Preamp issue on Fender Blues Jr.

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Rotus623

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Joined 2010
#1
I was wondering if you guys could give me a hand with an issue. I just got a Fender Blues Jr. and it sounded great. I have plenty of experience with Power Amp issues, but not much with Pre Amp.

There is a V1, V2 and V3 preamp slot, and when I turn the amp on, all pre amp and power amp tubes glow, except V1. Obviously I tried changing the tube (12ax7). There is no sound coming from the amp. Could you guys point me in the right direction? Here is a link to the schem.

Blue Guitar Schematics
 
#3
If one valve doesn't glow then there's a fault in the heater wiring. You can see the heater wiring in the schematic designated as H2 to pin 9 on V1A and H1 to pins 4,5. The voltage between H1 and H2 should be 6.3v AC, and since the other valves are operating correctly the fault is almost certainly a failed connection either in the heater wiring or at the solder joints. Locate and test and if necessary replace the heater wiring which will be a twisted pair daisy chained from one valve to the next.

DANGER OF DEATH! Do not work on the amplifier without disconnecting the plug from the mains. Valve voltages can be lethal.

Good luck.
 
#6
OK, we'll deal with the heater issue first. 6.8vac is within tolerance. Does the valve light up?

You should have ~240V DC at pin 1. If you can see more than a small AC ripple with the DMM this indicates a fault in the PSU, probably an electrolytic cap needs replacing.

First we want to know if the valve is still dark. You can swap it for the working one, this will give us a definitive answer since it is working in the other socket.

w
 
#7
wakibaki said:
OK, we'll deal with the heater issue first. 6.8vac is within tolerance. Does the valve light up?

You should have ~240V DC at pin 1. If you can see more than a small AC ripple with the DMM this indicates a fault in the PSU, probably an electrolytic cap needs replacing.

First we want to know if the valve is still dark. You can swap it for the working one, this will give us a definitive answer since it is working in the other socket.

w
Click to expand...

There is 317VDC at pin 1. The multimeter is reading 231VAC and fluctuating to zero and back very quickly, but the 231VAC is pretty consistent. I changed the tubes around and still the same problem. The first valve does not light up.
 
#9
If you have 317VDC on pin 1 then the tube is not conducting. Check for heater voltage AT the tube socket. Pull the tube and check for 6.3VAC on pins 4/5 and pin 9, remember to count CCW from the top side. Does this amp use a ribbon cable from the main PCB to the tube PCB for heater voltage? If so check the ribbon cable. If any previous work has been done (probably) could have lost one of the conductors right at one of the PCBs.

Craig
 
#10
llwhtt said:
If you have 317VDC on pin 1 then the tube is not conducting. Check for heater voltage AT the tube socket. Pull the tube and check for 6.3VAC on pins 4/5 and pin 9, remember to count CCW from the top side. Does this amp use a ribbon cable from the main PCB to the tube PCB for heater voltage? If so check the ribbon cable. If any previous work has been done (probably) could have lost one of the conductors right at one of the PCBs.

Craig
Click to expand...

Hello again llwhtt!! Thanks for all you help with my previous projects. I am getting 7.13ACV from pin 4/5 to pin 9. I think that the link is to the incorrect schematic, despite what some have to say, I am looking for the updated (mexican) version of the schem for the Blues Jr. and I can't find it.
 
#12
I'm using the schematic off of Schematic Heaven, all the way to the bottom of the Fender page. If your input (wall) voltage is high everything else will be high too, don't worry about it. Sometimes it's hard to see if a tube is lit or not, are you using at a tube that is easily seen? If you have AC voltage between 4/5 and 9, the tube is KNOWN good the only thing left is the socket. Try twisting the tube slightly in the socket to see if it lights up.

Craig
 
#13
Once again Craig, YOU ARE THE FREAKIN' MAN!!

This makes me sound like an idiot, but I twisted the tube around in the socket, and the heaters lit up when I put tension on in. I plugged the guitar in and SOUND!!!

Man I feel like an idiot, but it is funny how sometimes we overlook the little things, and them someone like you figures it out by what would seem like a silly suggestion. Just never seems to be this easy, ya know?? So Craig should I try to add bend some tension to the input pins and re-solder where the jack sits on the PCB and call it a day?
 
#15
I see this all of the time on several forums, 5 minute problems that take days and days to solve. Sometimes I'll chime in and sometimes not because nobody ever seems to have any test equipment and I'm not there to actually see what's going on. I guess you have to learn somewhere and now that you've seen this I'll bet you'll never forget it, correct?

Craig
 
#16
Forgot to mention the tube socket, replace it and order a few extras while you're at it. Many new manufactured tubes have skinny pins and cause problems in marginal sockets. That amp should have light brown/tan Beltons in it, I think "Tubesandmore" carries those.

Craig
 
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