Why is one el84 running much hotter in my Blues Jr.

[rosssurf]

I am learning to work on and repair to amps. I'm mostly a paint-by-numbers kind of guy, but I am graduadually leaning how circuit design variables effect an amps sound. I am aware of the safety precautions that are necessary and I'm learning more each day.

A friend brought me his Blues Junior 3 after it had stopped working. Upon inspection I see it blew a fuse inside because one of the el84's went bad. After replacing the fuse and putting in a couple of random el84 is I had knocking around. The amp seemed to be fine, but one of the sockets tended to get that tube very hot. I figured perhaps those 84's were not matched closely at all so I ordered a new set of matched el84 s. After putting this new matched set of el84 in, I'm still noticing that one of the tube sockets makes its respective 84 much hotter than the other one. I swapped the tubes to different sockets and the problem stays within one of the sockets and does NOT follow the tube. Although the amp now seems to be fine, I'm concerned that this failure will simply repeat itself until I find out why one of the power sockets is getting so much hotter. I am aware that many of this series of amps by Fender are often biased very hot. I plan to address that later by adding a pot in place of R51 and R52 for adjustable bias. As for now my concern is simply this mismatch of heat in the sockets.

Can anyone steer me toward which component I need to check to fix this overheating issue?

[Pedro Vecino]

What bias measurement system do you use? The Blues junior is known for the tendency towards oscillation on one side when the connections of the plates are extended outwards.
To avoid this, simply unplug the 12AX7 that corresponds to the phase splitter (the one next to it).

More info, here:

https://music-electronics-forum.com/...ad.php?t=47663

[Randall]

https://www.manualslib.com/manual/86...page=11#manual

Lets check your bias voltages. Measure from pin 2 to ground on each socket, with tubes out. They should be pretty close, if not you can measure across R33/34 (1.5K) and R31/32 (220K) in circuit, amp off of course. They won't be exact, becuase they are in circuit, but they should be pretty close to each other.

[Leo_Gnardo]

What bias measurement system do you use? The Blues junior is known for the tendency towards oscillation on one side when the connections of the plates are extended outwards.
To avoid this, simply unplug the 12AX7 that corresponds to the phase splitter (the one next to it).

If removing the drive tube as Pedro suggested, settles the bias currents in each output tube to some reasonable level, that's an indication that ultrasonic oscillation is indeed the culprit. Here's where having an oscilloscope would be very handy. You might be able to push the wires between the main and tube boards into a position where there's no ultrasonic pests, and you could see that on the scope.

FWIW Bill M mods website offers as part of their mod kit, a 100 pF cap to install parallel to one of the plate resistors on the drive tube. Of course you don't have to buy the cap from them, any 100 pF cap with a sufficient voltage rating, say 500V, will do perfectly well. Which plate resistor? Hmmm, I forgot. You'll have to snoop the Bill M site to find out.

OR use my method, which clobbers ultrasonic oscillations just as easy. A 470 pF disc cap, 1000V, across the plate terminals of the drive tube socket, pin 1 to pin 6. Do that, and if your bias current mismatch persists, it's the tubes themselves way out of match. I'm not as famous as Bill M, so my method isn't as well known. Theoretically this would muffle the amp's high frequencies, but my customers & I have never noticed any such effect. Either we're old and deef, or those frequencies are way up in the zone where bats and dogs might be entertained by them. Or both.

[drewl]

They run the EL84's very hot in these amps.

After replacing blown output tubes I install a bias adjustment pot to set the level where it should be.

That Bill M site will show you how.

And oscillation from the right EL84 is common when trying to measure bias as mentioned.
Pull phase inverter and it will be stable.

[Enzo]

So perhaps the problem is not that one tube is so much hotter than the other, and maybe the hot one is normal for the amp, and the other tube is running colder than usual? MAke sure both tubes have B+ on their screens.

[Leo_Gnardo]

They run the EL84's very hot in these amps.

After replacing blown output tubes I install a bias adjustment pot to set the level where it should be.

Definitely a good idea, I also do this.

That Bill M site will show you how.

Well, sort of. Last time I looked, the Bill M method allowed bias voltage to be adjusted all the way down to zero volts. I'm not too fond of that. Plus they sell you a 10 turn pot. Way too fiddly, and overkill for the purpose. Bill M has lots of good ideas. This isn't one of them.

What I do, is insert a 10k trimpot set up as a rheostat - variable resistor - between ground and the resistor in the bias supply normally connected to ground. 5k is enough in fact. I often wind up with a bias voltage around -12 and 26 to 29 mA bias current. Instead of the 40 mA often found in Blues Juniors. Same solution in a Pro Junior. Bourns single turn "blue cube" trimpot, Mouser has 'em, $1.60.

652-3386W-1-103LF Mouser part # for the 10K trimmer. Change the last figures to 502LF for the 5K part.

And oscillation from the right EL84 is common when trying to measure bias as mentioned.

Also during regular operation, it's unfortunately one of the amp's common problems.

[drewl]

Aside from pulling the phase inverter, if you twist the tranny leads that also kills oscillation so you don't even have to remove the PI tube to measure without inducing oscillation.

[Leo_Gnardo]

Aside from pulling the phase inverter, if you twist the tranny leads that also kills oscillation so you don't even have to remove the PI tube to measure without inducing oscillation.

I'll have to try that next time I run across the problem.

[drewl]

I find that twisting the different PT wires together and running them neatly where they have to go cuts down on hum and noise also.

Plus it looks so much nicer!

All part of normal service of these amps, checking & reflowing pc boards, especially the tube socket connections & adding bias pot

[Helmholtz]

Twisting OT primary plate wires is one of the most effective measures to prevent parasitic oscillation. After all the plates carry signal voltages of several hundred volts and a tiny parasitic capacitance suffices to produce a positive feedback loop. Twisting the wires with opposite phase signals compensates/reduces emitted fields.

[Leo_Gnardo]

Twisting OT primary plate wires is one of the most effective measures to prevent parasitic oscillation. After all the plates carry signal voltages of several hundred volts and a tiny parasitic capacitance suffices to produce a positive feedback loop. Twisting the wires with opposite phase signals compensates/reduces emitted fields.

One would think, putting those leads in such close proximity adds capacitance with respect to each other. OTOH it would minimize radiated electromagnetic field from both to other circuitry in their proximity. In any case, I'm itching to try it in a misbehaving Blues Junior or Pro Junior. Or any other amp for that matter.

[Helmholtz]

One would think, putting those leads in such close proximity adds capacitance with respect to each other. OTOH it would minimize radiated electromagnetic field from both to other circuitry in their proximity. In any case, I'm itching to try it in a misbehaving Blues Junior or Pro Junior. Or any other amp for that matter.

It actually adds something like 10pF across the primary. But this just adds to the 100pF to 500pF primary self-capacitance of the OT. These are numbers from real measurements.
So its effect on upper frequency limit and phase lag should be negligible.

As with twisted plate wires the electric field is confined between the wires there is much less unwanted coupling to other circuit points.

[The Dude]

I've also had Blues Juniors where the ribbon cables to the socket board are too close to the OT wires. Carefully bending them away from each other will often cure oscillation and hum.

[Mick Bailey]

I've had Blues Juniors where even hand proximity will cause oscillation. The 100pf cap in parallel with R30 (cream board models) is a good and certain fix, though I don't know what the resistor designation is on the later models. I've had a few amps where one tube pulls more current and the capacitor has fixed those where oscillation has been a problem. There are thousands of these amps and I'm sure most are fine, so it's difficult to generalize and say any particular fix will magically cure all ills, as there are many different causes for the same symptom.