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

john trials

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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:
 

Scooby9261

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Hmmm... seems like something is out of spec or on its way out if those numbers are correct. The best thing to do is take out the 22k R52 I think and place with a 27k or 32k. That will cool the bias and it may or may not solve the higher reading your getting. But for a quick mod, id try this and then check what things look like after. Ive worked on and modded a bunch of Blues Jrs and the bias is waaaay too high even for EL34's. Luckily its a quick fix.
 

Scooby9261

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The caps are also sh**ty IC caps. Order some new F&T caps and replace them. Your amp is about as old as those IC caps will last. Ive seen those caps go out on 3 year old Blues Jr's.

Fromel makes a recap kit and a deluxe kit that has the bias resistor as well as the caps and some other parts. But if you source those parts out, they will be much cheaper. Their is directions to show you how to do all of this on the net and its really not that hard as long as your good with a soldering iron. :)

My Blues Jr i always liked but with doing the mods i like it a lot more ;-) If you dont like anything about he Blues Jr stock, dont do the mods imho. ;-)
 

tubedude

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If your tubes didn't become bottle rockets I suspect a measurement error. Does your cathode current change?
Does it sound good? If so that rules out an oscillation related measurement error.
I'd take Scoobys advice on the cap replacements. That's needed anyway. Where are you grounding the meter for the plate measurement?
Try disconnecting D12 and re-measure.
 
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Scooby9261

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Even so i still think he needs to recap that amp. That will be the next issue down the road if its ignored and the bias needs to be cooled or else you will be spending $ on extra output tubes often. These imho are great little amps, but this is a going on ten year old amp. Very lucky you have gotten 10 years without a fail yet. But hey thats just like... my opinion... man... :)
 

corliss1

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10 years is no big deal - I start pushing for recaps at 15. Blues Juniors do run hot though, so it's a good idea to at least be aware of where the bias is.
 

john trials

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Thanks for the replies.

I have been using the chassis as my ground reference for my measurements.

I agree that 1300 Vdc sounds impossible, but I watch the normal voltage of 330 Vdc on the brown wire for about five seconds after turning the amp on, and then it ramps up to over 1300 Vdc.

I repeated that observation multiple times and it’s repeatable. I have an EE degree, and am mostly familiar with analog synthesizers (much lower voltages than tube amps), so I am not entering into totally foreign territory here.

I am not saying that I am perfect. I definitely could have made a mistake, so I will repeat the measurement tomorrow.

Is it useful to measure the AC voltages (ripple) at the filter caps to see if they are bad?
 

Scooby9261

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The caps in these are notoriously bad, as is all of the entire HOT ROD series. Im not kidding, as i said above ive seen these IC caps go in under 3 years in Blues Jrs, Hot Rod devilles, Blues Deluxe etc. Ive worked on a LOT of these amps and modded them. If hes already working on the amp and is going to do the bias mod, spend another 30 mins or so and recap it with F&Ts. If you do all this now, the amp wont need anything done for a LONG time. Just going off experience with these. Also they have a 5watt touching the board and after 3-4 years ive seen boards burn up from this. When you put the new one in make sure you dont have it touching the board. Look into Hot Rod Deluxe capacitor issues haha. Its one of the #1 problems these amps are known to have. That is all ;-)
 
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john trials

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Also, is there any value in making the DC voltage measurements with the output tubes removed, or is that not a smart idea?
 

Burning Fingers

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Also, is there any value in making the DC voltage measurements with the output tubes removed, or is that not a smart idea?
Measuring voltages across the OT with no power tubes installed is fine.

What is not ok is to have the speaker disconnected with the power tubes in and the amp turned on...that can cause high voltages to develop in the OT and lead to the OT failing and killing the output tubes.
 

ElPositivo

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Since you say the amp sounds fine this is most likely an error in measurement. I’ve had similar things in the past and realized either my dmm was malfunctioning or the battery was empty. Try doing the measurements with a different multimeter, with different leads and with new battery first.

In case you get the same results I wonder what the other voltages on the tubes are doing. Does the same thing happen on the screens? And what if you change tubes around in the sockets? Does it stay on the same side?

But first things first: try a different dmm. And after that: like scooby said change the filter caps as many blues jrs have these going bad sooner or later.
 
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Scooby9261

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I think his prob only goes to 500 and thats the issue. If ya got a Fluke meter, ^ def as bebop says then you shouldnt have an issue measuring. :)

I always have my meter and my fluke and i check voltages often with both and i write everything down in a notebook and make notes etc. This way I dont forget where I am, and when i ask a question myself on here I always write down stuff. Theres a lot of smart folks on here who know a thing or two ;-)

-Walt B
 

john trials

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Okay...some more data.

I use a Fluke 23 multimeter, which is listed as 1000 Vdc max, but it did not get fried when I measured over 1300 Vdc. Very strange. This has been my faithful VOM for 40 years, and I'd hate to fry it.

