Preamp issue on Fender Blues Jr.

[Rotus623]

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

[SY]

CAREFULLY check the AC voltage between pin 9 and pin 5 (or 4, they are connected together) of V1.

[wakibaki]

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.

[Rotus623]

Thanks guys. This is a great starting point!!

[Rotus623]

Wiki, voltage to pin 1 is going ape**** on my multimeter.... Also, the voltage from H1 to H2 is in fact 6.8VAC.

[wakibaki]

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

[Rotus623]

Originally Posted by wakibaki

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

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.

[Rotus623]

Waki, should the power transfomers output be fluxuating up and down drastically since it is pure AC current?

[llwhtt]

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

[Rotus623]

Originally Posted by llwhtt

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

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.