Jump to content
Gibson Brands Forums
Sign in to follow this  
apdale

Valve Junior is Broke

Recommended Posts

Yeah, R14's the next likely suspect. If not, the only suspects left are the big filter caps, the OT, and of course, the tube.

 

Gil...

Share this post


Link to post
Share on other sites

changed out r14 to 2 470ohms in parallel to give me 235ohms and changed the el84 it fried the tube when i turned up the volume, still the same problem after reverting to the old tube. Any other ideas? Should I add the grid screen resisitor?

Share this post


Link to post
Share on other sites

Yes, definitely add the screen grid resistor. I thought you already had. It'll help stabilize the screen voltage and may help prevent that runaway condition for future tubes. And it can help save the tube as well as other various components if it does happen.

 

And definitely get a stash of some decent power tubes. Don't ever rely on getting just one tube. Get a few. Just because a tube is fresh out of the box doesn't guarantee it works. Guarantees cost twice the price and create companies like Groove Tubes. I buy regular, off the shelf, JJ brand el84 tubes because the overall expense is less, in spite of the occasional dud. Besides, many tube dealers will accept returning duds if you bring 'em back within a day or two of the purchase, but check the dealer's policy on that to be sure.

 

Gil...

Share this post


Link to post
Share on other sites

The current seems to be stabilized....it never tops 9V on the cathode of the EL84 that I've noticed. But B+ keeps dropping steadily, of note I fired up the amp w/o the power tube and the B+ was rock solid. When its put back in then it steadily drops until I crank it til it distorts and cuts out when it jumps up to ~400V then keeps dropping again.

Share this post


Link to post
Share on other sites

well without the tubes in you stated the voltages are solid but probably high.

When you put the tubes in then the readings start getting strange.

The amp will work just fine without the screen grid resistor in it, but it does help the screen but this is not the end all to fix your problem.

The catch is you have an amp that seems to like having a voltage issue and will fry up your tubes.

So about the only thing you can do is to pull a component check it and put it back in, making sure that everything is in spec since a visual check hasn’t been able to do that.

 

Don’t forget to scrape the fuse holders from time to time as they get an oxide build up

Share this post


Link to post
Share on other sites
what do you think Gil... bad output transformer.

When it starts to heat up it slowly shorts internally?

Just a guess

 

But it's a good guess, because it's quite possible that is part of whatever the problem is. However, without using a power tube that has been proven good (in another amp for instance), trying to sort out an OT issue by simply plugging a different OT could be little more than an exercise in futility.

 

Gil...

Share this post


Link to post
Share on other sites

The tube I've been using I've been switching in and out of my working VJ whenever I feel like playing instead of tinkering with something...it works fine. So, I've got a known good tube with a known good OT and I've still got the same output.

Share this post


Link to post
Share on other sites

Well, that eliminates that. Somewhere is a leak. But where's the current going? And why is the B rail voltage dropping when the grid voltage increases? What's left? I think we've pretty much eliminated everything but the PT. If both of your VJr's still have the quick connects, maybe try making some HT, heater, and ground wire jumpers so you don't actually have to pull the PT's to test this theory.

 

Gil...

Share this post


Link to post
Share on other sites

The PT holds up fine when the power tube is pulled...it stays right where its supposed, theres not much current flowing but theres definitely some passing through the 12AX7 and across the bleed resistor.

Share this post


Link to post
Share on other sites

Yes, without current draw, the PT has no trouble providing the higher voltage, but as the signal voltage to the tube's control grid increases, the "valve opens" and current draw at the plate increases. And the voltage will drop as the current demand exceeds the capacity of the PT. This causes excessive heat, and we all know what too much heat will do to a transformer winding's thin insulation. You'll get a taste of this playing with the bias, but you'll see it even more dramatically if you do the octal mod and measure the voltages and current with different tubes and different bias settings.

 

Without that screen grid resistor, the PT could've easily been damaged by a runaway tube. This is why the screen grid resistor mod is necessary in the first place. I don't have a clue why Epiphone chose not to use one in the VJr. All the other Epiphone amps from the Valve Special on up use 'em, and they're shown in the schematics for all the other amps I've seen so far. But not the VJr. So if you haven't already added a screen grid resistor, this is a good time to add one.

