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simple SE blowing fuse when 6.3v attached with no tubes in place.

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#1
Hi, I have built up the PCB and case put together finally for testing, and it blows a fuse.

my transformer is an edcor xpwr059

I have tested it with all secoundarys unattatched and all voltages are fairly normal with no fuses blowing, attatched them seperatly and both the 5v & 750v line were fine when attached to the PCB however when the 6.3v line is added the fuses almost instantly pops

all tests where done with no tubes fitted.

if any one can suggest any ideas? I can only presume my wiring to be correct.

cheers,
 
#2
update:

stuck a fresh fuse in and 2 kt88's & a 12at7

switched on the fuse didnt blow instantly but looking at the base of the 12at7 1 leg was glowing brightly (this is my first valve amplifier so not sure if this was normal but I presumed it not to be so turned off)

didnt see any noticable glow on the two kt88's however.
 
#3
update: 3 - is there no way to edit your posts?

disconnected all secoundarys and attatched the 6.3v winding to the heater (pin 4&5 of the ecc81 / 12at7 valve

I place 2 meters to measure the voltage and current

switched the transformer on without the valve connected (i.e. completely unloaded) and it made 7v turned off connected the valve, and then the instant I turned it on the fuse blew.

I am leaning to the problem being the transformer, as it dies when under any load (though I struggle to see how a valve-less PCB board would overload it?) but is fine when disconnected.

I'll probably buy another 12at7 just be safe.

any idea as to how I can test the transformers 6.3v winding using a variable load of some sort?

cheers.
 
#4
I suggest a problem with the PCB, shorting the 6V line. Check your grounds and make sure you have no shorts. Disconnect the transformer and measure resistance at the PCB on the heater line. And the other lines as well. You might have a reversed capacitor or diode.
 
#5
Pull all the tubes, and disconnect the power transformer from the T1-GRN connectors on the circuit board. Put your ohmmeter across the terminals on the board. You should read open circuit (infinite resistance). You can also check between either terminal and ground. I think one terminal will show open, and the other should should 10K ohms.
 
#7
disconnected all secoundarys and attatched the 6.3v winding to the heater (pin 4&5 of the ecc81 / 12at7 valve I place 2 meters to measure the voltage and current switched the transformer on without the valve connected (i.e. completely unloaded) and it made 7v turned off connected the valve, and then the instant I turned it on the fuse blew.
Click to expand...

I suggest a problem with the PCB
Click to expand...

Are you saying that you connected the transformer directly to the 12AT7 without connecting the PC board and the fuse blew? If so then the PC board isn't the problem.

What size/type fuse are you using?
Click to expand...

That would be my first question. On UK power you should be using a slow blow 1 amp fuse.

The second thing to check is the primary side wiring. You should have the black/white and green wires connected together with no other connection to them. One of the mains inputs should be connected to the black wire, and the other to the green/white wire.

The grey wires, the white/yellow, and the white/brown wires should have no connection made to them. They are not used in this application.

I am leaning to the problem being the transformer, as it dies when under any load (though I struggle to see how a valve-less PCB board would overload it?) but is fine when disconnected.
Click to expand...

I have never seen a transformer do this but I suppose that anything is possible. If the transformer acts up when connected to the 12AT7, what happens when connected to another tube? You were connecting the yellow wires to pins 4 and 5 of the 12AT7 when the fuse blew, with no connection to any other secondary wires? Tape up (seperately) the red leads so that there is no possibility of contact with them when testing a transformer like this since the high voltage on these wires can be lethal. Try connecting the yellow wires to one of the KT88's. Does it light up? Try connecting the brown wires to a 5AR4 with no other secondaries connected. Does the tube light up?
 
#8
tubelab.com said:
Are you saying that you connected the transformer directly to the 12AT7 without connecting the PC board and the fuse blew? If so then the PC board isn't the problem.



That would be my first question. On UK power you should be using a slow blow 1 amp fuse.

The second thing to check is the primary side wiring. You should have the black/white and green wires connected together with no other connection to them. One of the mains inputs should be connected to the black wire, and the other to the green/white wire.

The grey wires, the white/yellow, and the white/brown wires should have no connection made to them. They are not used in this application.



I have never seen a transformer do this but I suppose that anything is possible. If the transformer acts up when connected to the 12AT7, what happens when connected to another tube? You were connecting the yellow wires to pins 4 and 5 of the 12AT7 when the fuse blew, with no connection to any other secondary wires? Tape up (seperately) the red leads so that there is no possibility of contact with them when testing a transformer like this since the high voltage on these wires can be lethal. Try connecting the yellow wires to one of the KT88's. Does it light up? Try connecting the brown wires to a 5AR4 with no other secondaries connected. Does the tube light up?
Click to expand...

yes I connected the power straight to the tube, I used a 2A fuse(s) although I dont know if they where slow blow or not.

Primary windings are correct. and the unused secoundarys are also fine.

I'll see If I can talk my dad into running a couple of the tests you post (I am a student and all my electronics gear at home)

namely the 6.3v winding to the kt88's. I should also probably check the idle current draw on the primary 240v and 6.3v secoundary side (with the transformer fully detatched) which might give a few more clues.

