Tweedle Dee causing rectifier arcing

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Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

So the build I just did was working wonderfully for a while, but now it is not so happy. I plugged it in today to play and started hearing arcing like mad. I shut it down, brought it down to the shop and put it back on and can see the rectifier arcing, if you recall on the video, I had it arc a few times only so I swapped it with another tube I had on hand and it worked fine for a lot of playing.

I had a 5u4GB extra here (have several) and was wondering if it was just the 32uF caps being too much for the rectifier I had in, so I know the 5U4GB can take it, and it immediately started arcing as well. I've now been through 3 tubes, and don't want to put in another until I figure out what's up. Could it be one of the salvaged 6V6's? I also tested those in my tester and they both test okay. I noted as well that the first dropper resistor, 4.7k 2W is getting quite dark like it's being pushed too much.

Ideas?

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M Fowler
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Re: Tweedle Dee causing rectifier arcing

Post by M Fowler »

Increase that 4k7 to 5w that's the screen circuit and it definitely will burn up that 4k7 resistor it did in my Tweedle Dee.

Mark
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Re: Tweedle Dee causing rectifier arcing

Post by martin manning »

pompeiisneaks wrote: Sat Jan 13, 2018 10:30 pmI've now been through 3 [rectifier] tubes, and don't want to put in another until I figure out what's up. Could it be one of the salvaged 6V6's? I also tested those in my tester and they both test okay. I noted as well that the first dropper resistor, 4.7k 2W is getting quite dark like it's being pushed too much.
Try backing up the vacuum rectifier with a couple of 1N4007's. You can put them on the rectifier tube socket using pins 3 and 7 for tie points (move PT leads to 3 and 7, and add diodes from 3 to 4(k), and 7 to 6(k)). Easy and cheap, then see if you can understand why its doing this. The 5AR4 and 5U4 sheets say 40uF, so the reservoir doesn't sound excessive. What are your plate voltages and idle current, and what is the OT primary impedance?
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Re: Tweedle Dee causing rectifier arcing

Post by 67plexi »

1, What power transformer did you use it should be 354-0-354 to get 398.1 V B+1 under load. I used a 3 watt 4k7 no issues.
2, In the past I had a bad batch of Belton octal tube socket's I will never use them ever. They were arcing inside the socket.
Look for carbon traces on the Micalex plastic if so replace the tube socket. Turn off the lights and power up and pin point the arc.
I might ruffle some feathers with this one My last repair I ordered a Heyboer PT and received a Magnetic Components it failed in 1/2 hour so hot it would burn you.
that's # 7 failure on Classic Tone PT out of 14 used 50% failure rate. I used a Mercury Magnetics it's running ice cold. :D
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Re: Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

martin manning wrote: Sun Jan 14, 2018 1:23 pm
pompeiisneaks wrote: Sat Jan 13, 2018 10:30 pmI've now been through 3 [rectifier] tubes, and don't want to put in another until I figure out what's up. Could it be one of the salvaged 6V6's? I also tested those in my tester and they both test okay. I noted as well that the first dropper resistor, 4.7k 2W is getting quite dark like it's being pushed too much.
Try backing up the vacuum rectifier with a couple of 1N4007's. You can put them on the rectifier tube socket using pins 3 and 7 for tie points (move PT leads to 3 and 7, and add diodes from 3 to 4(k), and 7 to 6(k)). Easy and cheap, then see if you can understand why its doing this. The 5AR4 and 5U4 sheets say 40uF, so the reservoir doesn't sound excessive. What are your plate voltages and idle current, and what is the OT primary impedance?
I'll give that a go, and see. Will the diodes stop the back arcing? I guess I can put in one of hte rectifiers taht's been arcing already. I thought I read that once they arc they're toast and you shouldn't use them anymore, but I'll see. I'm using the heyboer output for tweed deluxe, from CEDist, It runs at 8k https://www.cedist.com/products/transfo ... der-15w-8k

I'll have to get voltages once I power ti back on, I don't think I wrote them down on this one. I'll be posting the PT values next once I get to 67Plexi's question.

