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apdale

Valve Junior is Broke

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Somehow I've broken my Valve Jr. Now when I poke around inside the power supply voltages are way high. They were previously biased to 300V at B1 now its up to ~390V which I'm pretty sure isn't good. I get some noise when I play my guitar through it but its nothing that resembles what I'm playing. I've tried changing out tubes and checking for any bad connections. Could I have fried the PT?

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If the PT's got 390v on the HT, it's alive. You've got something in there that has fried and there is no load on the PT, which explains the higher than expected voltages. Guess it's time to unplug, drain the cap's charges, and check those individual components.

 

Gil...

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I'd unplug the tubes and start with checking the B rail resistors, R10, R11, R12, and R13, and plate resistors R3 and R4. Technically, you would need to remove one leg of each component to get an accurate measurement. However, with caps attached, the resistor measurements will be close but a little off to begin with, and then drift as the meter battery slowly tries to charge the caps. Once you figure out (through experimentation) what to look for on your meter, you should be able to tell if a resistor has shorted or burned open (which I suspect) and if a cap is working or not without unsoldering anything.

 

Look closely for scorch marks on resistor color bands and other obvious discolorings. For caps, look for any sign of bulges, especially on the ends. For radial caps, this would require removal, so if you just want to cross your fingers instead of inspecting for end bulges, we'll understand. Besides, the ohmmeter drift effect will be a sign that they're still functional (somewhat).

 

Gil...

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So, I tried changing out the power tube again after finding 0V across R14. Then it was awesome cause sound came through and it was great then after turning the volume above 9 o'clock it started to oscillate and then at about 12 it had a defined oscillation and then cut out after a few seconds. After turning it back down it played for about 60 seconds until I decided to check on here to see what was wrong now. I'd guess that when I tried the new tube previously it was at a high enough volume to not have any sound...So now I've got a new problem but at least I've got sound. Any ideas now....

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You're just gonna have to go over it part by part. And check the OT for damage. Just a 120v neon lamp, a 9v battery, and maybe a few alligator clips are all you need for that. And this link.

http://www.premierguitar.com/Magazine/Issue/2007/Apr/The_Super_Secret_Transformer_Tester.aspx

 

 

Also, these links might help chasing down the gremlins.

 

http://www.paulrubyamps.com/info.html#FirstPowerUp

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

http://www.pacificrecone.com/JackDarrBook.html

 

Gil...

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I swapped the preamp tube again, then tried the OT from my Valve Special and it still does the same thing... Anything I should pay specific attention to?

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Yep. I can think of a few things. But I've already thought of 'em. See previous posts regarding checking each component and the troubleshooting links.

 

Gil...

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None of the resistors looked shorted or open. As far as I could tell none of the caps had any bulges and they charged up when my meter was across the resistors. Another observation I made was that the voltage at pin 9 of the EL84 started out at ~310V then after a minute or two started dropping pretty fast after another minute it went down to about 160V at which time I shut off the amp because I figured something was getting really hot.

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Have you tried measuring the heater as well as the wall voltage when this happens? If the heaters drop too far below 5.7v, the tube can stop working. If the wall voltage drops, both the heaters and HT will drop.

 

Gil...

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So, I had time to open it up again....The heaters are ~6.7 on pin 5 and 0 on pin 4 of the EL84 pins 4 and 5 of the 12ax7 are at the same before during and after the cutout/distortion. I didn't check the wall voltage, but my other Vjr works fine so I'm assuming thats not the problem. When it distorts the voltage at pin9 jumps from ~300 to ~380V I'm assuming this means the tube is turning off (open circuit) causing this current spike then if I quit playing the voltages slow goes back down to ~300 until I cause it to distort again. The amp distorts much earlier than the other amp, the problem gets worse as the tubes heat up. I was able to play for 10 min at a low level ~8 o'clock without any problems, then I turned the volume up a little and it cut out. Of note, previously I attempted to move R1 across the input and then when the amp quit working I moved it back, could I have messed up the level of the signal coming in so much that it can't handle that amount of current? I've double checked R1 and R2 and as far as I can tell they're where they are supposed to be. It does seem like the volume is louder than my other VJ, I only have one cabinet so I can't try them at the same time unfortunately.

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Got me stumped, but something is causing the screen grid to go into runaway.

 

I'd start swapping parts at this point. Like the bias resistor R14 and bypass cap C5. I recommend 240 ohm, 5watt. You can use a 3 watt or 2watt metal oxide, but don't go lower than a 2 watt rating. For C5 I recommend at least 50uF/35v for a loose feel, or 1000uF/35v for a tighter bottom.

 

You should add the screen grid safety resistor on the trace to pin 9 on the el84 to help stabilize the screen voltage.

That's usually a 1k/1w, but current thinking now says a 1k/3watt resistor is the better choice so you won't have to replace it as often.

 

It would also be good to get the rest of the rail resistors balanced as long as you're in there. It's best if you have about 310v on pin 7 plate, no more than 300v on pin 9 screen grid (after the 1K safety resistor drops a little), and about 10v on pin 3 cathode.

 

On the bright side, your heater voltage is okay and doesn't need to be messed with.

 

Gil...

