23% bias difference between tubes not being cause by tubes

[LPSGME]

I'm getting a 23% bias difference between tubes [40mv versus 31mv] and it doesn't seem to be a bad tube - because I swapped them and the higher bias reading stays with the same socket.

Right now I have the bias voltage set on the conservative side, so one tube reads 40mv and the other 31mv. I think the JJ 6L6GC tube specs show 450v/30 amps; so 65% of 30 divided by 475 comes out to 41mv.

I think the problem happened after the amp blew it's .5 amp fast blow fuse while I was pushing a probe into one of the bias test sockets. I discovered there was a drop of solder sitting between the wiper of the bias pot and the chasis - so I assume that caused it.

However the amp keeps blowing new fuses, unless I use 1 amp. I believe Nik shows a 1 amp on his OTS schematic but a .5 on the HRM (?).

Anyway I don't see any problem with any the resistors on the power tubes. The only disparity I see is in the output trany windings carrying the (475v) B1 supply to the output tube plates. The winding from B1 supply to the tube drawing the higher current measures 31.5 ohms while the winding to the other tube is 33.5 ohms. But that's only a 6% difference, not 23%.

The amp sounds OK. But I'm wondering if it would sound better with the tubes evenly biased.

Anyone have any thoughts or suggestions?

[Wayne]

It's quite normal for the two halves of the HV winding to have different resistance.

The first thing that comes to mind is your bias measuring resistors - With power off and the output tubes removed, make sure you have 1 ohm from each of the cathode pins to ground.

Another thing that can cause unequal bias is a leaky coupling capacitor out of the PI. With power on and output tubes removed, make sure you've got equal voltage on each grid pin of the socket.

Good luck

W

[bluesfendermanblues]

In a dumble style amp, where distortion happen in the preamp, I would prefer pretty balanced output tubes (with not too much difference in bias mv)

However, in a crancked fender amp, like a 59'ish Bassman, where the distortion happen in the output section, I've had lovely assymmetric overdrive, from unbalanced overdriven output tubes.

But if your amp sounds good as is, why worry too much about it?

[Bob-I]

Wayne said it, my suspicion is that something damaged either the OT, a coupling cap or one of the bias resistors when the fuse blew. Fuses don't usally blow for no reason.

I hope that it's just a coupling cap or bias resistor.

[Structo]

Is the plate voltage the same on both?

Check the Pin 3 voltage on each tube, then check the screen voltage after the choke or choke resistor.
Then check the voltage at Pin 4.

Check to make sure both 220K bias splitter resistors are correct and within tolerance.

Make sure both swamp resistors are correct and within tolerance.

[talbany]

I'm getting a 23% bias difference between tubes [40mv versus 31mv] and it doesn't seem to be a bad tube - because I swapped them and the higher bias reading stays with the same socket.

Right now I have the bias voltage set on the conservative side, so one tube reads 40mv and the other 31mv. I think the JJ 6L6GC tube specs show 450v/30 amps; so 65% of 30 divided by 475 comes out to 41mv.

I think the problem happened after the amp blew it's .5 amp fast blow fuse while I was pushing a probe into one of the bias test sockets. I discovered there was a drop of solder sitting between the wiper of the bias pot and the chasis - so I assume that caused it.

However the amp keeps blowing new fuses, unless I use 1 amp. I believe Nik shows a 1 amp on his OTS schematic but a .5 on the HRM (?).

Anyway I don't see any problem with any the resistors on the power tubes. The only disparity I see is in the output trany windings carrying the (475v) B1 supply to the output tube plates. The winding from B1 supply to the tube drawing the higher current measures 31.5 ohms while the winding to the other tube is 33.5 ohms. But that's only a 6% difference, not 23%.

The amp sounds OK. But I'm wondering if it would sound better with the tubes evenly biased.

Anyone have any thoughts or suggestions?

