6505+ blows output fuse on standby. Does not with no power tubes. (

[Biffstar]

G'morning folks and hello. I'm new to this forum, so bear with me. I'm looking for advice/info on what's going on with my Peavey 6505+. I've already scoured the forums (and all of the internet for that matter) for anyone who has come across the same issue, and I've found absolutely nothing. I've found similar issues that had a simple solution, like dead tubes, sure. But nothing like what I'm encountering.

Just a quick disclaimer, I'm no stranger to fixing electronics, but this is the first time I've really dabbled in repairing a tube amp. I'm well aware of the high voltages and dangers present, blah blah blah. Don't worry; I'm aware of draining capacitors, etc.. not my first rodeo.) Also, I'm not interested in hearing any replies suggesting that I take the amp to a tech... I am a tech, so to speak... Just not for tube amps necessarily, but I'm more than capable of fixing it with a bit of further direction. I wouldn't be asking for help if I were looking to just send it to a shop.

So let's go!

I received this 6505+ on a trade, and the previous owner said the only issue was a blown output fuse, so I thought cool, not a big deal. Received the amp, and sure enough, the output fuse was indeed blown. I swapped in a new one and powered on the amp. Within 10 seconds, still on standby, a single power tube arced white light, big pop from the speaker cabinet, and the few blew again. Hmm... Bad power tube? Tested all the tubes, all checked out fine. Put in a new fuse again, and powered it up.. Within 10 seconds again, single random tube arced, loud pop, blown fuse. Tried again with an entirely new set of known good tubes; exact same condition.

So here's the scenario in point form:

-With power tubes in, single tube (seemingly random) arcs within 10 seconds, and the output fuse blows. This is while still in standby. I never get to the point of being able to switch it to play condition. See photo:



-After fuse blows, all tubes heaters stay lit and look normal; amp will not come off standby (since fuse is blown). See photo:



-If I replace the fuse and remove all power tubes and power it up, standby can be switched off and on absolutely normally. Note picture:



-Tried running with 2 tubes, same issue... random tube arcs, blows fuse.
-Built-in bias voltage test points read 55 volts (which is where it should be for these amps).

Here's what I've deduced so far, with no luck:

Because the amp powers on, doesn't blow fuses, and seems just fine with no power tubes in, I figured the issue would have to be at the power board. So I removed the power board and tested all the resistors, caps and diodes on it. I found one dead screen grid resistor on the board:



I replaced this resistor thinking it was the cause of the short/arc... Nope. Still exact same condition. Recall, the arc still happens in a random tube, not necessarily the tube that this resistor was connected to. Same random arc before/after resistor swap.

So the power board all seems fine. I tested the rectifier diodes on the main board (visible near the fuses in the photo above), and those check out ok. There are no visible burned/scorched spots or components anywhere on the board. Note that I'm only talking about the top/visible side of the main board. I tested the output transformer, and the windings all seem fine. Found no shorts on any tap.

And again, this happens after around 10 seconds of power on. It's not an instant short/blow (the fuse is a fast-blow, btw).

I haven't yet taken out the big main board, but before I do, I'm hoping somebody here might have some insight on what step(s) to take next. Ideally, I'd love if one of you experts can chime in and be like "oh! that's a dead ______" and have it be a simple fix, but I'm not holding my breath.

Please help! Thanks!

[Enzo]

I would not pull the big board unless I had a darn good reason. You think the problem on the power tube board? OK.

A bad screen resistor is a pretty sure sign the tube for that socket was bad. Or the tube was replaced after a bad one blew the resistor.

When you say you tested parts, may I assume you did it with your hand meter? A meter uses a volt, maybe two, for any tests, and does not put real world conditions on parts. A cap that doesn't leak at 2v might well leak like a screen door at 200v. The miniscule meter currents will not expose something that breaks down at 2 amps or even 200ma.

Same with transformers. Your meter will tell you if two windings are shorted together or to frame, but how would you know if two turns shorted together? You won't. Nor will a meter expose failed insulation in a transformer that arcs. You might tell if the primary of the output transformer was shorted end to end or in some other gross fashion, but that is about it. Arcs can only be detected in operation.

Go to RG Keens Geofex web site and there is a tech tip of a simple transformer tester you can make. It won't make it arc, but it will at least detect shorted turns. Really, justa battery and a neon bulb, it is very simple to make and use.

When HV fuses blow, we often suspect the flyback diodes, but if they are failing, it blows fuses without tubes too. So I don't suspect them today.

ANy tube that arcs inside - white flashes - is instantly on my bad tube list.

Take your four tubes and one at a time, sub them into some other amp, preferably one with similar B+ voltage. I won't assume they are good at this point.

[Biffstar]

Hi Enzo, thanks for the response.

You are correct, I've been using just a multi-meter to test components, including the transformers. I agree, it's not the most reliable method, but I still don't suspect the transformers. They do not/have not ever smelled or emitted smoke. Sure, not a reliable method either, but I've yet to see any other evidence that they might be at fault. Again, I'm not a tube amp expert (so correct me if I'm wrong) but if a tranny were internally shorted, wouldn't fuses pop regardless of whether or not the tubes were present?

