Blues Junior - extremely high voltage problem (1300 Vdc on EL84 plate)

King Fan

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Glad this is getting sorted. Good job, @john trials . Super job, consultants.

Broad question: Anyone know a clear, simple web page (or *short* video) that tells us how to do a voltage table? Safety, what to measure, VAC v. VDC, pin-to-pin vs. pin-to-chassis, and so on? Like most folks here, I tend to suggest "do a complete voltage table" to folks having amp problems -- but I seem to recall that was mysterious when I was starting out... and shoulda been scary, me, a noob, working in a live amp.
 

peteb

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The tube gets hot, and either due to bias being too much or the tube being old, it partially (or fully) fails and the elements short out. Enough current gets drawn through that resistor to make it get hot and usually it will fail open.

thanks Corliss.

i have been trying to understand screen and screen resistor failure.

Fender specs similar size and wattage cathode and screen resistors. 1 W 470 ohms, carbon comp.

the cathode in a bf/sf champ can exceed 50 mA.

let’s say a bf/sf Bassman is biased at healthy 35 mA of plate current, the screen will then idle at 5 mA or less.

I see 10 times the current on the cathode resistor, over the same ohm resistor, that is 10 times more volts. 10 times more volts times 10 times more current is 100 times more power.

it partially (or fully) fails and the elements short out. Enough current gets drawn through that resistor

I know we are all speculating when it comes to failed screens and or screen resistors, so please don’t think I am being critical. I am just trying to understand this.

you are saying the tube fails and takes out the screen resistor. That would mean the tube is at fault or is the weak link, and not the screen resistor.


I believe on John Trials Blues Jr, the amp worked and the problem stayed with the tube socket location and not the tube.


if in fact the tube failed and caused the screen resistor to fail, then switching tubes would result in one bad tube in one spot and one open screen resistor in the other spot. I don’t see that as the problem remaining at one location.



here is a question.

when screen resistors fail, how often does the tube also fail?
 

corliss1

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Yes, in my scenario (and likely most Blues Juniors) it's the tube failing. Usually when resistors fail, they are a symptom as something is drawing too much current through them. Rarely are they the lone problem.
 

peteb

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Broad question: Anyone know a clear, simple web page (or *short* video) that tells us how to do a voltage table? Safety, what to measure, VAC v. VDC, pin-to-pin vs. pin-to-chassis, and so on? Like most folks here, I tend to suggest "do a complete voltage table" to folks having amp problems -- but I seem to recall that was mysterious when I was starting out... and shoulda been scary, me, a noob, working in a live amp.
Good question king Fan.

there are many ways to do it and personal preferences play a big role.


I like to make a one page word doc.

make paper copies.

I like to leave a blank for each tube and write in the tube type and brand.

number each pin

i think nomenclature helps.

3 plate
4 screen
6 grid

for example.

it is nice to have a place to write the voltage from the schematic next to the measured and recorded voltage for reference, maybe a place to also write down the % difference.

measure all pins to chassis ground
heater in AC
cathode, control grid, screen grid, plate are all DC.

calculate cathode idle bias current if there is a cathode resistor.

prepare to calculate plate current using output transformer primary resistance.

either before the amp is turned on, or after the amp is warmed up and turned off, measure the resistance of the OT primary. In push pull amps, measure resistance of the blue wire to the red center tap wire and also measure the brown wire to the red wire. Single ended is blue to red.

measure cathode resistance

with the amp idling, measure the DC voltage across the OT primary’s, either blue to red and brown to red, or just blue to red on single ended amps.

calculate currents using ohms law.

calculate power dissipation at the plate

calculate % of max plate dissipation.


I copied ten copies. When some new need arises, on a new sheet, I write the date and the reason or the current state of the amps.



some people like the spread sheet format and enter the values right into the spreadsheet on the computer.


I am old fashioned and prefer the hard copy ‘boiler plate’ method.
63810B83-7545-4927-B3EE-1BCD92D9A218.jpeg
 
Last edited:

peteb

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Yes, in my scenario (and likely most Blues Juniors) it's the tube failing. Usually when resistors fail, they are a symptom as something is drawing too much current through them. Rarely are they the lone problem.

thank you Corliss.
 

