MEDIC ! My Carvin just crapped out on me

[JohnW63]

I have a Carvin Vintage 16. It's become my main amp. I has had no symptoms, until tonight. Fired it up and is warmed up with a hum and inconsistant volume and just sounded dirty. I unplugged the input and turned it back on, to remove anything my guitar and pedal board would be adding. It came up with a hum, and as it warmed up, a bit of a pop and the same hum got louder. I unplugged it from the surge strip , unplugged the power cord and plugged it back in and put the amp straight to the wall, to remove a bad surge strip from the mix. Still hums. My first though is that it developed a bad ground, but the hum did not go away with changing the master volume or " soak " knobs. I haven't moved it in weeks or months. No new gear or cords. I haven't tried changing the powwere Did I just get a power amp tube fail ? I have the factory ones , still in a box, if I need to swap something.

 

[gilded]

Change out your tubes and see if that fixes the problem. If not, Google 'Carvin Amp Problems' It will pull up an old 2014 thread from talkbass.com that talks about issues with dirty ribbon cables on Carvin Bass Equipment.

Now, I don't know if your amp even has ribbon cables, but if the idea of cleaning ribbon cables is a new thing for you, call around and see if you can find a tech that will do it for you. That's what I'd do, anyway.

Best of luck, gilded

 

[Nuuska]

Hello

I just looked short info about this amp in net - no personal experience. But ampsare more or less all the same, when it comes to troubleshooting.

Trying new tubes is a good idea - but before that - there's the little switch in the back - selecting between 5/16W - try switching that one back and forth about 20-50 times - POWER OFF when doing that - if that is of no help - then you can make the tubes have better contact to their sockets by pulling them off their sockets and pushing back few times - alternative way is to tilt them slightly and "turning them around" - I do not know how to describe this in english - but you tilt it a bit and by rotation change the tilt direction - so all contacts retract and go back a bit - you just have to be careful not to tilt too aggressively. Again - POWER OFF while at it.

After this one have to open chassis and examine inside connections - if any connectors - open-close repratedly - if no help after that - qualified service person.

 

[JohnW63]

I can try contact cleaner on the 5/15w switch at the back. I'll pull the business part out and look for anything obvious. I'll see if there is a Carvin amp forum. I took digital and analog electronics in college and did small audio / visual electronics repair as part of my job until that sort of thing was replaced by computer based technology in our school district, so I'm not so adverse to checking on things in the amp. I know some voltages can be quite high, and I'll look into what to avoid.

 

[JohnW63]

This looks helpful.

 

[JohnW63]

I pulled it apart and with my wife's craft detail circular light/magnifying glass , I checked all the solder spots. I didn't see the tell tale half smile face or tiny cracks around the circle of the solder spot. So, I sprayed DeOxit in all the pots, and the amp wattage selector. as well at the ribbon cable from the amp selector circuit and in the tube sockets. . Problem still exists. Tone pots have an effect of the hum. Mostly the treble. My Amp has a reverb tank, as opposed to the one in the video, and the reverb has no effect. The soak ( preamp ) has the most effect. The volume seems to have much less effect.

I replaced the 4 main tubes some years back, but not the preamp tube. I wonder if that one has gone bad. I'll see if I can find a replacement.

 

[JohnW63]

Looks like the preamp tube is a 12AX7 . Two of the 4 I changed out are the same type, so I put one of those in. Problem is still there, but maybe less dramatic. I'm going to swap the power amp tubes and see what happens.

 

[Stuball48]

Waiting for next report--very interesting trip.

 

[JohnW63]

Well, it's none of the tubes. I cycles a known good original through all 5 tubes and still get the hum. Even tried a different power cord. So, the issue must be on the board, but not as obvious as I hoped. I'll have to take the board back out and take it to work, where I have a bigger magnifying light. I may warm the solder on all the pins for the tubes, in case they got micro cracks I didn't see. Since it's a hum, I may have to check the " input transformer " or what ever it's officially called. More Google research....

 

[JohnW63]

Here's a link to the schematic, if anyone wants to look:

https://www.thetubestore.com/lib/thetubestore/schematics/Carvin/Carvin-Vintage16-Schematic.pdf

 

[kakerlak]

Have you tried cleaning the tube sockets, like @Nuuska described? Radio Shack used to sell something called "TV Tuner Cleaner" that was great for pots/jacks/sockets. Hose it down with the aerosol and scrub the socket with the tube, rocking it around and in and out. Alternately, try tapping/jiggling on the tubes one by one while the amp is on and see if anything changes. Could have a pin or two making inconsistent content. Past that, I'd visually inspect any caps and see about replacing the (probably 1 watt) resistors going to the power tubes. (Disclaimer -- I'm no amp tech and my advice is probably bad)

You guys ever miss old Cap?

 

[Nuuska]

What happens to hum when you remove tube V1 ? If the hum goes away and returns with a new good tube - then I would check the anode supply voltage - 325V at point C - there should be no AC. Maybe one or more of the capacitors C26-27-28-29 have lost some of their capacity.

 

[JohnW63]

Nuuska,

I didn't try running it without any tubes. I can try. I have to see what the max DC volts range my multimeter has. Does the little "flags" marked C in all the spots indicate one common location that they are tied to ? V1 is up by the top and the caps are down near the bottom, so I suspect this is the case.

kakerlak,

I hosed down every socket and knob with DeOxit electronics cleaner, when I had the board out. I tried taping around and nothing changes. Since this came out of the blue, I suspect a component failure. Maybe.

 

[Tom O]

It could be a bad capacitor in the power supply. The electrolytic cans don't have much more than a 10 year life. We changed them every ten years in power plant equipment.

 

[JohnW63]

It hums whether the volume is up or down. The master volume seems to have zero effect on the hum volume. The tone controls do change the sound. Makes me thing it's after the volume, but before the tone circuit.

 

[DThomasC]

Try removing the phase inverter (V3, probably closest to power tubes.) If the hum disappears, then we know it's coming from the phase inverter.

If the hum does not disappear (my guess) then the hum is coming from the power stage. Hum from the power stage of a push-pull amp can be caused by one of the two tubes failing. If you can find a pair of known-good EL84 to substitute then try that.

Other possibilities are that the bias supply has failed or one of the coupling caps to the phase inverter is leaking electrically. Do you have a DMM and feel comfortable using it inside the amp when its powered up?

 

[JohnW63]

DTC,

"If you can find a pair of known-good EL84 to substitute then try that. "

Been there, done that. Swapped each of the 5 tubes, one at a time.

" Do you have a DMM and feel comfortable using it inside the amp when its powered up? "

Yes and yes.

 

[JohnW63]

Nuuska,

Pulled V1. As it warmed up, it started to hum, then dies down. Then I heard a pop, like before and the hum came back with a vengeance which included a very static like noise.

 

[DThomasC]

Did you try powering up the amp with V3 pulled? Don't worry, it won't hurt the amp.

" Do you have a DMM and feel comfortable using it inside the amp when its powered up? "

Yes and yes.

Then, with the amp powered up, measure the DC voltage between pins 2 and 3 of each output tube (V4 & V5). They should both be in the neighborhood of 15 to 20 VDC. Most importantly here, they should both be the same. What we're looking for is a leaky coupling cap.

 

[Nuuska]

So what about anode voltages and their filter capacitors C26-29 ?