Carvin 50 Tweed Help!

randymelnar · Post by **randymelnar** » Mon Jul 11, 2011 2:34 pm

I use this amp a lot, especially when I can't use my super lead. It has never failed me, until last week. I heard a buzz, then the lights went out. The fuse blew. I put a fuse in, removed the power tubes, turned it on. It lit back up. So I ordered new power tubes, El 84's. 4, each were marked 21/5.3. I turned it on to see without biasing it what we had, it was a super distorted sound and very little power, even the clean channel was distorted. I don't know a tech here, I've only been here a year, I came here with 4 good amps in case I couldn't find a tech. Could this extremely distorted and powerless sound be because I didn't bias the amp or is there something wrong? Help! Thanks

toner · Post by **toner** » Mon Jul 11, 2011 4:26 pm

Aren't those cathode biased anyway? Hard to say what's wrong without more info. You may have a fried bias resistor. ?

randymelnar · Post by **randymelnar** » Mon Jul 11, 2011 4:57 pm

I just took it apart, no obvious things appear to me wrong inside. I cleaned it some, put it back together, changed a few pre amp tubes, nothing has changed as far as sound. It does seem to have that "fried smell" a little when it warms up, but it kind of always did. The Carvin manual didn't say anything about bias adjusting after tube changes, they said 'change' the tubes. I would question my new tubes next? It sounds like a 2 watt amp now, with all tube distortion. That's the best description I can give.

syscokid · Post by **syscokid** » Mon Jul 11, 2011 5:40 pm

toner wrote:Aren't those cathode biased anyway? Hard to say what's wrong without more info. You may have a fried bias resistor. ?

They're Fixed-Biased. You're probably right on the fried bias resistor. A damaged Screen resistor can do this, too. I use to have a Carvin Legacy Combo. A power tube went bad, and made a mess of the pcb for the power tubes. One of the bias resistors were obliterated! A nice sounding amp when working right. Could have been designed better for reliability, though.

randymelnar · Post by **randymelnar** » Mon Jul 11, 2011 6:19 pm

Would the bias resistor be the one in the center of the circle made from the power tubes soldered to the pcb? I can't find a complete schematic. And thanks,

syscokid · Post by **syscokid** » Mon Jul 11, 2011 8:21 pm

Here's a schematic link from the Carvin Museum. I'm assuming your amp is a VT50 or similar:
http://www.carvinmuseum.com/pdf/amps/11 ... REV-E1.pdf" onclick="window.open(this.href);return false;
Check for R37 & 38 (Bias), R60 R61 R64 R65 (Grid Stoppers), R62 R63 R66 R67 (Screen grids). Even if they look OK, check for continuity and resistance values.

randymelnar · Post by **randymelnar** » Mon Jul 11, 2011 8:30 pm

Thank you sooo much, I'm on it, I will follow up when I get it going again.

randymelnar · Post by **randymelnar** » Tue Jul 12, 2011 9:27 am

R37 & 38 (Bias), R60 R61 R64 R65 (Grid Stoppers), R62 R63 R66 R67 (Screen grids) are perfect or damn near +-%5. There is a bias adjustment I see. It says set for 75ma on standby. Is there simple things that could have fried with the power tubes? Preamp tubes or the speaker or something that would make sense to you guys? I've never dealt with this so all suggestions are welcome.thanks again. I miss those old tube testers that used to be in grocery stores and such. Makes me wonder if my new tubes are any good.

syscokid · Post by **syscokid** » Tue Jul 12, 2011 5:31 pm

Hopefully toner will come back here an offer more suggestions...or anybody else!

The obvious, is making sure you've got good tubes in there.

I don't think the bias setting has anything to do with your amps current situation.

Inspect the sockets...especially the power tube sockets. From the outside of the socket, where the tubes meet the socket, look for traces of electrical arcing.

From the PCB side, look at the soldering points from all the pins on those sockets. Insert a tube on each socket and gently wiggle it. Try to notice if any cracks appear in the solder which would be an indication of a cold solder joint. Use a magnifying glass for this.

Clean the sockets out with elctrical cleaner. Use the tube pins, in and out, to promote the cleaning.

Also, with the amp on and tubes installed, tap the all those soldering joints with a wood or bamboo skewer, and listen for noises related to bad contacts. Hell, tap and try to move those resistors, too! Be very careful when working on live amps...

How old is the amp?

What area do you live in?

randymelnar · Post by **randymelnar** » Tue Jul 12, 2011 6:19 pm

I am in FT Myers FL, I cleaned the boards, sprayed, wiped, and kind of got the residue crap off, especially where the tubes heat up the sockets. I inserted the new tubes, sprayed and cleaned, pulled in and out. I bought the tubes from jlassen36wy on ebay, he has 100% feedback. I got some Mullards before, though from someone else, on ebay still, and they had to be microphonic, they caused every amp to feedback I put them in, usually I use Doug's or others. That's what makes me wonder about the tubes..., but, these tubes don't blow my fuse, etc. I'm really sick of crappy electric at these gigs here in these old sea side bars, I blew a solid state- newer bass amp, an ashdown at one gig, that makes this amp #2 in a year or less. This amp may be 2008 at best. 2nd set of tubes. Everyone else I see uses cheap solid state stuff here, now I see why. Most, not all my pre amp tubes are new, and the speaker is a new british MOJO. That's why I want help. It's long term help here.

Froumy · Post by **Froumy** » Thu Jul 14, 2011 1:19 am

Aww crap. I didn't even look at the schematic.

Don't know if this helps, but the older Carvins(80's) would fry on the right side of the board. -15V? supply to the channel switching chips is poorly designed.. Dead without 'em. Stupid logic switching. Pisses me off every time. Sorry if I butted in without a schematic check-but it's relevant to any one working on an older one. The diodes aren't the only thing that fry, the chips will too.

randymelnar · Post by **randymelnar** » Thu Jul 14, 2011 7:27 am

I talked again to the Carvin tech guy, he said to plug the guitar into the return on the effects in the back, it was exactly the same, nice distortion, no volume, so he sent out a OT. Let's hope it's the OT! Thanks everyone, I will post if it works in 5 days when it gets here. I would have loved to take the time to upgrade, if I knew it was the problem, to a MM, but boy, it happens to be the most expensive OT MM sells.

randymelnar · Post by **randymelnar** » Tue Jul 19, 2011 2:09 pm

OK, the OT came in today, direct replacement, ten minutes to put it in and voila. It works like it used to!
Thanks guys

syscokid · Post by **syscokid** » Tue Jul 19, 2011 4:52 pm

Wow! At least the amp is fixed, but...

What could have caused the damage to the OT?

Carvin has some pretty good Customer Service. Did they give you a little break on the OT?

randymelnar · Post by **randymelnar** » Wed Jul 20, 2011 7:53 am

The OT was full price, 95. I had a Marshall do the exact thing before, blew a power tube and turned the OT to dust. I guess the second before the fuse blew, the ground short hit the ot? Then the little tiny output side of the Ot wires fried. I took the old OT completely apart. I couldn't find the cause of death, it was deep n the windings, but it had a smell to it. This makes me worry about the durability of this amp. With a MM OT I wouldn't worry, but they wanted 270 for their OT. This amp sounds so good too. It's a shame it isn't tougher.

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