fender hot rod deluxe issues

[Curranproducer]

I have a friend who has dropped off a fender hot rod deluxe that is in very bad shape!!! He had a bunch of NOS 12ax7 tubes in there that are all microphonic and have burn and carbon char marks around the base of the tubes and on the pins. The 6L6's where JJ's and noisy as all get out. So my 1st move was to go through and clean and re-tension the sockets and re-tube the entire amp. it now sounds better, it has a bad treble pot which need replaced BUT the real issue is a bit deeper!!! The pilot lamp is flickering and dim, I grab my DVM and each heater wire is only giving my 3.3vAC at the lamp. So I go through and pull the voltages from every pin in the amp and there is some shenanigans happening! here are the voltages!

V1

P1 235v
P2 0
P3 1.73v
P4+5 3.3vAC
P6 229v
P7 0
P8 1.82
P9 3.3vAC

V2

P1 223v
P2 0
P3 1.89v
P4+5 3.2vAC
P6221v
P7 0
P8 1.94v
P9 3.28vAC

V3 this is where things get wonkk

P1 264v
P2 0
P3 37.5v
P4+5 3.3vAC
P6 236v
P7 21.8vDC and checking AC it goes between 238vAC and 10vAC
P8 goes from an Open Circuit to 37.4mVDC
P9 0.3vAC

6L6
V1

P1 2.8mVDC
P2 3.3vAC
P3 435vDC
P4 435vDC
P5 -47mVDC
P6 19mVDC
P7 3.2vAC
P8 48mVDC

V2

P1 2.5mVDC
P2 3vAC
P3 439vDC
P4 429vDC
P5 -47mVDC
P6 2.2mVDV
P7 3.2vAC
P8 0vAC -47mVDC

I also noticed more burn marks on the tube pcb boards with some bare wire jumpers going around the power tube sections. This looks to me like a PCB failure and a tech did some work to jumper the connections for the amp to function. There is no new burning marks or anything that I can tell around the amp sections I have looked and The amp is playing and working. I am thinking the power tranny is gonna let go pretty soon here.

[xtian]

I grab my DVM and each heater wire is only giving my 3.3vAC at the lamp

Exactly as it should. If you measure from heater to heater, it will read 6.6v.

Sounds like you've got a time bomb, though, with all the scorch marks and gerry rigging. If it's working and sounding OK, let it alone, and send it home with a, "sorry, cannot warranty this amp."

[Curranproducer]

I am confused then, I thought the heaters would each need 6.3vAC on each heater? thus making 12.6vAC for the 12AX7 to function properly? The lamp is dim and flickering, and the VAC on each blade of the lamp is only 3.3vAC. I agree with you on the time bomb issue as well.

[romberg]

I am confused then, I thought the heaters would each need 6.3vAC on each heater? thus making 12.6vAC for the 12AX7 to function properly?

The 12ax7 has two triodes and two 6.3VAC heaters. If you wire the two heaters in series you need 12.6VAC (where the 12 comes from in 12ax7). Most guitar amps wire them in parallel and thus they need 6.3VAC.

Mike

[Curranproducer]

correct BUT the power tranny is only giving me 3.3vAC on each heater wire.a little more than HALF of what one of he heater lines should be giving me

[xtian]

OK, let's try this another way. You have two choices for powering your preamp tubes. Use a 6v supply or a 12v supply. Most amps we deal with have 6vAC heater supplies, like your Hot Rod. You have a 3.15 - 0 - 3.15v coil on your power transformer. The center tap is usually bonded to earth, so has 0 volts, and the two legs have 3.15v each. Measure across the legs, and you get ~6.3v.

Wire the 12AX7s as spec'd in the schematic, with one heater leg to pin 9 and one heater leg to pins 4+5, and you're set.

Measure either pin 9 to ground, or pins 4+5 to ground, and you will see 3.15v. Measure pin 9 to pins 4+5, and you will see 6.3v.

[Curranproducer]

OK GOT IT, I was making assumptions about the heaters all being 6.3 vac all the time. and seeing the super dim lamp I was thinking bad tranny. I dont have a schematic for the amp yet. So why wold the bulb be so dim and flickery then? I did swap out the bulb but with one of my #47 type bulbs that work on 6.3vAC

[xtian]

Seen plenty of dim bulbs (ahem!) that were just because bad contact in the socket. Paint the bulb with some Deoxit.

[Curranproducer]

well I know I am a dimwit! hahaha never been called a bulb before though...............hehehehehe just a noob when it comes to things and i haven't pulled the schematic for this thing yet

[TimmyP1955]

I have encountered a number of recently manufactured Fenders with a flickering and/or dim pilot bulb. In every case, the bulb was not seated properly, and tape was required for me to get a sufficient grip to turn it after I pushed it down all the way.

[xtian]

That is a problem with those bayonet bulbs! You can't get enough grip on the glass with your fingertips to turn them. A little Deoxit works wonders.