Tubelab Simple SE - fuse blowing

[afrench]

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/ss260/afrench034/Top.jpg

http://i581.photobucket.com/albums/ss260/afrench034/Under.jpg

http://i581.photobucket.com/albums/ss260/afrench034/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

 

[DimZ]

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)


http://www.diyaudio.com/forums/showthread.php?s=&threadid=132308&perpage=25&highlight=&pagenumber=3

 

[DimZ]

btw nice job ..!!

 

[adamus]

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).

 

[Ty_Bower]

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/showthread.php?postid=1532365#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.

 

[Ty_Bower]

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...

 

[adamus]

edit - beaten to it

 

[n_maher]

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.

 

[Ty_Bower]

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.

 

[afrench]

Hi, Thanks for all the replys!

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

http://www.diyaudio.com/forums/showthread.php?s=&threadid=137329

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.

 

[afrench]

I'm at home now and have found some RCA 6L6GC tubes.

What is the best way to check the output voltages of the transformer, should it have a load on it ie be connected to the board with valves installed?

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.

Ok so maybe using the metal cased 6L6 has shorted / damaged either the FRED diodes, power supply cap or a short between the windings of the output transformer?

How do I check these items for a short?


Please excuse my ignorance on these but am quite new to this.

Thanks again

 

[Ty_Bower]

What is the best way to check the output voltages of the transformer, should it have a load on it ie be connected to the board with valves installed?

Ok so maybe using the metal cased 6L6 has shorted / damaged either the FRED diodes, power supply cap or a short between the windings of the output transformer?

How do I check these items for a short?

I'd start with the amp switched off and unplugged. Set your multimeter to read ohms. Use the lower settings of the ohmmeter to check for "small" ohm values, and the higher settings to check for "large" or infinite ohm values. Check the following on the power transformer:

Primary windings should be a small number of ohms, end to end. Either end of each primary should read infinite ohms to any end of any of the other windings.

The heater windings, 6.3 volt (green) and 5.0 volts (yellow), should be less than an ohm. They are very short (few turns).

The high voltage winding (red) is relatively long, and should measure more than primary winding and much more than the heater windings. It may be several tens of ohms.

The primaries on the output transformers have many turns, and will likely be a couple hundred ohms from end to end. If you have a UL tap, the ohms from the center tap to either end of the winding may not be symmetrical. This is perfectly normal. On a side (and completely unrelated) note, for some push/pull output transformers it is usual for the center tap to have asymmetrical DC resistance with respect to either end of the primary winding.

The secondary windings of the output transformer should have a very low ohm reading - less than one ohm, usually. Again, the ohm reading between any tap on the secondary and any tap on the primary should be infinite.

Looking at your "under" photos, it appears you may have installed the FRED diodes. If you intend to use the 5AR4 (or any other vacuum tube rectifier), I'd recommend just clipping the FREDs out with the side cutters. There has been far too many (recent) reports of shorted FREDs, and your life will be less complex without having to deal with them. It seems there was a bad run of FREDs or something, and the current crop just doesn't hack it. My FREDs are still sitting in the bag - I never installed them. BTW - a shorted FRED will blow the fuse, even if the "solid state" switch is not installed, or never closed.

You can try to check an electrolytic cap for DC leakage using an ohmmeter, but sometimes they only leak when high voltage DC is applied. For the low voltage DC check, just stick your ohmmeter probes across the cap. I try to make sure I keep the polarity on the cap correct - use a second voltmeter to check. The high voltage leakage test is trickier. I suppose you'd need to get the circuit up and running, and check for a voltage drop across the cap. This may be hard to do if the fuse blows instantly.

If I were a gambling man, I'd probably put my money on a shorted FRED diode. If your meter has a "diode test" or "continuity beep" setting, try checking the FREDs. They should conduct in one direction only. Put the red probe on one end and the black probe on the other. Note the results, then reverse the orientation of the leads. It should beep one way and not the other. There's no need to remove the FREDs from the circuit for the test, but if they fail I'd cut them off the board immediately.

 

[afrench]

Looks like i put an incorrect link to how I wired the 374BX. Here is the correct link:

http://i581.photobucket.com/albums/ss260/afrench034/374BXconnections.jpg

Thanks again everyone.

 

[Ty_Bower]

Your wiring of the 374BX appears correct to me.

 

[afrench]

It Goes!!

Thank you to everyone who replied with ideas!

As suggested by Ty I checked the FREDs and one had no "continuity" both ways, and the other had "continuity", both ways so I removed them.

I decided to check the transformer as well, all appeared ok here.

I then hooked it all up and switched it on, and the fuse went again, so then on the off chance I swapped out the "5U4GB" rectifier tube with another "5U4GB" and it went! So may be a faulty rectifier tube?

I am guessing using the metal cased 6L6's caused the damage to the FREDs and 5U4GB?

Any way I am very happy with the sound, and looking forward to adding the motor run cap, and trying different modes etc.

Thanks again to every one!

 

[n_maher]

I don't think the damaged FRED is really tube related, mine did the same thing after a couple hours of use. So following the same advice you got I yanked them only to find (like you) that fuses kept blowing. Turns out that I too had a damaged rectifier tube so here's what I think happens. For reasons unknown the presence of the FREDs can cause an issue which results in damage to the rectifier tube. The reason I believe this is that the rectifier that I used (a 5AR4) had been inservice for ~6 months in an amp that runs it a lot harder than the SimpleSE does.

Bottom line, I'm glad you were able to find the problem and get your amp working. Congrats!

 

[DimZ]

thats good news ....

btw after what happened with mine JJ5AR4 (sparks) I bought a sovtek (which worked just fine) and I also ordered last week a weber WZ34 copper cap (http://www.webervst.com/ccap.html) from ebay, to give it a try

seems quite good , anybody tried it ??

 

[chrish]

The combination of blown FREDs and sparking/destroyed JJ GZ34 happened to me too. The two failing appear to be related. Removing the FREDs permanently and replacing the tube rectifier has solved the problem for me now for over a year.

Chris

 

[coldcathode]

You have the primary wired incorrectly.

Accounding to Hammond you should wire it like the attached for 240 AC

 

[afrench]

You have the primary wired incorrectly.

Accounding to Hammond you should wire it like the attached for 240 AC

Standard line voltage in New Zealand is 230v not 240v (i think Australia is 240v). I checked this with Hammond when I brought the transformer, and they advised how to wire it (they don't have a wiring diagram for 230v on there website).

Thanks

P.S. Amp is still going great!