Hi all,
Just wanted to mention that I have a Chinese BOYUU A9 SEP (EL34's) using an old-stock 5U4B. After running it for several hours, it would arc with a big BANG! After that it would arc about every half hour. I tried Eli's fix with the diodes and all works perfectly now.
I first became acquainted with Eli back in the eary/mid 2000's. He's helped me off and on on many projects. He's always been spot on, so I've learned to trust his judgment. Lately he's been helping to improve the BOYUU. It's a much better amp now than it was. Thanks, Eli.
Dave
If a diode bandaid makes you feel better, and covers a problem with your 5U4 and you aren't interested in knowing why your amp could have created an arcing 5U4, then, carry on. Did Eli walk you through figuring out what was going on, or just offer a quick pet fix?
Eli has a copy of the schematic and checked it all out. Did a few other mods to get hum lower and make a better input impedance match for my tube preamp. Any way, here's the original schematic. Funny how the amp would run most of the day before it would arc. The new mods aren't shown. Some day I'll draw them in.
An externally hosted image should be here but it was not working when we last tested it.
Oops, photobucket decided it wants to charge me for direct links. Time for a change.
https://www.flickr.com/photos/12271467@N03/35558029374/in/album-72157601754510993/lightbox/
https://www.flickr.com/photos/12271467@N03/35558029374/in/album-72157601754510993/lightbox/
Haven't figured that one out yet. Seems every forum is different. I'll play with it a bit. Thanks.Well we do encourage you to post your pictures directly to the forum.
Dave
Perfect, vastly superior to the hosted site. The image is much cleaner and renders faster.
Okay, so as long as your current draw doesn't fall below the magic minimum for your choke, the 5U4 doesn't "see" the 150 uF capacitor. All would be well. However, if the current draw does drop too low, then that 150 uF capacitor could possibly give the 5U4 a headache. Can you keep a watch on total current draw? Maybe an ammeter temporarily can help you out.
-Chris
Okay, so as long as your current draw doesn't fall below the magic minimum for your choke, the 5U4 doesn't "see" the 150 uF capacitor. All would be well. However, if the current draw does drop too low, then that 150 uF capacitor could possibly give the 5U4 a headache. Can you keep a watch on total current draw? Maybe an ammeter temporarily can help you out.
-Chris
I could probably hook something up. Thought I should mention that before I put the diodes in it, it acted the same way with a 5U4GB.
Since I put the diodes in, it's been purring along without a hiccup.
Dave
I could probably hook something up. Thought I should mention that before I put the diodes in it, it acted the same way with a 5U4GB.
Since I put the diodes in, it's been purring along without a hiccup.
Dave
Just going by the diagram, it looks like there is no unusual stress on the 5U4GB. The peak inverse voltage would be @ 800-850v (the peak negative AC + the B+) depending on your actual B+. That's well under what a 5U4GB is rated for. You have normal to light 1st cap value so that's not a factor. A nice choke. Your arching would seem to be a failure of the rectifiers themselves, or a 1st cap that has a possible heat related problem or other downstream intermittent large current draw. Maybe an intermittent EL34.... But a normal good quality 5U4GB should have no problem handling the draw of your amp or the PIV on it. The diodes could be just masking the real culprit, and just crutching your amp.
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Well, I've ran this amp twos days in a row without anything happening. You'd think just adding the two diodes wouldn't make a difference with the capacitors. The voltage would be close to the same, the DC was good (very low hum level, so caps were doing their job.) The heat is the same either way (the transformer isn't real hot for a tube amp). I have 3 old stock 5U4g's and two 5U4GB's here, all used. I don't have a tube tester, so can't test the tubes. They all acted the same way before adding the diodes, and now all act great with the diodes. Really don't know what to test. I do plan on putting a surge protector and 1/2 amp fuse on the input transformer's center tap for a bit more protection. Parts should be here this week.
Dave
Hurdy, no mention of any arcing rectifiers in your other posts about this from last month.
When did the arcing begin?
When did the arcing begin?
Trying to remember. Maybe three or four weeks ago. Not sure. It was just a mild irritation at first.I have installed a couple 100k resistors on the small tubes heater circuit to cut hum, bypassed the volume control and replaced grid-to-ground resistor with 100k resistor. I am not sure, but the popping may have started soon after that.
I'm going to check solder connections later today. My vision is a bit blurry because of a stroke back in December. It's better now, but still hard to focus perfectly. It's conceivable I have a bad solder joint. Before the stroke I soldered well. Been doing it since early 70's.
It gives a loud hum for a few seconds at turn-on, then goes silent. Sounds just like an open input ground when it does this.
Dave
Hi Dave,
In servicing, we try to create the fault in order to find the cause. I'm afraid that you won't be able to solve this without reverting back without the diodes. Your rectifiers shouldn't be arcing in that circuit. So something is wrong and you need to fix the problem at the source, not just by bandading it.
-Chris
In servicing, we try to create the fault in order to find the cause. I'm afraid that you won't be able to solve this without reverting back without the diodes. Your rectifiers shouldn't be arcing in that circuit. So something is wrong and you need to fix the problem at the source, not just by bandading it.
-Chris
When you say, at turn on, is that right from time 0 switch snap, with a cold start? Or is it a few seconds after, but during the normal 5-10 seconds of initial warm up? Or are you also describing a warm or hot start? Also, is this a factory assembled unit, not second hand kit? Still have the original China glassware?
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I haven't tried any JJ octal tubes. I have some JJ EF806's in two condenser mics and they are quiet. I don't have a lot of hours on them.
GZ34 (5AR4) should handle that power transformer and load. The old RCA color TV sets had two 5U4's in parallel off the power transformer and furnished 400 v under load at 400 ma to the 36 tubes in the TV set. Can't use the TV anymore without a converter box.
With a 150 mA load, has anyone tried a 5R4GY? This is a military tube used primarily in transmitting equipment using 807 and 6146 tubes. My Hallicrafters HT37 ham transmitter uses a 5R4 for the 750 volt 6146 plate supply and a 5V4 for the low voltage supply. Transmitter still works fine and is over 55 years old..
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