No sound, hum or buzz from Custom Kraft N100X

[Resonator Guy]

I have been rebuilding a Custom Kraft N100X by Sound Projects. I have replaced every cap and resistor in it. I got a very weak sound out of it for a moment or two and then nothing.
While reading in another thread about a similar sounding ( or not-sounding) problem, it was pointed out that the OT might be the problem.
I measured at the speaker terminals as instructed and got:

With both speaker wires unhooked - 6.6 ohms
With one speaker wire unhooked - 6.6 ohms
With both speaker wires connected - 0.5 ohms

Does this look like the OT is shot?

Although this is just a 3 tube beauty with an isolation transformer, I am determined to get this and three other amps languishing on shelves in my work shop running this winter.

Any assistance would be appreciated.

[Chuck H]

While reading in another thread about a similar sounding ( or not-sounding) problem, it was pointed out that the OT might be the problem.

Oy... Everyone always thinks the output transformer is responsible for "no sound". I can't say your OT is OK but it's not the first thing I would suspect.

I measured at the speaker terminals as instructed and got:

With both speaker wires unhooked - 6.6 ohms
With one speaker wire unhooked - 6.6 ohms
With both speaker wires connected - 0.5 ohms

Does this look like the OT is shot?

It looks like you weren't sure what to test or what you were testing for. That 6.6 ohms is the speaker resistance which would be the only thing your measuring at the speaker terminals with either or both speaker wires removed. Your last measurement would be the OT secondary resistance parallel with the speaker resistance and due to the extremely low resistance of the OT secondary should basically read like a dead short. So don't worry about your OT yet. Instead go here:

Tube Amplifier Debugging Page

Be careful. There are lethal voltages inside tube amps. Sometimes even when their turned off and unplugged. It's important to know some specific safety precautions when dealing with tube amps. Look here: (some humor, but take it seriously)

SafetyTips

[Resonator Guy]

Thanks Chuck. I was just following instructions from another thread. I must admit that I don't know what a dead short would read like on my multimeter. Having reviewed the Tube Amplifier Debugging Page, it leads me to tests on the OT, as everything else checks fine except for the PT, it doesn't have one, just an isolation transformer, though that might not be working. I will work my way through the stuff there and see what happens.

[Chuck H]

Having reviewed the Tube Amplifier Debugging Page, it leads me to tests on the OT, as everything else checks fine except for the PT

Actually it leads you to voltage testing first. Starting with "does the pilot lamp come on?" You also need to KNOW that all your tubes are in proper working order. If your PT is suspect that's where you should start. How much sense would it make to replace an OT in an amp with no HV on the tubes??? Point is that your low reading on the speaker terminals is not at all suspect and you need to test actual working voltages and circuit operations before suspecting the OT at all.

For now it might be best to just take the OT out of the issue all together. Forget about the OT. It's clouding your thinking. There was a thread here about a month ago where a guy had a similar issue. 'Amp doesn't sound right=OT must be bad'... Fortunately he had a couple of correct OT's on hand and started swapping them in and out and in and out add nausium. One of the OT's came out of a GOOD sounding amp, otherwise I think he may have assumed all his OT's sounded bad. Having put in so much time already he gave up and ultimately ended up where he started without an answer as to why the amp sounded bad. His fixation on the OT being the only possible answer kept him from finding an answer.

[Resonator Guy]

Chuck, your statement that the low reading on the speaker terminals is not at all suspect was what I wanted from my first post.
Nothing is clouding my thinking, I just wanted to get rid of the obvious stuff first. That's something I learned 35 years ago when I started in on-line computer system analysis. A straight forward answer to a question always helps. I am rather new at amps so I have to learn as I go, but I will always attempt to eliminate the easy stuff first.

[Enzo]

Just being direct.

Transformers are the last thing on any list. They do fail, but a lot less than all the rest of the stuff. Yours is fine.

A dead short will read about the same as a good part when checking DC resistance of that output winding. That transformer winding is after all made of a relatively small number of turns of heavy gauge wire.

You replaced a lot of parts, and it tried to work then quit. Rather than suspect something you didn't change, I'd suspect something you did. A cap installed backwards, a 470 ohm resistor installed in a place where 470k was supposed to be. No shame, everyone has done this.


Iso transformers are just 120v in 120v out, or whatever your mains might be. Eaasy enough to tell if it works with an AC voltmeter.

Can you link or post a schematic?

Your tubes light up or they don;t. Voltages are present or absent.

And if your amp uses tubes like 50C5 and 35W4, then if there is also a pilot light, it actually is important that the pilot light bulb be there and working.

[Resonator Guy]

Thanks Enzo. I never could find a schematic for this amp. I just referred to "similar" ones. When I got the amp it didn't work at all and it had already been "repaired" by someone else. I have run into a fair few like that from ebay. I have repaired about 35 amps now by recapping, replacing ripped speakers, retubing etc but still have three, maybe four, that are beyond me.
This one has a 12AU6, a 50L6, and a 35Z5. They all light. There is no pilot light.
There were jumpers, black tape and other stuff inside when I got it but I will keep trying. The others are real amps and will take more learning, but one has to keep busy at something and I don't see myself playing golf or curling.