No sound - no hum - no good - please help

[warrenh]

I am building a ceriatone 36w EF86 (uses 4 EL84 in power amp) using Shinrock trannies and I am getting no sound - no hum either. Does anyone have a way to check the OT to see if it is defective - I fear the primary leads may have been colored wrong. I had the red wire as center tap on the primary. But using an ohm meter I get equal resistance across coils on a different wire. Does anyone have a way to check using an ohm meter or otherwise whether the OT is blown?

Thanks all in advance.

[martin manning]

Lift the lead that you have connected as the primary center tap. Put one end of a speaker cable in the speaker output to open the shorting contact (leave the other end unconnected). You should now have the OT isolated, with only the secondary grounded.

Start by verifying that there is no continuity between the primary and the secondary. Just pick one lead from each and measure resistance, which should be infinite. Then make sure that there is continuity between all primary wires and measure the resistances. Pick one lead and measure the resistance to the others, recording the readings. Do the same for the secondary, and you should be able to identify all the leads. Expect 100's of ohms on the primary and as much as 10% difference between the two halves. Expect just a few ohms on the secondary.

[warrenh]

Here is what I have for readings and thanks for your reply.

Primary - anode to anode = 89 ohms
CT to anode #1 = 40 ohms
CT to anode #2 = 48 ohms

Secondary - ground to 16 ohm tap = 1 ohm
ground to 8 ohm tap = .7 ohm
ground to 4 ohm tap = .4 ohm

I have no shorts between primary and secondary windings.

Does this sound normal or is my OT a goner?

Thanks for your help.

[Phil_S]

Ingo probably winds the best transformers on the planet. Apparently, people have been known to wait many months for a set. I feel certain they were good when you got them.

Your readings are certainly reasonable. On the primary, that imbalance is normal. To make the same number of turns on the outer requires a longer length of wire than the inner.

I think you should look elsewhere for your problem, provided you've got it hooked up correctly.

The advice I usually give for situations like this is to check the schematic to the build two times. Once from the input jack(s) to the output jack, and once in the reverse direction. Use a highlighter to mark the schematic as you go along. Where you can't get an in-circuit read on a resistor, you may want to lift one end to check it with the meter. It won't be the first time someone used a 1K where a 100K was needed or something like that. Check all the coupling caps, too. Caps should read open on the Ohm meter.

There is a good chance this is something simple, like a misconnect, a missed connection, or bad/missing ground. Go over all of it very carefully.

Good luck.

[martin manning]

Those readings don't look unreasonable, and certainly you should get some sound out. I'd look elsewhere for the problem.

Start by powering it up with no tubes installed and check the power supply node voltages, which should measure high and nearly all the same. Just ground the black lead from your meter and probe the positive ends of all of the power supply filters. If that doesn't present any anomaly, then put the tubes in and make a voltage chart that has the voltage at each pin on all of the tubes, plus the power supply node voltages. AC for the HT on the rectifier and all the filaments, DC for all the others.

[sliberty]

when there is no sound, I like to probe the amp from input jack forwards, to determine when the sound disappears. I use the probe that Weber descibes in one of his books, along with an iPod plugged into the amp's input as a signal source. It is essentially a coupling cap connected at one end to a cable that can be plugged into another amp (or even powered PC speakers). Then, by using the exposed end of the cap as the probe, I start at the input jack, then move to the first stage plate, then the second stage plate, etc, following the sound until it stops. Along the way, I make sure that the pots to what they are supposed to (once I am probing after them).

This technique saves a lot of time that would be otherwise wasted playing around in stages that DO work, and it takes very little time.

Oh - moun your cap onto a non-conductive chopstick for easy handling. Make sure that the connected lead of the cap is taped over so you don't hurt yourself.

[KT66]

The signal tracer idea is a good one, it's an easy way to isolate noise issues, or no sound issues as in this case . I built mine out of an old boom box I got at Goodwill and a few bucks in parts at Radio Shack :

[warrenh]

Thanks all for your replies and help - I will try the probe troubleshooting this weekend if I have time and see if I can figure it out.

Thanks again,
Warren