Music Man Bias Problem

[opacheco]

Hi,

I am in the restoration process of a Chassis 1650-RD, 110 RD 50 MM Amp and I changed the 2x6L6 Power tubes the problem is I can't to get the same bias current in the (idle current) although I wait at least 15 minutes for warm the tubes. I think the drivers transistors JE1692 could be mismatch before many years working and this maybe producing a deferents reading of Bias current but I am not sure.

Someone can help me or have had this Bias problem before??

Thanks a lot anticipated,
Opacheco.

[Enzo]

Did you take readings with the old tubes? Or can you return the old tubes to the amp and see how they behave?

Are the drive transistor emitter resistors both at value?

There is voltage dropped across those resistors to determine current, but there is also the voltage on the cathodes, what is there?

Also, make sure the positive voltage is the same on both control grids.

[The Dude]

Are you saying the 2 tubes don't bias the same or that you aren't getting any bias current? A bit more information would help. Maybe even post your actual bias current measurement(s).

[opacheco]

Enzo,

Thanks a lot for your response!.

I don't take any reading with the old tubes cause the owner pull out the tubes and trashed them.

Yes the emiter resitors are same value (aprox.)

Let me to check again and I will return with cathodes and Grid current and voltage values respectively.

-----------------------------
The Dude
Thanks for your comments!

The 2 tubes have Bias current but diferent value no the same although the power tubes are MATCHED TAD and new tubes. I will post values tomorrow.

Opacheco.

[The Dude]

Ok. Read the bias current for each tube. Then switch them from socket to socket. Do the current characteristics follow the tube or the socket? In other words, does one socket always have lower current or one tube always have lower current? They don't need to be and likely won't be exactly the same...... More on that when you tell us what your readings are......

[opacheco]

The Dude,
Ok, I will send the values later!

Thanks again
Opacheco.

[g1]

The factory did not expect the readings to be exactly the same, and it will depend on how well the tubes are matched.
Factory bias info allowed for up to 30mA mismatch, not really close at all.
If the tubes are a matched pair, and there is substantial difference in the currents, then there may be a problem in the circuitry.

[opacheco]

Ok. Read the bias current for each tube. Then switch them from socket to socket. Do the current characteristics follow the tube or the socket? In other words, does one socket always have lower current or one tube always have lower current? They don't need to be and likely won't be exactly the same...... More on that when you tell us what your readings are......

Hello again, I was late for christmas time, sorry!!.....

Well here the measurement:

Tube Socket 1/Tube 1
Ramdom Measurement: 5.133mA
Min. Value: 4.918mA
Max. Value 5.224mA
Window Time: 5 Minutes

Tube Socket 2/Tube 2
Ramdom Measurement: 7.17mA
Min. Value: 7.018mA
Max. Value 7.226mA
Window Time: 5 Minutes

Tube Socket 1/Tube 2
Ramdom Measurement: 5.85mA
Min. Value: 5.749mA
Max. Value 5.870mA
Window Time: 5 Minutes

Tube Socket 2/Tube 1
Ramdom Measurement: 7.17mA
Min. Value: 7.018mA
Max. Value 7.226mA
Window Time: 5 Minutes

I waited 15 minutes for warm the tubes for bias current measurements and the voltages in the tubes were:

Vp = 562 V
Vscreen = 281.72 V
Vg1 = Vg2 = 31.70 V
Vk1 = 63.55 V
Vk2 = 62.37 V

Vk1-g1 = - 31.23 V (negative voltage!)
Vk2-g2 = - 32.41 V (negative voltage!)

V emitter 1 = 36.91 mV
V emitter 2 = 53.7 mV
V emitter1(+) to emitter2(-) = -16.63 mV (negative voltage!)

Look like the bias measurement follow the Tube Socket no the Tubes to me!!.

I think the Transistor 1 could be bad, What do you think??

Any comment about?
Thanks a lot anticipated
Opacheco.

[Enzo]

OK,remember these are just guitar amps, not lab equipment. SUper precision is not necessary or expected.

You have two tubes with current varying from 5 to 7 milliamps. The fact even that varies an extra tenth of a milliamps is not an issue, your mains voltage isn't steady either, and as it rises and falls, all the measurements inside the amp will as well.

You have 60v on the cathodes and 30v on the grid, so of course the voltages are negative. 30v is 30 less positive than 60, hence there is a negative 30v from cathode to grid. No different at all from when the grid is at -30 and the cathode grounded.

That 60v on the cathode means ther is about 500v across the tube.

