Blues junior plate voltage issue

[Colas LeGrippa]

Hola compañeros,


I have a blues junior reissue on my bench.
One of the 2 EL84 shows 0V on its plate. The screen grid is ok at 300V. The other tube 's plate and screen are fine. When I pull the  tube showing 0 V on its plate,  I get regular voltage at the corresponding socket, plate and screen. I get back to zero V on the plate when I put back the EL 84, or any other one. Seems like the output transformer can't face no load, on one side. I plugged directly the blue lead from the OT to the plate of the el84
 Same result. While the tube is heating I get 324V, then it drops to 0 when its hot.
I don t see nothing but a bad OT.  But I may be wrong
 What do you think ?

[Colas LeGrippa]

Picture

[Colas LeGrippa]

Othernpic

[Colas LeGrippa]

Lookingat the schematics, we can clearly see two diodes to gnd marked tp18 and tp19 ( test point ) that indicate 29vac......these diodes are connected to theOT plate supply...what are they for ?

[Colas LeGrippa]

Heres a pic

[sluckey]

Flyback protection diodes. In case you play a set with the speaker disconnected.

[Colas LeGrippa]

OMG playing a set with the speaker disconnected.....can one guitarist be drunk a this point ?  I remembet my first dtummer throwing drum sticks at me because I was playing   "play that funky music" half tone up from the other musicians......


Ok  Sluckey now that I got you, I wont let you go before you tell me about this f......( fender )
amplifier. What would cause a voltage drop to zero on a plate of a single power tube once this tube is hot  ? 


Regards

[tubeswell]

What's the bias voltage on that tube socket?

[Colas LeGrippa]

According to the schem, minus 10 V, on both tubes

[Colas LeGrippa]

Here's a pic of  bias

[pdf64]

According to the schem, minus 10 V, on both tubes

What about the voltage on your amp?
The amp in the schematic is working right, whereas your’s perhaps isn’t. 

What is the anode or cathode current of the affected valve?
ie when the amp is powered up and the problem anode is measuring 0V, what current flow do you measure?

[JB]

Are you sure it’s 0V? I’ve had issues with blues juniors that don’t like meters on their EL84 anodes. Maybe it’s oscillating and giving you a false reading. Maybe check with another meter?

[pdf64]

Removing the valve in the phase splitter socket would eliminate oscillation.
That’s the only condition in which I’ll probe an output valve anode.
Otherwise, to me, it’s akin to asking for trouble.

[Colas LeGrippa]

Ok, sorry I said it bad..


1- On the amp, I have measured -10 V  bias on both tubes while the issue is going on


2- The plate of the tube on the faulty side is not 0 V.  When the amp is cold, my meter shows the right plate voltage, 2-3 minutes later I can hear a high pitch sound when my meters probe touches the plate's solder, while the.reading disappears from my meter but there is no OV indicated, the meter seems like shaky with no digit marked


I think this explanation is clearer


The oscillation theory makes sense ! I ll remove the PI and take measurements when I get back to my shop later on today


Thanxxxx


 

[shooter]

did you swap PA tubes to see if the problem follos the tube or stays put?, i didn't read everything
those amps had so many sucky solder joints, bad/cheap parts I quit even looking inside.

[Colas LeGrippa]

No I did not swap rhe preamp tubes. I swapped the PI tube wirh same issue
 
I think theres an oscillation problem affecting the first power tube


I keep on making tests


Thanks

[shooter]

sorry, meant PA tubes, that's a quick n dirty when one side ain't playing

[sluckey]

sorry, meant PA tubes,

And that's exactly what you said.

shooter 1, colas 0   

[shooter]

 
getting out my cheerleader uniform....with cleats!

[uki]

I get back to zero V on the plate when I put back the EL 84, or any other one.

did you swap PA tubes to see if the problem follos the tube or stays put?,

Looks like he sort of have done that, with different tubes,

those amps had so many sucky solder joints, bad/cheap parts I quit even looking inside.

Maybe a bad solder in the socket, once tube is in it lost connection...

[Colas LeGrippa]

Finally got rid of the problem. I had to hookup a 280 pF ceramic cap accross the PI 100k.plate resistor in order to get rid of the oscillation. Thanks to JB for the hint.


Final score: JB 1, Sluckey 0 ....

