My Vjr's losing power. What gives?

[Auherre756]

Amp starts a'sputterin' the tone breaks down, then the volume starts dying -- as it warms up. I swapped preamp and output tubes with "good" ones. Everything glows yet, nothin' seems to change. DC V at pin 7 of the EL84 reads 350V. Can somebody shed some light on my situation?

[Jaybird]

Here is some info from a helpfull website....."Faulty Preamp Tube

Faulty Power Tube(s)

Bad preamp cathode resistor

An unbypassed cathode resistor has drifted upwards (to 5K-10K or over)

Faulty phase inverter

If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.

Open cathode bypass capacitors in preamp

if they go open, the stage they're in loses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.

Faulty vibrato circuit on neon/LDR vibrato Fenders

If there is a dummy plug in the footswitch hole, or a bad footswitch so the vibrato is always active, sometimes the vibrato tube turns on and stays on, not oscillating. This keeps the neon bulb on all the time, shunting lots of signal away. Same thing can happen if there is a shorted vibrato tube (rare) or a bit of wire or solder shorting the vibrato tube. Check the plate voltage on the vibrato tube to be sure it's oscillating.

High voltage isn't high enough for some reason

Failing rectifier tube - try swapping in another one

Failing power filter capacitors

Failing or open series dropping resistor in the bypass networks leading to preamp stages

Failing bypass capacitor - treat as in power filter caps.

Open screen resistors on power tubes

Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note

You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics. "

http://www.geofex.com/ampdbug/lowpower.htm

[strangedogs]

I think he's sayin' it's broke

[cGil]

Yep. I feel pretty confident that we can rule out the possibility of trouble with the phase inverter. It hasn't got one! Chances are, it's just a bad solder joint underneath one of those PT or OT quick connectors. They're prone to causing problems like that. If you can't find one that's obviously loose, the easy way to fix it is to simply reflow the solder to all the solder pads.

 

Gil...

[Eracer_Team]

also you said the voltage on Pin7 is 350, that seems really high on one of these little guys; I wonder if its stripping the materal off inside your tubes.

 

I would suggest to print off the schematic from www.sewatt.com and check all the voltages (safley of course)

you should see voltages something like, 12ax7 pin1 160-200vdc, pin3 .9-1.7vdc, pin 6 160-200vdc, pin 8 .9-1.7vdc.

on the el84 pin 7 305-320vdc - good (but usually seen as high as 330vdc), pin 9 290-305vdc (best not to go over 300vdc)

on R10, r12, r13 is all dependant on the values of each resistor but mostly what is set at R10. to bring the voltage's down inside this guy you may need to change it to a higher value somewere around 1.2k ohms at 5watts.

[Auherre756]

Well, friends

 

First, I appreciate all your input. Thanks. Here's what I've accomplished so far. I compared my tube voltages against the ranges you gave me, Eraser_Team. My OT voltage at pin 3 reads 9.6 VDC - a bit high; pin 7 reads 334 VDC which exceeds the 305-320 range provided; pin 9 is coming in at 310 VDC - also above the acceptable 290-305 range you noted. All the PT voltage readings appear to be in the ball park albeit on the high end. Based on your response, it sounds like I might need to replace R10 with a 1.2K ohm resister. Is this correct?

 

Now, armed with that information, I pondered Jaybird's comment about the open cathode bypass capacitors in the preamp: the fact that if they short, it dramatically shifts the bias point, and may cause distortion as well as low volume. THIS IS WHAT MY Vjr IS EXPERIENCING. But, I have 2 issues...which of the Vjr's capacitors is the 'open cathode bypass capacitor'? And, more importantly, is there a way to test a capacitor with a multimeter that doesn't measure capacitance? :D Any and all advise is welcome.

 

Gil, frankly, I'm a bit hesitant to start reflowing the solder joints on the PT and OT quick disconnects yet. All the joints appear solid, shiny, and totally stable (I'll save this until some of my other observations are discounted.)

 

Again, thanks everyone. I look forward to hearing from you.

 

Auherre

[cGil]

C3, C4, and C5 are the bypass caps. You can lift 'em and the amp still plays fine. The bias on the tubes does shift a bit, bit nothing dramatic or beyond operating spec by any stretch. We do this for tone tweaks around here. If they're shorting to something else, well, that would depend on how and to what they're shorting to. Normally they just connect between the cathode and ground, in parallel with the cathode bias resistor that does all the heavy lifting in terms of a tube's bias. And even the cathode can be connected directly to ground. All that does is force you to push your grid signal that far negative to "bias" the tube the old fashioned way to get the tube to "play".

 

Your symptoms sound more like something isn't connected properly. Possibly a lifted bias resistor, instead of a lifted bias bypass cap? And the troubling thing about amp troubleshooting is, you gotta park your ego to survive the fact that Murphy's Law rules supreme inside amps! That means whatever causes the most trouble is invariably whatever you're absolutely positively the most sure could not possibly be wrong and therefore avoid to the very end. Sometimes you literally gotta go over the entire amp three or four times with a magnifying glass to spot the layout and lead dress mistake(s).

 

But I'm sad to say a visual/chopstick inspection of solder joints is no guarantee of anything. Looks can be deceiving. The pcb needs to be mounted securely in the amp for chopsticking because it requires power and a speaker attached, and you sure as heck don't want to be handling the pcb with your hands during any power up test. A pair of meter clips will help you monitor suspect voltages as you poke around with the chop sticks and listen for the telltale crackle of bad solder joints. You've got power problems? Follow the power around every corner.

 

Gil...

[Auherre756]

HAPPY NEW YEAR TO ALL!

 

Thanks, Gil, for your sage advice. Personally, being a novice at amp repair/mods, I'm pure id -- no ego here. I'll do a total recheck of the circuit...yes, and even the tubes.

