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

andrewRneumann

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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).

Heh.... yeah they get hot. Just the heater itself is putting out 5W of heat. The plate is dissipating about 8W and the screen another 1W or so. So that's equivalent of two 14W incandescent bulbs burning inside the amp. Going to get hot!
 

Scooby9261

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After the other parts are replaced and you have bias set using a 27k or even cooler 33k in the bias you wont be having issues with your power tubes burning up. As i said before im using 27k in mine. Some people add a bias pot, but i have not had an issue with the bias on any of the amps ive worked on (that i know of) or my own amp simply installing a different resistor. But yeah tubes are gonna put out heat, some heat is alright, too much be bad. :)
 

ElPositivo

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@Scooby9261 just out of curiosity: how many mA are your tubes biased at with R52 @ 27K and at what plate voltage? How big was the difference in mA between the JJ EL84’s vs 6p14p’s with R52 at 27K? Im curious if the 6p14p is drawing more current than other tubes?
 

Lynxtrap

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Unless it's a fuse and not a stopper.

Bad design either way, if you ask me.
Without getting into a "resistors as fuses" debate, it should be said that resistance should not be increased without upping the resistor wattage.

Personally I see 1.5k at >2W as minimum for EL84's. That will in itself protect the tube, and you might like the change in sound and response of the amp.
 

peteb

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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).

thanks for the full disclosure John.

the amp looks to be running properly.



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 believe one power tube is going to sound a lot like two power tubes.
 

peteb

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I bought this amp used a few years ago.
Both tubes (same ones that have been in the amp)

there is more to this than I thought.

we still don’t know if a tube failure caused the screen to fail.

we don’t know when the screen failed.

what are the power tubes?

groove tube?


Both tubes (same ones that have been in the amp) are equally hot now. I wish I had a thermometer to measure them.

I think if they are too hot to hold and not red plating, they are probably fine.
 

john trials

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PeteB: The EL84s are JJ brand. V1-3 are Groove Tube (V1 & V2 are 12AX7, and V3 is an ECC83).

Also, I figured (maybe incorrectly) that the open screen resistor meant that one EL84 probably wasn't working (or not operating very well, if at all). Wouldn't the output audio be like a half-rectified signal? I would think that would sound drastically different then when both EL84s are operating properly.
 

john trials

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I just ordered the parts! All new filter caps, upgraded rectifier diodes (1N4007), and the screen grid resistors. Hopefully this will make the amp more robust, and eliminate any future problems.
 

corliss1

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Usually when I encounter a failed screen in a Blues Junior it's because the tube died and took it out. The amp sounds alright, but distorts much earlier than normal. Unless you were bringing up the volume to something other than bedroom levels, you might not notice.
 

peteb

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PeteB: The EL84s are JJ brand. V1-3 are Groove Tube (V1 & V2 are 12AX7, and V3 is an ECC83).

likely replacements. It looks like Corliss nailed it.

Also, I figured (maybe incorrectly) that the open screen resistor meant that one EL84 probably wasn't working (or not operating very well, if at all). Wouldn't the output audio be like a half-rectified signal? I would think that would sound drastically different then when both EL84s are operating properly.

at low volumes the single power tube amplifies the entire signal, with no cut off, which would imply classAB.

at low volumes it may or may not be class A, but it is single ended for sure.
 

B.J.TELE

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I have a totally stock Blues Junior 3. I was going to check the output tube bias, and do the "bias cooling mod" that I have seen a few techs illustrate on YouTube. I was probing voltages in the amp, and most look fine, except the voltage on one EL84 plate.

Voltage on one plate is 1323 Vdc. No, I am not missing a decimal point.

The amp works fine. There is a very low background hum from the speaker, but not much at all. Both output tubes (new, matched JJs two years ago) look the same...no lightning or overly bright glowing in either of them.

Other voltages:
B+ 315 Vdc
Z 295 Vdc
Y 258 Vdc
X 235 Vdc

Build date on the amp is 2013, and the electrolytic caps and everything else inside looks immaculate, like brand new. I've had it for two years, and it seems to works fine. The only reason I found this high plate voltage problem is because I decided to check the bias on the El84s.

The plate voltage that I am measuring is at the BROWN wire from the output transformer. When the amp is first turned on, the voltages on the OT wires (RED, BLUE and BROWN) are all near 330 Vdc initially, for about five to ten seconds. Then the BROWN voltage ramps up to over 1300 Vdc in just a few seconds. BLUE wire is around 314 Vdc and changes to 420 Vdc, and RED is around 307 Vdc. This voltage ramp-up is repeatable every time I turn the amp off, then back on.

When I measure the relative DC voltage between OT wires, RED-BLUE is oscillating, so I cannot measure it with my VOM (digital Fluke...the voltage never settles to anything that the VOM can read).

Any tips for what I should check? I am baffled, since I cannot figure out what would pump the voltage up so high, when the power section appears to be operating fine. And baffled that I can still play guitar through the amp. It sounds fine.

Any help would be greatly appreciated.
John

Schematic:
I had a similar model year Blues Junior. A friend wanted to try some russian industrial grade EL-84s in my amp because he didn't like the way they sounded in his. Put them in and played them about 15 min. After he took his out and went home I put mine back in and one of them instantly red plated! I switched them and the other one did the same thing in the same socket. Long story short after talking to techs, the general consensus was a bias issue called runaway bias. Those tubes of his knocked the bias for a loop evidently. Blues Juniors are known for having loose tube sockets as well. Hope this might lead you to find your Blues Junior gremlin. Good luck!
 