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Re: Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

67plexi wrote: Sun Jan 14, 2018 6:10 pm 1, What power transformer did you use it should be 354-0-354 to get 398.1 V B+1 under load. I used a 3 watt 4k7 no issues.
I used the Hoffman sold Heyboer listed as "Heyboer Tweed Deluxe Power Transformer" Here's the link to the datasheet: http://el34world.com/Transformers/files/MOJO756.pdf

Says it's 384-0-384, maybe it's running too hot and I need to drop the power?
2, In the past I had a bad batch of Belton octal tube socket's I will never use them ever. They were arcing inside the socket.
Look for carbon traces on the Micalex plastic if so replace the tube socket. Turn off the lights and power up and pin point the arc.
I've got a ceramic socket in there. I don't see signs of arcing, but I'll look, the only arcing I saw was on the inside of the rectifiers themselves.
I might ruffle some feathers with this one My last repair I ordered a Heyboer PT and received a Magnetic Components it failed in 1/2 hour so hot it would burn you.
that's # 7 failure on Classic Tone PT out of 14 used 50% failure rate. I used a Mercury Magnetics it's running ice cold. :D
No Classic Tone here, Heyboer.

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Re: Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

I re-measured the dropper, I'm going to replace it anyway, but it reads 4.6k I think, in the ballpark and in spec. I read voltages without a rectifier and I get 390 VAC.

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Re: Tweedle Dee causing rectifier arcing

Post by martin manning »

The OT impedance sounds fine. Measure the plate voltage at idle to get actual DC voltage. The spec sheet says the 5VAC winding is 2A, so I don't think it will support a 5U4's 3A draw for long. You'll have to use 5Y3 or 5AR4, 5Y3 will drop more voltage, maybe about 30V more. The 1N4007's will prevent current from flowing in the wrong direction, but the arcing is likely the result of excessive forward current.
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Re: Tweedle Dee causing rectifier arcing

Post by 67plexi »

I think Martin nailed it 5U4 3A draw.
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Re: Tweedle Dee causing rectifier arcing

Post by martin manning »

I can see where installing a 5U4 could cause the PT to overheat, and sagging rectifier filament voltage, but not the arcing. I think that would have to be either very large inverse voltage, or very large current draw from a short or perhaps a failed reservoir cap.
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Re: Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

That was only added as the last step in troubleshooting, the previous two or three were 5ar4

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Re: Tweedle Dee causing rectifier arcing

Post by 67plexi »

First step building the amp
next step debugging the amp
next step fine tuning the amp that would start with a voltage chart from the power transformer to V-1 in that order then adjust the dropping string for optimum voltages.
Now you have to isolate the problem 1 disconnect your high voltage B+1 to the first filter capacitor power up if the rectifier still arc's go backwards towards the power transformer make sure the rectifier is wired properly. If it's OK reconnect B+1then disconnect B+2 the next filter capacitor and so on until you find the issue.
Voltage Chart.JPG
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Re: Tweedle Dee causing rectifier arcing

Post by pompeiisneaks »

Thanks, I really do need to do that 67plexi.

Update, though, I think I do have a cause, but not yet sure of the root cause. Today I put back in one of the other rectifiers with the diodes still out to start again (I was sick for a few days, thus the lack of updates). As I was testing the voltages etc, I all of a sudden heard a massive pop and one of the 32uF caps blew it's top. Either I got a dud from Kemet, OR the other option I'm debating, is that they're 450V caps, and when I first powered it up it went to about 490V, I wonder if that was enough, even at that short of a time frame, to cause some damage that just took a little more play time to cause failure? At any rate, I do know now that the power supply is sending a bit more than the tweedle dee schematic I used was expecting, so my best bet here is to maybe make that first one be 32uF @500V if I can get one easily. Then I am also going to remove that 4.7k 2W and put in a 5k 5W (all I could find at the local shop, but since it's already over voltage, dropping it a bit more there will put the rest of the amp likely in better range.)

I have yet to put in the diodes, which I'll do, nor have I replaced that resistor, but I'll do that too, after the new cap comes in. Here's a picture to show the gory details.

https://photos.app.goo.gl/OtB7uBvsNIb1oAL13

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Re: Tweedle Dee causing rectifier arcing

Post by xtian »

pompeiisneaks wrote: Wed Jan 17, 2018 12:12 amHere's a picture to show the gory details.
Why, that dirty bastard.
I build and repair tube amps. http://amps.monkeymatic.com
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Re: Tweedle Dee causing rectifier arcing

Post by Aaron »

Hi Phil,

I don’t think it would be a voltage spike with a valve rectifier, silicon yes.

But I had a quick look at the specs for the Kemet capacitors and they have a tolerance of -10% - +30%.
So even though it’s rated for 450v, the capacitor you have might have been only rated to 405v and still be in spec.

Thanks,
Aaron
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