Give this a shot..
First thing I would do is pull the tubes and make sure both sides are giving you the same amount of negative voltage...If they are different.. There is your problem...Trace it back all the way to the PI tube..Maybe a grid resistor gone south pi coupling cap or even a PI tube gone bad will do that...Check the voltages on both sides of the plates of the PI tube..
If your getting the same amount of negative voltage on both sides.. Check the screen voltage and resistor If your certian it's not tubes wiring issue bad solder joint then take a look at the OPT...Is this a 50w or 100?

Happy Thanksgiving... Good Luck

Tony

[LPSGME]

It appears I found the problem but I still don't know why it would have resulted in a higher rather than lower bias reading.

The faulty (high) reading appears to have been only when read from the bias test jack. Although I know I read the bias voltage at Pin 1/8 of the tube 'several times' and it was also high. Go figure.

So I re-soldered the wires to that test jack, and now the bias reads fine at the jack.

But wouldn't a cold solder joint on the bias test jack have delivered a lower reading rather than higher one?

I also noticed the B1 plate voltage is now reading around 460 rather than 475 last night. I assume that is from electric line drop from everyone cooking their turkeys.

Thanks for replies. This (Ceriatone HRM) is my first dumble clone. Although a recent factory build, I ended up modifying it with some kind advice from Henry and thoughts of my own gleaned from this great group.

[Wayne]

Glad you nailed it!

Whether your bias measurement should have gone up or down with a cold solder joint would depend, among other things, on whether your 1 ohm resistor was located at the tube socket or at the test point. Not sure what to say about that.

Your change in B+ is about 3%. That could be holiday turkey-induced, or it could be from a change in current draw now that your tubes have a proper path to ground. Either way, it's 3%

W

[Bob-I]

But wouldn't a cold solder joint on the bias test jack have delivered a lower reading rather than higher one?

If you increase the resistance and have the same current flowing, the mv drop will be higher. Think about it, 1 ohm with 32ma through it will drop 32mv, 2 ohms with the same 32ma through it will drop 64mv.

[Wayne]

Bob- I had all that typed in, but I'm not sure if the OP's current measuring resistors are at the test points, or on the sockets - hence the uncertainty.

W

[talbany]

Bob's right measuring the voltage drop( this case mv) across the resistor..


Tony

[LPSGME]

Agree - as one end of the 1 ohm resistor goes to ground, the voltage appearing at the other end is equivalent to the voltage drop across it.

However, I was reading 40 mv at the resistor but 50 mv at the test jack that is attached to that resistor by a wire.

Shouldn't a bad solder joint between the wire (going to the resistor) and the test jack should cause a lower reading rather than higher, by reason that the bad joint is adding resistance between my probe and the resistor?

Otherwise, if the reverse were true, I should get an even higher voltage reading when I disconnect my probe and even further increase the resistance between my probe and test jack.

[Structo]

I would measure the continuity from the jack to the resistor.
You should read zero ohms.
If there was a bad solder joint there that would increase the resistance value from the cathode to the ground point on the test jack.

E/R = I

So an increase in R will result in a lower current.

[talbany]

[

quote="LPSGME"]Agree - as one end of the 1 ohm resistor goes to ground, the voltage appearing at the other end is equivalent to the voltage drop across it.

However, I was reading 40 mv at the resistor but 50 mv at the test jack that is attached to that resistor by a wire.

Shouldn't a bad solder joint between the wire (going to the resistor) and the test jack should cause a lower reading rather than higher, by reason that the bad joint is adding resistance between my probe and the resistor?

Reread your post and got what asked

Your reading will be higher because your still adding resistance across your meter...Like Tom said

T

[Bob-I]

So an increase in R will result in a lower current.

Not exactly. The current here is not a factor of the voltage across the resistor, but the conductance of the tube. The 1 ohm resistor is a convienent way of measuring the bias current. Some amps use 10 ohms. 1 ohm, 2 ohms or 10 ohms won't change the current through the tube measurably.

Back to ohms law, if the current is the same, but the resistance increases, the voltage across the resistor will increase.