JCM 800 2203 dead...

[MitchK]

this is a little mashed together but help is greatly needed and appreciated I have a gig tomorrow night...

So. I bought a jcm 800 2203 original issue one of the earlier ones. the other day it started making some bad noise as if there was a bad connection in the input jack, with a little wiggling it seemed to be fine but then eventually it started cutting in and out and then died. the tubes all looked fine in the back but no sound. left it and came back a few days still nothing, moved the post gain knob a little and it cut in payed at low volumes it worked fine, went to turn it up to playing volume and it cut out again. I fiddled with it a little and when i moved the post gain from high to low back and forth it would cut in again for about 5-10 second and then cut out again. noticed when I was adjusting the volume up and down that I'd get a humming/hissing noise from the power tubes, this wasn't coming from the speaker cab but form the tubes themselves, they also had some blue in the glow of the tubes. eventually turned it up fairly high and let it for a few seconds and one of the tubes failed and turned off. So was it just that that tube was messed up? or is it something else. The big test was that I then tried it with just two power tubes. this works fine in my 5150 and as far as I know should just run this one at half power but still nothing. I can get replacement tubes tomorrow in fact I wanted to put in kt88s/6550, but I have some questions about that two, the first of course being will it actually run because by the looks of it it might be something else and also should I only run 2 kt88s as opposed to 4 el34. This is what I do with the 5150, run 2 kt88s instead of 4 6l6gcs because wouldn't 4 kt88s equal like 200 watts and overheat and wreck the transformers??

[stokes]

You could run the KT88's but you would need to rebias.Although they are capable of putting out more wattage than the EL34's,in that circuit they wont automatically put out their maximum wattage.Running that amp with just 2 tubes could be bad unless you check the bias too.Also if 4 tubes are pushing 8ohm spkr load 2 tubes would need a 16ohm spkr load.Most amps can safely handle the mismatch but Marshalls are not as tolerant of an impedance mismatch.I wouldnt do it.Your initial problem sounds like it could be just a tube going south but I would have it checked out to make sure something else didnt go with the tube or actually caused the tube failure.In any event the bias will need to be checked and most likely re-set with whatever tubes you choose to replace the old ones with.Unless you were using GT coded tubes or the like and replace with the same-and are sure the old ones were biased properly.Dont just drop a new set of power tubes in there.

[MitchK]

yeah I'm going to be rebiasing no matter what at this point really.

a few questions:

the most important being, I tried the marshall with just two el34s and it would still not work, this is leading me to believe its more than just a bunk tube since the bunk tube was out of the amp at this point.

second if the amp can run off of 2 kt88s is there any real advantage to having 4 in there then?

and finally I don't quite understand the change needed in speaker load with switching the tubes down to 2 instead of 4 I wasn't aware of this. I've been my 5150 into a 4 ohm cab and still had the amp set to 4 ohms as well but with only 2 tubes does this mean I should technically have a 2ohm cab at that point? also running this on a marshal I've heard as well that they aren't so accomadating to wrong ohmages but would it make it just not run at all?

thanks so much hopefully I can get some new tubes and rebias this before tomorrow night

[tim]

I can't remember if that amp has an fx loop. if it does it may have anp open return jack or some spooge in the return jack tip contacts which would cause no signal out. jam your guitar cord into the return jack and see if you're getting any sig at the speaker. if so, I would suspect the jack.
Blue in tubes isn't necessarily bad. ALOT of blue would be but there will those tubes that have it and they are fine. I suspect you may have more than one problem with your amp. IF there is something wrong with the output circuitry (bias/output tranny, etc) you won't be able to bias it from the get go. You need to figure out why you aren't getting a signal out before you attempt to try biasing it as it will be an exercise in futility.
Just my .02

[MitchK]

there is no fx loop.

my main thing at this point is why I can't get signal with the 2 tubes. I'm worried I'm going to go shell out the cash for new tubes and they aren't going to work either.

[tim]

mitchk...I guess it's time to drag out the scope (or at least the voltmeter) and see if you have correct voltages/signals in the right spots. I usually pop a signal in the input jack and work through the signal chain until I don't see the signal. It's easy to check to see if you have signal on the control grids of the power tubes (pin 5)...if you have signal there then it has to be something either in the voltages (probably lack thereof) on the power tubes or your output circuit is crapped out somewhere. If you don't have a scope or access to one I guess you try and do it with a meter but it makes it real hard to know for sure if there are signals where they are supposed to be.

[Amp Kat]

The one I had did the same thing and it was the power tube sockets. Take some tech spray and shoot them down and see if it comes back. Wiggle the tubes around at low volume and see if you can get the signal back. It will pop when you do it but that's normal and so is the blue glow but deep purple means they are near the end. KT-88's will break up later than the 6550's and this amp doesn't break up until 10 anyway and you best not be in front of it when it does or it will hurt you. Four 88's would only give you about 3dB of increase and you can keep the switch on 4 and your cool. I don't think there are loop jacks on this amp and it doesn't have a master volume either.

