouncing around in there and that crap hurts! Or, you can get a bias adapter an dread directly.

[tmac]

you could get a bias meter (or adapter head if you want to use your own meter) from Weber

https://taweber.powweb.com/biasrite/br_page.htm


so how can I bias my amp in a better way ???

[VikingAmps]

If you were to read the cold resistance (amp off) from where it says D3 on the board to pin 3 of the outer 2 tubes (standby on) and use those numbers to divide the DC voltage readings by at the same points with the amp on and warmed up you'll get the total plate current at idle from each side. If you start by pulling a tube from each side you can figure out the bias current of each tube by taking the DC readings with 2 then 4 tubes and subtracting. This method is very accurate but also somewhat dangerous. It does give you the actual plate current and not the combined plate and screen currents which can throw you off by 10-20 percent with true pentodes. It's not nearly as dangerous (or troublesome) as breaking the circuit and measuring current and is more accurate since you can have connection/meter problems with that method. Using a bias probe that measures cathode current is OK but you should subtract the screen current to get the true plate current. That's easily done by checking the drop across the 1K screen resistors. The DC voltage there is the current in ma.

[antosimoni]

thanks a lot redman...
so wheres this 1K res are ???
I'llget the probes and try your suggestion but I need a bit of enlightment

[Jon C]

I agree that based on the question, if you don't know where it is I wouldn't be comfortable telling you where to poke around in there, the downsides are too drastic.

I used a BiasRite til I was familiar enough w/ the innards to do the bias shunt method, it's cheap life insurance.

[Clutch21286]

Just an FYI: You should not get a 5 minute tutorial off the internet and then play around inside a tube amp. You have to totally understand the whole procedure.

Make sure someone is around to apply CPR.

[Blue Strat]

Any of the methods that involve probing around inside the amp are viable, but offer no advantages and several disadvantages (blown up meters, shocked operators, etc) over using a bias probe.

I've worked in electronics for 30+ years and now use a bias probe.

[VikingAmps]

thanks a lot redman...
so wheres this 1K res are ???
I'llget the probes and try your suggestion but I need a bit of enlightment


They're the big white cement resistors on the power tube sockets. Be careful!

[walterw]

They're the big white cement resistors on the power tube sockets. Be careful!
:eek: "the pointy end of gun goes away from you. be careful!":eek:

[Rosewood]

:eek: "the pointy end of gun goes away from you. be careful!":eek:
Unless you've had a really bad day and...:Devil

[JamesHealey]

take it from me you need to understand what you're doing, and you obviously dont..

500v shocks hurt, luckily amps tend to draw more current when being played so amps on idle give a "less" lethal shock, I think thats the only reason Im still here to tell the tale. I've learned a lot since then and don't plan on making anymore stupid mistakes like that.

[ted01]

Dude,

I know other keep saying this, but FOR GOD'S SAKE, DON'T DO THIS. I know you just want your amp to sound awesome, but the risks are just too great. Ya' can't play when you're DEAD. If you really want to learn more about these things, take your amp to a tech this time. Tell him you want him to teach you what he is doing. Some may be resistant. They may think that you'll be taking away business. Keep looking. Many tech's are way cool and will be more than happy to teach you. PLEASE DON'T DIE!!!

[phsyconoodler]

I totally agree with the bulk of the guys here.Get a bias meter and save yourself headaches and possibly worse.At the very least you'll blow up your meter by touching something you shouldn't.I've been zapped a few times at 500+ volts and it's not pleasant.I also watched an amp go up in smoke when a guy tuched the heater pin to the plate.It kept arcing until the socket burnt up.
Do yourself and your amp a big favour and find a better way to bias.

[Bonenut]

You should have your affairs in order (final will, life insurance, cemetary plot, etc.) before you start poking around with live voltages, just in case... Hooking up a current meter in series with the PT center tap scares the crap out of me, and I've been doing this stuff for years!

[Blue Strat]

You should have your affairs in order (final will, life insurance, cemetary plot, etc.) before you start poking around with live voltages, just in case... Hooking up a current meter in series with the PT center tap scares the crap out of me, and I've been doing this stuff for years!

LOL! Well put!:BEER

[antosimoni]

taken!!
thanks a lot for the advices

[VikingAmps]

take it from me you need to understand what you're doing, and you obviously dont..

500v shocks hurt, luckily amps tend to draw more current when being played so amps on idle give a "less" lethal shock, I think thats the only reason Im still here to tell the tale. I've learned a lot since then and don't plan on making anymore stupid mistakes like that.

If the amps drawing more current youre less likely to draw some of it.

[rockon1]

Any of the methods that involve probing around inside the amp are viable, but offer no advantages and several disadvantages (blown up meters, shocked operators, etc) over using a bias probe.

I've worked in electronics for 30+ years and now use a bias probe.

+1000 I slipped with a probe bising my Supersonic(measuring across the resistors) Fortunately I only fried a couple of safety resistors and learned quickly how to desolder componants from a board! I highly recommend a bias probe as Mike and others have stated! I could have distroyed something far worse or got the shock of my life. Get a probe. Bob