Lynxtrap

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My going hypothesis is that the presence of the Fluke 23 on the plate circuit induces oscillations between the OT and the DMM. I didn't find much on the internet, but I read something that said DMM's sample voltage by rapidly charging and discharging a capacitor. Maybe there is something just right about this BJr circuit--the exact right amount of inductance and capacitance--when attached to the Fluke 23 sampling circuit, it just starts oscillating. Nothing wrong with the circuit or the DMM--just a freak coincidence. (And this would explain why it only happens on one side--both sides are not perfectly identical.)

The way I understand it, oscillations can cause the impedance of a circuit to skyrocket at the resonant frequency. If this was happening, I think it is possible to see massive voltages due to the energy stored in the OT. In this case the power tube sees a massive increase in impedance. This is like pulling the speaker and just letting the amp run all out until something breaks.

Another thing I don't know for certain is whether the blown screen resistor a) caused or exacerbated the oscillations or b) is a result of the oscillations.

I might lean toward a) because I'm imagining (speculating, guessing) the plate resistance dropping dramatically if the screen is allowed to float. (This is the how I'm connecting all of this to the screen.) If that happens, there would be less dampening in the circuit, making the oscillations possible or maybe just worse.

This is all rank speculation. I'm hoping someone smarter lays my ideas in a well-deserved grave or gives them some credence and I can earn a pat on the back. Hopefully my ideas are entertaining at least.

I don't really see that the screen would be "floating" if the resistor fails. If the resistor is shorted (at zero ohms), I figure the screen voltage would be pretty much constant no matter how much current the plate draws.
If the resistor is open (not conducting, infinite resistance) it would be as if the screen is not there at all. IIRC, screen grids were invented to prevent RF oscillation, so that could be an issue if the Fluke is creating oscillation.
I'm not sure about impedance, your theory seems plausible. But if oscillation forced the plate to draw lots of current, I see how it could take out the resistor or even cause a screen meltdown, but in that case you'd expect plate voltage to be lower than normal.
 

Scooby9261

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Good question king Fan.

there are many ways to do it and personal preferences play a big role.


I like to make a one page word doc.

make paper copies.

I like to leave a blank for each tube and write in the tube type and brand.

number each pin

i think nomenclature helps.

3 plate
4 screen
6 grid

for example.

it is nice to have a place to write the voltage from the schematic next to the measured and recorded voltage for reference, maybe a place to also write down the % difference.

measure all pins to chassis ground
heater in AC
cathode, control grid, screen grid, plate are all DC.

calculate cathode idle bias current if there is a cathode resistor.

prepare to calculate plate current using output transformer primary resistance.

either before the amp is turned on, or after the amp is warmed up and turned off, measure the resistance of the OT primary. In push pull amps, measure resistance of the blue wire to the red center tap wire and also measure the brown wire to the red wire. Single ended is blue to red.

measure cathode resistance

with the amp idling, measure the DC voltage across the OT primary’s, either blue to red and brown to red, or just blue to red on single ended amps.

calculate currents using ohms law.

calculate power dissipation at the plate

calculate % of max plate dissipation.


I copied ten copies. When some new need arises, on a new sheet, I write the date and the reason or the current state of the amps.



some people like the spread sheet format and enter the values right into the spreadsheet on the computer.


I am old fashioned and prefer the hard copy ‘boiler plate’ method. View attachment 1012474
Thank you so very much. I am watching my printer make copies of the document i typed up with yours as the basis. Greatly appreciated.
 

andrewRneumann

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I would like to point out that many problems can be determined with an ohmmeter and the amp off. This minimizes the risk of shock, short, and further damage to an amp. In this case the problem could have been observed while reading resistance from screen grid pin to B+. Consider taking resistance readings before turning the amp on and taking any live voltage readings. Peace out.
 

andrewRneumann

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It has behaved in a similar manner as @john trials Fluke 23 on several push pull amplifiers.

