What causes this?

[stokes]

Just had another thought.Measure the voltage on your heaters.Make sure you are getting the 6.3vac you are supposed to have.Lower voltage here would indicate a problem.You have to measure this with the tubes installed.Have a feeling this aint the problem,but it cant hurt to check.

[496dart]

i checked a very similar amp , a 50w version. It got warm after an hour , but not hot.


On the problem amp, i get 5.7vac on the heaters. seems low .

[stokes]

Theres your problem.Either something is shorted to ground in your heater circuit or the current rating is too low on that PT.Pull your 2 preamp tubes and measure it again,if it comes back to 6.3v you most likely dont have enough current rating with that tranny.Also check the resistance to ground on each leg of your heaters.If it is center tapped disconnect the CT and measure you should get zip.If it is the 2x 100ohm "fake" CT you should see 50ohm on each side.That is without a doubt the problem,anytime you see less than 6 vac on the heaters there is too much current.That is the test to see if an unknown PT can handle the heater current.Ya know I was gonna suggest this a long time ago but with that 27v drop,I just put it out of my head.If there is no short to ground in the heater then you will need an aux.filiment tranny.But i cant imagine a PT that is made for that amp not having enough for those tubes.Unless it is rated for 6L6's,the EL34 have a higher current in the heater.An extra 2amps actually for 4 tubes.Check for shorts,if you dont find any then it is likely that the EL34's are too much for that PT.

[496dart]

Its definately the correct tranny for 4-EL34's. Heres something i dont think i mentioned: Ive added 1 extra stage(1/2 a 12ax7) in the preamp. However ive done this mod to lots of Marshalls in the past and never had a problem. I was told by seveal amp guys that 1/2 a 12ax7 should not load down the stock tranny at all.

So now for some reason im getting higher readings today ??

starting with 0 pretubes, i get 6.21v
1 tube 6.19
2 tubes 6.16
3 tubes 6.11
4 tubes 6.09

This amp doesnt use the CT. There is no resistance from the heaters to ground.

[stokes]

When you say the amp doesnt use the CT,do you mean it doesnt have one or it is just not connected?I looked at the tranny on tubesandmore.com and unfortunately it doesnt give the current rating for it.The fact that it is only giving 6.2v with no preamp tubes,and I am assuming you have the power tubes in,tells me that it is not delivering enough current to handle even the power tubes alone.If there is a center tap connect it to ground,if there is no CT you should have a 100ohm on each side of the heater to ground,.dont know if this will help the volt problem,but it will eliminate hum.Right now you have no ground reference so it might help to stabilize the voltage readings.The extra 12AX7 is only adding .3 amps to the heaters so it shouldnt affect it,if that was the problem your volts would have come up to par with 3 preamp tubes.See if the CT or ground reference helps,if not I suspect that the tranny is defective.Even if you were to add an aux filiment tranny it seems the main tranny cant even handle the power tubes.The slight increase you are seeing in the volts today could just be a fluctuation in the house AC,not uncommon,but it could also be the tranny is faulty.If the ground reference doesnt help,and I think it most likely wont, I would contact AES and get a replacement.By the way if the volts you just listed are without the power tubes installed,forget it and go right to returning the tranny,it is defective.But always use the CT or the resistors on the heater supply.

[496dart]

It has a CT (yellow wire) , but if i ground the CT, it instantly blows the main fuse.

[stokes]

Are you sure this is the CT for the heaters?It doesnt sound like it.A CT for the heaters is usually a green and other color striped or checkered lead.Didnt this tranny come with any literature?There are standard color codes for tranny leads and I realize that not all manufacturers follow them,but if that is the 6.3v CT then I would send the tranny back,it is defective.Do you know who made this tranny?Are there other yellow leads?Yellow is the color for a 5v tap on most trannies.If there is another yellow lead then I suspect you are grounding a "live" lead which would result in a fuse popping.Give me a rundown on all the color leads you have and how they are connected,maybe we can sort this out.Something is either wrong with the tranny or the way you have it wired.With no tubes you should get close to 7v on those heaters.

[496dart]

The trans is made by CE or New sensor ?. I got it from tubesandmore.com . part # p-tm-p100 is stamped on it.



gray and blue wires are 350v/ct @ 290ma with a yellow CT that is not grounded. Thats the one i tried to ground and it blew the fuse.

heaters: black and black wires with green CT. This green IS grounded.
6.3v@7A

bias: green and white wires 98v@50 .. the green is grounded

This all matches up with my Marshall 2203 scem from the Marshall book.