I'll give an update as soon as I can, but it might be aslong as 3-4 weeks for me personally to test it out.

cheers.


p.s. Ty_Bower I recall there deffinatly being an infinite resistance between the t1-grn connections but I cant test for the resistance to ground as I dont have the PCB with me to test
 
#9
Hi
OK I'm having a look at the amp whilst my son is away at uni.
I don't know a lot about electronics am ok around cars etc.

so I have got it running (well not blowing fuses) reworked the earth and bought some more slow blow fuses .so it runs.

sound is only from one speaker sounded nice very slight hum from other speaker (constant irrespective of volume)
I have read some posts that I thought where pertinent to my problem and placed a croc clip from c21 - c11 volume came out of both speakers at a lower volume ( I guess it's halved by sharing) so faults on the input side. the 12AT7 tube was a second hand one so I ordered another new old stock tested tube but when fitted same symptoms I have rechecked joints / earth swapped tubes btw the tubes get really hot
so can someone give me some test points / voltages ect so I can pin it down (I am now about to pull hair out!)

power amp is edcor xpwr059
opt xse25-8-3k
R1 fitted
no freds
in fact removed vol pot so its just as simple as I can make it
voltage at the plug is 249 at the moment

cheers in advance
 
#10
hi
well I just looked through the pcb again with a meter and soldering iron reworked a few of the joints and removed some resisters to test there values, and one of the resisters was wrong r25 was 220ohm not 220k changed it and low and behold music from both speakers and sweet music it was too, but then rectifier valve flashes bright and blown fuse (lots of swear words). only other thing I found was resister r3 should be 10k 1 watt we used 10k 3 watt I didn't think that would make a difference. I all so noted that the 5 watt resisters where getting very hot, to hot to touch in fact. so my post above is kind of solved but now blowing fuse again when fuse is replaced it powers up looks ok music starts to build up volume and then flash blown fuse probably about 10 seconds
Any "hand holding" help appreciated
cheers
 
#13
Some 5AR4's are prone to flash-over when run close to the max peak current rating (which I think is the case with the SimpleSE) -- even though they supposedly operate within the specified limits. I've used some Sovtek 5AR4's with good results. They seem pretty rugged.

~Tom
 
#15
proview1122 said:
but then rectifier valve flashes bright and blown fuse (lots of swear words)... when fuse is replaced it powers up looks ok music starts to build up volume and then flash blown fuse probably about 10 seconds
Click to expand...

Once a rectifier arcs over for the first time, it is virtually guaranteed to arc over again in the future. May as well just give up on it now, and stick it in the bin. You need a new rectifier, sorry. 🙁
 
#17
just doing some testing
voltage at wall socket 248v

using test photos at Checkout pictures by Ty_Bower - Photobucket
thanks Ty it should maybe be a sticky


The electrolytic caps in the power supply I'm showing a final resistance of ~80K ohms = 76.2k
Check the 220K grid resistor on the 12AT7 (pin 2) = 217.7k (pin 7)=217.3k
Check the 220 ohm cathode resistor on the 12AT7 (pin 3) =220.5ohm (pin 8)= 202.6ohm
Check the 220K grid resistor on the power tube (pin 5). right=219.8k left 219.8k
Check the cathode resistor on the left power tube (pin 1)=566.3ohm pin 8 566.4ohm
Check the cathode resistor on the right power tube (pin 1)=570.3ohm pin 8 = 570.3ohm
Check the strapping from plate to screen. should be 100 ohms (pin 3-4) left 121.2ohm right 121.4ohm is this a bit high?
Check 12AT7 socket for 6.3 VAC heater voltage (pin 4 and pin5) =7.3vac same voltage on pin 2 of power tubes is this a bit high?
Check the rectifier for 5 VAC filament voltage (pin 2) = 6.1vac is this a bit high?
Check rectifier for HIGH VOLTAGE (pin 4) = 404.3vac (pin 6) 404.9vac


thanks
 
#19
proview1122 said:
Check the strapping from plate to screen. should be 100 ohms (pin 3-4) left 121.2ohm right 121.4ohm is this a bit high?
Click to expand...

No problem here. I added an extra 250 ohms to the screens on mine. I was having issues with cheap tubes and glowing screens. The extra 250 ohms didn't fix my problem with the cheap tubes, but it doesn't seem to affect the normal operation with good tubes. I haven't bothered to take out the extra resistor. You won't hear any difference from the extra 20 ohms on yours.

Check 12AT7 socket for 6.3 VAC heater voltage (pin 4 and pin5) =7.3vac same voltage on pin 2 of power tubes is this a bit high?
Check the rectifier for 5 VAC filament voltage (pin 2) = 6.1vac is this a bit high?
Check rectifier for HIGH VOLTAGE (pin 4) = 404.3vac (pin 6) 404.9vac
Click to expand...

These seem a bit higher than I would like to see. They will come down a bit once you load up the windings (by plugging in the tubes). Still, these unloaded voltages seem higher than they should be. Sometimes cheap multimeters give bad readings on the AC scales. I checked the specs on the Edcor XPWR059 power transformer, and it does say it has a 240 volt primary. Odd.
 
#20
Ty_Bower said:
No problem here. I added an extra 250 ohms to the screens on mine. I was having issues with cheap tubes and glowing screens. The extra 250 ohms didn't fix my problem with the cheap tubes, but it doesn't seem to affect the normal operation with good tubes. I haven't bothered to take out the extra resistor. You won't hear any difference from the extra 20 ohms on yours.
Click to expand...

that's good to know



Ty_Bower said:
These seem a bit higher than I would like to see. They will come down a bit once you load up the windings (by plugging in the tubes). Still, these unloaded voltages seem higher than they should be. Sometimes cheap multimeters give bad readings on the AC scales. I checked the specs on the Edcor XPWR059 power transformer, and it does say it has a 240 volt primary. Odd.
Click to expand...

Well the meter is a new its a true rms auto ranging al singing all dancing job and just to check I tried another meter and got the same results the PT was bought from edcor in the U.S. mains voltage here is high 248vac - 249vac as the supply transformer is only 10m away and I am first house on the line so does that may explain it?
will it be OK with the amp? is there any way of reducing the voltage or should I just ignore it.
cheers
 
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