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Old 26th January 2009, 02:31 PM   #1
afrench is offline afrench  New Zealand
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Default Tubelab Simple SE - fuse blowing

Hi everyone, I have been reading these forums for a while now, and decided to have a go at building an amp.

I chose to build a Tubelab simple SE which I have just about completed a , but am have some trouble with fuses blowing.

I am quite new at this so need a bit of guidance!

The configuration is as follows:


Hammond 374BX Power transformer (we have 230V mains here) Hammond 193H 5H 200ma Choke
80 UF 500volt motor run cap (not installed yet)
Edcor GXSE15-8-5k output transformers

RCA 6L6 output tubes (they are metal case type for testing etc)
GE ECC81
and a Zeninth "5U4GB" rectifier tube (have not been able to get a 5AR4 locally yet which doesn't break the bank!) I understand the 5U4GB is ok in this design.

It is wired up for Triode, and no feedback at this stage.

I have built the PC board up using the standard components on Georges web site.

Photos below.

http://i581.photobucket.com/albums/s...nch034/Top.jpg

http://i581.photobucket.com/albums/s...h034/Under.jpg

http://i581.photobucket.com/albums/s...h034/front.jpg

I fired it up for the first time last night, and although it only worked on one channel it sounded great. I started looking for any lose connections etc and after about 10min of checking, and switching it back on a few of times the fuse went. (I am using a 1amp fast blow as recommended by George). There was a faint smell of burning to.

Now it blows instantly when i switch it on. I can't see anything obviously burnt of incorrectly connected.

So far this is what i have tried:

1 Checked Primary side of Power transformer for shorts etc this appears to be ok.

2 Disconnected all the secondarys and switched on - fuse did not blow.

3 I decided to check the secondary voltages before I reconnected them one at a time, this is what happened:
Filament 1 (green wires)= 5.5v
Red wires = 828v or red wire and red/yellow 413V & 411V
Filament 2 (yellow wires) = Blown fuse when connected to DMM.

4 Tried connecting Filament 1 (green wires) to board (no tubes attached) - all ok.

5 then added red wires - Blown fuse.

Stopped testing at this point.

I am hoping it is just a simple wiring connection I have got wrong, but after checking and double checking I can't see any thing obvious. May be some one out there can see something I can't? Possibly just some silly wiring fault? Any help would be welcome! I really want to get this thing going!

Thanks
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Old 26th January 2009, 04:59 PM   #2
DimZ is offline DimZ  Greece
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i blown some fuses too,

finally it was a faulty JJ5Ar4 (there were sparks), when i tried a GZ34 it worked fine , later I got a sovtek 5AR4 and it worked fine too

so try another rectifier tube, maybe it'll save you some time

look what others suggested to me (post 47 and down)


tubelab se
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Old 26th January 2009, 05:02 PM   #3
DimZ is offline DimZ  Greece
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btw nice job ..!!
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Old 26th January 2009, 05:06 PM   #4
adamus is offline adamus  United Kingdom
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hold on, you disconnected the secondaries....


the green should be 6.3v?

it shouldnt blow a fuse when you measure the yellow.

Sounds like somethign is wrong with the transformer. how have you wired it up (include primaries).
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Old 26th January 2009, 06:42 PM   #5
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Default Re: Tubelab Simple SE - fuse blowing

Quote:
Originally posted by afrench
Hammond 374BX Power transformer (we have 230V mains here)

RCA 6L6 output tubes (they are metal case type for testing etc)
The Hammond 374BX will put about 450 volts on the B+ rail. I believe this is much too high for the metal cased 6L6 tubes. See here: http://www.diyaudio.com/forums/showt...65#post1532365

If the Simple SE is blowing fuses, there is a short somewhere. It could be shorted FRED diodes or a shorted power supply cap. You might have a short inside one of the 6L6 tubes, or a short between the windings of the output transformer.
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Old 26th January 2009, 06:48 PM   #6
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Quote:
Originally posted by adamus
hold on, you disconnected the secondaries....

the green should be 6.3v?

it shouldnt blow a fuse when you measure the yellow.
As adamus says, the green should be 6.3 VAC and the yellow should be 5 VAC. I'm assuming you made a typo in your original post, or incorrectly wrote down the results of your test?

Neither should blow the fuse if you measure them with your volt meter. Perhaps you accidentally set the meter to measure amps? That would blow a fuse...
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Old 26th January 2009, 06:49 PM   #7
adamus is offline adamus  United Kingdom
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edit - beaten to it
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Old 26th January 2009, 07:09 PM   #8
n_maher is offline n_maher  United States
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Quote:
Originally posted by Ty_Bower
Neither should blow the fuse if you measure them with your volt meter. Perhaps you accidentally set the meter to measure amps? That would blow a fuse...
Not contradict Ty's excellent advice but I've had the same thing happen before trying to measure the filament voltage of an unloaded trafo before. And there was nothing wrong with the trafo, it's been working for 2 years + since then.
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Old 26th January 2009, 08:25 PM   #9
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Fair enough. I'll admit I don't think I've actually tried measuring the no-load voltage on the heater windings before, at least not with full line voltage on the primary. I'm too scared of the loose leads of the high voltage winding which are running around nearby.

When I'm testing unknown power transformers, I stick it on the variac and feed maybe 12 volts AC into the primary (24 VAC for you guys overseas). I've also used a small AC wall adapter at times when I didn't have a variac handy. I robbed it from an answering machine, I think. It puts out about 13 VAC. With a relatively low voltage on the primary, I feel more comfortable taking measurements on the secondary windings. Of course, with only 12 volts in I'll get 0.63 volts on the green wires, and 0.50 volts on the yellow. Still, it's enough to prove the transformer is more or less working, and avoids having 800VAC around.
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Old 26th January 2009, 09:50 PM   #10
afrench is offline afrench  New Zealand
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Hi, Thanks for all the replys!

Below is a link to how I wired the 374BX:

Tubelab Simple SE - fuse blowing

I will change out the metal cased 6L6 tubes as segested by TY, I will also recheck output voltages tonight (I am at work now).

Thanks again for all the help.
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