Look at the service bulletin G1 posted. Note it says adjust for 25mv on the 3.9 ohm resistors. And if they are different, set the lower one to 25mv. And the other tube should not be more than 55mv. You are at 36mv and 54mv, darn close I'd say. If you adjust the bias for a few milliamps cooler to drop that 36mv to 25mv, I bet that 56mv will also drop to like 45mv, and at that everything will be happy and within the factory specs. Yours appears to have 6.8 ohm resistors, but the same idea applies, your differences are not major.

Try thiss, with no power, measure the resistance of the output transformer primary on each side. If they are not the same resistance, that alone can cause a difference in the two tube currents. Not all transformers are wound with even resistance.

So I suspect nothing is wrong with the transistor. There can be slight variations in the transistors, or in the measured values of any involved resistors, or even the OT.

[opacheco]

OK,remember these are just guitar amps, not lab equipment. SUper precision is not necessary or expected.

You have two tubes with current varying from 5 to 7 milliamps. The fact even that varies an extra tenth of a milliamps is not an issue, your mains voltage isn't steady either, and as it rises and falls, all the measurements inside the amp will as well.

You have 60v on the cathodes and 30v on the grid, so of course the voltages are negative. 30v is 30 less positive than 60, hence there is a negative 30v from cathode to grid. No different at all from when the grid is at -30 and the cathode grounded.

That 60v on the cathode means ther is about 500v across the tube.

Look at the service bulletin G1 posted. Note it says adjust for 25mv on the 3.9 ohm resistors. And if they are different, set the lower one to 25mv. And the other tube should not be more than 55mv. You are at 36mv and 54mv, darn close I'd say. If you adjust the bias for a few milliamps cooler to drop that 36mv to 25mv, I bet that 56mv will also drop to like 45mv, and at that everything will be happy and within the factory specs. Yours appears to have 6.8 ohm resistors, but the same idea applies, your differences are not major.

Try thiss, with no power, measure the resistance of the output transformer primary on each side. If they are not the same resistance, that alone can cause a difference in the two tube currents. Not all transformers are wound with even resistance.

So I suspect nothing is wrong with the transistor. There can be slight variations in the transistors, or in the measured values of any involved resistors, or even the OT.

Dear Enzo,
Fantastic analysis and I am agree with you!....But if I want to have more bias current for better tone, are agree with a change of the zener diode controlling the bias or more specific the grid voltage i.e. 27V (1N4750A)??

Thanks a lot!
Opacheco.

[opacheco]

Enzo,

I forgot!; this amp don't have any pot for bias adjustment in each tube......how do you can adjust each bias independently?

Opacheco.

[opacheco]

I had measurement the DC resistance of the two half the Output Transformer and found the followings values:

Brown to Red (Tube 1) = 52.27 Ohms

Blue to Red (Tube 2) = 55.53 Ohms

Opacheco

[opacheco]

Well I am to finished to repair the unbalanced bias current between the to 6L6 idle currents plates by one simple method showing in the following pic.

I got the following bias idle current measurements in a 10 minutes wundow after of a 5 minutes of warm time:

Ic1: 6.48mA ~ 6.73mA
Ic2: 6.43mA ~ 6.82mA
Vg1=36.70volts
Vg2=31.49Volts

I will be pending for any comment about!!

Thanks a lot
Opacheco.

[Amp Kat]

Enzo,

I forgot!; this amp don't have any pot for bias adjustment in each tube......how do you can adjust each bias independently?

Opacheco.

Without mods you can't because the transistors are basically tied together. The circuit was never really meant to be biased as tubes back in the day were pretty closely matched in NOS. So the amp was designed as a plug and play amp. Without changes to isolate the grids and cathode transistors the bias is for the combination of tubes. I've never really seen one of those 1692 transistors fail or even be way out of spec because there really isn't any stress on them to change their characteristics even over time.

[J M Fahey]

But if I want to have more bias current for better tone, are agree with a change of the zener diode controlling the bias or more specific the grid voltage i.e. 27V (1N4750A)??

Forget it, this is not a conventional tube amp by any means.

2) you are applying the treatment to the wooden leg, not to the real one, grid bias voltage is irrelevant (some use 24V, others use 30V , none of them adjustable for a good reason) all the grid bias does is guarantee overabbundant POSITIVE voltage at any current the grid may demand, if necessary to pull down to the last mA from worn and dying tubes.

3) this is actually a cathode driven Class AB2 amplifier.

4) 5mA vs 7mA? that's no mismatch at all !!!!!!
If the OT can survive 30mA mismatch (like many OTs) and you actually have 2 to 4 or 5 mA ... where's the problem?

And the proper matching trimmer, if any, applies to the IC or get supermatched driver transistors.