where the heck is the cathode bypass cap on my marshall jtm60 combo

[sandford]

I have been looking at this amp which I loved at one point in time for like a year. I've had it open so long Its like part of my home. I need to get it fixed. It drops volume, bias is way off can't seem to figure it out. I cleaned the effects loops, checked and changed tons of components. I have come to checking the cathode bypass cap because it could be the answer although I've been this sure before about numerous problems with the thing that lead me to nothing. Anyone out there been here before?

[simonallaway]

If this is your amp schematic (from this site's archive):

http://www.el34world.com/charts/Schematics/files/marshall/jtm60_60w_jtm600.pdf

..then you have several. Two that jump out are C114 and C117. But those are on cathode biased preamp tubes. If you suspect those are shorted, you could at least remove them if you can't replace them.

When you say "bias is way off" do you mean the output tubes? Have you tried measuring pin voltages without the tubes in their sockets?

[phsyconoodler]

Those amps have DC heaters on the preamp tubes and those little bridge rectifiers they use often overheat and go south.
  But because your power tube bias is wonky,you may have a bias supply problem.Did you replace the bias supply caps and diodes?

[sandford]

No I have not tried checking those pin voltages. Its the power tubes that are out of wack. The amp got really hot and melted the plug jack so It was replaced kept dropping volume afterthat one output tube pulls over a hundred ma while the other is pulling 40ish tested on a .  I'm kinda taking a crash course at troubleshooting since I payed the last guy a ton of money for absolutely no results. The old jack worked just as well I wanted the amp to be playable.  I have a number of fender amps but this marshall plays the guitar for me. I have replaced many a cap on this thing trying to figure it out. I'm not a complete retard just new to it. I'm a second semester elt student trying to be a great tech one day. but that day is far off. I say this cathode bypass cap because Iv'e tried every other thing I could find on the internet about this amp. Bad design I guess but the tone is like no other I really want her back. The Rectifier between the two sets of heaters on the schematic has darkened the board underneath a bit but it tests out fine I pulled out of the board At one time I should of just replaced it then. What voltages should I be seeing at the pins of the Power tubes without tubes in em.

[sandford]

Hey sandford here again anyone know how i test a transformer safely out of circuit?do i just throw a resistor in series with each of the secondaries and measure what it drops? I don't want to die. And nobody at school seems to be of any assistance. I really need to get this jtm60 fixed I have no idea what else could be wrong with it. maybe someone can help. the thing is keeping me up at night and has been for like two years. This amp has sparked my intrest in pursuing a tech degree from a local college in which I'm finishing this semester. but a tech I am far from at this point, I need some hands on experience with an amp tech. Also if there is anyone looking for cheap labor I am offering my time in trade for some amp building/troubleshooting experience. I will relocate within the US to do so. I have been succesful in repairing a ton of amps for friends and family. Most of which are visible problems. I have learned quite a few tricks and techniques through reading posts and I am driven to succeed. I can't seem to figure out why one power tube is wicked hot the other normal. How do I know if the power transformer is not working correctly?

[PRR]

> one power tube is wicked hot the other normal.

That's not a power transformer issue.

I have two dogs. They both eat the same food. One is sick. I'm 99.44% sure it isn't the food.

Oh, OK, one dog could have an allergy.... tubes don't.

> What voltages should I be seeing at the pins of the Power tubes

Again: you have two. Compare and contrast.