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Old 8th July 2009, 12:16 PM   #111
rknize is offline rknize  United States
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Quote:
Originally posted by oldmanStrat
I would agree with you that it could be the mosfets, but since there is no B+ I just don't see a current path there...
There is a path to ground through the diode that you strapped across the B+ bleeder resistor.

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My meter says that they are both still diodes. Although one is dropping more than the other (.8v vrs .6v). The caps appear to be still caps, since there are no shorts.
Well if the diodes are good, you should be able to apply power to B- with R5 out of the picture.
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Old 8th July 2009, 12:46 PM   #112
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I have seen these diodes measure good with an ohm meter but still break down when high voltage is applied. With R5 removed and the 5AR4 removed but the transformer reconnected, a blown fuse would indicate bad diodes.

Shorted mosfets will generally make R6 hotter than normal but not bother R5. It will put positive bias on the output tubes causing them to get upset and possibly blow fuses.

Test as above (no tubes installed). No R5. Blow fuse, replace diodes. No blown fuse, Replace R5 and try again. Blown fuse or smoking R5, replace caps. No smoke, verify negative voltage present. Voltage OK, install 5AR4 (no other tubes). Measure voltage at output tube grids. It should be adjustable via the bias pots. If one channel does not adjust and is at a positive voltage, replace the mosfet for that channel and check the grid stopper resistor.
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Old 8th July 2009, 02:02 PM   #113
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Quote:
Originally posted by tubelab.com
I have seen these diodes measure good with an ohm meter but still break down when high voltage is applied. With R5 removed and the 5AR4 removed but the transformer reconnected, a blown fuse would indicate bad diodes.

Shorted mosfets will generally make R6 hotter than normal but not bother R5. It will put positive bias on the output tubes causing them to get upset and possibly blow fuses.

Test as above (no tubes installed). No R5. Blow fuse, replace diodes. No blown fuse, Replace R5 and try again. Blown fuse or smoking R5, replace caps. No smoke, verify negative voltage present. Voltage OK, install 5AR4 (no other tubes). Measure voltage at output tube grids. It should be adjustable via the bias pots. If one channel does not adjust and is at a positive voltage, replace the mosfet for that channel and check the grid stopper resistor.
Thanks George - sounds like a plan. I'll get to some of this today, but replacing parts - yeah, I didn't order extras.

One thing - I was using a 5U4-GB/A old used tube for the rectifier. Any chance this had something to do with smoking the diodes/caps etc (assuming that is what the problem is)
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Old 8th July 2009, 02:23 PM   #114
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Quote:
Originally posted by rknize


There is a path to ground through the diode that you strapped across the B+ bleeder resistor.
I did? You lost me there Russ.

not hard to do these days...
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Old 8th July 2009, 03:06 PM   #115
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Look at the R30 section of the Resistors page in the assembly manual. It talks about putting a diode across R30 to prevent (slight) reverse charge on C5 on startup.
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Old 8th July 2009, 03:59 PM   #116
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Quote:
Originally posted by PaulyT
Look at the R30 section of the Resistors page in the assembly manual. It talks about putting a diode across R30 to prevent (slight) reverse charge on C5 on startup.
Err, yeah, No diode. I remember seeing it but I already bought my parts and I don't have an equivalent one lying around...

You know - I do have an inrush current limiter in circuit... this takes a few seconds to warm up which delays B+ even longer - perhaps that slightly negative voltage is more damaging that it would be normally...
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Old 8th July 2009, 09:41 PM   #117
rknize is offline rknize  United States
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Oh yeah, sorry. i thought I saw the diode on your board, but it was probably one of the other TSE threads.
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Old 8th July 2009, 10:32 PM   #118
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Quote:
With R5 removed and the 5AR4 removed but the transformer reconnected, a blown fuse would indicate bad diodes.
This is exactly what happened. Took out D2 D3 and the fuse is spared.

So now I want to know why!

I'm placing a digikey order tonight - with all the cap's/mosfets/diodes in the bias path.
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Old 8th July 2009, 10:49 PM   #119
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Quote:
Originally posted by rknize
Oh yeah, sorry. i thought I saw the diode on your board, but it was probably one of the other TSE threads.
It will be by the weekend.
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Old 8th July 2009, 11:16 PM   #120
rknize is offline rknize  United States
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Quote:
Originally posted by oldmanStrat


This is exactly what happened. Took out D2 D3 and the fuse is spared.

So now I want to know why!

I'm placing a digikey order tonight - with all the cap's/mosfets/diodes in the bias path.
I used the same Fairchild Stealth diodes here that I used in my SSE. They are similar to what George recommends for the SSE.
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