Carvin Series III X-Amp

[lowell]

edit: actually this is not series III... it's the version w/ the 82k/100k PI resistor combo.

This amp came to me with all sorts 'o problems. I've managed to bring it back to life... somewhat. The "power amp in" test ok. So I've narrowed it down to the preamp.

The amp is motorboating. The motorboating is only there when volume and master volume are turned up. There is no signal when plugging guitar into input, just motorboating. I removed V1 and the motorboating went away. I'm guessing that the filter caps for the preamp tube plates are bad. The schematic I have does not seem to fit this version of the amp. What would be the best troubleshooting procedure here? I might just replace the filter caps and see, but am open to suggestions.

[jrfrond]

I have seen those amps with multiple cold solder joints. Sounds like you might have an open or poor ground or SOMETHING causing instability. I would give the PCB a close look.

[Gtr_tech]

One of my main gig amps is a circa 86 X-amp. I always hated the way they sounded (overdrive channel especially) so I made some changes to smooth things out. After modding the dirt channel I had problems with motorboating and the signal modulating the pwr rail. I ended up adding pwr supply decoupling in the form of a 10k in series with the plate load (100k) and a 22u cap connected to gnd at the junction of the 10k and 100k. Gives the supply wiggle somewhere to go and keeps the rail stable. Tightens up the overall response as well......less mush. I can't remember if I did all 3 stages or just the first 2. Its been so reliable I haven't had to open it up in years.

Its all in the design of the front end. Some amps need decoupling....some don't.

[lowell]

thanks guys I'll check those out.

[lowell]

yup... in fixing a missing ground wire I mistakenly wired it to the wrong solder pad... boy those traces can play tricks on you eyes!!

[lowell]

Ok so the amp is now working, however the rhythm channel is dirty. It sounds VERY much like a blown speaker but I tried a known working speaker and it was still distorted. It is that fizzy distortion that rides on top of the signal. I've seen this before from bad solder joints but I think I've checked for that. I also checked tube voltages and nothing reads outrageously off. The "power amp in" produces good clean tone. If this was a bad solder joint and I cannot "see" it what would be the most likely place to cause this?

[Gtr_tech]

Check the Jfet switch associated with that channel's switching....common problem.

[lowell]

hi I don't see any switching Jfet in this schematic. There is a square w/ "switching logic" written inside it. In the amp I have there is a relay for the channel switching. Is this what you mean?

[BackwardsBoB]

You can find schematics for several generations of the X-Amps at Carvinmuseum.com in the "technical documents" section. They made the amps for at least a decade, and changed the design many times, but the more it changes, the more it stays the same. Stick with it. The old 6L6 based versions have a marvelous clean channel. Infinite headroom, fast response, a bit of warmth, versatile tone controls...

Those dinky screen resistors and the one feeding them could use a prophylatic beef-up. I have several x-amps that took moderate damage to the screen circuit from tube failures. Carvin greatly increased their wattage and value in the later EL34 based amps, and they're still kind of wimpy.

I've got a private stash of x-amp manuals with additional schematics too, though I only have access to some of them. Do you have a board number?

[lowell]

Hi thanks for help w/ this. I cannot see any board # except for the X-100-B on the board. I can tell you that it only has 1 input jack. All schematics I find have at least 2 input jacks. Another unique quality is that the Rhythm channel 12ax7 has the passive tone controls between the 2 halves of the first triode. This is not the case in any schematic I've seen. I also see w/ o-scope that it seems the 8-pin dip op-amp after the rhythm channel 12ax7 produces a hard square wave on pins 1-3. Could this be the problem?

[Enzo]

Did you talk to Carvin? CALL them and be ready to describe the amp so they can send you the correct schematic.

[lowell]

I called Carvin and explained. The guy sent me 3 solid state schematics that in no way represent the amp I have. Maybe I'll try to get someone else.

[Gtr_tech]

You prolly have this version:
http://www.carvinmuseum.com/pdf/amps...mp-3-28-82.pdf

The "switching logic" circuit is just a black box on the schematic. You're gonna need a scope to trace the signal from the triode stage through the EQ to see where the signal is corrupted. It may not be the switching at all, but it doesn't detail what the hell it is......

There are likely op amps in the EQ section (rotary EQ, the lead ch doesn't go through that) and they'd be likely to peter out.

[lowell]

Gtr_tech thanks, but that's not it either. My amp has only 1 input and the passive tone controls are between the first dual triode per Fender classic circuit. I seem to have narrowed it down to being around the relay switch. The preamp out produces corrupted signal and power amp in is good. Now, when I put a signal tracer on the outputs of the PI all sounds good to me...?? Doesn't match w/ the corrupt preamp out sound. If I trace the preamp out jack it goes back to the relay switch which is controlled by the push/pull "LEAD ON" switch. Is is common for relays to corrupt a signal? I cannot figure out how the relay/switch circuit is functioning as of yet. I figure in the meantime I'd ask of anyone's advice on relays causing distortion artifacts.

[Enzo]

Maybe you have the 1981 version, it seems to match your description.

I posted the preamp and power amp schematics for this over at www.ampix.org in the Enzo gallery. See if it is what you need.

Relays are nothing more than switches. The same things go wrong with their contacts as with toggle switch contacts.