Resistance issue [resolved]

[Paul Birkeland]

Did you pass the glow test?

[Paul Birkeland]

Well, if you don't pass the glow test, none of those DC voltages will be correct.  So neither tube glows?  When you replaced that STP cabling, you disabled the heaters.  This is the wiring that feeds pins 1 and 12 on each 12 pin socket.  Follow that backwards and you'll be at the 10,000uF cap and 0.1 ohm resistor, then backwards from there you get to the H terminal strip with the big rectifier diodes that mount to the chassis.

Something is not connected or broken along that path.  You can forget about doing any other debugging of any kind until your amp passes the glow test again. 

[soundfel]

Both tubes are off. All the wiring looks fine. I replaced  the wires that run 31/32 and resoldered but still no glow. Since I'm getting no voltage at H4, could it be a problem with the power transformer? I ran the transformer secondary test on the manual again and it's OK, the AC voltage are also normal. Do you think I should replace the capacitor or the 0.1 resistor?

[Paul Birkeland]

I think you should stop replacing parts without actually knowing they are bad.  You had glowing tubes, now you don't.  All that you've changed is the STP wiring that heats the tubes.  If the capacitor is bad, it will pop its top and blow out its goo, and you would be telling me about that.  The primary way that happens is if you install it backwards.  You wouldn't have ever passed the glow test if you had put that cap in backwards.  Likewise I couldn't figure out a way that you could kill the 0.1 ohm resistor.  If you start pulling these parts out and replacing them without any reason to do so, you will continue to make new problems that you will have to track down.

With one probe on H2 and the other probe on H4, what DC voltage do you get?  How about with one probe on H5 and the other probe on H7?

[soundfel]

You are right Paul, I get impatient but that only makes more trouble. Voltages are -18 on both those readings. Thank for your help!

[Paul Birkeland]

Voltages are -18 on both those readings. Thank for your help!

No, they are not.  Maybe it says -18mV? Or Maybe -0.018 or -0.18?  If in doubt, you can post a photo of what your meter is showing for one of those readings.

[soundfel]

When I turned the amp to test these voltages again, tubes were glowing. I tested the voltages and they were OK, but afterI flipped the amp to test it with my headphones, tubes went back off and the voltages are wrong again. This is what I get between H2 and H4. It´s the same on H5-H7.

[Deke609]

It could just be an effect of the photo, but in the first pic you posted in this thread it looks like one of the ends of the thick solid green wiring isn't connected to a terminal of the power transformer (or is poorly connected).

You may also want to take a look at the condition of at least one of the 1.5uF capacitors (big black ones). It looks like it got pretty toasted. From the first pic you posted it looks like the internal metalized film is exposed and possibly damaged.

cheers and good luck, Derek

[Paul Birkeland]

What is your incoming AC line voltage?  Having 20V there makes me think you are outside of the US.

When you flip the amp over and that changes the glowing behavior, that means you have a loose connection.  This could be in the green twisted pair of wires going from the power transformer to the H terminal strip, on that terminal strip itself, or back up front where the 10,000uF cap and 0.1 ohm resistor are.  This is not a bad component.

[soundfel]

Thanks foryour reply Derek. Those capacitors have suffered my clumsy soldering indeed, but I’m guessing that wouldn’t have to do with my tubes not glowing, so I could deal with them later. Am I right? And Paul, I am outside of the US but we have 120v. The AC on the IEC measured 123. I’m going to check those connections and will report back. Thank you.

[Paul Birkeland]

I would be inclined to tell you to change the batteries in your meter too.

The power transformer has a 6.3V winding feeding those terminals.  You shouldn't see anything above 10V there. 

[soundfel]

Hello there, I just wanted to report back. I checked the soldering and now all voltages are normal. I’m glad to inform that sound is awesome on both channels so I think I’ll try to install the C4S upgrade. BTW, I checked those H voltages before resoldering with brand new batteries on my meter and I got the same 20v. Now, I get 6v.

Thanks for all your help!

[Deke609]

The issue is "academic" now that the amp is working, but I find it really curious that there was 20V DC on the heater wiring. How could that happen? I thought that maybe B+ and the heater wiring were accidentally connected, which at the high current draw of the heaters might produce serious voltage sag on B+ and result in an additional 10 or more DC volts on the heaters. But if that were the case, I would expect B+ to be tiny  (as a result of the very same voltage sag), but that doesn't appear to have been the case.  Weird.

@PB or anyone else: any guess as to how this happened? Just curious.

cheers, Derek