View Full Version : Squeeling JCM800, bad tubes?
12-26-2007, 12:19 PM
My JCM800 combo is squeeling like a pig at times. I know this was discussed before with but than it was solved as it was then the power supply for the pedals. Now I checked that already, not it.
It does it lately most of the times when I turn it on (and a ts-9 or distortion pedal on). Today it was fine at first but halfway playing it came back. Also I seem to detect a little more background hum when it occurs and I think the tone gets somewhat thinner or maybe more treblier...Could it be a bad (pre)amp tube or maybe a bad contact ?
Thanks for the tips in advance fellows.
12-26-2007, 01:22 PM
Asssuming the tubes all look good, and that you ruled out an outside source such as pedal, cabels, pickups, etc.., bad tubes go microphonic. The way to tell is to tap each tube with a pick. The bad tube will transmit and amplify the tapping sound. The bad tube usually has an unusual look to it (brighter or dimmer, uneven glow, etc.) whether the amp is on or off.
12-26-2007, 02:02 PM
Try replacing your first preamp tube with a known good 12AX7?
That might be a cheap fault/fix.
12-26-2007, 02:22 PM
Try replacing your first preamp tube with a known good 12AX7?
That might be a cheap fault/fix.
Yep, what 'bolt said; that's usually the one! The worse the squeal, the higher up the cascade it is! :thumbsup
Yes, it's probably a pre-amp tube problem. But it could be a result of other issues in the amp...bring it to a qualified repairman. Just my .02
12-26-2007, 03:14 PM
Good tips. Well I tried tapping and moving each of the tubes, no result. They look fine , but I got some fine preamp tubes heading my way already, so think those are right in time for me to check this noise.. if not the repairman (again takes me a step further away from the Ceriatone custom amp I am trying to buy since summer) .
How old is your amp and your tubes? Pre-amp tubes last a fairly long time.
A re-tube and a re-bias won't break the bank.
12-26-2007, 08:44 PM
Are you sure that your amp squeals even if you plug it in at another house or studio? I can make my tube amp have this same type of squeal if I plug certain electrical devices (a ceramic heater for example) on the same circuit.
12-26-2007, 10:36 PM
Looks like everyone else had the same thought as me.
Pre-amp tubes get microphonic when they start getting real bad.
12-27-2007, 03:53 AM
hey Remy which model JCM 800 combo do you have? Mine squeeled too
12-27-2007, 08:04 AM
I think it is called the 4140? Let me post the pics again.
12-27-2007, 09:27 AM
It could be that you have it plugged into the same circuit as your tv and your DVD of Deliverance is bleeding thru to your amp. HAHAHA. Squeeeeeel like a piggy haha
12-27-2007, 12:20 PM
Sounds like bad tubes. When the noise happens, tap the side of the amp. If the noise briefly goes away, it is the tubes for sure. Probably a good time to swap them out. If you have more that a year of heavey playing or 2 years of good playing, they need to be replaced.
01-31-2008, 04:36 PM
Ok. We made some progres shere.
In short what been discussed, amp has not enough gain (i feel akak like the Randy Rhoads RI tread in Tone Talk) and squeels like a pig half of the times.
(pre)amp tubes has been all replaced in different orders as well all of them-i think they're good.
It needs also a different value presence pot-has not arrived yet, doubt it ever will, but my conclusion is I need pro-help::
To get more gain I turned the JCM up to about 8 on the master and about 8,5 ont he preamp...well gain there is..gladly my attenuator (Weber Mass) can handle it..but it's turned open only to 1 ..(of scale 10)
SO now..what happens, gain, squeeling still there at times but somethings I noticed which sound as (more) trouble. When my distortionpedal is on (into lead input) the amp seems to want to explode..it's noisy, it is hot, it wants to rock! However there's a pretty nasty hiss also..I knew that, the pedal kicks hard..well..when I unplug the guitar..I hear the hiss continue and some vague sounds..but sometimes silence and quietness...so that leads me to think there's maybe still a tube wrong or bad pots, condensators or circuitry..but the the bad thing is when it is plugged, pedal on settings like above...and I turn the guitar's volume down..it works..till it is off/closed...than pssccchccchchchcchchhchhhhhh the most crazy hizzing nasty noise.. :scratchhe :-x
This leads me to think the ground of the amp or maybe the hi input jack has bad issues..cause when it just is a tad open..that nasty noise is gone (the other noises come back :) ) So...all together I think it needs maintenance but but I am curious why the booster pedal makes unearthly noises when I shut the guitars volume down...it's weird men...
Well so far it is now..tbc!
01-31-2008, 05:13 PM
I heard that if the Tube Marshalls have low numbers on master volume, it will reduce the quality sound of the amp. Is that true?
01-31-2008, 07:47 PM
If your amp sounds like its about to explode I wouldn't turn it on again and I'd take it to a tech. The best sounding amp I ever heard was a Marshall JTM combo that sounded just like that. A minute later, the tranny blew. It was awesome sounding though for a while before it went.
I think you need to get yours checked.:bananahea
01-31-2008, 08:51 PM
I've been having the same problem with my 84' 800 Remy. Advice, The problem with these amps and why they notorious have these issues is that they have such High value treble and presence pots that they squeal like anything with too much treble. Cut the bright cap on the amp and see if that helps. Mine is in the shop with the same issue.
02-01-2008, 04:48 AM
Otherwise one word.....DrTube!
If you want to keep it, give DrX a call or mail :thumbsup He's got the strobes etc. to do a proper check...
What I would do is seperate the preamp stage from the main stage and test them. You can hook up a cd player and feed it directly into the powersection and see what happens...and hook the preamp section to a poweramp. It does sound like a earth issue the way you describe it, this was often the case with older Marshalls (and other tube amps for that matter!)
