Bias problem in JVM410H

[Malima]

Hi,

My JVM410H started making some strange noises. I thought it was a bad cable but it was not.
After checking it out, one of the EL34 was the culprit. Nothing strange so far.

Then I went to replace the tubes and check BIAS. Thats when it happened, one side the bias was around 20V (yes volts, not millivolts), the other around 40mV. Half of the tubes were too hot, much hotter than the other half.

After some digging I found that R27 (1R, 1W) was blown and that was driving the BIAS problem. I replaced it with a new one (10W this time) and proceeded to set the bias.
Everything is OK now.

I'm posting this to find if someone has already bumped to this issue.
I also plan to replace also R26 to a 5 or 10W resistor because it has the same function of R27 on the other half of the power amp.

 

[Spanngitter]

Those resistor normally do not fail until they either got tampered mechanically or a valve crapped out due to an internal fail or loosing Bias.
I would absolutely not recommend to replace them with a higher wattage as they also act like a fuse and prevent a core meltdown in case there is a catastrophic fail....or do you troubleshoot russian style by replacing fuses with going higher in rating until the root cause identifies itself with the BTSS* Signal ?




*BTSS = Black, thick and stinky smoke

 

[Malima]

No, of course not.
It took me a day to find the problem and I did not find any reference to this resistor breaking in the net. So I decided to post it. Not only to find any person with similar problems but also to get some advice.

I understand what your saying but I also ran into some problems with a NAD power amp in the past. A resistor wattage was very close to tolerance values and, depending on other components values it could burn. So, it was clear a design issue and a higher wattage resistor was needed. I though this might also be the case.


As far as I know there was no tampering on the amp. A faulty tube, maybe. I got it second handed so I don't know how long those tubes were there.

IMHO, if that resistor purpose is to act as a fuse, then put a fuse there. At least it is clear when it's blown.

Anyway, thanks for the sharing.

 

[Chris-in-LA]

That 1R resistor is there so that you can bias the amp easily. In the old days, that was simply a wire to ground.

 

[Ken Underwood]

The 1w resistors are rated like that for a reason, to protect in case of an internal short, which in this case the reason why it went O/C.

By increasing the rating to much higher then if either O/P tubes go internal S/C then you will pay the price of the extra damage caused.

Its very basic electronic design.

What you are saying is that "I Replaced a 1amp fuse with a 10amp fuse" that is just a total bodge and very dangerous, i don't think you should be attempting to go inside your amp with that very basic knowledge, be warned.

 

[Pete Farrington]

Bypassing the 1ohm cathode resistor with a diode limits the voltage across it (and hence its dissipation) to 1V 1W absolute max. See the Super Reverb RI R72//D4 & R73//D5 https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf

A suitably rated, fast acting type HT fuse is very beneficial in mitigating collateral damage when a valve shorts.
With solid state rectifiers, such a fuse should probably be best inserted after the reservoir cap.
Another HT fuse with a delayed action, in series with the HT winding, to protect it from rectifier / reservoir cap / wiring faults, might also be considered. Such a fuse should be mounted internally, as these events are more unlikely than valve shorts.

The point being that valve shorts are to be expected, so the likelihood of collateral damage arising from such incidents should be minimised.
Hence a reasonably competent user should be able to replace valve and back panel fuse, check bias, and put the amp back in operation, no tech intervention required.

 

[Malima]

Ok. Just to understand :

Some say putting a 10W 1R resistor is bogus design, others say in the old days was just a wire. Others suggest a diode.

And to clear somethings

The purpose of this post was to help others in debugging this issue.

Of course the common replacement is with the same specs resistor (in stock the only I had was a 10w)

It is yet to be discovered why it blew. Was a bad tube? Still don’t know.

I also understand with a higher wattage of R it will not work as a fuse. But believe me. With this R open the amp was working but the tubes were too hot. Some disaster was about to happen. So wish is the lesser bad? I don’t know.

 

[Malima]

The 1w resistors are rated like that for a reason, to protect in case of an internal short, which in this case the reason why it went O/C.

By increasing the rating to much higher then if either O/P tubes go internal S/C then you will pay the price of the extra damage caused.

Its very basic electronic design.

What you are saying is that "I Replaced a 1amp fuse with a 10amp fuse" that is just a total bodge and very dangerous, i don't think you should be attempting to go inside your amp with that very basic knowledge, be warned.

