# JJ EL84's Incompatible with Blues Jr?



## ampaholic (Sep 19, 2006)

I just completed the Bill M bias mod on a friends Blues Junior and am experiencing a weird problem.
After reassembling the amp I installed a brand new matched pair of JJ EL84's in place of the original Sovteks. To be safe I brought the amp up to power using a variac, checking voltages as I increased the AC line voltage.
Everything was perfect. Plate voltage 332 V. Screen voltage just a bit less. Grid voltage on both EL84's is now -12.5 versus the stock -10.9 which should cool down the power tubes slightly. When I checked bias current (transformer shunt method) however V4 was running at a comfortable 25 ma but V5 is running at 44ma!
I double checked all of my voltages, pulled the board back out to check and see if I could see anything, touched up solder joints, looked for potential shorts, put it back together and tried 3 different sets of JJ tubes. All doing the same thing- V4 is fine but V5 running way too hot.
Here's the weird part; if I put the original Sovteks back in both tubes are nicely idling away at 24/25 ma. I threw in another matched pair of Sovteks and they are running at 21/22 ma however when I put any of the JJ's back in the V5 current goes to 40 - 45 ma. Triple checked all of the voltages on each tube and everything seems to be right where it should be. The other weird thing is that the V5 tube does not exhibit any sign of red plating in spite of the high current draw (or at least what appears to be high current draw).
I've used lots of JJ's before and not had any problem but to be honest I much prefer the Sovteks for tone.
Am I losing my mind??


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## PaulS (Feb 27, 2006)

I use to have a Blue's Jr with the Bill M mods and JJ EL84's and had no problems.


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## ampaholic (Sep 19, 2006)

Yes, I have done the mod to other BJ's without issue, which makes this stranger still!
PS: I don't do the bias pot mod, just the higher value dropping resistor.


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## WCGill (Mar 27, 2009)

With 3 different sets of tubes? Very strange. If it's always the same socket, it's in the amp. If the same tube over-dissipates in the other socket, it's the tube. The fact that the Sovteks (yuk) idle at the same point makes this very tough to logically deduce. Did you try switching them also and measuring?


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## ampaholic (Sep 19, 2006)

I know! Doesn't make any sense. I've tried reversing the tubes and the problem stays with the V5 socket, hence it's the amp not the tubes. But- it is fine with Sovteks yet runs rampant with every JJ that I put in there.
Plus the voltages. Would a tube that is drawing excessive current not show some difference in voltages as compared to the good side? They are identical. Measured the resistance of the output transformer primary. 94 ohms on 1 side and 95 on the other.
I even suspected something crazy with my digital meter but I checked everything with an analog meter and the numbers are the same.
It doesn't take much to stump me on this stuff but I'm having a hard time figuring out where to go with this. Other than just sticking with the Sovteks I guess. But of course I'm worried that there is still some underlying problem.


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## WCGill (Mar 27, 2009)

Have you checked your screen resistor value? Do the Sovteks measure the same when you change their positions? This is beyond weird.


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## ampaholic (Sep 19, 2006)

I sure appreciate your help with this!
100 ohm screen resistors measure 98.9 ohms on each side.
Checking the bias on the Sovteks V5 measures 21.8 ma and V4 = 20.6 ma. If I pull the tubes and reverse them I get V5 now at 19.7 ma and V4 at 20.3 ma. (Should these numbers have stayed exactly the same as before but in reverse or is it normal that the numbers would change a bit?).
Plate voltage is 333 V on each tube, screen voltage is 313 V on V5 and 312 on V4. Grid voltage is -12.13 on V5 and -12.14 on V4.
All seems pretty normal doesn't it?


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## WCGill (Mar 27, 2009)

Your numbers will change a bit because the tubes are slightly different, the screen resistors are slightly different and the windings on the OPT vary. Missing something here but I'm not sure what. If it's in the amp, ie switching the JJ's causes no differences between V5 and V4 dissipations, then try reversing one connection at a time from your circuit to the output tube sockets and see if it changes anything. Grasping at straws here.


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## ampaholic (Sep 19, 2006)

Sorry, how would I go about reversing connections?
When I had pulled the circuit board out you have to disconnect the wires from the Power and Output transformer. They are colour coded plus I marked them but would it make any difference for example if I had the two brown wires from a power transformer secondary reversed? I'm assuming not.
I'm thinking of borrowing a friend's Blues Junior and see if I can make any kind of comparison between the two.
Totally at a loss here!


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## WCGill (Mar 27, 2009)

Sorry, I'm thinking about a hand-wired build. Don't even go there. My bad.


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## WCGill (Mar 27, 2009)

Measure the hot-running tube plate voltage. It should be lower than the colder-running tube. If not, then your meter might be giving you grief because of some unknown flux capacitance (?) that's throwing it off.


