Valvking 212 Blowing Fuses in Power Stage

[gusmanchu]

Just got my amp shipped down to FL from MA and went to finally use it again yesterday. Put the amp on standby to warm up the tubes and then finally flicked it to Power and heard a speaker crackle sound and instantly the fuse blew and the amp turned off.

Any ideas what this could be? All the tubes look to be ok. I re seated all of the power tubes and am about to grab some more fuses. Again, seemed fine until I flicked it out of standby, any ideas?


Thanks in advance.

[Jazz P Bass]

When you get the new fuse, try it without the power tubes installed.
If the fuse holds, try one of the output tubes in one socket.
If that holds, using the same tube, check it in the other (3) output tube sockets.
If that holds, try each output tube one at a time.

[gusmanchu]

The amp will work without any power tubes?

Ok, so I'm looking for a bad tube or socket?

[Jazz P Bass]

Yeah, your trying to get the fuse to hold.
The amp will "work" with only one tube in.
Won't sound good.

[Enzo]

Um... The tube heaters in that amp are all wired in series. Well actually the four 6L6s are in series, then the three 12AX7s are in parallel themselves with the result in series with the power tubes. Pull one or more power tubes and the whole string goes out. Pulling one or two 12AX7s wouold allow the amp to function, but would upset the heater voltages.

But do try this with the tubes out, because that will tell you if anything else is blowing your fuse.

[Jazz P Bass]

Thankyou Enzo.
I did not realize the 6L6 heaters where in parallel.

Peavey Classic 30 & 50 are the same I take it.

[Enzo]

Nah, series. Look on the power tube page of the VK212, upper left corner, there they are. Two more 12AX7s are lower right corner.

6L6 draws 0.9A heater current. The 6v-wired 12AX7s draw 0.3A, so three of those in parallel makes 0.9A, same as a 6L6. SO put all that across 30VDC and the same 0.9A heats them all.

The Classic 30 has two strings. The power tubes - four - are wired in series across 24VAC. The three 12AX7s are wired in series across -36VDC. So if you needed to, on the C30, you could pull one small tube and leave the power tubes hot, or pull one power tube and leave the small tubes hot. But of course within the two grounps, any missing tube darkens the rest.

SOmeone, either here or over at Peavey forum, made some dummy EL84s. SInce you can;t pull two tubes to drop your power, he made up a couple EL84s with tube element pins cut off, leaving the heater pins and any unused pins. He plugged those into the amp in two spots to complete the heater circuit, but the missing pins prevented those two tubes from acting electrically. So instant C30 on only two power tubes.

The Classic 50 has four EL84 heaters wired in parallel across 6VAC, the 12AX7 PI tube is also wires for 6V in parallel with them. The two remaining 12AX7 preamp tube heaters are wired in series across the -27VDC supply.