[stokes]

Marshall amps should not be run with an impedance mismatch in the output.The OT could fry.When you run 4 Power tubes ,lets use 5881's for example.Your power tubes are running at 2100ohms plate to plate primary impedance.Lets say you are running this into a 4ohm load.Your OT has a certain turns ratio that matches the primary load to the secondary or spkr load.When you pull 2 tubes you now have about a 4200ohm plate to plate primary.Your OT still has the same turns ratio.right?In order to match the primary to the spkr load you need an 8ohm spkr load.Running the same 4ohm load puts you halfway between the proper 8ohm load and a dead short.Even if the OT can take this mismatch (which Marshall OT's cannot),your tubes are going to run hotter and die sooner.A mismatch in the other direction is not as bad,i.e.running an 8ohm spkr on a tranny that wants to see 4ohm,but you are running 4ohms into a tranny that wants to see 8ohms.Many amps can tolerate a 100% impedance mismatch but unfortunately Marshall OT cannot.Another thing to consider when pulling 2 tubes is that you pull the right 2.If you pull the 2 that are wired together on one side of the OT you only have one half of the push pull OT working.You have to pull 1 tube from each side.But in any case you should not run a Marshall with two tubes pulled unless you match the spkr load.As far as the KT88's go,if you use them you are not going to get 200watts so there is no need to run 2 only.A KT88 is rated for about 42 watts per tube.But that is its design maximum.You will not get that out of that circuit.Also you will most likely have to change the screen resistor as well as resetting the bias.You will also have to change the feedback wire connection from the 8ohm leg of the impedance switch to the 4ohm leg of the switch.

[Steve Conner]

I don't see why you can't use two KT88s in an amp that used to take four EL34s. As long as you get them in the right two sockets so you're not running single-ended of course

The impedance match, if you leave the speaker plugged into the correct impedance tap, will be about right to extract 100 watts from either four EL34s or two KT88s. The power is really determined by the B+ voltage, speaker load and OT turns ratio, more than the tubes, as long as you don't ask the tubes to do more than they're capable of. (If you pulled two EL34s without changing the impedance tap, the remaining pair would try to deliver 100w, and end up giving about 60w and burning up prematurely.)

By the same line of reasoning, even four KT88s would still only give about 100w, unless you changed the loading by connecting a 8 ohm speaker to the 16 ohm tap. Then you'd probably get almost 200w, but at risk of burning your transformers out. I've managed to increase a 50w amp to 100w by this method, but can't vouch for the long term reliability.

The 42W max plate dissipation rating of the tube is NOT the same thing as the maximum power output. Plate dissipation is the amount of power wasted during the amplification process, not the amount delivered to the load. This is a common mistake. In practice a pair of KT88s can easily deliver 100W of audio power without exceeding their ratings.

[stokes]

Maximum plate disipation is maximum plate disipation.You will not get 200 watts out of that circuit regardless of what speaker load you put it into.Cutting the speaker load in half will not double your output watts.It will run your primary impedance into that much closer to a short,thereby causing your tubes to run hotter and die sooner.You will not double the power output,that is a common misconception.To significantly increase the power a tube puts out out you have to use a circuit that will drive the power tubes harder,which will include a heftier PT and a driver circuit that will push the tubes harder.You can run the amp with 2XKT88's but you need to match the spkr load or you will burn out your OT.And you wont get 100 watts from the pair,in that circuit.Steve you say you managed to get 100 watts from a 50 watt amp by changing the spkr load?All I can say about that is you have a magical amp.You will get a couple of more watts because you are running the tubes a little hotter,but that is all.If you dont believe me put it on a scope and you will see.It aint that simple.If it were that simple why would any body buy a 100 watt amp?We could just change the spkr and double our output.We could build 100 watt amps with much cheaper 50 watt OT's.If you were actually putting 100 watts into that 50 watt OT it would at best saturate and distort like hell at about 50-60watts.Most likely you would sooner or later end up with the worst case scenario and fry the OT.That said,I want to repeat my earlier warning on this.While Fender and other OT's can tolerate a 100% impedance mismatch,Marshalls OT's are not so tolerant and should not be run at a mismatch at all.In over 25 yrs repairing amps the biggest cause of and probably the only cause of OT's failure in Marshalls was because the user had the switch set to the wrong impedance.

[wizard333]

My best guess, without seeing the amp, is that you had a power tube problem to begin with, but cranking it and causing a tube failure or short in the socket may have taken not only the tube but a few friends with it. Hopefully, one of those friends wasnt your OT, but its possible. Its one of those situations where, if something is stuck, applying more brute force may not be the best thing to do. If the amp is intermittent, cranking it up and forcing a failure isn't really the best thing to do.

You need to buy new tubes anyway so you may as well do that and try it; its not like you can get the amp to work with your current bad tubes in any case.

What was mentioned above about the tube sockets is good advice when putting the new tubes in, clean the contacts, re-tension if necessary, and check the wiring to the sockets to make sure something hasnt come loose. If you do that and put new tubes in and its still dead, start looking other places.