As for the tubes, I originally started with a used set of four that came out of a working amplifier. Stuck them into the 6505+, and a random one arced. I tested them again in the other amp, all worked fine. Stuck them back in this amp, same order for consistency, and again, one arced in a different position. For good measure, I bought four new tubes, and again, a random one arced. So I really don't think that all 8 of them are bad when only one tube in a random position arcs. It's never the same tube, nor the same position that arcs before the fuse blows. There is no discernible pattern.

My guess is just that one spot happens to overload and arc at random before the rest, blowing the fuse. It did the same thing when I ran it with only two tubes.

The only thing I haven't done yet is try a single tube at a time in each slot. For argument's sake, let's say it does work/stays stable with a single tube... Where do I go from there? What do I check next?

[alexradium]

i did some work recently on one of these,same problem,popped fuse at standby off.
I found an open screen resistor,changed it,then proceeded to clean very well from dust,both compressed air and contact cleaner on all sockets.
Then i started reforming capacitors because the amp was on a shelf for quite some time,after this procedure everything went fine.
I've seen in other cases that molex connectors go bad,heater circuit draws lots of current and heats up the connections,potentially leading to conductivity or interruptions,if you unmount the main board i'd check out those lines and eventually make a mod to connect those lines to the sockets without passing on connectors.

Where do you see the arcing,inside the bulb or in the socket?

[Biffstar]

Yeah, I considered some of the ribbon/molex connectors too, and I even tightened a few down to give good connections. Continuity is present and strong in all of them, so I've ruled that out.

I also don't believe it's the heater circuit because after the fuse blows, the tubes all sit there just fine with the heaters glowing at normal brightness (i.e. pic above).

The arcing is within the glass bulb. I've checked the sockets themselves for carbon traces or any signs of shorting from pin to pin, and they're all fine. If any one socket had a problem, I imagine the same socket/tube would arc over and over... but that isn't the case.

[alexradium]

are those tubes from Ruby? (chinese str)
when a tube arcs inside its either too much voltage on the screen grid (and this amp has over 500V,easy) or it has metal flakes that fall down and short elements,which is easy in that position.

[Biffstar]

The tubes I've been using are 4 Mesa and 4 Electro-Harmonix. Not sure who the original manufacturer was before they rebranded them though. If a tube was internally shorted, would I not see arcing immediately on power-on?

No matter what I try, it always takes around 10 seconds from power on before a random tube arcs and blows the fuse. It's never instant.

[g1]

Hello in the 'Peg and welcome aboard.
Do you have a load connected when you are running the amp? This can be critical.
Otherwise, alexradium brings up and interesting point about arcing inside the tube being most likely caused by excess screen voltage.
If a power supply resistor supplying everything downstream of the screens opened, I wonder if the screen voltage would go high enough to cause your arcing.

As far as I know, this should be the same schematic:

[Biffstar]

Yes, of course I have a load connected. Haha I'm not that daft. It's a 16ohm Orange 2x12 visible in one of the photos above. Cabinet works fine, impedance reads correctly. No issues with the cab (before somebody asks!).

If the screen voltage is too high, how would I even go about correcting it? By upping the screen grid resistor values? Right now they're the stock value at 100ohm.

[alexradium]

tubes are not predictable,they can develop shorts after some minutes,anyway they don't need a real short,even a resistance of less than 300 kohms is enough for a tube to be defective.
tube tester can detect that behaviour but not always because they operate at 30/40 Vac in that test,and when you heat up with 500V in the amp,boom!
If the standby is really open,check that,i'd get a new set of Ruby's STR,or Sovtek WXT+ or JJ 6l6gc

[Biffstar]

Having arcing at random tubes in random slots really doesn't point to bad tubes IMO... especially when I've tried two different sets. But I'm going to try one single tube at a time tonight after work to see if they are indeed the problem. I'll be extremely surprised if they are. 8 bad tubes that worked perfectly fine before sticking them in this amp is kind of hard to fathom.

[Jazz P Bass]

How about a full on picture of the tube sockets.

It looks like there is something going on at the bottom of the holes.

[The Dude]

How about a full on picture of the tube sockets.

It looks like there is something going on at the bottom of the holes.

Agreed. It looks like some kind of corrosion or crud in the socket pins.

[TimmyP1955]

If the schematic posted above is the correct one, the standy switch opens the screen supply, not the B+ as is common, so any tube issues will still be in play (though the screens are not powered, they are still connected together via their 100 Ohm resistors).

Have you marked the tubes so you know that they are always going back into the same socket?

[Biffstar]

Here are the four tube sockets. The pictures might be a bit deceiving, but they are clean.






However, after doing a bit more searching on the internet, I came across this repair page:

PEAVEY 6505+ | PCBA Removal

Curious, I took a closer look at the pins under my connector, and noticed discoloration there similar to that repair site:





Sure, the burning isn't as bad as what's on that page, but still skeptical, I ran a tester across the pins... 100% shorted out between the pins. I don't believe that's normal...

Thing is though, that site says that these two wires/pins are for the filament, and would prevent the heaters from firing if they're shorted.. But my heaters seem to work fine... So something still seems off.