Just to be clear. I did not replaced the resistor with a fuse. I replaced with another 1R resistor.
What I said was if the purpose of this resistor is to work as a fuse, then a fuse should be placed (not in spite of…)

 

[Spanngitter]

What I said was if the purpose of this resistor is to work as a fuse, then a fuse should be placed

It acts as a 2nd line of defense in case the HT or Main Fuse do not blow because somebody installed an incorrect rating or a nail...
Overall said I have to echo Ken as your statement does clear show a lack in understanding of electrical circuit so I would recommend to work on your basic electronical knowledge before conductiing any further work.
As an assistance here some hints:
Pin 8 is cathode connection of the Power Tube and it needs to go to ground in these circuits. In the past this was done by a simple wire but you can also insert a Resistor which then will allow you to determine the current flowing thru it by the voltage drop generated and applying ohm's law. This is done on these amps due to 2 reasons:
1. To enable the feature that you can measure the Bias (Current) without using an external adapter
2. As a 2nd line of defense in case of a valve failure (as stated above)
The value of 1 Ohm had been determined to be the easiest one to calculate with and having less impact on overall cathode current but you could also use a 10 Ohm or any other value in between, however then it gets more complicated to calculate. 1 Ohm will give you 1mV per 1mA running thru it so it's really simple, if you would have a different value then you always need to calculate and this is a possible error source.
Theoretically it would be possible to replace that Resistor with a wire but then it would render the Bias Measurement Feature no longer working.
What Pete mentioned is a diode in parallel to the resistor which will protect the resistor and short out to GND if an excessive current is given but then it all depends on correct fuse rating be used (and I can tell you that >50% of the amps I get in for repair have incorrect fuses fittet).

 

[mickeydg5]

The problem with 1W bias monitoring resistors is that they take a jolt every time you turn the amplifier ON, especially so for higher wattage amplifiers. Over time this little power resistor will act up, usually heating up and causing bias to go higher as it heats up. Sooner or later it will burn and open under the right conditions.

THIS HAPPENS REGULARLY AND MANY THREADS/COMMENTS CAN BE FOUND ON THE TOPIC.

Unbeknownst to many is that this can actually happen to fuses as well.

It is always good to check tolerances of these resistors and fuses periodically.

@Malima and any others needing to know
These bias monitoring resistors should have a resistance value tolerance rating of 1% or better since it needs to be and stay more accurate.

 

[MonstersOfTheMidway]

Greetings to all.
I have an original 410H with original R26 and R27 installed. For the sake of reference, OP can look at mine to see yours is similar, which could give you some idea if the amp has been modified/tampered at those points in the circuit.

If all look ok/similar, then it's most likely a tube failed. Hopefully this can get you closer to finding out why your amp malfunctioned.

 

[Pete Farrington]

The problem with 1W bias monitoring resistors is that they take a jolt every time you turn the amplifier ON, especially so for higher wattage amplifiers

By what mechanism does that ‘jolt’ occur, what’s its typical magnitude?
It’s just I’ve never noticed it.
The only amps that I can think it would happen and be significant are the early - mid 70s 50W JMPs, where standby cut the feed to the HT AND bias supplies.

 

[Malima]

It acts as a 2nd line of defense in case the HT or Main Fuse do not blow because somebody installed an incorrect rating or a nail...
Overall said I have to echo Ken as your statement does clear show a lack in understanding of electrical circuit so I would recommend to work on your basic electronical knowledge before conductiing any further work.
As an assistance here some hints:
Pin 8 is cathode connection of the Power Tube and it needs to go to ground in these circuits. In the past this was done by a simple wire but you can also insert a Resistor which then will allow you to determine the current flowing thru it by the voltage drop generated and applying ohm's law. This is done on these amps due to 2 reasons:
1. To enable the feature that you can measure the Bias (Current) without using an external adapter
2. As a 2nd line of defense in case of a valve failure (as stated above)
The value of 1 Ohm had been determined to be the easiest one to calculate with and having less impact on overall cathode current but you could also use a 10 Ohm or any other value in between, however then it gets more complicated to calculate. 1 Ohm will give you 1mV per 1mA running thru it so it's really simple, if you would have a different value then you always need to calculate and this is a possible error source.
Theoretically it would be possible to replace that Resistor with a wire but then it would render the Bias Measurement Feature no longer working.
What Pete mentioned is a diode in parallel to the resistor which will protect the resistor and short out to GND if an excessive current is given but then it all depends on correct fuse rating be used (and I can tell you that >50% of the amps I get in for repair have incorrect fuses fittet).

I get that.
BTW I have a master in electrical engineering. I’m quite familiar with ohm law, Kirchhoff law and Maxwell also.
I’m not very experienced in tube amps though.

Why 1R is easier, I don’t get it. As ohm law says I=V/R. So doesn’t matter what R value is, it’s pretty straight forward to get the current once one measures the voltage.

What also I don’t get it is why the resistor blowing is considered a protection as it will put an extreme overload on tubes. Doesn’t seem to be protecting anything to me. A protection circuit is supposed to prevent a failure, not to redirect it to other components.

I put new set of tubes and almost destroyed a pair due to the blown R. I just found the failure because the bias probe was on 20V and 2 tubes were too hot.

