D.i.y. Bias Probe

[chocopower]

Hi there!!

Recently i finished my bias probe.

I use a tube socket conected to a tube base via a PVC section, with the wires going from pin1 in the socket to pin1 in de base, pin2 to pin2...etc..

In Pin8 a 1 ohm, 2 watt, 1% resistor is intercalated.
A 30cm wire at each side of this resistor is send to a plastic box.
and another wire is send from pin3 for Plate Voltage measure....

Ok.... I chek it 1 hour ago.....and it don,t work!!!
My amp is a Pro-Reverb clon with diodes rectified...
When i swicht the power on the tube don´t light at all and if i swicht the stanby a stange and dirty sound go out from the speakers!!!


Searching the interne i found this:

http://www.geofex.com/FX_images/biasprob.gif

1 ohm resistor in pin3???? Shouldn´t it be in pin8?

Whats wrong in my BIAS TESTER?¿?¿?¿

[stokes]

Since the tubes dont light up,I would suspect your heater wires are not connected properly.Check the continuity between each pin and the socket.You can use the 1ohm on either the cathode (pin 8) or the plate (pin3).

[chocopower]

I checked pin conecctions and everything is ok..

I had installed the 1ohm resistors inside the amp too.
Could it be the reason off the problem?


bufff...... i will check again.....


thanks!

[Steve Conner]

Maybe you have some sockets with the pin numbers printed wrong? The only explanation is that you wired it wrong.

[Don Moose]

You can use the 1ohm on either the cathode (pin 8) or the plate (pin3).

But if you use the plate, the measurement terminals will be floating at B+ - VERY dangerous - please only do this to the cathode.

[stokes]

But if you use the plate, the measurement terminals will be floating at B+ - VERY dangerous - please only do this to the cathode.

True,if you want to use the plate to measure current it is better to have an ammeter connected in series with the plate.

[R.G.]

But if you use the plate, the measurement terminals will be floating at B+ - VERY dangerous - please only do this to the cathode.

It is very dangerous, hence the note on my schematic. However, the cathode has both plate and screen current in it, and if you measure the cathode, you're relying on tube characteristics to make the screen be similar on both tubes. That's not that bad, but it's not plate current.

True,if you want to use the plate to measure current it is better to have an ammeter connected in series with the plate.

I believe that measuring the voltage drop across a low resistance IS an ammeter, isn't it?

The bias probe as shown is shown as fully cased in plastic and not ground referenced at all, just for those safety reasons.

[chocopower]

Hi again!

I check again all the pin conections and everything seems to be ok.....

The link i posted, don,t have a plate voltage measurement wire.
I installed the plate wire for external measument.
Do you think is too dangerous? i think is more dangerous open the amp every time and check the voltage on the pins....

Some times, the danger is hidden....waiting for us
Yesterday, with my Pro-Reverb clon full mounted and closed, i tried to tweek the external bias adjustment..... Accidentaly i touched the choke terminals.......and... 475 dc in my F****G hand!!!!!!!!

Silly me.....

[Enzo]

Please be careful, not only can it hurt, but it can KILL YOU.

He is not measuring current at the plate, he wants to bring out a plate terminal to check voltage there so his current readings can be used for power calculations.

If it still doesn't work, let me ask this, do the wires from the top socket run DiRECTLY straight down to the plug pins below? They do not appear to cross to the opposite side or anything?

Here is another way to check the wiring. PLug the adaptor into a power tube socket, but do not plug a power tube into it. Turn the amp on as if to play. Now stick your meter probe down into each hole of the empty socket and see what voltage is there. Is there B+ on both pins 3 and 4? Is the bias voltage, -50 or whatever, on pin 5?

If the tube heater lights when the tube is installed with the adaptor, fine, but if the heater stays dark, then remove the tube and measure AC voltage between pins 2 and 7. If your heater supply is center tapped and grounded, then there ought to be 3v or so from each pin 2 and 7 to ground. One of those missing would darken the tube.

And with power off, Measure resistance to ground from pin 1 and from pin 8. Pin 1 should be zero ohms, and pin 8 should be 1 ohm. If pins 1 and 8 are wired together inside the amp, then you need to break the pin 1 wire between plug and socket and wire it over to pin 8 of the socket. In other words, your adaptor should run both pins 1 and 8 through the 1 ohm resistor.

