Bias problems.

[Snake5150]

Was attempting to bias my jtm 45 build. And ran into a snag.

I have 366 plate at each tube.

But when I measure across the 1 ohm resistor pins 1/8- ground I get no readings in ma or mv.

What are the possible causes of this?

[mresistor]

no current flowing in the tube     black meter lead no connected to a good ground   meter in wrong mode selection


is the amp operating normally?

[Snake5150]

no current flowing in the tube     black meter lead no connected to a good ground   meter in wrong mode selection


Haven’t gotten that far. Was still trying to bias the new build. Starting to think I got solder in the tube socket pin receivers. They didn’t wanna push in right.

I have good continuity from the resistors to ground. 

[Calboy]

As an example, does the meter DCV Have 600v
200v
20v
2v
200m
The meter needs to be set to 200m.

I have not heard of 5w 1ohm resistors.

[Snake5150]

As an example, does the meter DCV Have 600v
200v
20v
2v
200m
The meter needs to be set to 200m.

I have not heard of 5w 1ohm resistors.

Yes in dc is has 200m. That’s what I was using.

I simply can’t seem to get any change across the resistor. And the resistor and its connection to ground seem to be ok. The bias pot changes nothing at both extremes

[labb]

Take the tubes out and see what bias voltage you have on the grids.

[Snake5150]

Take the tubes out and see what bias voltage you have on the grids.

With out the tubes in I could get grids from -30 to -49 , it currently set at cold max. But sweeping the pot does nothing at the tube. I checked with tubes removed and have the same grid sweep.

[pdf64]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

[acheld]

Hmmm, with those voltages, you should be able to measure some current from cathode to ground.

Three suspects:   1)   Your multimeter battery is weak.
2)   Your tube sockets are miswired. 
3)   The tubes are bad.   

I'm excluding bad multimeter leads, or error in placing the probes, but both have happened to me. The real mavens will have other things to think about, I am sure.

Since you are looking for way less than an amp flowing through your power tubes, I'd suggest a smaller (1watt) current sensing resistor.  A good 5 watt resistor shouldn't cause a problem, however, it's just not needed.

[Snake5150]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

Don’t know what ur referring to “g2k”
And “a-k”

[Snake5150]

Hmmm, with those voltages, you should be able to measure some current from cathode to ground.

Three suspects:   1)   Your multimeter battery is weak.
2)   Your tube sockets are miswired. 
3)   The tubes are bad.   

I'm excluding bad multimeter leads, or error in placing the probes, but both have happened to me. The real mavens will have other things to think about, I am sure.

Since you are looking for way less than an amp flowing through your power tubes, I'd suggest a smaller (1watt) current sensing resistor.  A good 5 watt resistor shouldn't cause a problem, however, it's just not needed.

Deff gonna check the battery.
Gonna re socket the tubes
Gonna change the 1 ohms to 1 watt

A guys was saying to check screen voltage

[mresistor]

Try a different meter?

[Snake5150]

Try a different meter?

Tried 3. Pretty sure I’ve got it buttoned down to the sockets being jacked up. They were cheap and I resoldered them a few times.

[SEL49]

Other than not being able to adjust the bias, are there any other issues? Does the amp work?

[Snake5150]

Other than not being able to adjust the bias, are there any other issues? Does the amp work?

I amp is a new build and I haven’t put audio to it yet.

[Snake5150]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

Don’t know what ur referring to “g2k”
And “a-k”

It ran away all the way to like 4 volts before I shut down

[pdf64]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

Don’t know what ur referring to “g2k”
And “a-k”

It ran away all the way to like 4 volts before I shut down

JTM45 are traditionally used with KT66 output valves -

So 'a' refers to the anode, 'g2' the screen grid, 'g1' the control grid, 'k' the cathode, and 'h' the heater.
Hence 0V g2-k means zero voltage (DC) between the screen grid and cathode.
And 0mA a-k means zero current between anode and cathode.