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## ampaholic (Sep 19, 2006)

Plate voltage on both tubes running identical at 336 V. I was always surprised too that the hot side wasn't red-plating running at 44ma and 336 Volts on the plate. That has to be well over 100% plate dissipation?
I've had the wonky meter thing in the back of my mind too and to be honest I'm using a cheap Canadian Tire meter while my good meter is in for warranty repair. That having been said I checked everything with a second cheap meter and it acted the same. (I've got to break down and by Fluke!).
I'm going to grab my buddy's Blues Junior and check it. If it acts the same (hopefully!) then I'll know that it is the test equipment.
Running out of other explanations!


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## WCGill (Mar 27, 2009)

Another way to measure dissipation is to install a 1 ohm (1% tolerance if possible) resistor between the cathode and ground. Measuring millivolts across the resistor gives you a direct ma reading. A 10 ohm resistor will also work so then divide your reading by 10.


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## ampaholic (Sep 19, 2006)

An excellent suggestion but if only this damn thing would be wired point-to-point! I'd somehow have to break the cathode ground connection that is there now. If it was my amp I'd cut the trace on the PCB but not on someone else's amp.
I think I've got to get a meter I can trust and try this again. I see a Fluke on Kijiji locally. Hmmm...


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## dtsaudio (Apr 15, 2009)

Another thought is that you may have an oscillation somewhere. While it should affect both power tubes, if the oscillation wave is asymmetrical it might offset the current in one tube. Do you have an oscilloscope?


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## ampaholic (Sep 19, 2006)

Unfortunately not, but I've been keeping an eye out for one (after I get a good meter!).
Interesting that you mention an oscillation. Last night I checked the bias 4 different times with the exact same set of tubes and every time I get completely different readings.
What started out as a simple tube change and bias check is turning into a nightmare. This amp is still under warranty so my suggestion might be that he take it to an authorized Fender repair shop that is (hopefully) more knowledgeable than me!


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## Emohawk (Feb 3, 2006)

Is it possible that maybe the pin spacing on the JJ tubes are slightly out of alignment with the V5 socket, and maybe one of the pins isn't making a clean good connection, or is shorting or something like that? I've seen this with pre-amp tubes.


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## ampaholic (Sep 19, 2006)

Thanks for hanging in on this one guys!
Another good suggestion however I have previously popped out the little board that the tube sockets are mounted to and checked all of the solder joints, inspected the pins and made sure they are well tensioned. The amp is only a year or so old and the pins are sparkling clean.
Plus, I have 5 brand new matched pairs of EL84's and have tried them all numerous times (granted 4 of them are a matched quad that would have very similar characteristics). The only ones that are stable at all is the original Sovteks.
Crazy!!


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## sscups (Aug 21, 2010)

I have the billm mods and Bill emailed me back when I asked questions. Definitely email him. 

He has a youtube channel, just a few vids. Here's one on the pro jr bias setting.
http://www.youtube.com/watch?v=KzetPGnmcXQ

He has a whole website that should help also, www.billmaudio.com

Good luck with your amp, I'm sure it will get squared around.


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## ampaholic (Sep 19, 2006)

dtsaudio said:


> Another thought is that you may have an oscillation somewhere.


BINGO!
I found the problem. I'm certainly pleased to have found it but a little bit sheepish that it took me so long.
If you look at the photo below there are two ribbons that connect the output tube pins to the main board. The blue wire that is running across the 2 ribbons on the left is the connection from the output transformer primary to the plate of V4. The ribbons were curved up so that this wire was laying right on top. It might have been that way all along or more likely that the ribbon got moved slightly out of place when I tipped the board down to do the bias mod.
Regardless I tucked the ribbons down out of the way, fired things up and with 2 brand new JJ EL84's in place and the bias is rock solid at 24 ma in V5 and 23.5 in V4. Plate voltage increased slightly to 340 Volts (understandable now that the current is lower and under control). Screens measure 320 Volts and the grid voltage is -12.44 Volts.
The odd thing is that the oscillation was created in V5 - the opposite tube!
Anyway, I think I'm good to go and truly appreciate everyone's help and advice. I hope I didn't waste your time. As always, I learned a lot on this one!


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## WCGill (Mar 27, 2009)

I didn't think these amps had such problems. Good call on the oscillation DTS, you can come and work in my shop anytime. I guess Sovteks are impervious to oscillations. Go figure.


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## ampaholic (Sep 19, 2006)

WCGill said:


> I guess Sovteks are impervious to oscillations. Go figure.


Yes, you have to scratch your head on that one. I'm thinking that it wasn't so much the brand of the tube but perhaps the tube characteristics. All of my JJ tubes came from 2 sets of matched quads with nearly identical ratings (one set labeled 5.3/21 and the other set labeled 5.4/21). I'm thinking the parameters of the Sovteks might have been the difference.


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## nonreverb (Sep 19, 2006)

Funny, I'm just reading this thread for the first time...I had a BJr with the same problem after I did the mods as well and yes, it was exactly the same fix.


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