Valve Junior V3 Head input mod gone wrong...

[rgvkiwi]

Hi Guys

I have a Epi Valve junior Head (v3) that i replaced the R1 resistor as per most suggestion to a 1M (from 68k).

I have done this mod on a V2 combo with success so didn't think i was going to have a problem and to my ears improved the response and sound nicely.

Unfortunately in the process i seem to have damaged something as when i turn it on it powers up fine, but get no sound whatsoever. Not even a crackle when touching guitar lead etc.

The only thing i can think of is that the resistor is larger than the others and is a 1w not 0.5 that i presume the originals are....(although electronics shop said this wouldn't be an issue)

Can anyone suggest where to look, i have tried replacing the resistor in case i overheated it but no luck. During removal of the original R1 i may have damaged the circuit board tab for that component (it is really dark and hard to see the tracks on, unlike the v2 board) so i soldered the leg end of the R1 resistor to the end of R2 and connection 3 of the valve (as per the schematic) with same result.

Any further is a little over my head to diagnose.

In addition i would have thought the amp would put some sound to the speaker even IF the input circuit was a bit screwed up.

I believe with these symptoms some of the gurus here will have an idea of what part of the board to look at and help out a 39 year old, (now) sheepish beginner modder......?

Regards

Mark

[tubeswell]

Welcome to the forum.

When you do a mod and the amp stops working, and it was working fine before, its because of something you did. But the first thing I would check is your soldering. Did you actually test the fitted new resistor with your R-meter (going back one solder point either side of the resistor each way)? You might not have made a proper solder joint - and possibly you need to go back and re-flow the connections with a bit more solder. Check that you plugged in all the jumpers properly and that you didn't break anything. If in doubt test each connection with your R-meter

[rgvkiwi]

Thanks Tubeswell, good point.

Unfortunately i didn't unplug anything, managed to turn the board over simply by unscrewing from mounts.

Can you/someone confirm that the 1w resistor is or isn't a potential problem...

I finally found some info on the V3 board and it turns out the bloody resistor is already changed to the suggested mod anyway. bugger.

I have tried resoldering, but the solder doesn't seem to have very good flux as it wouldn't silver up or stick to the board like it should. Will also try new solder.

[tubeswell]

Did you test the connection with your R-meter? If the solder isn't 'sticking to the board', then chances are you haven't got a connection. Trace pads on most modern pc boards can't take much abuse. If you buggered up the trace pads when you took the old components out, you will need to expose some of the good trace copper by cleanly scraping off some of the trace insulation near where you want to resolder each component lead. Be careful not to create any unwanted shorts across to other traces when re-soldering the component. Test the connection with your R-meter by measuring the resistance 1-pad back along each trace, to make sure your connection is secure. FWIW I use de-soldering braid when removing old components, to cleanly get all the old solder away from the board without damaging the traces. If there is any residue solder still connecting the component leads to the trace pads when you try to remove the component, chances are you will tear the trace pads off.

The other thing you should check is the resistor value, but assuming it is the correct value, it is unlikely to be the resistor, and more likely to be the soldering I would say.

[rgvkiwi]

Thanks Tubeswell. I appreciate your time. Fortunately I "discovered" the problem.

One of two things happened (neither show me in a good light )

1/ When i removed the original R1 it stuck and when i used force to pull it out damaged the trace on the PCB. So when i replaced the first one I soldered in I skipped the track and soldered straight to the leg of R2 and Valve 3 as per the circuit diagram.

2/ Upon completion of the above i noticed the top of the 12ax7 valve was smokey. Turns out it was broken towards the base. I may have broken the 12ax7 valve when manouvering the board so as not to unplug any leads ( this would surprise me, as i am not generally that ham fisted!).

Could having the track of the input resistor cause the valve to blow?


Thanks for your help. I'll try be more carefull next time.

[Enzo]

You two had to come all the way over here for this? You could have met at Lake Taupo and discussed it over a beer.



(Nah, I cheated and looked at a map)

[tubeswell]

You didn't take the tubes out when you worked on the amp? Get some new tubes and see if it works. Can you post a pic of what you did?

Taupo is a whole day there and back. I can get beer from my fridge in 5 seconds (altho' I'll concede that 30 years ago I'd have said in 2 seconds flat, but now I think - what's the rush? Right?).

[rgvkiwi]

Yeah i did take the tubes out, that is the part i don't quite get. Perhaps i broke it putting it back in.

Seems like that would be pretty hard to do though.....I have removed my share of tubes over the years without ever doing that!

It's a bugger because the v3 tubes are definately nicer than the v2 tubes (from what i hear/can tell anyway)....

Any suggestions on NZ based Valve suppliers, there was one i had used before but lost the details (it was a few years ago...)

Enzo, Taupo is a lovely place in the summer, big lake, road race and motocross tracks (my other hobbies) all sorts of adventure sports etc BUT one of the coldest, wind swept places in new zealand during winter! (the army have base not far from there for the "harden up" training.

I too am quite happy sitting here with my Beer and various toys.

Cheers

Mark

[tubeswell]

Well here in the tube-amp forsaken rump of the world, you just to get tubes where you can. Ryan from CustomAudioBoutique in Grey Lynn is a very useful resource and he is a nice guy too. Unparalleled (on this forum) is another great chap in your neck of the woods