Am I Crazy? About Bias Resistors

[heymymy]

So I normally use the transformer shunt method to bias my amps, where i set the meter to 200mA and put my negative lead in the ground of the meter and the positive lead in the Amps input and measure.  So I tried installing 2 1ohm resistors from pin 8 to ground on my 5E4, but I'm not getting a reading when I put one lead to the resistor and one lead to ground.  Not sure what's going on, I've read all over the forum and on the internet, but it's just not clicking in my head how to set the meter, or maybe I'm measuring it in the wrong place? What gives?

[Willabe]

I've read a number of times not all meters are able to read (--down-- ? to) the 1 ohm. What kind of meter do you have?

You could try a 10 ohm instead, and see if you get a reading with them instead of the 1 ohm resistors.

 
        Brad      

[Iannone]

It is generally helpful when measuring the voltage drop across the 1 ohm resistor to connect the negative probe directly to the low side of the resistor, rather than to the transformer bolt or to the chassis lip. 
Try this and you should get a reading.  (Set the meter to the lowest DC voltage scale.)

[phsyconoodler]

It also helps by turning the meter setting to milli-volts and not milli-amps when measuring VOLTAGE across the resistors.
  The transformer shunt method was current in milli-amps not volts.

[stingray_65]

+1 set meter to read mV

What you are actually reading is mV drop.

because of the math of ohms law and using a 1 ohm resistor mV drop is equal to mA draw.

Ray

[Leevi]

Have you checked the battery of your meter?
Check also the meter setting which you are using with some other voltage source
that it's working. The cables can also be broken (which not always visible outside)
If you can hear the sound from your amp then there has to be some voltage over the cathode resistor
or at least over one of them.
/Leevi

[heymymy]

Ok, I checked the meter and it's working, I set the meter for 200mV and measured across the Resistor and still nothing.
Checked the OT method and I get -27.5 on 6V6's.  Don't know why it's not reading it. 

Also i checked normal voltage through the amp with the meter and I'm getting it where it should be. 

[phsyconoodler]

I bet you didn't disconnect the ground wires to pin 8 and added a 1 ohm resistor to ground as well.You need to REPLACE the cathode wire with the 1 ohm resistor.

"Checked the OT method and I get -27.5 on 6V6's"
What does that mean? -27.5 what?

[heymymy]

"I bet you didn't disconnect the ground wires to pin 8 and added a 1 ohm resistor to ground as well.You need to REPLACE the cathode wire with the 1 ohm resistor.

"Checked the OT method and I get -27.5 on 6V6's"
What does that mean? -27.5 what?


I did disconnect the ground wire, it's on pin 2 on my super, pin 8 and pin 1 are connected and then the resistor goes from pin 8 to ground,
is that the right way to hook it up? 

it's -27.5mA on the 6V6's.

[phsyconoodler]

Quote:"I did disconnect the ground wire, it's on pin 2 on my super,"
 
  What do you mean on pin 2? That's where the heater wires go on the power tubes,pins 2 & 7.
 Turn on the amp,connect the positive lead to pin 8 of one power tube,and the negative lead to ground.There is no way you can't get a reading in millivolts.Are you absolutely sure you are using a 1 ohm resistor and not a 1 k or something else by mistake?
Black,brown,black for color code on the resistor?
  Reverse the leads on your meter to get positive readings using the transformer shunt method.

[heymymy]

Ok, I'll reverse the leads.


Both power tubes are grounded from pin 2 to the chassis.  the wires originally went from chassis ground to pin 2, 1, and 8.  they were tied together originally.  the heater wire is only one one side, it's just like this...

http://www.turretboards.com/layouts_schematics/fender/layouts/super_5e4-a_layout.gif

i cut the ground braid from pin 2 and 1 which left me with a ground braid on pin 2 to ground.  then pin 1 and 8 tied together and a 1ohm resistor going from pin 8 to ground.

all the preamp tubes have this as well, pins 4&5 are connected to a green heater wire, but each tube at pin 9 on the preamp tubes is going straight to ground at the tube socket.