Here's a chart from the GE 6L6 GC info which illustrates that -

Unfortunately they use a different nomenclature, 'Ib' means anode current, Ec1 means control grid voltage, Ec2 means screen grid voltage (all voltages are with respect to the cathode.

So, as you've only got 4V on the screen grid (I think that's what you mean?), no anode current can flow.

You need to find out what's the problem with the HT supply to the screen grid (HT means high tension, an old term for high voltage).

[Snake5150]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

Don’t know what ur referring to “g2k”
And “a-k”

It ran away all the way to like 4 volts before I shut down

JTM45 are traditionally used with KT66 output valves -

So 'a' refers to the anode, 'g2' the screen grid, 'g1' the control grid, 'k' the cathode, and 'h' the heater.
Hence 0V g2-k means zero voltage (DC) between the screen grid and cathode.
And 0mA a-k means zero current between anode and cathode.

Here's a chart from the GE 6L6 GC info which illustrates that -

Unfortunately they use a different nomenclature, 'Ib' means anode current, Ec1 means control grid voltage, Ec2 means screen grid voltage (all voltages are with respect to the cathode.

So, as you've only got 4V on the screen grid (I think that's what you mean?), no anode current can flow.

You need to find out what's the problem with the HT supply to the screen grid (HT means high tension, an old term for high voltage).

Screen wire was disconnected at the board. Now have 460 at the screens and and getting current of 112ma across the cathode resistors.

However the bias pot still doesn’t move the current draw.

[AlNewman]

Do you have a light bulb limiter?

[HotBluePlates]

... the bias pot still doesn’t move the current draw.

Remove the output tubes.

Measure the voltage at Pin 5 of the output tube sockets.  You should have a negative voltage.

Fiddle the bias pot.  You should see the negative voltage at Pin 5 changing.  If you do not see a changing voltage, look carefully at your bias pot.  There's a wiring mistake somewhere right there.

[pdf64]

...
 current of 112ma across the cathode resistors.
 ...

How did you make these readings?
ie what were the meter settings, where did you connect the probes?

[Snake5150]

...
 current of 112ma across the cathode resistors.
 ...

How did you make these readings?
ie what were the meter settings, where did you connect the probes?

It was actually mv across the cathode resistor.

[wdelaney72]

Separate question....I'm tapping my HT Secondary for bias voltage. Looking to the Visio / JCM900 pdf Sluckey attached and hoping this would work.


PT Secondary are 275v. There is a center tap, but it's not grounded....I have it as the voltage doubler circuit via Marshall 100W superlead in between the 2 mains filter caps.


Can I use this bias circuit? I believe the 56K resistor to gound serves to half the voltage, so 275 would drop to about 137. I need to drop it further to 60vac....can I do that by simply raising the 56K or do I need to add another resistor to make a true voltage divider. I am using a .047uF cap as described in layout.

[pdf64]

It's by far preferable to start a new thread here, there seems no relevance to snakej's thread?

[pdf64]

That bias circuit will work but it really needs a bleed resistor on the HT, so that the bias voltage can charge up and be ready for when the valves are conducting.  Merlin explains the issue http://www.valvewizard.co.uk/bias.html

A voltage doubler will turn 275V AC into a very high V DC, maybe 600, and draw a lot of AC current from the winding, much higher than the DC current. Are you sure it's rated sufficiently?

[wdelaney72]

Apologies for hijacking the thread. Back to the OP....


I would suspect you have something still wrong with your sockets.

[mresistor]

What is the screen grid voltage pin4?
0V g2-k results in 0mA a-k.

Don’t know what ur referring to “g2k”
And “a-k”

G2 is the screen grid,  K is cathode,   a is anode      so he's saying Ov between screen and cathode results 0 milliamps between anode and cathode or   across the whole tube
Yes "a" that he typed is anode or plate anode     


well pardon me  I see that this was explained by pdf64 way back on 31 Dec!  please disregard this post of mine  thanks

[Willabe]

Anode = plate.