1) Today, for a test, I changed the output tube bias (which was my original plan in the first place, before I saw these strange voltages). I added a 68K resistor in parallel with R51 (33K). Result: 22K

C+ was -10.3 Vdc stock and is now -13 Vdc.

I made measurements with this changed C- voltage.
B+ 328 Vdc
X 248 Vdc
Y 274 Vdc
Z 313 Vdc

Now the BROWN wire only goes to 850 Vdc, not 1300 Vdc (like yesterday, with stock bias). BLUE is at 379 Vdc and RED is 333 Vdc.

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.

3) I don't know if this information is any help, but the B+ voltage drops to less than 15 Vdc in about ten seconds after turning the amp off. Is this faster than a normally operating amp? Does this support the theory that the filter caps are bad?

This is my son's amp, and I was planning on doing the "cooler bias mod" while he is on vacation. He will stop by my house tomorrow on his way home from vacation, and I will update him on what I have discovered. I think he will agree to have me replace the filter caps, and probably new EL84s, since one is slightly cooked. I will probably do some other maintenance upgrades as well (tips from Psionic's YouTube videos about Blues Juniors, and how to make them more robust).

Do you guys still think that this super high voltage will be fixed with just the filter caps replaced? I am concerned about replacing the filter caps, and installing new EL84s, and cooking them (wasting time and money).
 

Scooby9261

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Replace the caps, including the 22uf 25v at C1 and C4, cool the bias and get some new tubes. Also their is a white 5w 390ohm that needs to be replaced. As it is now that 5w is actually touching the board and the heat will eventually burn the board and kill the traces. Youll need a new board if this happens. When you install the new one make sure you have it off the board a bit so air flow can move the heat and it will NEVER burn the board. I really think the issues with this amp are just the normal issues that most of these Blues Jrs experience. Sometimes the amps **** the bed within 3 years, sometimes they will last 8-10years. But a majority never last more than 3-5 years. I know this because i see it. Just google like you have and see all issues these amps have in videos, message forums etc. When you do these mods/repairs the amp will sound better than it did new.

Message me if ya ever want to ask me about anything. I love working on these amps, because i love taking the potential this amp had and turning it into something it has ;-). These amps could be very nice amps straight from Fender, but nope. Fender wants to limit their cheaper gear so you will buy one of their more expensive amps down the line. These are just my observations and opinions but for the little bit itll cost to do this, its def more than worth it. As long as your good with a soldering iron, the nice thing is how well everything on the board is marked as well. Makes this easy imho.

Hope this helps and when looking into tubes, look into the 6p14p-k matched Reflektor tubes. They are awesome and cost much less than standard EL34s. After you cool the bias you are opening the doors to so many more tube options. This thing stock will eat EL84s like crazy, only new EL84s that dont burn up as fast are the JJ's. But i dont think they sound as good as the 6p14p-ks and those tubes are military spec and the -k in the name is because they were designed for lots of vibration. This is the type of tube you want in a small tube combo amp like this.
 

peteb

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Is it useful to measure the AC voltages (ripple) at the filter caps to see if they are bad?

sure.

Also, is there any value in making the DC voltage measurements with the output tubes removed, or is that not a smart idea?

I was going to suggest that, just in case the tube is really seeing high viltage.

get the voltage right, then add the tubes.


reading this, I can’t tell if the high voltage is transitory (better) or the voltage stays high (worse).
 

andrewRneumann

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What have we learned so far? I've learned that no one has a reasonable explanation on the amp side for your strange voltage readings. One would think if it was your meter, you would see the same bogus readings on both sides of the OT. If it was the amp, one would think the EL84 would be arcing and/or you'd have tons of noise/hum/poor audio performance. I don't know what one should think.

If you measure Vac at all the B+ nodes, you should get a good reading with your true rms meter of the ripple voltages. If you don't have much Vac there, you can be pretty confident the caps are still working.
 

john trials

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With no EL84s in the amp, the voltages at the OT primary wires are rock solid (red 336 Vdc, blue 336Vdc, and brown 337Vdc). The voltages never change (brown never ramps up to 900 volts).

Ripple on the B+ is only 0.2VAC...I don't know if this is high, but I plan on replacing the filter caps anyway.

Rapid B+ voltage discharge: could this be due to a bad EL84?
---If I turn the amp on and then off within a few seconds (before the BROWN wire has a chance to ramp up to 900 volts), the discharge of B+ is extremely slow (almost no discharge).
---If I turn the amp on and let it sit for 15 seconds (so the BROWN wire ramps up to 900 volts), then shut the amp off, the B+ discharges to less than 15 volts in about ten seconds.

How long should the B+ voltage discharge without assistance (like a discharge cap, other manual means) in a normally working amp? Seconds? Minutes? Hours?
 




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