What causes this?

[stokes]

Thats different than the one on Schem.Heaven.I dont see anything that would be loading down the power supply and causing a voltage drop in that schem.I assume the extra 10k you mentioned was in series with the other one,which would only cause a lower voltage supply to the next cap when the tubes are installed,shouldnt cause your PT to overheat.I just looked at another schem that shows 2 10k in series after the choke going to the next junction,so I am pretty sure that isnt the cause.The fact that the PT is getting hot with no load (tubes) is troubling.With no tubes the only thing in the circuit is the PS caps,and they should not draw any appreciable current.I know the 5-6volts doesnt seem like much,but in my experience,it should not be there,and indicates some draw.I assume you wired in the new cap in place of the one at the point where the voltage drop appears,right?If that is the case and you are sure the new cap is good,I am at a loss here.There apparently has been an improvement over the 27 volts you were losing before,but the PT should not get hot with no tubes in it.I cant tell you to ignore it and use the amp,over time the extra heat could burn it out,as I would suspect is what happened before.If it were a case of it getting hot with the tubes installed I would suspect the PT were not rated for enough current in the circuit,but that is not the case here.Sorry for the long post,but I am mostly just thinking out loud here.Try checking the resistance to ground at each junction in the power supply,with no tubes and the amp unplugged.

[496dart]

Ok--i really appreciate your knowledge Stokes ! Yea, like i mentioned, the 1st can cap is the new one , and the other 2 i un-wired and clipped NEW JJ can caps in place.

Ill measure that resistance tonight .

Weird how those 2 schematics are different ? The jcm800's 2203 ive worked on in the past were just like the scem i posted. Maybe the one you had was a newer jcm800 version

[stokes]

Yeah, I've seen alot of discrepencies in schems.Could be newer versions,but it could be a case of for all the info there is on the net,there is as much misinfo too.It's rare that I really compare a schem to an amp I happen to be working on,usually just trouble shoot and fix the immediate problem.Post those resistances,maybe we will see something there.

[496dart]

nothing--no resistance to ground on the B+. the grounds are all good however.

[stokes]

I was hoping we would see some resistance at the cap that is dropping the 5-6v's.If you look back at the earlier posts,I think it was Ken who suggested connecting only the first cap,checking to see if the PT gets hot and then reconnecting each cap down the line one at a time ,checking as you go.I suspect when you get to the 3rd cap the tranny will get start to get hot again.There is something there drawing current,if it is not the cap itself,I cant imagine what it would be without having the amp in front of me.You mentioned earlier doing the 9v test.Although it is true it is not the best way to check for leaky caps,if you see the 9v drain off very fast compared to the other caps,you can be fairly sure it is leaky.That is sure,what is unsure about this test is that if it doesnt leak very fast it could be a case of not enough volts,but if it cant hold the 9v it is certain the cap is leaky.Another suggestion would be to move the cap that is showing the voltage drop down to the last position in the line and see if the voltage drop moves with it.One more thing,when you checked the resistances you didnt see a very high reading?Are you using a DMM?I usually see an unsteady,very high,like in the 8-10 meg range.

[ken]

I bet another one of your JJ caps are stinky... it's not unheard of. The problem is that most cap checkers simply won't generate enough volts thru the cap during a test to make the thing leak, some caps will be just fine up to a certain voltage and then as you go higher it will leak like a sieve. I had a very expensive tube home stereo amp do just that, if you checked the caps with a meter, they were great. Pull the tubes out, get 500v at the plates and grids of your EL34's. Put in the tubes and current draw made the plate and grid volts drop to less than 300v and the amp wouldn't work well at all and the trans got hot. Autopsy - Caps wouldn't hold a charge under load.

Anyway, switch the caps around and see if the problem 'follows' any one cap,
if it does that cap is bad.

Ken

[496dart]

OK, today i tried this:
removed ALL tubes , un-wired the 50/50 caps and started with the 1st one hooked up. Tranny was cold after 30 min.
wired up the 2nd cap -- tranny got "a little " warm after 30-40 min.

wired up 3rd cap--- tranny got warmer--but not HOT at all .after about 30-45 min.

Now im getting these voltages down the rail:

517/ 517 / 517 / 515 / 515 / 514

[stokes]

Those voltages dont look too bad.How is the tranny?Is it still getting unusually hot?Are these caps pre-formed?Do you "form" them before installing them or just firing them up cold?If the caps are not formed properly it could explain the slight voltage drop you are getting now.Just a thought.

[496dart]

Last night i tried this: hooked up the caps one at a time . the trans got warm by the 3rd cap but not hot after an hour. I put the power tubes back in and the trans got oretty damn hot after about 30-40 minutes. hot enough that i could only keep my hand on it for about 5 seconds .


????

[stokes]

Assuming your power tubes are good and biased properly,it still appears there is extra current draw,most likely at the third cap.Are your voltages still the same as you last posted?Without the tubes,I mean.It appears now that adding the power tubes current is taking the total draw over the edge.Are any of the power tubes redplating?Post the plates idle current and the volts on the plate,lets try that angle.I am sure this is getting kind of frustrating for you,but something is drawing too much current,and from here it is tough to pinpoint.Stick with it we'll get it,eventually.

[496dart]

The volts are 520/520/520 /518/517 / 515 .

With the tubes in i get 466V at the plates and 36ma idle current (per tube).

I dont know the age of the tubes--probably 6 months or so .

Right now i have it on and idling with a different set of EL34's in it. I will post back here in an hour or so.

[496dart]

Idling with a different set of El34's----Tranny still gets too hot after an hour .

[stokes]

What kind of PT are you using?I know that may seem like we are going backwards but it looks like the volts arent too bad,your bias current is okay,and I dont think it was asked yet.I know you said you got a new one,I have been assuming it is the right one for that amp.Do you know the current rating for the PT?

[496dart]

It is a replacement pt for a marshall JCM 800 - 100W . got it from tubesandmore.com . Made by CE ? Not sure on the exact specs, but this amp has an identical power section and output section as a 2203 model.

[stokes]

I thought so,just thought it wouldnt hurt to ask.Since you got rid of the 27v drop you had earlier, is there any improvement in how hot it is getting?Is it still getting stupid hot with no tubes?Do you happen to have another JCM 800 you can compare it to,the heat issue,I mean?I'm starting to wonder if the heat you are now experiencing is closer to normal then it was when you had that bigger voltage drop before.I'm sorry for all the questions,its just that I got a couple other projects going on right now so if I'm asking things we covered before.... I'm gonna email a friend of mine who is more of a Marshall expert,maybe he's got an idea.