 

Here's a observation that may be related, maybe not. With the amp powered up and the volume cranked, there was a noticeable hum that immediately disappeared when I poked the R7 resistor. (honestly, I think the hum disappeared when I touched R7 and the adjacent jumper wire at the same time). Aside from the hum going away, no other changes in the amp's performance were noticed. Could this be related to gremlin related to my power loss problem?

 

Your continued insight is appreciated.

 

Auherre

[Eracer_Team]

these little guys hum with wire routing, if by say over time gravity had a wire slowly bend over you may notice a hum ‘suddenly appear’

that’s why Gil always recommends taking wooden chop stick and pushing wires around (not your finger, not your screwdriver).

 

The voltage ranges I suggested were with the R10 increased, but 334 on pin 7 seems a little high even for someone that has high a/c outlets, what’s your wall outlet at?

Remember the voltage will very a bit with what the wall a/c is doing.

 

Sometimes these guys also suffer from poor solder flow. You probably want to flip the board over and re-touch up all the solder connections too.

Don’t forget that as the unit heats up so does the board, if there is a bad solder joint on it there maybe an opening and closing of certain components.

[cGil]

At least when it was humming, there were signs of life. Reflow the solder to R7 and see if that helps. And look for a lifted solder pad or trace in that area while you're at it. And as long as you're in there, I still recommend reflowing solder to the transformer quick connect lugs. Or better yet, remove the lugs and quick connects and hardwire those transformer leads straight to the board. Those are the worst for problems like yours. And clean the pressure contact connectors inside the orange wire's fuseholder. Can't stress that enough, either; cuz they've also been a constant source of problems with these amps as well, until their properly and thoroughly cleaned, that is. When it is clean, I recomend treating it with ProGold (from Rat Shack) to keep it in tip top shape. ProGold is essential for maintenance on your 1/4" phone jacks, too.

 

Gil...

[Auherre756]

Well, friends here's the latest...

 

I reflowed connections and retested the components. Hmm, R7, R1, and R2 are each reading half their rated impedance. So, I reflowed the connections on those resistors and, voila, the amp worked like a champ (no pun intended). After installing the chasis back into the cab, enthusiastically connecting my speaker cab, grabbing my axe, plugging it in and twisting the volume to 11 o'clock, I was rewarded with the most delicious, creamy tube-driven tones. Feeling good about myself (and my ability to follow this forum's participants' expert advise), I swung the volume knob to 3 o'clock to savour some overdrive. On my first pick attack, I hear this 'tick' (kinda like the sound an incandescent lightbulb makes when its filament fails). The next thing you know, I back to an significant loss of output with an extremely distorted signal. I retested R1, R2, and R7, and wouldn't you know it, they're reading half their impedance rating again!

 

Please provide some insight as to likely causes and some recommendations for next steps. Besides fixing the problem, maybe I'll learn something. Thanks.

[cGil]

If the amp still sounds right when you roll the volume down below the tick/harsh distortion point, it could either be one of two things. Bad tube. (Easy enough to fix with a good tube) If rolling the volume back down doesn't get the good tone again, it's most likely the tubes. If you're gonna have a tube amp, ya gotta have extra tubes. And expect the occasional DOA right out of the box, so buy extra. In spite of the DOA's, it's still much more cost effective to buy 'em off the shelf, rather than pay the upcharge for pretesting by GT, Ruby Tubes, or The Tube Doctor.

 

If not a bad tube, I suspect blocking distortion, which is usually instigated by jumpering R6, which can cause so much signal to hit the grid of the power tube at higher volume settings that it pushes the tube beyond it's max operational bias point (and max distortion point) and it goes into cutoff. Solution: Use some value of R6, like 100k, with R7=1M (for gobs of gain to play with); or simply use a lower gain preamp tube.

 

Also contemplate the fact that the factory bias is above the 12watt max specified for the el84. The stock amps EAT power tubes for breakfast if the wall voltage is 120vAC and above. The power supply/bias mods lower the el84's plate voltage so it's within specs and sets the bias for 90% of max dissipation or slightly below (the sweet spot). Even then it can still be driven into cutoff, so for the best tones the preamp signals do need to be under control as well.

 

Gil...

[Eracer_Team]

Also watch out for shorting something out when you put it back into the chassis..

Also checking resistors that are soldered on both ends to the circuit board will not yield the correct ohms. The only true way to test ohms is with the resistor out of the circuit.

If you created a solder bridge across R6/r7 you could get R6 jumper-ed (6&7 are right next to each other), 6&7 connect at one end and not the other; that could cause the blocking distortion that Gil is talking about since there might be too much gain with r6 solder jumper-ed.

Although some on this forum have claimed very long life out of their stock VJ’s, don’t rule out a tube gone bad; which is why many recommend adjusting the bias on the output tube.

[Auherre756]

I believe I got everything under control thanks to y'all. Although I was convinced that my new JJ EL84 was a good tube, I took Gil's advice about checking and rechecking things, and low and behold the tube must've been DOA because I replaced it with another new JJ EL84 and all is well. Go figure. While I was on a roll, I deox'd and treated all contacts.

 

That said, I have a question for you gentlemen regarding power supply/bias mods to extend tube life. Although my home outlet voltage reads 115V, that doesn't mean that some of the venues where I play maintains 115V. Here are 3 questions:

1. What's the best way to make power supply/bias mods to lower the el84's plate voltage so it's within specs?

2. Does changing the value of the R6 to 100K address this or is there more to it than that?

3. How do I know that I've set the bias for 90% of max dissipation or slightly below "the sweet spot"? Kindly remember, I'm a novice at this so you'll need to put your recommendations in layman's terms.

You're continued support and patience is greatly appreciated.

 

Cheers,

Auherre