[Steve Conner]

Wait a minute, how can you touch the terminals on your choke with the amp "full mounted and closed"?!

If it has live terminals sticking out even when it's fully assembled, that's not good. You ought to put something over the choke, even if it's just a yoghurt pot and some duct tape! :P

As an aside, I wonder how amp makers even manage to sell gear that has exposed tubes sticking out, in this era of health and safety madness. Not only are they dangerously hot, but if you shattered one, it would expose live parts...

[MWJB]

"As an aside, I wonder how amp makers even manage to sell gear that has exposed tubes sticking out, in this era of health and safety madness. Not only are they dangerously hot, but if you shattered one, it would expose live parts..."

Probably why some manufacturers now stick those grotty metal cages round all their power tubes, so you have to pull off the back panel to change/check/bias your tubes...seems to me that it creates more risks than it solves?

Perhaps similar cages for lightbulbs are next on the agenda?

[Steve Conner]

Well, that cage does have one use, it stops you from whacking the tubes if you store your footswitch inside the back of the combo.

Personally I like my tubes sticking out in the fresh air

I heard there is legislation underway to ban incandescent lightbulbs in favour of energy-saving CFLs... How long before they ban Ampeg SVTs

[chocopower]

Please be careful, not only can it hurt, but it can KILL YOU.

He is not measuring current at the plate, he wants to bring out a plate terminal to check voltage there so his current readings can be used for power calculations.

If it still doesn't work, let me ask this, do the wires from the top socket run DiRECTLY straight down to the plug pins below? They do not appear to cross to the opposite side or anything?

Here is another way to check the wiring. PLug the adaptor into a power tube socket, but do not plug a power tube into it. Turn the amp on as if to play. Now stick your meter probe down into each hole of the empty socket and see what voltage is there. Is there B+ on both pins 3 and 4? Is the bias voltage, -50 or whatever, on pin 5?

If the tube heater lights when the tube is installed with the adaptor, fine, but if the heater stays dark, then remove the tube and measure AC voltage between pins 2 and 7. If your heater supply is center tapped and grounded, then there ought to be 3v or so from each pin 2 and 7 to ground. One of those missing would darken the tube.

And with power off, Measure resistance to ground from pin 1 and from pin 8. Pin 1 should be zero ohms, and pin 8 should be 1 ohm. If pins 1 and 8 are wired together inside the amp, then you need to break the pin 1 wire between plug and socket and wire it over to pin 8 of the socket. In other words, your adaptor should run both pins 1 and 8 through the 1 ohm resistor.

Yes, I want to bring out a plate terminal to check voltage. This way, i can calculate tha max. static disipation...

"...do the wires from the top socket run DiRECTLY straight down to the plug pins below? They do not appear to cross to the opposite side or anything?"

mmmm..... yes.... i think so. Pin 1 in the socket to pin 1 in the base....

I,m gonna try the check you told. Mounting the probe in the amp without valves.....


And yes.... the f***g choke have live terminals.....just 10cm close to the bias pot...i´ll try to cover them with heat srink....


By!!!

[Paul P]

Accidentaly i touched the choke terminals.......and... 475 dc in my F****G hand!!!!!!!!

I'm kind of glad to hear of people getting zapped. It shows that it's possible
to live through the experience. Being new to tube amp building I'm really
quite concerned about making some stupid mistake so it's nice to see that
there is a least some leeway.

I'm too old to live dangerously. The last exciting thing I lived through was
learning to operate a skid-steer loader on the side of a mountain. I thought
my heart would stop a couple of times. So I plan on being real careful when
I build my amp, I'm going to follow all the rules and I'm going to wear gloves .

Paul P

[Enzo]

On the other hand, some people die too. Don't plan on surviving, plan on not doing it at all.

A friend was working on his AMpeg a while back, and touched his finger to something B+ inside and couldn't let go, it had him. In reacting, he threw the live chassis across the room. Now has a nice V4B shaped hole in his wall, and a new respect for voltage.