So that would blow my "just so" theory out of the water. The problem manifests itself on different circuits with presumably different inductance. I also presume these amps did NOT have blown screen stoppers when you measured them? Also another knock on my theory. Thank you for sharing your experiences.

Usually when resistors fail, they are a symptom as something is drawing too much current through them. Rarely are they the lone problem.

Agreed. The 100Ω flame proof screen stoppers in the schematic look more like fuses than any sort of screen protection. I was examining a datasheet that said the maximum continuous dissipation on the EL84 screen was 2W. If that screen is dropping 310V at an estimated 4mA, then that's 1.24W just sitting at idle. We don't subject resistors to that kind of abuse and expect them to last. If those screens are driven hard, then screen current could peak up near 40mA (20mA RMS for a square wave) and that would be 6.2W... well above the 4W maximum peak value. (Enough to damage the screen, but still not enough to blow a 100Ω resistor.)

If the resistor is open (not conducting, infinite resistance) it would be as if the screen is not there at all. IIRC, screen grids were invented to prevent RF oscillation, so that could be an issue if the Fluke is creating oscillation.

I don't know. I was thinking a disconnected screen would behave similar to a floating control grid. Does it just match the voltage of the field between cathode and plate and stay there? I'm not so sure it does. (When we want to turn a pentode into a triode, we connnect the screen and plate... we don't leave the screen floating. There must be a reason for that.) I think it gets hit by electrons and takes on a charge of its own. Does this increase or decrease plate current? I probably need to get into some of those old textbooks on www.tubebooks.org to find an answer to these questions.

I'm not sure about impedance, your theory seems plausible. But if oscillation forced the plate to draw lots of current, I see how it could take out the resistor or even cause a screen meltdown, but in that case you'd expect plate voltage to be lower than normal.

I'm imagining the plate current staying relatively constant while the voltage swinging wildly from 0V to 2000V. When the plate is 0V and the screen is 310V--all the current is going to go through the screen, the screen fails short, the 100Ω FP blows open and there you go. (Or maybe the 100Ω FP blows open before the screen fails.)

Once again I have strayed into murky waters, but it is fun to toss ideas back and forth. Does the OP know if the tube is cooked or just the resistor?
 

john trials

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I am just posting these video links here for reference. My son’s Blues Junior wasn’t distorting, but did have an open screen resistor.



I plan on doing the changes that Psionic recommends. These are not tonal mods, but reliability mods. They seem like really good advice. I plan to order the parts this weekend and get the amp back to my son soon.



Hopefully these may help others.

Psionic recommends changing the screen grid resistors from (100 ohm 1/2W) to (1K ohm 2W). I can understand increasing the power capability, but what does changing the resistance do?

Is this good advice, or should I stay with 100 ohm?
 

Scooby9261

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I am not disagreeing with Psionic, but there is a gentleman names Bill M who spent over a decade coming up with Mods for this particular amp. I would go off what he says, because he specialized in this. He passed away recently but you can still find his info on the net. He is the one who started all these mod kits for Blues Jr. Psionic is recommending some parts i wouldnt get. Like get 1watt resistors instead of 1/4watt etc.


Psionic knows his stuff def, but id advise ya go off Bill M's stuff and i think theres others that would too.

-Walt B
 

john trials

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I remember BillM’s website. I wish it was still up and running. It was nice having that info in one convenient place.

never mind…I didn’t see the link to the archive!
 

peteb

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Did the tube fail first, taking out the screen resistor?

or did the screen fail on its own?

Still not good.

One of the EL84 tubes (nearest the BLUE wire) has slightly cooked paint label. To the touch, it is hotter than the EL84 nearest the BROWN wire (which confuses me, since the BROWN wire is the one going way up in voltage, not the BLUE wire).

2) I swapped the positions of the EL84s. BROWN wire is still misbehaving (907Vdc). The super high voltage problem didn't follow the EL84 swap.

it looks like the tubes were switched back and forth and the problem stayed with the brown wire tube socket, and it appears the tubes were not failing, while the screen grid resistor did.

John already noted that the tube in the brown socket was cooler to the touch than the tube in the blue wire socket, even though the brown tube had the incredibly high voltage on it.
 