[phsyconoodler]

Ok,that's just the old way to do the heater wiring.That's fine.You are not doing something right,there HAS to be a reading from the cathode across the resistor to ground.

[heymymy]

I've got two other 1 ohm resistors, I'm gonna swap them in and make sure it's not the resistors.

thanks for your help!

[Willabe]

I've had one of the two fuses in my meter blown, and didn't know it. I was still able to get some readings, but not able to get other types of readings. I couldn't figure it out at first. You might want to check them. IIRC it happened when I was checking the bias with the trans. shunt method.   


         Brad         

[HotBluePlates]

Is this an old 50's 5E4, or a modern copy?

I see possible headaches in your future, as you sort out the wiring to function like you think it should.

Let's say it's an old 5E4. The filament wiring was done by running a single wire to each tue socket and the pilot light, with the other side of all (including the PT filament winding) connected to the chassis in some way. Reworking the filament wiring will likely prove a frustrating experience, because you will almost certainly miss the tricky grounding of one side of the pilot light. I speak from experience with these particular amps.

If it's a modern copy, you may have some difficulty sorting out how the modern builder attempted to copy Fender's 50's-era techniques.

If you get no voltage reading across the 1 ohm resistor, then it is probably being shunted by some alternate path for current to flow to ground, rather than going through the resistor.

The problem here is (assuming everything is wired perfectly, and according to the old method) that because the pin 2 side of the socket was wired with a short pigtail to the chassis, you might have difficulty breaking the connection over to pin 1 (not needed, except maybe in a metal 6L6) and pin 8 (the cathode). You could run the resistor from pin 8 to pin 2, which is then connected to the chassis, but the goal of that is to avoid soldering to the chassis (if that is how the connection is presently made).

A better choice would be to run the 1 ohm resistor from pin 8 directly to ground. My thinking with that is to not have the cathode connected to the filament circuit if possible (not really a problem *if* wired 100% like 50's Fender, but not ideal). If you attempt to solder to the chassis though, it's very easy to create a high-resistance cold solder joint, and still look sorts like a good solder joint. If you have a cold solder joint to the chassis and push against it with a flat screwdriver, it will pop right off with only mild pressure (I found that out the hard way by making poor solder joints to the chassis in a couple amps). If this is the case, and you still have the connection from pin 8 to pins 1, 2 and then on to ground/chassis, then the good ground through the filament  ground at pin 2 will shunt current away from your resistor, leaving no reading.

So if you have no reading (in mV's) across the 1 ohm resistor, and the tube is actually passing current, then there must be some wiring error or shunt path for tube current to flow. The trick is to figure out where the problem lies.

[heymymy]

oh yes indeed, it's the old style with pin 2 on each tube straight to chassis ground and only pin 4 + 5 has a heater wire.

i just quit on the resistor bias idea, took it out, rewired pin 1 and 8 back to the ground on pin 2 and biased by transformer shunt.
it just works better for me.  just for fun, I tried the resistor method on my jtm45 build, same deal no reading, my meter's drunk.
I don't understand.  I wish someone could just do it in front of me so I could see it. 

The Super is too much for me I've come to realize., I might be letting it go, I can't deal with the vintage piece, I'm too freaked out to tour with it and too young to just let it sit there at home collecting dust.  I have more fun with my Marshalls, Supro's and builds I do from scratch, and if they blow up or get stolen I'm not gonna cry.  I think owning this thing was a valuable lesson to me in what not to do for me personally.  Plus, I just have no use for a tweed fender in what I play, every time I think I love them, I end up hating them.

[phsyconoodler]

hbp,
  "i cut the ground braid from pin 2 and 1 which left me with a ground braid on pin 2 to ground.  then pin 1 and 8 tied together and a 1ohm resistor going from pin 8 to ground."

tHIS SHOULD WORK.You have still done something wrong.A picture is worth a thousand words.I'm betting you don't have your meter set right or the leads are still in the 'shunt' position.

[sluckey]

I'm betting you don't have your meter set right or the leads are still in the 'shunt' position.

Yep.