What causes this?

[496dart]

with no tubes, the trans does not get hot.

the resistors are 10k

[stokes]

So if you are dropping 27volts across that resistor you are drawing 72ma with no tubes- that just aint right.With no tubes you should not draw any significant current if there is nothing in the circuit but the caps.But in your case it is like you have a pair of 6L6 type tubes in there when there are none in there.The tranny aint gonna get hot with a pair of tubes but when you replace your tubes you are now drawing what amounts to 6 power tubes into what should be 4.If you disconnect the cap with 494v and measure that side of the resistor with no cap and no tubes you will get the same 520+ volts you see at the first and second cap-you removed the draining cap,see?I would replace the one downstream as well- the 6v drop there is suspect with no load on the circuit.I think you have found your problem.You say the amp sounds good and it will probably start sounding better just before the new PT goes again.It could last a long time but the extra heat will win out.If the caps blow they will act like a fuse and stop any draw they were creating,possibly saving the PT from total instant destruction, but the longer the circuit is allowed to draw excess current the more the PT will stress and eventually fail.

[Mars Amp Repair]

Boy,
If it churned out that much smoke before the fuse blew, makes me wonder if the fuse hadn't been replaced at some point with a wayyy overrated one.

First of all as always mentioned in this forum, there are LETHAL VOLTAGES in these amps & if you are not familiar, stay away...better to hurt you in the pocketbook, than your life.

Lots of smoke could be even just from a couple of resistors...as a tech, you kinda get used to what smells like what when it burns. Personally, I would doubt the transoformers would do that these days....they typically just short & then blow fuses.

A good check would be does it blow with the standby switch in standby, or only when you throw the stby switch. If it blows with only the power switch on, then it could be the power transformer, but I wouldn't assume.
I would remove the output tubes 1st & see if the fuse still blows. Most of the trouble in tube amps comes from the section that draws the most current, hence the output section.

If the fuse doesn't blow with the power tubes out in stb or otherwise, probabaly one of the output tubes shorted & took some resistors with it...really not a job for a novice at this point.

Hope that helps...glen Mars Amp Repair

[stokes]

Glen, if you read the post from the beginning you will see some facts that are very pertinent here.For one he stated that he built the amp so it safe to assume he is capable of probing with out getting dead.Second the problem isnt that he is blowing fuses,he has already blown a PT and replaced it and the new one is getting unusually hot,not blowing fuses.He has also shown that the B+ is drawing current,72ma,with no tubes installed.Nobody is assuming he has a problem with the PT yet.But if the excess current draw isnt remedied he will most likely blow the new PT again.I think he has taken the diagnosis beyond the possibility of a power tube being a culprit,since he is drawing current in the B+ rail with no tubes.If you look at the posts he is dropping 27 volts across a 10k dropping resistor in the B+ with no tubes.

[496dart]

ok--thanks. i will order some new caps and let you know what happens

[Tom Phillips]

...72ma,with no tubes installed.... dropping 27 volts across a 10k dropping resistor in the B+ with no tubes.

That would be 2.7ma by my calculation Right?

[stokes]

Oops sorry,I used the formula for watts not amps-volts sq divided by resistance instead of... well you get it.Always did hate math.But 27 volts is still to much of a drop with no load.Voltage should remain the same down the rail if there is nothing drawing.Still think the caps are leaky.

[496dart]

Ok, i found an extra 10k resistor on the B+ rail that shouldnt be there. I cant believe i never noticed it ! I must have overlooked it when i built this thing ! I removed it ,now i get (starting at the rect.) 517v , 517v , 512, 512 down the rail . This is without tubes.

The current draw at the standby switch is .06ma with no tubes. With tubes in it jumps around from .09 to .69 ma.

I have it idling right now and i will report back after about an hour

[496dart]

The PT is still getting fairly hot after about an hour or so . I cant find anything wrong so maybe its just this brand of PT ?

[stokes]

Where was that "extra" 10k resistor and how was it wired?Check the voltage again with the tubes removed.You went from 521v to 517v on the first cap.The 5v drop you have now between the 2nd and 3rd caps is questionable.I have never seen more than 1 or 2v drop in a "clean" power supply- with no tubes to draw current there shouldnt be any draw,and 5v indicates some draw.Another check for bad caps is to check for AC volts on the filter caps.Your main and screen supply caps can have 2-5vac but if you see more than 1 volt in the preamp caps,that would indicate a bad cap.The ac check can be done with the tubes installed.

[496dart]

it was right before the 1st 10k. I cant believe i never noticed it..

I checked again today with new 50/50 caps clipped in.(no tubes) I get the 5-6 volt drop right after the 1st 10k on the B+ line. If you look at a 2203 marshall scem, you will see where im talking about. The 10k tests good.

[stokes]

On the schem I am looking at I see a 50mf- choke-50mf-10k*-50mf+50mf (100mf)-10k-50mf-10k-50mf.Is that what you have?What I am reading here is that you have only 4x 50mf's.The schem I have is from Schematic Heaven.That first 10k* is unclear on the schem I am looking at.In any case the problem I see is the 5-6v drop.I just double checked 2 amps I have on the bench right now and opened my MKII Boogie,and I dont and have never seen more than 1-2volts drop along a B+ rail with no tubes to draw current.That cap that now shows a 5-6v drop,and showed a 27v drop before is suspect to me.If the cap is indeed leaky just putting a good cap across the bad one will not help,since there is still a leak and the current will take the easiest path to ground which is the leaky cap.Take that cap out and try it with the fresh cap.One other thing to consider when "clipping" in a cap across another.If you are using can caps you have to be sure to ground the can of the new cap.

[496dart]

i did un-wire the other caps before i clipped in the new one. They were not in the circuit.
my amp is rect out > 1st 50/50 cap > fuse > choke > 2nd 50/50 cap >
10k/1w > 3rd 50/50 with 10k across the positive terminals.

[stokes]

The way I am reading what you have is your first 50/50 cap is actually 100mf if you dont have the 10k resistor across the terminals on that can.Then you have a choke going to another 100mf ( 2 50mf caps tied together) then a 10k to a 50mf to a 10k and finally a 50mf.Depending on what you have connected to those caps you could have it all wrong.What you should have,by the schem is rectifier to the first 50mf connected to a fuse then your plates and one side of the choke, the other side of the choke goes to the other 50mf and your screens then a 10k connected to 2 50mf's together to make 100mf connected to the plates of your phase inverter then another 10k and the plates of the tube feeding your tone circuit and the next 50mf cap with a 10k to the next cap for the plates of your first preamp tube.If,as I suspect,you have your plate to that first 50 and your screen to the second 50 you better go back and start again,your stages are not decoupled properly.

[496dart]

i double-checked again . my amp is wired just like the 2203 scem in the big Marshall book .
This is scem is found here: http://www.marshallschematics.com/