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So, I plan on adding the screen resistor and balancing the others as soon as I can get to radioshack...but in the meantime I continued poking around. I noticed that the voltage at pin 7 on the EL84 (after it cut out and went to 384V started dropping steadily, it continued to drop all the way down to 160V, so I checked at R10 and there was still 345V there, so I knew somewhere there was a large voltage drop that should not be happening and shut the amp off. Hopefully this helps, this definitely rules out a power supply/PT problem not that that was suspect before. Hopefully this is enough to track down the problem. Thanks for all the advice so far Gil, with your help I'm definitely a lot closer to having a fully functioning VJ than I was by myself.

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I'm thinking there could be something going wacky with the cathode bypass cap, C5. That might explain the wild voltage swings at the plate. The cathode resistor R14 certainly needs swapping for something a little higher, regardless; and as long as you're in there for the cap swap... Anyway, I typically use a 1.2k/5W in R10, and a 240R/2W+ in R14, which gives me around 300v on the plate and 90% of max anode dissipation. Some folks like a 270R for a slightly cooler bias.

For R12 and R13 I typically use 2.2k/3W, and that extra 1k/3w on a cut trace leading to the screen grid.

 

Gil...

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It doesnt seem to be C5, I put a 470uF cap in parallel with r14 physically attached on top of r14 and its still doing the same thing, since caps add in parallel its ~500uF cap now. Same problem....I dont have the resisitors to change out just yet.

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apdale... earilier you reported 5v on pin3 of the el84 that should be around 9~11v.

 

Take the R14 right out and measure it and install a new one like gil suggested something between 240~300ohm's 2/3/5watt will do.

 

you should print out the schematic and check all the voltages r10/r12/r13, pin 3,7,9 of the el84.

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It doesnt seem to be C5' date=' I put a 470uF cap in parallel with r14 physically attached on top of r14 and its still doing the same thing, since caps add in parallel its ~500uF cap now. Same problem....I dont have the resisitors to change out just yet.[/quote']

 

Adding a good cap in parallel with a potentially bad factory cap won't eliminate the possibility that the factory cap is bad. You've simply got to remove that suspect cap from the circuit. And testing loose parts with a meter doesn't necessarily recreate the live circuit conditions under which a cap can get funky.

 

If you have a bench with lots of test gear, it's not so hard to check these things directly. For us regular folk armed only with a VOM and a hot soldering iron, sometimes it just boils down to a process of elimination by swapping a lot of parts. That can mean pulling the board out, and putting it back in... a lot. It's just one of those things that goes with the territory. But after a while, you get pretty quick at it. And as you learn the ropes of troubleshooting tube amps, you'll find ways to speed things up just by knowing how things work and what to expect.

 

That's why I highly recommend reading Jack Darr's book for some great troubleshooting tricks.

http://www.pacificrecone.com/JackDarrBook.html

 

And if you seriously want to learn about tubes from the inside out, the Valve Wizard site is one of the best resources there is.

http://www.valvewizard.co.uk/

 

Gil...

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I took out the old C5...still no run to get replacement parts, no change on the circuit....disappointing but not unexpected. However I noticed that the constant voltage dropping--about a volt/sec or so--goes all the way back in the power supply chain to R11, which was also steadily dropping at the same rate....so thats probably not good, I spot checked the voltage at D3 and D4 on the line side of them (anode??) was ~145V not sure if thats right or not it seemed low I'm guessing they should be closer to 175V if B1 is ~340V. So, maybe it is a PT problem, I can try to hook the circuit into the PT of my Valve Special if that becomes suspect..it might be a stretch but i should be able to figure out something. Just to recap, amp still works fine at low volumes, still distorts at a low volume then starts oscillating and progresses to cutting out when volume is turned up. Immediately after the tubes warm up I can play some notes and everything seems to work fine, but 20 seconds or so later the cutting out begins and gets worse with time. Thanks for the help guys.

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If something in the power amp or even more forward in the preamp is sucking current, the available voltage drop will be felt all the way back to the power transformer. I wouldn't expect a problem with the preamp tube to suck that much power, but you never know.

 

Try watching the tubes as you play low, and then crank it up. If one of them starts glowing, that'd be a good indicator of where to look for the problem. Also, take measurements of the preamp cathodes when the volume's low. If your amp is stock with 100k plate and 1.5k cathode resistors, you should measure around 1.5v on the 12ax7's pins 3 and 8 at idle. Then measure it when the volume's up. If it goes down with the plate voltage, that's good and would be consistent with voltages reported by guys using a VVR circuit to intentionally lower voltages from the power supply.

 

If the cathode voltage increases when plate voltage drops, swap the bypass cap on the gain stage that goes up in cathode voltage. And maybe that resistor, too. Also check the input grid on the next stage. If there's more than 1vDC, it means the coupling cap just before it is failing. That could also suck enough current to draw down the available voltage.

 

Gil...

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With an 820R cathode resistor you should probably measure about 1volt. I used an 820R on a VJr I just finished modding and got exactly 0.999v on it. That'll vary a tiny bit as the wall voltage goes up or down.

 

Gil...

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I turned it on, checked the 12ax7, voltages looked fine - about .9V on pins 3 and 8, then I got to the EL84 and it had 16 V across it!!! 7.2mA and it was going up so thats not good. I never even played it where it cut out. Sounds like that resistor - R14 which I still haven't replaced might be the culprit since C5 has been swapped. This would explain the drop in the B+ voltage, right? Definitely narrowing things down...I can't figure out any way of tests to see whether that resistor is bad or not, it tests fine with my DMM while in the board.

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