Amps Carvin BelAir in the shop.

[CapnDenny1]

A guy brought this by last week. Nice amp. It is almost identical to the Nomad which I have, except the BelAir has a 2x12 instead of the 1x12 in the Nomad.

He reported the amp had an issue where it was fine for a while, and then the volume would drop off, to like half volume. He said he had it somewhere else, and they couldn't find it. That's always scary. intermittent issues are hard to find, and it's hard to know if you really fixed it, depending on how hard it is to get it to act up. If it ain't broke, you can't fix it.

I started by monitoring the bias. The schematic says to set the bias by measuring the current across the Stand By switch. That only works as long as the standby is in the off position. It measure the sum of all 4 power tubes. The schematic says to set it to 75mA. I believe it was closer to 90mA, or even a bit higher. So I cranked it down pretty quick. It seemed pretty unstable at lower current settings, but I later decided it was OK, just really sensitive.

I played it for a good 30 minutes through my test speaker on my bench. An Emminence Legend speaker. I was watching to see if the bias would creep up, or something, but it was pretty stable. I really couldn't find any issues other than the slightly high bias. Which could cause it to overheat with heavy playing.

So I thought, well maybe there is an issue with the speakers in the actual 2x12 cabinet. I looked at the cab and gave the cable with the phone plug on it that connects the speaker to the amp and little wiggle. I didn't like the way one of the push on connectors wiggled. It was too easy to make it change directions. It was just loose. So i investigated and found the speaker tab was bent way back, almost folded against itself. I also saw that the crimp connector on the end of the wire was bent open a little. the result was a really loose connector. I verified this using an ohmmeter.

You do NOT want to test a loose speaker wire by wiggling it when the amp is in use. For a solid state amp it's ok, but in a tube amp, you can ruin the amp by removing the load. It causes arcing inside the transformer and can ruin the output transformer. Bad tube sockets on the power tubes can do the same thing.

So i straightened the speaker tab and put new crimps on the wires. I checked it with the ohmmeter, and verfified the rest of the speaker connections were OK.

Then I went ahead and used my extender cable to allow me to connect to the cab on the floor, with the amp still on the bench. The cab has 2 Celestion Vintage 30 speakers, which I decided I liked. But guess what, I didn't like them. Too shrill and ice pick'y. Pretty clear, but just too much treble, and not enough bass. They may sound great in a band setting, and cut through like nobody's business, but just there in front of me, nope!

No issues with the amp now, but I will test it out for a couple more days just to be sure. I checked the bias with a bias probe, and it's a bit low, so I will tweak it up a bit. That allows me to test the current through each individual tube, in case one is way hot, or one is way cold. There's no hum, so they are probably pretty close.

Here's a picture of the bad connector, and the bottom of my Nomad, which is basically the same as the BelAir. Nice clean layout.

IMG_4606 by Dennis Kelley, on Flickr

Chassis_inside by Dennis Kelley, on Flickr

 

[Rancid Rumpboogie]

Hey, Capn, my Carvin BelAir does EXACTLY THE SAME THING. Fire it up cold, let it warm up for a minute or so, start practicing, and pretty soon the volume drops by about half, the OD gets "funky", and, well, it's just a POS. I can't tell you how long it takes to start going whacko, haven't timed it, and also the time seems to be a little random. But it always does it, and at "about' 10 minutes. Also, I only use the clean channel, and volume around "3'. Mine doesn't have the Vintage 30's, it has Carvin's speakers that were stock for the amp. I have checked the speaker connections, which seem fine, all of the tubes seem to be mounted solid and not wiggling about, put a whole new set of tubes in it, and it still does the same thing. So whatever it is, it isn't just isolated to the amp you have in your shop ... mine does it too. My BelAir has not been banged about at all ... it has spent its entire life just sitting in my house.

I HOPE YOU FIND THE PROBLEM SO I CAN SEND YOU MY CHASSIS TO SPRINKLE YOUR PIXIE DUST ON.

 

[CapnDenny1]

At my real job we call those Features! That way we don't have to fix it.

This one has not done that yet.

I will keep trying.

 

[Rancid Rumpboogie]

At my real job we call those Features! That way we don't have to fix it.
This one has not done that yet.
I will keep trying.

Well, that "feature" is why my BelAir has sat unused gathering dust for over two years. I haven't sent it off to any "amp guru" for a fix precisely because he would probably plug it in, play it, and send it back telling me it was 'just fine'. It will probably drive you batsh*t trying to make it happen, the even more batsh*t trying to figure out if you actually have it fixed.

 

[mountain man]

Carvin vintage are great amps!! Before the Nace it was definitely my tone. I've got 3 different sizes (16, 33, 50)but don't have the Belaire. I've got 2 vintage tweed cabinets too. I'd be interested in a 50 watt Vintage head but haven't found one yet..........

 

[CapnDenny1]

Well, you have to be sane, to go crazy, so I’m not worried about that.

Whatis it Warf used to say on Star Trek, “The strength of my enemy does me honor!” Or something.

 

[Rancid Rumpboogie]

And the starship Entropy goes where no man has gone before! Into the guts of a Carvin BelAir!

 

[CapnDenny1]

So did you check the bias current on that amp after you got new tubes and before?

Or try to isolate it to the preamp or power amp using the send receive jacks?

