6.8k cap resistor blowing

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6.8k cap resistor blowing

Postby wungun » Thu Jun 25, 2015 3:58 pm

The 6,8k, 2W resistor joining one can cap section to the other blew on me the other day. Replaced it and that one popped. At 2W and 500V, its only capable of 4ma current...so I'm thinking somewhere down stream maybe be a partial shirt to ground or something....?
Down stream is the "K" connection on the Poseidon board...
Ive added (since practically day 1) a pair of 385V JJ caps in series to boost capacitance (each has a 100K resistor across each leg to balance).
I have no capacitor checker, but I have a working amp to compare resistance to ground, point to point on each.
Anyone else ever have a similar issue?
Any suggestions as to where to look or what could be causing the issue?
TIA
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Re: 6.8k cap resistor blowing

Postby wungun » Sat Jun 27, 2015 9:03 am

Bump.....?
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Re: 6.8k cap resistor blowing

Postby battradio » Sat Jun 27, 2015 9:49 am

Most likely a cap or a tube what caps did you use when you built it , photos are really helpful , the circuit board looked scorched on the pictures you posted at audio karma .
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Re: 6.8k cap resistor blowing

Postby wungun » Sat Jun 27, 2015 12:34 pm

Thanks for the reply....
Scorch marks from soldering iron.

Power tubes are Gold Lion KT88's.
On the Poseidon board I'm running ElectroHarmonix and Tung Sol.
Can caps were replaced when rebuilt with new Dynakit parts caps...the slightly higher capacity ones (than original).
Poseidon board uses Obliggato Platinums.
I swapped phase splitter tubes and there was no change...
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Re: 6.8k cap resistor blowing

Postby Geek » Sat Jun 27, 2015 1:10 pm

IME, Gold Lion tubes love to oscillate. Try bumping up the grid snubbers to 47K.
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Re: 6.8k cap resistor blowing

Postby wungun » Sat Jun 27, 2015 2:09 pm

So you think I lost a tube....?
Would it be safe to swap tubes from the good amp to verify, or do I risk losing another tube doing this?
Thanks
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Re: 6.8k cap resistor blowing

Postby wungun » Sun Jun 28, 2015 7:58 am

Replaced again the bad resistor...pulled both KT88's....still fried the 6.8k
Should I pull all tubes except the rectifier tube and try again?
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Re: 6.8k cap resistor blowing

Postby wungun » Sun Jun 28, 2015 8:13 am

....and pulled all tubes except the gz34....and still cooked the resistor.
So a bad cap somewhere? Please don't say its the quad cap!!
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Re: 6.8k cap resistor blowing

Postby 20to20 » Sun Jun 28, 2015 8:14 am

wungun wrote:Replaced again the bad resistor...pulled both KT88's....still fried the 6.8k
Should I pull all tubes except the rectifier tube and try again?


What's that unmarked black thing right against the choke?
Last edited by 20to20 on Sun Jun 28, 2015 9:40 am, edited 1 time in total.
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Re: 6.8k cap resistor blowing

Postby wungun » Sun Jun 28, 2015 8:54 am

The black thing is just a 3 way grounding point if I remember....comes with the Poseidon board I think.
This amp has run fine for many many hours prior to this...not sure which filter cap and you're referring to...?
This one.....?

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Re: 6.8k cap resistor blowing

Postby 20to20 » Sun Jun 28, 2015 9:30 am

wungun wrote:The black thing is just a 3 way grounding point if I remember....comes with the Poseidon board I think.
This amp has run fine for many many hours prior to this...not sure which filter cap and you're referring to...?
This one.....?



Disregard.
The black thing I saw in the first photo was a splice cover. Heatshrink probably.

Carry on
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Re: 6.8k cap resistor blowing

Postby 20to20 » Sun Jun 28, 2015 9:50 am

If you pull the "K" line from the board, read the resistance at that point to see if it makes sense. Does that stop the resistor from blowing? Trace everything going to the Q-cap. Take the lead off the Q-cap and just use your added caps.
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Re: 6.8k cap resistor blowing

Postby wungun » Sun Jun 28, 2015 10:20 am

If I pull the tubes from om the Poseidon board, is there anything on it that would cause the issue that would still be functioning?
Pulling the K line is easy enough I guess....desoldering the can cap is something else altogether! Lol
1 step at a time I guess...
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Re: 6.8k cap resistor blowing

Postby 20to20 » Sun Jun 28, 2015 11:46 am

wungun wrote:If I pull the tubes from om the Poseidon board, is there anything on it that would cause the issue that would still be functioning?
Pulling the K line is easy enough I guess....desoldering the can cap is something else altogether! Lol
1 step at a time I guess...


Desoldering the can cap (Q- cap) may not be necessary. Since I don't know which lug you have added the JJ caps to I'm assuming you could use them by disconnecting them from where they are now and tie them to the "K" pad on the board. Since you've replaced the 6.8K resistor you can just lift the output end off the Q-cap to make the new connection and disable that one Q-cap lug.

The board itself has a resonably high resistance path so a lower than expected resistance at the "K" pad would be a good clue as to the source of current draw. Could be just an errant solder splash...
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Re: 6.8k cap resistor blowing

Postby Geek » Sun Jun 28, 2015 1:24 pm

Shorted cap?
I've had 'em right from the box that way as a manufacturing boo-boo :$
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