Semi-Noob trying to make sense of my first project - diyAudio
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Old 10th June 2017, 04:55 AM   #1
diyAudio Member
Join Date: May 2011
Default Semi-Noob trying to make sense of my first project

Hey, first post - long time dabbler - trying to dive in a little harder to some repairs.

My first real project is this Contessa / Bartell CA 15.

It is just the chassis which was removed from the combo amp. Got it free and I've powered it up (tubes glow) but I get no sound.
I was going to fire it up again tonight and poke around in it, but I noticed some odd things which have already stumped me....

The speaker out jack has only the hot lead connected to pin 1 of one of the 6v6s - the ground of the jack is not connected. Shouldnt it be?
When I look up the pin outs for a 6v6 it says pin 1 is unused...

I've attached some pics of the innards - a little push in the right direction would be appreciated!
Attached Images
File Type: jpg KIMG0374.jpg (909.7 KB, 133 views)
File Type: jpg KIMG0375.jpg (856.1 KB, 128 views)
File Type: jpg KIMG0376.jpg (803.4 KB, 124 views)
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Old 10th June 2017, 06:41 AM   #2
Mooly is offline Mooly  United Kingdom
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Moved to I&A forum.
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Old 10th June 2017, 08:34 AM   #3
Enzo is offline Enzo  United States
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Pin 1 is not used by the tube itself, which leaves it free to use as a convenient solder post. Note your hot speaker lead connects to pin 1 of the lower 6V6, but so does one side of the output transformer secondary, the remaining side of that is grounded just above. The chassis is ground, so the cold side of the speaker jack is thus grounded by the mounting nut.

So the heaters work, fine, now is there high voltage in the circuit, say 300 volts or so?
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Old 10th June 2017, 08:45 AM   #4
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There are plenty of voltage readings to go by. I always find it best to work backwards, from furthest HT point to first HT point and the same with the signal line.
You will be well advised to remove the 0.02uF on the mains as it is affectionately known as the cap of death as they do go short circuit when old and fit 3 core mains cable to earth the chassis.
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Old 10th June 2017, 11:46 AM   #5
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I see a two wire power cord and a "death cap." This must go before the cap can live up to its reputation.

Remove the white .02 uF cap at one end of the board, and replace the 2 wire power cord with a three wire cord soldering the green wire to the bare wire going from the chassis ground to where the death cap was.

If that old cap shorts out or becomes leaky it can connect the wall outlet DIRECTLY to your guitar through the chassis ground.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 13th June 2017, 07:00 PM   #6
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Alrighty! Death cap is out - 3 prong is in - I assume I can run the green wire straight to a transformer bolt? Saw that recommended elsewhere. I need to go get a little wire fastener/eyelet to attach it properly.

Wish I had more time to work on this! Going to fire it up later and hopefully start checking voltages.

Thanks for the tip about the unused tube posts! There are a few places it seems they use them just as convenient solder points - clever, though it was throwing me for a loop!
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Old 14th June 2017, 12:25 AM   #7
PRR is offline PRR  United States
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> green wire straight to a transformer bolt?

National regulations often want you to go to a "dedicated" chassis screw.

Why? Some day your field service techs have to replace a power transformer. They disconnect the green, forget to re-connect it, unsafe product. So you install a screw that will "never" be removed (except perhaps for cord replacement).

In vintage amplifiers, yes, a transformer bolt is substantial and commonly understood by techs.
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Old 15th June 2017, 02:19 AM   #8
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Progress is happening!

Fired her up and started checking voltages. Got AC 6.3ish on all the tubes - got high voltage on all the tubes except for the first preamp tube. I think I should be seeing it @ pins 1 and 6 (they are jumped together) - the resistor they are connected to should be 220k I believe - though I cant get a reading off it. (most of the other resistors seem okay so far)

On the second 12ax7 pins 1 and 7 are jumped together... They read @ 125v and pin 6 shows 190v+

I tried swapping the 12ax7s to see if there was an issue with the tubes - the one in the first position doesnt show high voltage either way.

Having some trouble figuring out how to trace paths with this cardboard-like circuit board. Is it necessary to be able to see whats going on underneath?

(pointing probe at the resistor in question...)
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Old 15th June 2017, 02:55 AM   #9
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Sounds like you found your problem. Replace that 220k resistor at the first 12ax7
Dan Santoni
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Old 16th June 2017, 05:21 AM   #10
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Well I wanted to go through and more thoroughly check all the resistors and see how many I needed to replace -then I could buy em all at once.

Im having trouble getting consistent readings on a few of them.

The schematic shows .1uf caps connected to pin 5 on both 6v6s while the ones in the amp say .22 on them...
Also there seem to be some extra resistors coming off pin5 on both 6v6s.
The schematic shows 220k and 100k resistors around the .1uf caps (which are actually .22 on my amp) both of which I found. (though they all read about 10% high)
But there are also some 2k resistors attached right to pin 5. Would these be for bias?

Also, there should be 1meg resistors right on the input jacks - mine seem to read all over the place - most consistent reading I can get from them is ~10ohms - likely that these are just bad I assume?

Lastly - any light that can be shed on how this cardboard-ish circuit board is laid out? I keep trying to trace paths and running in to these little eyelets which make it hard to see where things are going.

here are the "extra" 2k resistors on the 6v6 pin 5
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