Valve Jr Fried?

[CJPreston]

Good evening folks,

A bud of mine was playing this amp wide open and smoked something. When I opened it up the first time I replaced the resistor that sat on the caps next to the burn mark. All of the color was burned off and I acquired the resistor value from schematics. I soldered the resistor in place and turned everything back on. I played through the amp for a few mins and heard popping noises and the resistor started to smoke again.

I do not recall the power amp tube looking the way it does now before I placed it all back together. I am confident I would have cleaned it to prevent anything from shorting out. Before I place anymore time in an attempt to repair this board what are the thoughts of the community on this one? I can provide more pics and hook this thing back up and take readings if needed.

Much respect and thank you for the input.

[The Dude]

When a resistor burns open it does so for a reason. Something else is shorted. My bet you have a bad output tube. The "popping noises" were probably from the tube arcing.

[CJPreston]

Ok - I will test the resistance on the resistor to see if it needs replacing. Based on your response I talk it I can clean up the traces on the tube socket and try a new tube and see if it repeats itself?

[The Dude]

Yep. That is what I'd do. Clean up any and all residue from burns. Check your parts that got hot and replace if necessary, and install new tube. That does it most of the time with these amps.

[CJPreston]

Awesome and thank you. Noob here with amplifiers and working slowly to build my chops.

[The Dude]

Come back if you still have issues and............almost forgot............welcome!

[TimmyP1955]

If there is any sign of arcing on the sockets (carbon tracks), you would be wise to replace the socket (or at least take a Dremel and grind off the offending area). Same goes for burning or carbon on the board if it is between traces.

[g1]

Yes, there appears to be a burn on the far left upper area of your first pic. Also the top 2 left solder connections need to be redone.

[CJPreston]

Yes, there appears to be a burn on the far left upper area of your first pic. Also the top 2 left solder connections need to be redone.

The first burn mark I cannot deduce what it is the result of. It was covered with black electrical tape and there were no obvious traces leading to that spot, just around it. The two that need to be redone is the "T3" connector and the trace has partly separated from the pcb. I'd need to think about how to properly fix it.

I did not upload the pic that shows the bottom of the resistor on the power caps. There is some damage to the trace (slight bubble like feature around the leads). I figured it would be ok to solder the resistor directly to the leads off of the caps or a shared connection underneath. But would like to confirm in the event I missed something.

In the mean time I will also share I've been perusing the forum and have come across a reference to Jack Darr's Handbook and will spend some time there too. I've built a few solid state devices and slowly inching into the Tube amp arena. I figured 5 watts are a good place to start. Thank you to the community and I look forward to becoming a regular student and participant. Merry Christmas Folks, I welcome your feedback and instruction.

[CJPreston]

Yep. That is what I'd do. Clean up any and all residue from burns. Check your parts that got hot and replace if necessary, and install new tube. That does it most of the time with these amps.

Cleaned up traces, replaced resistor, and no sign of arcing on sockets. Replaced tubes and all seems to be well. First replacement before tube change resistor burned in 3-5 mins. This time around I've played it for about 50 mins total and no magic smoke.

Next step is to try a few mods, I've grabbed the schematics and will study and compare a bit before the tinkering starts. Going to dig around busted electronics bin to see what parts I can source.

[lordraptor1]

upper left corner in first pic looks like it has a broken trace, second looking in the same pic looks like some burning around the solder joint of the tube socket, lastly ( again same pic) what is the purpose of the cut trace and resistor in the bottom of the pic?

[CJPreston]

upper left corner in first pic looks like it has a broken trace, second looking in the same pic looks like some burning around the solder joint of the tube socket, lastly ( again same pic) what is the purpose of the cut trace and resistor in the bottom of the pic?

The pic is a bit misleading about the trace on the socket all is well after I cleaned it up.

This amp was given to me as a "you can have it if you can fix it" project. So there are things I am still poking and prodding at to figure out what is going on. Regarding the resistor, I cannot definitively tell you what the purpose is. My first thought is a screen grid resistor. It is on my to-do list for today to figure out what the purpose of that resistor is. Even if it is a screen grid a 1/4 watt seems a bit light for the amp.

[Dave H]

Regarding the resistor, I cannot definitively tell you what the purpose is. My first thought is a screen grid resistor. It is on my to-do list for today to figure out what the purpose of that resistor is. Even if it is a screen grid a 1/4 watt seems a bit light for the amp.

It is in series with EL84 pin 2 so it looks like it's a stopper for the control grid. 1/4W should be OK there. Is it a factory mod? The schematic already has the resistor (R15) but I don't see it on the pcb screen print.

I have version 2. I think you have version 1. It doesn't have the pi filter on the B+ like the later versions. Does it hum? I've attached the stock V1-3 schematics below and the mods I did to mine. It sound great now, very similar to my 18W (it has the 18W normal channel tone control). Good luck with your mods.

StockVJ_1-3.pdf
DH VJ.pdf

[audiopete]

I would suggest you build a light bulb limiter for future projects - solid state or tube. That would have prevented the second resistor from frying when you powered up. As The Dude said, when something burns - fuses or resistors, there is usually a secondary cause.

[CJPreston]

It is in series with EL84 pin 2 so it looks like it's a stopper for the control grid. 1/4W should be OK there. Is it a factory mod? The schematic already has the resistor (R15) but I don't see it on the pcb screen print.

I have version 2. I think you have version 1. It doesn't have the pi filter on the B+ like the later versions. Does it hum? I've attached the stock V1-3 schematics below and the mods I did to mine. It sound great now, very similar to my 18W (it has the 18W normal channel tone control). Good luck with your mods.

StockVJ_1-3.pdf
[ATTACH]32121[/ATTACH]

Yes - it appears I have version 1 it only has the single rectifier. It does have a bit of hum to it. I am not sure where I am going to begin with the mods. My thought process is to find a grit sound that I like and use it as an expensive pedal for church service. I have thought about including it with my set up with an A/B/Y box to switch between and use for stereo effects. I am just playing with it to see if I can figure out what I like and dont like about it. I am not super picky yet but have liked what I've heard from other mods throughout the interwebz. The majority of the mods and examples I've heard or seen have been on V2 or V3 boards.

Again I am no expert and still do a lot of reading. Another question I have is there any impact of a single vs dual rectifier on the amp performance. The goal for today when I get back to the abode is to get voltage readings at the pins and see if they are where they should be.