[stokes]

The CT I was refering to is the CT for the heaters.I assume you got the current ratings with some sort of literature that came with the PT.The 7amp rating for the heaters is not enough.Your 4 EL34's draw 1.5 amps each for a total of 6 amps.The 12AX7's draw .3 each for a total of 1.2 amps.From the voltages you are getting on the heaters I can see this is the problem.Most trannies,like Hammond understate their ratings so as to be on the safe side.So with a Hammond you can run these currents and still be safe.Even tho you are only .2 amps over the rating I suspect that,this being an economical tranny,they overstated the rating.The fact that you were not getting the full 6.3v's with less tubes tells me that either the tranny is faulty, or from drawing the .2 extra amps the tranny is already starting to fail, either scenario points to the need for a better quality tranny.I asked in one of my earlier posts if you knew the ratings of this tranny and you said you didnt.Had you told me the heater was only 7amp we could have resolved this issue much sooner.Dont know if this was the problem with the other tranny going south as the 27v drop on the caps you described earlier would also cause this,but this tranny running the way it is is surely headed for a failure.I would try to return it,but i wouldnt mention the extra tube you are trying to run.Then get a better ( higher rated heater winding) one.

[496dart]

I didnt know the ratings of the trans because i couldnt find the scem. I found it today and posted them.

I guess i dont understand how or why Antique Electronics would sell a Marshall replacement trans that isnt rated for a Marshall circuit

If hammond makes one of these, which one do you recommend? I will go to their site and try to find one. Or maybe i can use a small trans to run the heaters , would that take enough of a load off the pt?

[stokes]

Are you quoting these specs from the amps schem or did you get it from the actual tranny's literature? I cant say for sure if the tranny is actually underrated or if it is malfunctioning.But you are not getting enough current even with a couple of tubes removed.It is possible that the tranny was defective when you got it.If the 7amp rating is the actual tranny specs and not a number you got from the amps schem,it is barely enough for the stock tube compliment in the JCM800.It is possible also that the tranny was okay until you ran it with the extra AX7 and if it was damaged by the extra .2amps then I would have to say it is not a quality component.If you exchange that tranny I would get an aux 6.3v at at least 3amps and use it to run the heaters on your preamps.The main tranny if it is only 7amps is just enough to run your power tubes,for those tubes I would feel safer with at least 8amps.I dont know why Antique Elec. would sell a tranny that is so marginal for that amp.As far as using a Hammond,you would have to go to the site and check specs.Best to look for a 10 amp rating on the heaters,although if they say 8amps you can usually expect to be able to run close to 10,they arent the best out there but their ratings are usually understated for safety.Try to return the one you just purchased and explain the voltage drop you have on your heaters,it is very possible it was defective to begin with.

[496dart]

The specs are from the trans scem.
i will contact them about returning it, but i really doubt they will.

I may just try that 6.3v/3a for the filiments. This would just run the preamp tubes ?

example = # 266K12

what rating am i needing, series or parallel ?

[stokes]

At this point I dont think an aux tranny is going to help.Look at the voltages you posted for the heaters.With no preamp tubes you still dont have the 6.3 needed for the power tubes.This is an indication the tranny is not keeping up with the current demand.Using the aux. with that tranny is not going to help,as it is still stressing and will fail eventually.Dont follow you on the series or parallel question.

[Steve Conner]

Hi 496dart,

5.7V is real low for heater voltage and it sure does suggest something is getting overloaded, like other posters mentioned. But there could be other reasons, like low line voltage or suchlike, and some transformers just do run hot under load, especially cheap ones. IME, if you can't smell burning varnish after a couple of hours it'll probably live If it is overloaded, taking off just .3 of an amp by moving the preamp tube heaters to another transformer probably won't make much difference. You probably want to move two of your EL34s instead.

What voltage do you measure for the heaters if you pull all the tubes? How hot does the PT run with only two power tubes installed?

[stokes]

Steve,if you read a little further you will see he is only getting 6.1v with no preamp tubes installed.Waiting to smell varnish burning is the wrong route to go.That heater winding is being stressed with only the power tubes installed.You wont ever smell anything burning,but sure as anything over time this stress will eventually cause a failure.Using an aux tranny along with this tranny is not the answer no matter what tubes you run off it.If it isnt holding up to the 6amps the power tubes require,then there is definately something wrong with it.If you were to use an aux for the preamp tubes you would be removing 1.2amps,not .3 as you said. .3amps is the rating for 1-AX7 not all 4.