King Fan

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Psionic recommends changing the screen grid resistors from (100 ohm 1/2W) to (1K ohm 2W). I can understand increasing the power capability, but what does changing the resistance do?
Is this good advice, or should I stay with 100 ohm?

A good read is Merlin on grid stoppers. It's short, and it's all good info, but you especially might skip down to the bit on “the other gain stages”, and oscillation, and then what he says about the screen grid:

“The screen-grid of a power pentode should also have a grid-stopper. This protects the screen from over-dissipation when the valve is overdriven, which causes the average screen-grid to draw more current. A screen-grid stopper limits the screen current to a manageable level, hopefully saving the screen grid from melting. This resistor should be at least 1k ohm, 2W or more. Some amps use 470 ohms, but I know from experience that this is not always enough.”
 

peteb

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I'm imagining the plate current staying relatively constant while the voltage swinging wildly from 0V to 2000V.

that is the right idea. The screen voltage is unaffected by the signal, because it is AC grounded, which keeps the plate current independent of the plate voltage, which swings low with signal.

When the plate is 0V and the screen is 310V--all the current is going to go through the screen,

this, I believe, is a mistaken perception.

true, the screen stays high while the plate swings low, however, it is physically impossible for all the current to flow through the screen. The screen is slight or skinny for a reason.


picture a cork dart board with metal wire dividing the sections. Every dart that avoids the metal wire and sticks in the cork, is like an electron drawn to the screen, missing it, and then running in to the plate, which is large, with no gaps.

only the darts that actually make contact with the wire, would be the electrons that are collecTed by the screen.


that is why consistently and predictably, only a small percentage of the tube current flows thru the screen.
 

john trials

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PeteB: I bought this amp used a few years ago. I have health problems, wasn’t feeling great the day I bought it, and therefore didn’t do a very good check of it before buying it. So I don’t know the history of it.

The amp sat at my house for a couple of years. My son has been using it for almost two years as-is. The amp sounded fine. The only reason I found this bad screen resistor is because I wanted to lower the bias current as many online techs recommend. I don’t know how it sounded so good without that screen grid resistor…and now that I replaced the resistor, it sounds the same to me…which also confuses me.

I soldered a good 100-ohm resistor across the open screen grid resistor as a quick fix (I will do a proper fix later, when I remove the circuit board).

Right now (with R51 lowered to 22K-ohm) bias for the EL84s are 20mA and 23mA. Both tubes (same ones that have been in the amp) are equally hot now. I wish I had a thermometer to measure them. Like I said, I am new to tube amps. i am not used to components getting this hot (I am mainly an analog synth guy).
 

Lynxtrap

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I can understand increasing the power capability, but what does changing the resistance do?

Is this good advice, or should I stay with 100 ohm?
The screen grid should be a bit more protected with higher resistance. In theory it might give you a little more compression or "touch sensitivity", but I think you'd have to play pretty loud to notice any difference in sound.
IMO 100R is too low.
 

andrewRneumann

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IMO 100R is too low.

Unless it's a fuse and not a stopper. If the goal is to have a fuse that protects the rest of the amp, I wouldn't up it 1W--I'd leave it the same wattage. I'd keep it the same and just run the whole screen circuit at a lower voltage... say 280V instead of 310V or whatever it's currently at. This would involved increasing the value of R47 and decreasing the value of R48. As a side effect, the bias current would come down when reducing the screen voltage with no change to the bias circuit.

IMHO lowering the bias current by lowering the bias voltage does not protect the screen during full/overdrive. In fact, more current may be drawn from the screen as the load line hits farther below the knee. It does reduce the idle screen current, which I have mentioned before, is starting out pretty high... so not completely ineffective as a screen protective measure.

Upping the 100Ω to a real current limiting value like 1K/2W would also protect the screen nicely, but it does change the dynamics of the amp as @Lynxtrap notes. It's definitely not theoretical--it's quite easy to hear screen sag. If you are cool with that, then that's another option and probably the safest option that I've seen proposed here.
 




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