Preamp tubes do not lit up!

[slidincharlie (Carlo P)]

This is the same Peavey Classic 30 amp of my previous thread on "hum and smoke".

The three preamp tubes do not lit up! The power tubes do, while the 12AX7's remain cold and lifeless.
What should I check?
I'm thinking of tracking down the heater circuit to see if there is a breakage at some point... any other idea?

TIA

[Enzo]

Of couse, what else would we do?

Those heaters are wired in series, look at the schematic. One goes dark, they all do. First, get under each and every little wire jumper between the board edges and tug up on each. If any come free, repair them. This is easier than trying to figure out just which one is bad from the schematic. besides, you might find more than one.

AFter that, make sure R66 is OK in the power supply.

[Daver]

I worked on one of these once for the same problem. Enzo is 100% correct. One of the bare jumper wires on the board edge had broken. Check them all!

[slidincharlie (Carlo P)]

I checked and probed each jumper and unfortunately they all seem okay.
R66 looks bad with rusty legs. I snipped it off and will be able to get me a replacement only tomorrow. I'll post at the end of this (endless?) job.

One question: why do the power tubes lit while the preamp tubes don't? Do they have separate heater supplies?

[stokes]

The Classic 20 has a dc filiment supply for the preamp tubes,could the 30 have the same set-up?

[Amp Kat]

I know the 30 has DC filaments for the preamp tubes but I'm not sure if the 20 has the series connection for the power tubes. The 20 is also the only Cathode biased amp Peavey made. The 30 has three fuses inside it and the heater fuse is a ceramic so you have to ohm it out. I would do what ENZO said and check R66 also and find out with out the tubes in if you get 30 volts across that winding. Each tube drops around 6.3 volts . If your 30 volts is there then it's a connection issue and those jumpers are right in it's path.

[Satamax]

Carlo, check for dead tube too!

[Enzo]

Carlo, READ THE SCHEMATIC.

The three 12AX7s are wired in series - 12v mode, not 6v - across the -36VDC supply.

The four EL84s are also wired in series - they are of course 6v heaters - and with added series resistors R71 and R72, they are across the AC winding of that same 36v supply.

SO yes they have separate heater circuits, though they all share the common 10A fuse.