How old are your pre-amp tubes? Your driver tube? Any of those could be your problem as well, try replacing and see if that helps. Also, from the sounds of it, check your input jack and make sure you have continuity from your input to your first gain stage. If all that checks out, then check the OT.

[MitchK]

alllright sooooo. I feel a like a bit of a tool saying this but I finally got to take it apart yesterday before the show and it turns out one of the fuses inside had just blown. so I replaced it and things were fine BUT since one of the tubes had already failed I rant it with just 2 tubes at the show, I matched teh impedance up right and everything as per what was talked about above but I had to crank it to the max to get a reasonable amount of volume out of it, and after the second set it craped out again. I was kind of expecting this but it had to be done at the time. Just got another look at it now and it was just another fuse that went, the only thing to see now is whether or not it keeps blowing fuses after I get some new tubes and so on in there.

One thing I've noticed though is that all the schematics I've seen for the 2203 show one fuse just in the mains power section but the amp has about 8 different ones on the inside, is it just assumed that I should know where these go and what they are for? some of the PCB says "center tap" and "bias" so that makes sense but then ones just say f5 for example and then says 500mA next to it but the fuse is actually a 5A. a little confusing for me.

What I'm wondering now is about putting KT88s in it because I'm definitely going to do that but I want to know if there is anything I should do? I mean I'm going to have to rebias of course, and I could use some help locating where I'll want to put in a trim pot for each socket so I can have an adjustable bias from now on, but someone mentioned changing the screen resistor? why would this be and what values should I be looking at? I think they are 5 watt 1K ohms on the schematic? With my 5150 I just dropped 2 kt88s in, rebiased, and it was fine?

just wanted to say that this forum is amazing, its a massive resource, I've been reading it on and off for almost 4 years and I would know squat (well I still know squat but I'm getting there) with out.

[Amp Kat]

In over 25 yrs repairing amps the biggest cause of and probably the only cause of OT's failure in Marshalls was because the user had the switch set to the wrong impedance.

I certainly expect your experience Stokes and have had many good conversations on amps with you in the many years on Ampage but I have to disagree with you on this one. Mismatching the speakers in any way will not harm the output tranny in any way and the taps are so close that there just isn't enough difference to matter. The amp will have dynamic effects on volume but that's it. Amp Co.'s would not put that switch there so end users could blow their amps. You can run an OT at zero ohms with correct bias and it would last and not blow however it may burn the tubes out faster which comes to my next point of why OT's in all amps blow. End users not changing the tubes soon enough no matter what the cause. Speaker cables that get twisted and OPEN up during high power conditions or any cause of an open whatsoever at high volumes. Bad speakers,running the bias too hot causing red plating. Loosing the bias totally and not changing the filter caps in amps with 40 plus years on them,power surges and ect.... I'll take a speaker cab at 2 ohms with good tubes biased correctly and can play till the cows come home wide open with no problems at all with the selector on 16 ohms. It won't sound as good as it on 4 but it won't do any damage to the OT either

[Tom Phillips]

One thing I've noticed though is that all the schematics I've seen for the 2203 show one fuse just in the mains power section but the amp has about 8 different ones on the inside, is it just assumed that I should know where these go and what they are for? some of the PCB says "center tap" and "bias" so that makes sense but then ones just say f5 for example and then says 500mA next to it but the fuse is actually a 5A. a little confusing for me.

Most Marshalls will have two fuses. The Mains fuse and the HT fuse. I think you just missed one of them. Looks like you have an amp fitted with the fuse board as used for the Canadian market where the code requires the extra fuses. In you example “F5” is the part designation meaning “Fuse Number 5” and 500ma is the value that should be installed in that position. All the other fuses should be similarly labeled.

HTH,
Tom

[stokes]

Although you can turn an amp on at zero ohms and let it idle,if you put a signal thru it,the secondary which is running at a dead short (zero ohms) will eventually fry.I gotta disagree with your that running a 2 ohm load on the OT set for 16ohms.That tranny needs to see 16ohms to run efficiently,2ohms is very close to a dead short.It will draw more current and get hotter.Gennerally an OT can stand a 100% mismatch i.e. 8ohm load into a 4ohm tap,problems arise when you go the other way.In my experience Marshalls dont hold up as well as say A Fender.This was also a topic covered ,I believe ,but have to look in one of Webers books.Another issue to look at when running a mismatch is that the spkr load changes with the frequency.As the freq. gets higher the load gets higher adversely it gets lower with lower frequencies.When you mismatch to the lower side you are asking for trouble.As you say "you can run an OT at zero ohms with correct bias...".You can rebias the tubes to run safer at an impedance mismatch,but how many end users know how to do this.Back in the old days the only amps were Fender or Marshall.And it was common knowledge among techs that Fender OT's could stand the impedance mismatch and Marshall could not take it as well.Sure the power tubes usually go first but they quite often take the OT with them.And it happens more with Marshall than any other amp.In any case I would not run any amp at zero ohms or an open load.And I would not run a Marshall at any mismatch.If I can find which book discussed this issue I will let you know,but its been a long time since I looked at a book,and now I gotta find my damn reading glasses!