unclej Posted June 29, 2006 Report Share Posted June 29, 2006 ok, i hate to be a thread hog and i'm getting to where i hate to ask for help but this carvin has got me stumped. i don't know what model it is but it's a tube amp on a pc board. it has a rhythm volume with a push/pull 1meg pot controling volume and bright. then it has the same type of pot that says master/boost. next is lead drive/on and lastly master/high lead. two of the pots were bad and after replacing them i get sound out of both channels but i'm getting a very high pitched squeal not. the volume control will raise and lower the volume of the squeal. it doesn't over power the sound of a guitar..it's more like it's in the background. one lead from each of the pots terminates into a small value resistor..is one of those the likely reason for this squeal? thanks john Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 29, 2006 Report Share Posted June 29, 2006 That sounds like a pair of wires that are too close too eachother and there is some parasitic capacitance happening or something. Try pushing those wires around with a chopstick or some other non-conductive object (not a pencil!) and see if doing that will make it stop. If it does, you have found the culprit. Quote Link to comment Share on other sites More sharing options...
unclej Posted June 30, 2006 Author Report Share Posted June 30, 2006 That sounds like a pair of wires that are too close too eachother and there is some parasitic capacitance happening or something. Try pushing those wires around with a chopstick or some other non-conductive object (not a pencil!) and see if doing that will make it stop. If it does, you have found the culprit. thanks paul..i'll give that a try first thing in the morning. Quote Link to comment Share on other sites More sharing options...
JoeAArthur Posted June 30, 2006 Report Share Posted June 30, 2006 The amp sounds like one I have, one of Carvin's older X-series from the early 80's. Mine is a head which is in a really huge case, has 3-12AX7s and 4-6L6GC outputs (making this an X-100, there was also a X-60 and an X-30 version). Besides the controls you mention, it has bass, mid, treble and presence... a graphic equalizer - with a eq in/out switch next to the power and standby switches, and the reverb knob is located on the back of the chassis. It has 3 footswitch jacks - vol boost, reverb/eq, and lead... preamp out, power amp in jacks. A 60% power switch, two 4 ohm and one 8 ohm speaker jack, and a slave output with level control. Does this resemble your amp in any way? If your amp has reverb and the pan is unplugged, make sure the reverb control is off - you can pick up feedback when the input to the recovery amp isn't correctly loaded by the pan. Those resistors coming off the volumes are for extra attenuation of the signal and shouldn't be causing your feedback. Off the lead master should be a resistor/capacitor combo - a 180K (brown, gray, yellow) with the cap being either a .001 or .0022 that goes to the switch - this provides the high boost. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 30, 2006 Report Share Posted June 30, 2006 If your amp has reverb and the pan is unplugged, make sure the reverb control is off - you can pick up feedback when the input to the recovery amp isn't correctly loaded by the pan. Good point! Quote Link to comment Share on other sites More sharing options...
unclej Posted June 30, 2006 Author Report Share Posted June 30, 2006 The amp sounds like one I have, one of Carvin's older X-series from the early 80's. Mine is a head which is in a really huge case, has 3-12AX7s and 4-6L6GC outputs (making this an X-100, there was also a X-60 and an X-30 version). Besides the controls you mention, it has bass, mid, treble and presence... a graphic equalizer - with a eq in/out switch next to the power and standby switches, and the reverb knob is located on the back of the chassis. It has 3 footswitch jacks - vol boost, reverb/eq, and lead... preamp out, power amp in jacks. A 60% power switch, two 4 ohm and one 8 ohm speaker jack, and a slave output with level control. Does this resemble your amp in any way? If your amp has reverb and the pan is unplugged, make sure the reverb control is off - you can pick up feedback when the input to the recovery amp isn't correctly loaded by the pan. Those resistors coming off the volumes are for extra attenuation of the signal and shouldn't be causing your feedback. Off the lead master should be a resistor/capacitor combo - a 180K (brown, gray, yellow) with the cap being either a .001 or .0022 that goes to the switch - this provides the high boost. joe..you are my hero for today. that's exactly what it was. i had pulled the chasis and had jumpers running to the speaker but the reverb was unplugged. dropped the chasis back in, plugged the reverb in and squeal gone. i did the happy dance. i really do appreciate it..this one was driving me crazy. i wonder why it's the obvious that always escapes me. by the way, it's an x-100-d. i finally found it printed on the circuit board. i still think it's a wierd way to run a company but whadayagonnado? thanks again to both of you for your help. john Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 30, 2006 Report Share Posted June 30, 2006 i wonder why it's the obvious that always escapes me. Maybe because you are always expecting the trouble to be something complicated?! Quote Link to comment Share on other sites More sharing options...
JoeAArthur Posted June 30, 2006 Report Share Posted June 30, 2006 Thank you unclej... glad it all worked out!! To paraphrase an old TV show - "I love it when an amp comes together!!" The board number is not the amp model - and that board was used on a number of amps with different model numbers. It's a good bet that the "D" portion is a modification or version number. If your amp is the head version like mine as pictured here: 1982 X-100B, X-60B Then it would be an X-100B. Or an X-60B depending on power output. The "B" stands for British because these amps were built upside down and therefore backwards. Not a slam on the British, just what happens when you take a fender amp like a 1959 bassman and flip it upside down, put it into a head and call it a Marshall!! If yours is a combo version, then check out this page: 1982 Carvin Combo's If you want a schematic, PM me with your mailing address and I'll send you a copy - it's from 1981 which was the only copy they had. I got mine from Carvin many years ago but it was like pulling teeth to do it. Apparently the existing staff doesn't know much about their history, so when I asked for an X-100B schematic from 1982 they sent me one for the newer version that uses op-amps in the preamp for tone control. I think that newer model came out in 1984, using a 3-volume instead of the 4-volume setup like our amps. BTW - this amp is very similar in design to the Acoustic 160 series of tube amps. Ok... end of rambling... for now at least!! Quote Link to comment Share on other sites More sharing options...
unclej Posted July 1, 2006 Author Report Share Posted July 1, 2006 thanks for the offer of the schematic joe but it's fixed, screwed back together and waiting for the customer to pick up in the morning so hopefully i won't be needing one. paul, you're exactly right..and i should know better. my dad was an auto mechanic and one of the first lessons he taught me was to look for the obvious and the easiest fix. 90% of the time that's it. anyway, another amp lesson learned and hopefully stored in the old brain pan. thanks again guys john Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted July 1, 2006 Report Share Posted July 1, 2006 I wouldn't have thought of the reverb pan, either. But then again, when I'm testing an amp out, I connect everything to the amp that's supposed to be connected to it... Quote Link to comment Share on other sites More sharing options...
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