[Colas LeGrippa]

Btw can you see it ????

[tubeswell]

If you had to slug the PI load resistor with a cap to get rid of the oscillation, it could be the resistor (bad resistor lead) or a dodgy PCB trace/pad for that resistor (or a related number wire on the same circuit).

[PRR]

If you had to slug the PI load resistor with a cap to get rid of the oscillation,...

Many NFB amps need a hundreds-pFd cap in the driver.

Guitar amps often not, because the NFB is small.

280pFd on the presumed 39k node impedance is 15KHz, little harm to guitar.

It did leave the factory without it, so I wonder what's gone wrong. Or is it just the folly of probing a power tube plate without a proper DC probe?

[Colas LeGrippa]

I thought these amps were naturally bad designed and this was the only solution to get rid of the oscillation, so I only cured the problem partially then

[Colas LeGrippa]

PRR, my probe was working on one tube but not on the other. More, the amp was not sounding right so when JB talked about oscillation I knew he had pointed out the issue.


Maybe the amp had left the factory as is. It was working after all and I m pretty sure no guitarist tested it before it was put  in a van to the warehouse


Anyway, I dont like the way these fender amps are made. Everything cheap for fast cash. But who gets the cash to buy a boutique amp ?
Very few musicians in fact.


Best regards folks, I appreciated your help.

[JB]

I wasn't suggesting that the amp was oscillating under normal conditions, just that you were making it unstable by probing that anode pin with your meter.  You may have found an underlying problem which that cap is now masking.  However I'd try maybe carefully bending the ribbon cables linking the two boards to reposition them and moving the output transformer wires a little to see if that is the issue. 

[shooter]

Everything cheap for fast cash.

yep
the last one I worked on, those ribbon cables JB mentioned, I took them out, rewired with individual wires, real wires, not leftover Studebaker iron oxide

[acheld]

There is always something lurking in those cables and the cheaply built tube board.   I've seen several loosened cold solder connections, lifted pads, and even burned traces in and around the EL84 cables.

+1 on shooter's approach.   Even better, Doug has a replacement tube board that works well.

Yeah these are cheaply built, but they (used to be) affordable, the form factor is nice and they sound pretty good.

I really want to see a schematic for the new  BJ design . . .

[Colas LeGrippa]

Happy friday,


JB, I  see your point , the probe causing oscillation,. It shouldn t be like that on a healthy amplifier.
 I had already.moved the ribbons unsuccessfully.


I don t understand though why an oscillation causef by the PI could be cured by  taking some action on a stage after.

[shooter]

by probing the PA tube plate you completely disrupted a "balanced/happy" circuit.  That disputation will radiate in BOTH directions by putting stress on the previous circuit

[Colas LeGrippa]

Ok I didn t know that.
I can see the  " balanced /happy"  circuit was not so well balanced, according to the tone that amp had before....


I have never experienced both directions oscillation. I know I have to lead dress an amp the right way to avoid oscillation and I have never, I say never, put a snubbing cap across any of the plate load resistors in my builds.


So if I get oscillation in the preamp section, I know darn well that moving the leads of the putput transformer won t cure the problem.

[PRR]

...It shouldn t be like that on a healthy amplifier.....

It IS like that. Even on healthy amplifiers. That is why 1950s VTVMs had a resistor and a shielded lead on the DC probe.

Attach a yard of unshielded lead to a 400VAC plate, near millivolt circuits in the same chain, you are down on your knees begging for some kind of oscillation.

[Colas LeGrippa]

Omg how I miss my VTVM !!!!!!


PRR, you re a God here.  So please explain to me why a plate will give a reading, and the other, no.


Tell me as well If I can buy good probes for my china meter . I can t afford  a Fluke.


From the other side,, don't you think that a cheap probe is able to detect oscillation while a good one will be "forced" to give a good reading?
That s exactly what happened here, my cheap probe revealed an oscillation


Colad

[shooter]

my fluke leads did the same thing, that's when the smart kids here said "Quit doing that!"  And I did, I measure the speaker side of the OT to see what's up with the primary.
the PI is part of the PA "circuit" so whatever happens there does not equate to doing something similar in the preamp section.  The PA can swing many hundreds of volts, the pre swings a few volts, inductance happens best with big volts.

[Colas LeGrippa]

I ll think about that. Thanks.