Wharfcreek

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Well, I know I'm coming in on this really late. Seems the basic problem has at least been identified, but 'WOW'.... what a process! I gotta say, going back through this whole thread, I think there are a few things that can maybe become 'lessons for the future'. For one thing, an amp with a B+ voltage of 328 (per the schematic), is not going to be able to spontaneously produce 1000+ volts! That's GOT to be a measurement error. And, in today's IC related world, it's no surprise to me that even the venerable Fluke meter could experience such a 'failure'. For another, the fact that the amp was working is not necessarily indicative of 'not' having a significant problem. Push-Pull tube amps will operate 'reasonably' well even when working on just the 'pull' or 'push' side of the transformer. And, if you have no basis for comparing a loss of performance with past 'good' performance (I think the OP mentioned having purchased this amp 'used' and not really aware of it's past?), you can run an amp like this on just one output tube or the other for years without ever really realizing you've got a problem, particularly if the amp design provides proper bias even with one side not working.

Just as a 'future' consideration to the OP, if you're world was low-voltage silicon devices and you're now venturing into the 'tube' world, perhaps an investment into a couple of 'bias probes' would be something to think about! This may be 'old fashioned' in some people's way of looking at it, but let's just say you'd plugged in those bias probes and checked current draw on both tubes initially; I think with the screen resistor 'open', the bias draw on that tube (regardless of what you were measuring as plate voltage) would have been WAY low!! So, that 'might' have made you check not just plate voltage, but screen voltage as well. Pretty common practice with tubes amps is to first validate B+ voltage, then validate screen voltage with tubes OUT of the socket. Likewise the Screen voltage, and if a 'fixed bias' amp, checking for a 'negative' voltage at the input grid is also a good practice. These may be off spec as related to 'operating' voltage levels, BUT, if the PS supply is working as it should and these readings are consistent from side to side, then that's a good start. At that point installing tubes, checking individual bias, and checking plate and screen voltages with tubes installed is a typical 'next step'. Generally speaking, if you have proper tube bias readings, proper plate and screen voltages, and 'no' output, you've got a problem elsewhere than in the 'power amp' section itself (phase inverter not included). This is true for standard pentode, Ultra-linear, and even a 'triode' type tube amp.

Just to complete my commentary, I've gotten so anal about bias that I now install a 10 ohm resistor off the cathodes of the power tubes on virtually every amp I build. I also install 'test point' jacks somewhere on the chassis. This permits a quick 'check' of bias on each tube, as well as an easy place to measure the affects of any 'adjustments' one makes to bias elsewhere in the amp circuit. A measurement of .35 volts across the 10 ohm resistor is a 35ma current draw. EAsy!!

Anyway, glad you found the problem......and good luck with your future 'tube' amp tweaking! They're a LOT of fun and it can get addicting, particularly once you build one from scratch...........

Tom D.
 

john trials

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Amp mods completed. What started as a simple “cool the EL84 bias current” turned into a longer project than I expected, but an interesting intro to tube amps for me.

I replaced the rectifier diodes with 1N4007, new filter caps (Merse), 1K ohm 2W screen grid resistors, and a 50K trimpot/47K resistor in parallel with R51 (33K) to allow precise adjustment of the EL84 bias current. I also did the BillM mid pot mod. I didn’t want to do any major tone mods, but just make the amp more robust (and hopefully trouble-free for a long time).

Unique “ending” to fixing the amp (funny in a sitcom, maybe?): after lunch on Friday, after I finished the mods to my son’s amp in the morning, I blacked out and had a near-death “I’ve fallen and I can’t get up” episode. While lying on the floor unable to get up (alongside lots of various bodily fluids), wondering how to survive this (my wife was away for the weekend), I began thinking about loose ends in my life. Since I didn’t put the rear cover on my son‘s amp, and the amp was still open on my workbench, he’d never know if I finished fixing it. I know that’s not the most important concern, but it still bugged me a bit (I had lots of time to think about stuff).

Looooooong story short: I got rescued a few hours later by a neighbor who called an ambulance. I am slowly recovering.

Go hug a loved one before it’s too late!
 

Wharfcreek

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WoW! Dare I ask if this was related to a 'shock' from the amp, or 'other'? Glad you're in recovery mode! I'm 70 and that crap scares the hell out of me! I had a similar experience taking a 600 volt poke out of an amp. Knocked me backwards off my work stool and into a motorcycle I was also working on in my shop. As I lay on the floor watching the motorcycle rocking back and forth on the jack I was wondering which was going to kill me; heart failure from the shock or being crushed by the bike as it fell off the jack? Fortunately neither happened, but I did get more careful with the amp repair.... Hope you recover soon!!
 

ElPositivo

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Looooooong story short: I got rescued a few hours later by a neighbor who called an ambulance. I am slowly recovering.
Wow happy to hear your neighbor realized what happened and called an ambulance. A few hours can be a long time.
Go hug a loved one before it’s too late!
This is very true and it’s something everyone tends to forget until it’s too late.

Wish you all the best @john trials !!
 

john trials

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Not due to an electrical shock. That might have been worse (I cannot imagine the emotional pain for my son, if I died trying to fix his amp).
 




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