 

And while we're back to square one... if you haven't already replaced your power supply filter caps C6, C7, C8 and C9, I strongly suggest you do so. Just because they look okay or check out with a battery operated meter doesn't mean they ARE good under operating conditions. It's possible that C8 might not have been damaged, but the rest would've certainly have taken a hard hit by a runaway power tube. So quit screwing around and swap 'em out.

 

Gil...

Share this post


Link to post
Share on other sites

Haven't had a chance to touch the amp but I'm able to read up at work -

 

looking here -> http://www.geofex.com/ampdbug/intermit.htm

 

"Amp stutters or cuts out when driven really hard: overdrive is causing the output tubes to go into grid blocking after being

over driven; this is caused by the signal causing a temporary bias shift."

 

which sounds exactly like what is happening, the only suggestion it says is to remelt every solder joint, this is a generic solution for all problems on the page. I can definitely do that next time I open it up, can anybody explain to me what a temporary bias shift means?

Share this post


Link to post
Share on other sites

Sure, but Merlin explains it so much better. Check out the info on triode gain stages.

 

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

 

As far as blocking distortion goes, the only time I've gotten that out of a VJr is when I jumped R6, and even then only when the volume was between 9.5 and 10. Sometimes a lower gain preamp tube is all that's required. I opted for tweaking the R6/R7 ratio to get the gain I needed without the blocking distortion.

 

Gil...

Share this post


Link to post
Share on other sites

I tried the EL84 out of my Valve Special this morning and the B+ voltage was rock solid with absolutely no input, I don't want to fry any more tubes if I can help it. The EL84 I've been using continues to work fine in my working VJ apparently it is the cause of the B+ leakage problem (not sure what thats called) However with the new tube there is no cathode bias at all, pin 3 of the EL84 stayed at 0V for about 5 minutes until I turned it off. The voltage at pins 7 and 9 were high, I assume this is because no current could flow. I double-checked the resistance through R14/C5 and it ohmed out to 235ohms from pin3 to ground.

Share this post


Link to post
Share on other sites

Nevermind that last one, (with new tube) once I turned up the volume B+ started dropping and a cathode-voltage became apparent. This seemed to happen only after I touched pin2 (twice!) on the EL84 to see if I was getting any input, maybe I imagined it but it seemed to have taken a really long time for things to heat up, then as soon as I tapped pin2 with my probe tah-dah sound started coming out and I started measuring a voltage on pin 3 also the voltage on pins 7 and 9 was dropping like usual and the voltage on pin 3 continued to rise. Because the cathode voltage started rising it looks like the extra current is going that direction, R14 and C5 have been double checked and swapped, any other reason for that current?

Share this post


Link to post
Share on other sites

Don’t take this the wrong way but... Gil has given you some good hints on fixing this amp, but you seem to be unwilling to take things apart and try and fix it. You just keep shoving another tube in it which as you seem to note, burns out.

 

Since you have a number of amps to rely on and this is a hobby amp. Take the board out and start pulling at least 1 leg of the resistors to measure correct ohms on each (can't be checked properly when its in the circuit).

You really have nothing to loose since you have backups available and the darn thing is only a $100 (+/-) amp.

 

Start hacking and you might find the issues

Share this post


Link to post
Share on other sites

I switched the board out with my other VJ and it still didn't work, so its definitely not the OT or the PT or the tube. Looks like I'll start pulling up resistors...

 

Thanks again for all the help.

Share this post


Link to post
Share on other sites
if you haven't already replaced your power supply filter caps C6, C7, C8 and C9, I strongly suggest you do so. Just because they look okay or check out with a battery operated meter doesn't mean they ARE good under operating conditions. It's possible that C8 might not have been damaged, but the rest would've certainly have taken a hard hit by a runaway power tube. So quit screwing around and swap 'em out.

 

This is where I'd be headed, if you haven't already.

Share this post


Link to post
Share on other sites
Which components should I check? Is it possible that the output transformer is bad' date=' or does some noise prove that its still good? [/quote']

 

Check R5

Make sure it is of correct value (220K) and soldered in properly and is grounded.

If R5 is not intact C2 will charge up and sound will all but shut off.

Share this post


Link to post
Share on other sites

IT WORKS! So it was a problem with R5, I pulled up the trace to ground when changing out C5 and it was floating. After installing a jumper it works great. Thanks for all the help guys.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

×
×
  • Create New...