Output Signal Lug Grounded

[biolite]

So I recently wired up my first amp build, a Two Stroke head, this weekend and have come upon a bit of a surprise as I was poking around with a multimeter checking continuity. The signal lug (tip connector) on the output jacks has a connection with ground. This doesn’t seem right to me. I’ve traced the yellow and white output wires back to the impedance switch and those are connected to ground as well. I’ve retraced all the connections with the schematic and everything seems correct. The only thing I can think of is if the wires are somehow grounded within the OT. Has anyone experienced this issue or have any guidance for me? Any help is greatly appreciated, thanks guys.

[beelzebum]

If you have already connected the ground lead for your speaker output, usually a black wire, to ground. All of your impedance leads will look like they are grounded. The output winding has an AC impedance of only 4 or 8 ohms, and would have a DC resistance of almost nothing. So checking with a multimeter for resistance, It should look shorted to ground. As long as you wired it correctly then you are getting the right readings for what you are checking.

[biolite]

Copy all. Thanks, beelzebum, it just seemed a bit odd to me, so I thought I’d check first. Hopefully when I fire this thing up everything acts as it should.

[Robin]

Using a shorting input jack, continuity to ground with nothing plugged into the input jack is correct. This keeps the amp quiet while it’s on with no input connected. If you still have continuity to ground with a cord plugged into the jack, this is incorrect and has been a somewhat common wiring issue with the 2Stroke as the instructions are vague.

Using shielded cable, the inner lead should go from the 68k resistor connected to the tip lug of the input jack to pin 2 of V1, the outer shielding should be connected to ground at only one end (to prevent a ground loop) and not connect to pin 2 of V1. You can ground either end of the shielding but be sure that the other end is protected from touching anything, including the inner lead. Because they are easy to read, you can reference any Fender schematic (the Tweed era amps are really good for this) to get a clearer view how an input jack should be wired.

[biolite]

Robin, thanks for the insight. I am using a switching jack on the input and it does short to ground when nothing is plugged in. The input is isolated when a plug is plugged in though. The issue I’m having appears at the output side of the amp between the OT black wire and, in my case, the yellow and white wires. From Beelzebum I understand that this is how those wires should read with a multimeter from the OT.

[Robin]

Your amp is not passing a signal? It it silent? Is there hum?

[biolite]

No, no; I haven’t powered the amp yet. I simply discovered that the output wires (8 ohm and 4 ohm leads) from the OT both appeared to short to ground as I was going back through checking the continuity of everything. From what I hear, this is expected. I’ll be working on a cab this weekend and hopefully get that finished and I can power on the amp then and see what happens. I’ll post the result in the case that it doesn’t work.

[Robin]

Using a current limiter, similar to this one:
http://tubenewbie.com/light_bulb_current_limiter.html
will avoid damaging any components if there is a short somewhere.

For additional protection, I use a Variac with new circuits (and cap jobs) to form new capacitors gently. Starting at about 40v for a few hours and then add 10v per hour until reaching 120v. It’s probably overkill but I know from working with large caps in photo studio strobe equipment that they work better and last longer if you give them a chance to form slowly rather than just ripping them up to full voltage right off the bat. On the other hand, not everyone has access to a Variac and I sure most manufacturers don’t take the time to form caps so going for it must work fine too. Please let us know how it works out for you, post photos if you get a chance. Good luck and congrats.

[Author]

Posts