02-01-2008, 08:27 AM
Warlord: Yes. Tubes need to be turned up.
Mr Crazyhimself: Good tip-with all the issues, it seems destined to go ampdoctor for fixing. I definately like to look into that capacitor change.
Nightwatch: Nah not like that explode..more like a racecar with the brakes hit and running full power..it weants to jump out..I can just feel the power of the amp being absorped by the attentuator..though all little noises, heatfrom the tubes..it's more like it is chained..:)
Mies: It's got no effectslink so no seperating of the pre/poweamp. Good tip though..a cd player ..than one can tell what is up.
Main problem is I have only 1 amp-so I cannot really bring it to Dr. Tube or Peter van Weelden..as it might be weeks before I get it back..but I should have it maintained...:-x :scratchhe
Anyway..we know it needs help and the tubes ar emost likley not it.
Thanks for the input-will keep you updated on how this ends. As I have retolexed it myself..I do like it a lot and I know it rocks and can even be improved.
02-01-2008, 09:56 AM
I didn't mean using an effects loop, but opening it up and connect/disconnect it right for that particulair test :thumbsup That way you can check the pre and power section as a whole :headbang
It looks like you have either a ground or a lead dress issue right there, means opening up and do some expert wiring or testing (with the amp on)...you should know what you're doing!
I assume you already opened it up and carefully inspected the resistors and caps for any damage?
02-01-2008, 10:58 AM
Just a quick advice, take all the knobs and tubes out, and get some contact cleaner and just hose the sucker down with it. 1. the spray is an electronic cleaner and won't hurt anything 2. sometimes a pot that doesn't scratch still may have an issue on the inside. You might be getting a ''not so clean'' contact with the tube and sockets. My tech said the first time my amp off ebay was doing that and some contact cleaner solved it. A cheap way to see if you can fix it. :thumbsup
02-01-2008, 11:48 AM
I expect you already have ,but have you tried the other input with the same setup?Does it squeel through both,could be the input jack needs replacing?It's worth relacing the precence pot with the correct value as i'm sure this would make a difference though
02-01-2008, 01:41 PM
Well I did not Wozzaroo...as I never use that channel before really..but now..I did and definately something with the high gain channel (besides the presence pot that needs replacement anyway)..so ..I tried to force the high input with the cable plug..I can tell it is a switched input (meaning it activates only when it is plugged) but it is more than a corroded contact. Well that teach me I need more spareparts..hmmm
Thanks man..sounds dumb I never tried it, but really I tried all besides that :stupid ..as you could read.
Anyways new input, new presence..
02-01-2008, 04:21 PM
jsut talked to my tech, problem with mine was that the cable connecting to the inputs wasn't shielded...could be your same problem. check it. :thumbsup
02-01-2008, 04:45 PM
Will do so too. Thanks ! :thumbsup
02-01-2008, 07:49 PM
it can take much time fixing it...and it can be so many things...just bring it in bros! And have a maintenance check on it :thumbsup
You can borrow my 40W Fender or 200W Valvestate in the meanwhile :thumbsup
02-02-2008, 12:20 AM
I had the Same issue with my 87 JCM 800, It was repaired. I Since sold it and dont have the paper work. but it is a couple of Capacitors, that go from the Input to the preamp. I don't know which ones , but those are notorious on 80's JCM's they last about 10 years or so.
02-02-2008, 03:27 AM
Dr Tube is nearly 200km's away from me, in Drenthe-thoughtthey were inRotterdam/Zuid-Holland?!!
Think Peter van Weelden is an easier option, to Apeldoorn.
Vinnie, interesting...I know i need to bring it away-interesting on you mentioning that. Honestly , Holland is like a desert concering Amp repiars...the ones here are good, but there's maybe 3 in the country...and those 3 know all..so that's good..I think I should learn this trade myself..save me some hassle...but being a Mechanic, Elctronics are a scary shit to me.
02-02-2008, 09:56 AM
Just pay me a visit then Remy, and we'll have a look :thumbsup
02-02-2008, 10:33 AM
First you havbe to come over to my place, march 8th. Food and drinks on me-birthday bash:thumbsup
02-03-2008, 06:30 PM
Its more than likely a preamp tube. Sub in a new 12AX7 in V1 or V2 and that should remedy the problem. You may want to replace the power tubes and bias the amp if you havent doen that in some time.
02-05-2008, 01:30 PM
Presence pot 22k Lin placed and old one trashed. Presence seems fully functional again. I like it..around 6-7 si fopr distorted sounds my sweetspot.
It's not the tubes, definately not. I did detect the V1 socket to be noisy/loose (I did spray all sockets btw) it works, I doubt that's the cause off all the noise, but when I tap the V1 tube, you can hear (through the speakers) the socket move, which v2 and v3 do not have.
V2's socket has been replaced also once in the past I can tell. (Again I changed all the tubes various times-they are not it).
Input jack is ordered and heading my place, while replacing the Pot I checked the board and such..nothing loose on sight, but did see 2 different inputjacks soldered to the board..assuming the housing on input hi and input low should be equal..I assume 1 of them also has been replaced in the past...this Marshall is not factory made :scratchhe , but like my Alfa Remeo and many of my Kramers a Mutt!! Damnit:D ;)
Well..will be continued after soldering in the new input.
Thanks for time and all advice, fellas!
02-05-2008, 02:34 PM
Next time ask me bros, I got all that shit on stock!
E1,50 for 2 inputjacks :thumbsup
02-05-2008, 02:38 PM
Switched ones with pin connection for soldering onto the board, not the ones with lugs? .
Anyway, is ordered now. Thanks for the offer though Mies. :thumbsup (remember march 8th)
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