My V4 has been having some weird issues as of late. It'll play fine for a while, then the power amp section will kind of die. It'll still make some sound, but its pretty quite. I was able to isolate the issue to the power amp section after trying another amp as a slave and it preamp worked fine. I got the power tubes changed out about two months ago and rebiased and they are all lighting up.

I noticed that the power tubes aren't held in. Most V4s I've seen have spring enclosures that hold them in securely. Could one of the tubes be coming loose and shorting something out in the power amp? If so, does anyone have any experience getting Ampeg parts from these guys?

I don't believe that there's any way an output tube could come out of the socket and short anything out. What it could be, though, is that it wasn't biased properly. Or, that the one or more of the tubes didn't have any problems immediately, but after a bit of use, it failed.

I think it's something like the first 20 or 40 hours of use is the most likely time for a bad tube to show its true colors. Not necessarily in the first 10 minutes, which is probably how long an amp is warmed up before being biased. I like to play the amp really loud for at least a good half hour or hour, to make sure it's gonna work like it oughta.

I would check the output tubes, to make sure that none of them have developed a short. Another thing is, the V4 runs tubes so hot... I would recommend having a fan installed, or if you're gonna be playing the amp with the volume at more than about 3 or 4, have a fan blowing on the output tubes, one of those reciprocating floor-standing fans or something.

Also, stock, the V4 has a single screen resistor for all output tube screens. According to Kevin O'Connor, this will burn through your output tubes unnecessarily quickly. Putting a dedicated screen resistor on each output tube will lengthen their lives. Hell, dropping the screen voltage would lengthen your tube life, I think.

Do you have access to a friend with a tube tester? Somewhere you can check the output tubes and see if one or more of them is shot?

Oh, and if you're worried about the tubes falling out of the sockets, all you need to do is make sure that the sockets themselves are adjusted to be tight. There are pieces of metal in each hole of each socket that grab onto the tube's pins and hold the tube in place. If those are properly tightened, the retainers (things that hold the tubes in place) aren't doing anything anyway. Unless you drop your amp from a few feet up or something.

Also, just because the tubes are glowing doesn't mean that they're working properly.

But then, you haven't blown a fuse in the amp, right?

Still, an amp getting quiet but still putting out sound... for me that's almost always been one dead output tube, in a 4-tube amp like the V4.

Scott,

Thank you for your post, it has helped me out a lot. A tech friend of mine in Seattle is taking a look at my V4 is going to help me install a fan with a thermal switch.

The oddest thing about this problem is it has happend three times, on two sets of tubes. After I got it retubed and biased, it happened again, and the guy in town I was taking it to got it going again. Suffice to say, now that the same damn thing has happened, that is the last time he is seeing my amp.

Something I would do, aside from adding the fan which should help a lot if you're running into thermal runaway in the tubes, is to replace the screen grid resistor with individual screen grid resistors on each socket, and also to drop the screen voltage down a little bit. I think the V4, like many other amps, runs with the screen voltage unnecessarily high, and lowering it could help things out in terms of tube life. And definitely the screen grid resistors. Good luck!

did this happen after new output tubes?

what "brand" of tubes are you using? quality control can be a mess these days w/ lower cost "alternatives.". i have played v-2's and v-4's for years w/o having to use an extra cooling method. plus i rock it loud and hard. (that last line is for quoating purposes only).

do a test, run the amp, when it starts to misbehave, see if the output transformer is noticably warm/hot, if so that may be what is going on, the output transformer...to my knowledge, this is not a known issue w/ these amps, it can be a possiblity...

i use j j 7027a output tubes...

hope that helps!

I put a brand new quad of JJ 6L6GC's (the same tube as a 7027, without the internal-wiring pin duplication of the 7027) in one of my V4's and it would start to red-plate within a few minutes of playing with the volume over 8 or so. I like JJ tubes for sure, and those are the only brand of new 6L6GC's that I ever really buy.

And hey, a fan is pretty much never a bad idea for a hot tube amp, and should always result in prolonged tube life. That's part of why amps like the YBA-1A, YBA-3, and YBA-3A came with fans built into the chassis. Because it's a good idea.

hiredgeek wrote:did this happen after new output tubes?

what "brand" of tubes are you using? quality control can be a mess these days w/ lower cost "alternatives.". i have played v-2's and v-4's for years w/o having to use an extra cooling method. plus i rock it loud and hard. (that last line is for quoating purposes only).

do a test, run the amp, when it starts to misbehave, see if the output transformer is noticably warm/hot, if so that may be what is going on, the output transformer...to my knowledge, this is not a known issue w/ these amps, it can be a possiblity...

i use j j 7027a output tubes...

hope that helps!

It happened with the output tubes that were in it when I bought it this past summer. Supposedly it had been retubed and worked on within the last year. . . but you can't trust everything a Canadian off Craig's List tells you.

Right now I've got a new set of Sovtek 6550WE's in there. The tech guy that worked on it recentely had biased it for the 6550's.

I'm lucky that I've got a friend who knows this stuff pretty well and is working on my amp for free.

So I'm the friend who worked on chris' amp for free. Luckily Chris is a patient man. I've been meaning to post this for awhile.

At first, the amp would screw up, Chris would take it in, the amp would work fine for the repair guy, chris would take it home and it would mess up a again. That's the basics.

Then the thing just stopped working all together. Kind of a fizzle/fart sound whenever audio was going through.

Turns out, one of the large resistors had broken loose on one end. I soldered it back in place and sent it home with Chris.
The large resistor has enough mass, though, so it broke loose again. Their legs aren't stiff enough. So I made a few bends underneath and above the circuit board to absorb some of the shock.

Shortly after I figured this out, Chris and I were buying some 15" EV's from a guy who had a SVT on his bench. The large resistors had a similar shock absorbing bend in them. So aparently Newton's law still works. Holy shit.

It should be noted that once onoroff (aka new Police Teeth second guitar player Adam, aka "The Bullshit Adapter") fixed my V4b for good in early June, I haven't had any problems with it, and I've put a lot more hours on it after recording a record, and a lot of practices and shows.

onoroff wrote:So I'm the friend who worked on chris' amp for free. Luckily Chris is a patient man. I've been meaning to post this for awhile.

At first, the amp would screw up, Chris would take it in, the amp would work fine for the repair guy, chris would take it home and it would mess up a again. That's the basics.

Then the thing just stopped working all together. Kind of a fizzle/fart sound whenever audio was going through.

Turns out, one of the large resistors had broken loose on one end. I soldered it back in place and sent it home with Chris.
The large resistor has enough mass, though, so it broke loose again. Their legs aren't stiff enough. So I made a few bends underneath and above the circuit board to absorb some of the shock.

Shortly after I figured this out, Chris and I were buying some 15" EV's from a guy who had a SVT on his bench. The large resistors had a similar shock absorbing bend in them. So aparently Newton's law still works. Holy shit.

Quick question:
Is it a Chinese V4 or a St. Louis Music (USA) model?
The former have had massive quality control issues.

-A