5150 filament string

[drewl]

Anyone ever draw out the heater string separately on 5150 etc. Style peavey's?

I found the problem, one of the 1000 uf caps was bad, fried the two 22ohm resistors.
I was just dicking around without the schematic, tomorrow I'll look up where they are etc.
Pain to track it down on the preamp tube board, everything jammed together.

I have to pull the main board. Looks like the amp was dropped or something.
One of the input jacks is smashed.
The main board screws often come loose, no lock washers.
When that happens all kinds of shit happens, the 1000uf that blew has a separate ground through one of the PC board screws.
Lose that and you lose filament voltage.

[Chuck H]

I'm nor sure there was a question in there, but nice report. It's always great when these things get documented here for reference

[tedmich]

I think its C9 1000uF 35V in the middle of the left edge of preamp schematic page, feeds R47/49 in middle top of next schem page I think. And C9 has fourth ground symbol not listed like the three joined by R76/77, the 47 ohm FPs. I always assume a bad tube killed the component but sometimes they go bad all by themselves

[drewl]

I was wondering if any of us here drew up a simplified version of the preamp tube dc heater circuit.

Aside from bad grounds I have yet to see one fail like this.
Looking at the schematic briefly it is a little confusing, having to flip through a couple of pages I printed out like ten years ago which have faded.
That's all.

[Enzo]

Just look at the schematic, this isn't as odd as it looks. Start page 1 upper left, that C9 cap connects to current source FIL and to pin 5 of V1a. Now follow across the page. Goes in pin 5 of V1a, out pin 4 of V1b and directly into pin 5 of V2a, out pin 4 and directly to pin 5 of V5. Coming out pin 4 of V5 it leaves as FILI. So all it really is is three 12v heaters wired in series. Now where do FIL and FILI come from? PAge 2.

FILI goes into pin 5 of V3, and out pin 4. From there the line goes to the right to -24v from the power supply. Note though the line passes through V4 on the page, there is no connection to that tube. So we have added a fourth 12v tube in the series string. SO we have left FIL. Look above V4 and see the two 22 ohm resistors in parallel. FIL is the left end and the right end is to +24v.

So all the more we have is four 12v tube heaters in series strung between +24 and -24 with 11 ohms resistance added in series. 48v of DC across 48v of tube heater.

.
I don't know why losing C9 would lose filament voltage - I mean lose its ground connection. But if it shorts, then it shorts -24 to ground through the 22 ohm resistors, And that can kill the heaters. So I pretty much expect a dead C9 and open 22 ohm resistors together.

Why does C9 fail? In my experience, I see it like this. Note in the power supply the 6VAC heaters - power tubes and PI - are elevated by the +24v. If a power tube shorts to heater, failure current finds its way down to +24, which puts way over voltage on that cap, so it blows.

Here is a reasonable copy of the drawings.
http://bmamps.com/Schematics/Peavey/...5150-EVH-1.pdf

C9 grounds to chassis. Note that same ground is found on the input jacks, speaker jacks, FS jacks. Note also at those jacks, other grounds are connected to chassis. Input jack and FS jack each ground a different ground circuit to chassis.

I haven't experienced a problem with the board screws coming loose.

[drewl]

I've had ones come in with half the board screws missing.
With the filament string ground missing, I had negative voltage across c9, that was killing it I assume.
It wasn't totally bad/shorted but bubbling out.

[tedmich]

Anytime Mfg. can avoid a board tap going anywhere (including the chassis) you save a little bit of $, and I'm sure Peavey engineers assume all the board screws will go in, but many a tech goes with "four corners" or less when they are in a hurry. PV did use FP resistors at R47/49 for a reason though, as these do also cost a little bit more too. A vibration loosened screw can lose the same ground and if a screw is totally loose is can bounce around and short stuff.

[Enzo]

There is no filament string ground, the heaters are wired between +24 and -24. Only that cap is grounded, and it is there as a filter.

If the screws fell out, they should still be inside the amp when you pull the chassis. I also vote someone else left them out when reassembling it.

If the ground screw is gone for C9, it doesn't have a circuit to anywhere, just a voltage on one end. However if it IS grounded and those 22 ohm resistors are open, then the +24 is gone from the string, so the whole thing snaps over to -24v. That will put -24v across the cap.

[drewl]

That's exactly what happened.
The screw was just loose, but not grounding c9.
I have found the screws floating around in other units.

Looking at a clear version of the schematic, the filament string is simpler than I thought, especially with that excellent description.
I had never had to fix this problem before.

The input jack nuts were a pain to get loose, totally seized up, rusted in place.

Some people don't take very good care of their stuff!