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Old 16th March 2013, 03:57 PM   #1
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Default Strange problem with Caps not blocking DC

Hey Im building one of NYD's 6SN7 One Bottle mic pres. I bread boarded it and
it is awesome!!! Then I made a turret board for it. Wired up another one, enclosed it. Fired it up. And began to debug.
Here's my issue. The 1uF cap between the stages used for coupling has DC voltage present before the next stage headed to the 100K pot. Its about 11vdc. One might think okay replace the leaky cap. Well I did and it's still there. I assure you I checked that turrets connections and it only has the cap, and the lead to the pot!! ???????
My breadboarded version has no DC after this cap. So I used that one in my build and sure enough still 11VDC.
Anybody got any thoughts on this?
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Old 16th March 2013, 04:24 PM   #2
DF96 is offline DF96  England
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You haven't built what you think you have built, or you have a faulty component.

As you have tried changing the cap, try changing the pot or the turret. Check what the other end of the pot is connected to: ground or a cathode?
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Old 16th March 2013, 04:42 PM   #3
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by DF96 View Post
You haven't built what you think you have built, or you have a faulty component.
Agreed. It sounds as though the grounded end of the pot is internally open circuit or goes somewhere else. If it was o/c though, the 1M resistor should ensure a good zero volts. So perhaps it's more likely that you think they're connected to ground when they are in fact connected to another point, such as the cathode of the second valve.
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Old 16th March 2013, 04:53 PM   #4
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Well I just went cap to pot and it's fine the turret measures 11VDC just sitting on the board with nothing attached!!!!!???????

My breadboarded version has about 5 more db gain when using the same tube in both units (swapping). The breadboarded version is the exact schematic. The enclosed turreted version has a phase switch post OT. The voltages are are about 10VDC difference at all Test Points. Is the extra wiring lowering the gain or just component tolerance differences?

The plus is that the turreted version is a bit quieter than the BB'ed version.

Another question is how to arrange the neg feedback to give me -5db in one position and -10db in the other position with On-OFF-On switch. Off being no negative feed back.

Thanks for all the help.
Can't understand why that turret is measuring 11 vdc with free standing on the Board
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Old 16th March 2013, 04:58 PM   #5
DF96 is offline DF96  England
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Reverse-engineer your circuit from what you have built. Don't refer to the original diagram when doing this. Show us the result, unless you spot the mistake yourself.

We can't diagnose a fault from a diagram of something different (e.g. phase switch - anything else). 10-11VDC difference suggests a wiring error.
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Old 16th March 2013, 05:13 PM   #6
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Sorry I posted reply but it hasn't shown up so here goes the quick reply:

Its working Fine if I just don't use that turret. It is sitting on the board all by itself with No Connections to anything, which I checked with Continuity, measuring 11VDC. Parasitic??

I am wondering if the component value tolerance differences are to blame since my Test points are about 10VDC lower in all the B+ areas. I will check all component values and see if I can raise the voltage a little and get back my 5db that I miss so Much!

I am also wondering how to implement Feedback to get -5db, and -10db attenuation. I get the switching just not the component values to achieve this.
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Old 16th March 2013, 07:35 PM   #7
DF96 is offline DF96  England
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You could have surface leakage from a nearby higher voltage point. Are you in a humid region?
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Old 16th March 2013, 08:03 PM   #8
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HA! I gave up on that turret and just removed it and went point to point cap to pot. Problem solved!
Now in regards to the output db drop. ROOKIE mistake. Edcor Swithched the primary and secondary on my transformer choice. I was using a 10k/600 on the breadboarded version and the new xfmr is a 600/10K. I wired them the same! Thats what I get for crying wolf as it were. Thanks so much. Now on to that NF implementation!!!!!
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Old 16th March 2013, 08:07 PM   #9
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Regarding surface leakage:
Does Pencil Lead/Carbon conduct?
There was a bit of residue from when I was drilling the board material.
Maybe?????
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Old 16th March 2013, 08:37 PM   #10
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Quote:
Originally Posted by Goldstache View Post
Regarding surface leakage:
Does Pencil Lead/Carbon conduct?
There was a bit of residue from when I was drilling the board material.
Maybe?????
that'll do it...
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