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Old 16th March 2013, 08:42 PM   #11
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Legend has it that during WW2, Forces wireless sets that suffered a burned out or broken resistor were repaired using pencil lead on paper to fabricate a replacement resistor.
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Old 16th March 2013, 08:45 PM   #12
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Good "stick-to-it-tiveness" Goldstache!

I happened to be reading another thread, another member had a similar problem with the Edcor wires on his Power transformer primary! The lead-wires were not connected per the color-code and had to be sorted out the hard way (many, many blown fuses).

Basic questions on configuring Tube AMP

Sounds like in general it would be a good thing to check the wires on a new pre-wired transformer to make sure the colored wires actually hook up the way the documentation says!!

It's always obvious after the fact, of course...

And even the "old hands" make the occasional "rookie mistake" As a good friend of mine was so fond of saying, " There ain't nothin' so simple you can't screw it up!"

I'm just starting my buildup and will check the power transformer wires on the Hammond PT I'm using. I'm still a few weeks from getting my Edcor OPTs, but those are the cheaper non-bellcap versions with solder tabs so if I screw that up I own the fault

Best luck on your amp checkout, Goldstache! BTW - the video was very cool! Thank you for posting your work!

~ Sam
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Old 16th March 2013, 08:57 PM   #13
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And yes, pencil lead will conduct. The following numbers are all very rough and approximate, just a quick Ohms Law exercise, but that could well have been your problem with that turret.

If you had 120V on the preamp plate, and about a megohm of resistance across the cap, you would get about a 10:1 voltage divider which would put around 11V on the top pin of your pot.

Current would be about 0.11mA, which might not be enough to blow out the pencil lead trace - the pencil-trace would be dissipating about 10mW, so that could well have been your leakage path.

Now, I'm wondering how conductive a sharpie would have been...
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Old 16th March 2013, 10:17 PM   #14
DF96 is offline DF96  England
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Pencil lead and carbon composition resistors are similar stuff, although with different proportions and purposes. Both are carbon with a binder.
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Old 16th March 2013, 11:40 PM   #15
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In high school auto shop class, lead pencil traces inside a distributer cap were successfully used to frustrate the shop teacher.
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Old 18th March 2013, 01:34 AM   #16
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Thanks for all the insight and help!
Still having some output issues with the turreted unit.
Still missing some db's and the volume is all the way up. Nowhere near where the breadboarded version sounded.
Also, when the turreted version is all the way up the unit is very microphonic. Infinitely more than the perfect performing breadboard.

I have troubleshot all the peripheral functions. Like -20db pad, phase, in xfmr and out xfmr, all jacks and connectors.

So Im looking at the circuit/turret board as the culprit.
Wondered if I could borrow you eyes and minds!
Take a peak at my layout of the board and see if you spot my error please.
The board is for the schematic at the top of this thread!
Attached Images
File Type: gif 6SN7 mic preamp turret board parts layout.gif (83.1 KB, 74 views)
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Old 18th March 2013, 01:37 AM   #17
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Also, the 4.7uF cap is going to the OT not phase switch.
Also wanted to note that all voltages are on spec.
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Old 18th March 2013, 09:25 PM   #18
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Bump sorry
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Old 18th March 2013, 09:32 PM   #19
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You say all voltages are on spec. You are describing DC voltages I presume.

Put an AC signal on the circuit and compare AC voltages to that of the breadboard unit. That should locate the gain problem.

You have two bypassed cathode resistors. Check those. They increase gain by reducing negative feedback.

Last edited by Captn Dave; 18th March 2013 at 09:36 PM.
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Old 18th March 2013, 09:37 PM   #20
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Thanks Dave!!!!
As soon as I get home tonight!!!!
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