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Old 27th January 2009, 03:37 PM   #11
afrench is offline afrench  New Zealand
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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?

Quote:
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
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Old 28th January 2009, 12:00 AM   #12
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Quote:
Originally posted by afrench
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.
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Old 28th January 2009, 12:56 AM   #13
afrench is offline afrench  New Zealand
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Looks like i put an incorrect link to how I wired the 374BX. Here is the correct link:

http://i581.photobucket.com/albums/s...onnections.jpg

Thanks again everyone.
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Old 28th January 2009, 01:04 AM   #14
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Your wiring of the 374BX appears correct to me.
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Old 28th January 2009, 12:46 PM   #15
afrench is offline afrench  New Zealand
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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!
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Old 28th January 2009, 02:52 PM   #16
n_maher is offline n_maher  United States
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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!
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Old 29th January 2009, 05:44 AM   #17
DimZ is offline DimZ  Greece
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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 ??
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Old 29th January 2009, 09:42 AM   #18
chrish is offline chrish  Australia
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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
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Old 29th January 2009, 04:21 PM   #19
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You have the primary wired incorrectly.

Accounding to Hammond you should wire it like the attached for 240 AC
Attached Images
File Type: gif 300s_240.gif (1.7 KB, 265 views)
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Old 30th January 2009, 12:25 PM   #20
afrench is offline afrench  New Zealand
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Quote:
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!
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