I ran this one for 90 minutes with no problems, but at low volume. My wife was listening to a live webcast, so I ran it thru the amp. Too much treble there too.

I will probably use the tone generator and a dummy load, and just watch it on the oscope and monitor the bias current. That way I can turn it up at higher power and not Have to listen to 1khz at 20 watts for hours.

 

[Rancid Rumpboogie]

No, Capn, I didn't do any of that.

 

[CapnDenny1]

You should start with that.

I’ve had a couple amps that red-plated on me.

One was an old Webcor that the bias adjust pot was bad, and if you bumped it the bias would drop to zero volts. It sounded ok at first but then the tubes just didn’t want to work very well, like they were struggling. Which I’m sure they were.

The other was an amp that had a coupling cap that would slowly leak dc from the phase inverter to the gate of the power tubes. The bias voltage would start at -25v or so, then would go to -20, then -10, etc. By that time the current was way too high. I think I saw 100mA in a 6v6. The owner kept using it anyway. When it got so high it quit working he would shut it down. Ifixed the cap pretty quick, but by the time he got it to me the tube sockets were ruined from repeated overheating incidents. Then the circuit would open up and caused the output transformer to fail. So a little $0.05 cap cost him the labor to replace the tube sockets, the tubes, and the output transformer. I think the final bill was almost $200, and I only charged him half the time I actually spent.

There are lots of things that can cause a signal drop in a tube amp, but red-plating can take out the amp, and make it not worth fixing.

That second amp was a Specter, which was bought out by Dean Markley. I liked the sound of it, so I have a busted one in my heap. It is after Dean Markley bought them, but it says “Specter by Dean Markley”.

 

[Rancid Rumpboogie]

Well, replacing the tubes was the last thing I did, and only turned it on once after that. It did the same thing as it had been doing, and I shut it down. That's what convinced me to hang up tube amps pretty much forever and I bought my Mustang III. I haven't really looked back since. That was somewhere around six years ago and I haven't had a single amp problem since. My Mustang III is still like new and my new Quilter has been going strong for two years without a glitch. That was after decades of constantly being plagued by one tube amp problem after another. I have only had one single tube amp that ever hung in there and kept working like a trooper should, and that was an old Ampeg 100W head I had back in the mid to late 70's that kept on truckin' gig after gig after gig, being hauled from one bar to another constantly. I saw this thread pop up and thought ... well, if CapnDenny can figure out what is going on with this BelAir, odds are it is the same problem with mine, and if I can get it fixed, why not.

 

[CapnDenny1]

I spent about 2 hours in the shop trying get it screw up, without success. I did get it to do this however. You will have to maximize it to see it.

It seemed to stop when I touched the bass or mid tone control. I couldn’t ge it to come back.

I had it running wit the tone generator and a dummy load. I had it from 5w up to clipping at 34w. Most of the time it was about 20w. Other than the weird stuff above it didn’t do anything.

My first guess aboutthe waveform above would be a microphonc tube, or at least one that had a tendency to ring. But there is th part of it starting at the peak of tge waveform, that says cap breakdown, or possibly a bad connection somewhere.

I will pull it back out and look at all the solder connections.

 

[Rancid Rumpboogie]

Capn, my guess is that you will try your level best to find the problem, never be able to make it do it, give it back to the guy, and have him calling you again telling you it still does it. There is nothing worse or more frustrating than trying to track down a problem that only happens when the amp makes up its silly mind to do it ... after 5 minutes, after 10 minutes, after a half hour of being played. It could be something that the vibration of actually being played causes ... in which case you will never find it with the chassis out of the amp.

 

[CapnDenny1]

I took the the two main pcb’s out last night and gave them a good look over. I soldered a few questionable spots, but nothing that was for sure an issue. They ran a 2 wire cable between the preamp and power amp pcb’s which carried the bias voltage. The cable used push on header connectors, on the preamp pcb where it was soldered I was suspect of one of the solder pads. It seemed like it moved when I wiggled it from behind.

I looked at it with the microscope, and couldn’t see it move, but the was a small little sliver of copper foil showing between the solder pad and the trace leading away. Normally you see either silver from the solder covering the copper, or green or red of the solder mask covering that keeps the wave of molten solder the pcb passes thru, from sticking to the entire trace. If you see copper then it could be an indication the trace s broken, and it pulled away from the pcb and pulled off the solder mask, and cracked the thin copper foil.

The pads appeared to have been soldered on before, since the amp was built. Perhaps there was an issue, or perhaps it was just cautionary. Someone like me guessing at what the cause is.

This type issue is hard to find. I will get the bias dialed in and try a test with some recorded guitar tracks. Maybe a nice Buckethead concert. That would be almost all guitar. Then I could listen and work on something else.

But I did find that bad speaker connector. That would have caused issues, so perhaps that was it?

 

[CapnDenny1]

The owner came and played it for about 25 minutes, and said that would have been long enough to get it to screw up. He was a pretty good player. He had that effortless touch, that the really good players had. He was doing swells with the volume control on his Strat better than I could do with a pedal. I gave him some advice to try and troubleshoot between the preamp and power amp sections, if it happens again. He said if it did he would try and capture it with his video camera on his phone. I only charged him $25 for the speaker terminal repair, which could have been the issue? He said he has several other amps stuck in the closet he is going to bring me to fix. We'll see. The Celestion Vintage 30's didn't sound as harsh with his guitar or playing style.