But, that’s just my opinion

 

[Malima]

The problem with 1W bias monitoring resistors is that they take a jolt every time you turn the amplifier ON, especially so for higher wattage amplifiers. Over time this little power resistor will act up, usually heating up and causing bias to go higher as it heats up. Sooner or later it will burn and open under the right conditions.

THIS HAPPENS REGULARLY AND MANY THREADS/COMMENTS CAN BE FOUND ON THE TOPIC.

Unbeknownst to many is that this can actually happen to fuses as well.

It is always good to check tolerances of these resistors and fuses periodically.

@Malima and any others needing to know
These bias monitoring resistors should have a resistance value tolerance rating of 1% or better since it needs to be and stay more accurate.

Thanks. I didn’t find any so it was a long troubleshooting session.

 

[mickeydg5]

By what mechanism does that ‘jolt’ occur, what’s its typical magnitude?
It’s just I’ve never noticed it.
The only amps that I can think it would happen and be significant are the early - mid 70s 50W JMPs, where standby cut the feed to the HT AND bias supplies.

I have never gone looking for or measured it either. I have read about it in some guru's books and have noticed that it happens regularly across user's comments/threads.
People also bring up flyback. Coils store energy along with capacitors. Flipping a switch like the STANDBY allows inrush. There is no soft start or delayed turn on circuits in these amplifiers. I believe this stuff takes a toll on low power rated bias monitor resistors.
I have also never looked at transient/instantaneous peak voltage/current during high demand playing situations.
Yes, I know, theoretically the low power rating for these should be enough but in reality it aint so. AND if using these bias monitoring resistors as a safety/fuse then why not use the standard 1/2W or 1/4W because theoretically they should handle it too.

I know as anyone else can see that even 1W resistors burn up sooner or later. That proves the theory. It is not my theory. I do however observe it.

 

[Malima]

Greetings to all.
I have an original 410H with original R26 and R27 installed. For the sake of reference, OP can look at mine to see yours is similar, which could give you some idea if the amp has been modified/tampered at those points in the circuit.

If all look ok/similar, then it's most likely a tube failed. Hopefully this can get you closer to finding out why your amp malfunctioned.
View attachment 103167

It looks pretty the same. I found no traces of mods or burnt components.

Maybe it was a faulty tube or some strange transient.

 

[mickeydg5]

I get that.
BTW I have a master in electrical engineering. I’m quite familiar with ohm law, Kirchhoff law and Maxwell also.
I’m not very experienced in tube amps though.

Why 1R is easier, I don’t get it. As ohm law says I=V/R. So doesn’t matter what R value is, it’s pretty straight forward to get the current once one measures the voltage.

What also I don’t get it is why the resistor blowing is considered a protection as it will put an extreme overload on tubes. Doesn’t seem to be protecting anything to me. A protection circuit is supposed to prevent a failure, not to redirect it to other components.

I put new set of tubes and almost destroyed a pair due to the blown R. I just found the failure because the bias probe was on 20V and 2 tubes were too hot.

But, that’s just my opinion

Not everyone uses 1R although it is a very common value in musical amplifiers like Marshall and other brands as well as bias probes.

 

[Malima]

I have never gone looking for or measured it either. I have read about it in some guru's books and have noticed that it happens regularly across user's comments/threads.
People also bring up flyback. Coils store energy along with capacitors. Flipping a switch like the STANDBY allows inrush. There is no soft start or delayed turn on circuits in these amplifiers. I believe this stuff takes a toll on low power rated bias monitor resistors.
I have also never looked at transient/instantaneous peak voltage/current during high demand playing situations.
Yes, I know, theoretically the low power rating for these should be enough but in reality it aint so. AND if using these bias monitoring resistors as a safety/fuse then why not use the standard 1/2W or 1/4W because theoretically they should handle it too.

I know as anyone else can see that even 1W resistors burn up sooner or later. That proves the theory. It is not my theory. I do however observe it.

For me it makes sense that inrush current can destroy an resistor overtime. Of course it depends on the amount of energy that inrush has.

Because this is a second handed device I don’t know the kind of abuse it had in the past.

 

[mickeydg5]

By the way, no designer has ever placed a low wattage resistor in the power tube cathode circuit of a Marshall as a safety precaution/measure. The resistor is only there for the sake of bias measuring. It is stupid that the wattage rating is so low for that specific task.
If they want a safety feature then put a damn real one in there.

 

[Malima]

By the way, no designer has ever placed a low wattage resistor in the power tube cathode circuit of a Marshall as a safety precaution/measure. The resistor is only there for the sake of bias measuring. It is stupid that the wattage rating is so low for that specific task.
If they want a safety feature then put a damn real one in there.

I fully agree. That what I was trying to say. Thank you for understanding