• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Kofi Annan in: "Push and Pull with Me"

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#202
SY said:


Yes, they have numerous meetings to negociate the wording of a warning that if the hijacking doesn't end, they will consider issuing a strongly-worded letter.
Click to expand...

Sticky problem of having to consult with various affected parties. Not to worry, the benevolent grand leader, consulting only a "higher father", will come to the rescue, shooting at the hijackers through the windows before charging in guns blazing. Turns out, many of the passengers had some sympathy for the hijackers, some hated them, some hated their rescuers, they all mistrusted one another and they had knives and stuff. That's cool though, they ain't like us, and besides, the wing tanks are still full of fuel.

Sheldon
 
#208
Last heater question (I hope). Is there a way I can test the heaters once they're wired to make sure they work? I have the two EL84s daisy chained with the 6.3VAC between pins 4 and 5 (one wire on pin 4, one on pin 5) and the E88CC chained after that, also to pins 4 and 5.

Pin 9 of the E88VV is grounded to the power star ground and all center taps (6.3V, 5V, B+) are also grounded there.

When I check the AC voltage between the pins, there is almost none-- .73VAC I think. I checked the line and theres 115VAC out of the wall and into the amp, but not enough voltage for the heaters.

I'm sure something else needs to be grounded here to complete the circuit and give me the full 6.3VAC, but I'm not sure what. Maybe I need to ground the cathodes of the EL84s and the E88CC first?

Help?

Kofi
 
#210
If that 5V winding is heating the rectifier tube, you don't want to ground its center tap - it's sitting at B+! Remember, even though the GZ34 is indirectly heated, the cathode is still connected to the filament. At best, you'll blow a fuse. More likely, you'll take out the tube or the power transformer.
 
#212
It's possible, but you didn't get any smoke or fireworks, so maybe you're fine. Disconnect all the secondaries, put the primary on a variac and measure the voltages if you want to be sure. Otherwise, just try it again with the 5V winding floating. Grounding the ct of the 6.3V winding is fine. Oh, and i'm assuming you have a fuse (not more than 2 or 4 amps) in series with the hot leg of the primary? That is not optional.
 
#213
Kofi Annan said:
Last heater question (I hope). Is there a way I can test the heaters once they're wired to make sure they work? I have the two EL84s daisy chained with the 6.3VAC between pins 4 and 5 (one wire on pin 4, one on pin 5) and the E88CC chained after that, also to pins 4 and 5.

Pin 9 of the E88VV is grounded to the power star ground and all center taps (6.3V, 5V, B+) are also grounded there.
Click to expand...


All filament should be in parallell (with daysichain I understand series connection).
This is, all EL84 and E88CC pins 4 go together, the same for pin 5s. Then, you apply 6.3Vac between pin 4s and 5s.
Then, if you want, you ground the CT of the 6.3Vac secondary or leave it open and put a 100 ohm rheostat between secondary ends and ground the cursor of the rheostat. You can then find the point where the amp is less noisy.
In the future you can lift that ground and put some 30-40V positive relative to ground to minimize parasitic rectification effects between filament and cathodes.

Center tap of 5Vac has to be left open.

I would connect pin 9 of the E88CC to the star ground of the signal input. This amp is not particularly sensitive, but shields can induce noise if not grounded correctly, and I'd rather be over zealous of noise than hunt noise around.

Gaston
 
#214
So, you're saying that I could have accidentally blown the transformer by grounding the center tap of the 5V heater for the GZ34?
Click to expand...

If you did that, then you shorted out the B+. This would have damged either the B+ secondary or the GZ34 or both.

Note that, in a GZ34, one end of the heater is connected internally to the cathode at pin 8. Your 5v winding needs to be well insulated from the other windings, so that it doesn't get accidentally shorted to ground within the transformer. If the 5v winding does have a centre tap (which would be for use with directly heated rectifiers like the 5U4), don't connect it to anything. Take the B+ from pin 8 of the GZ34.
 
#215
OK, got it. But let me explain further.

I never had the GZ34 in the socket. In fact, I never had any tube in the socket except for one EL84 for a moment to see if the heater would glow. It didn't, so I started testing.

The supposed 6.3VAC is measuring less than 1V and the 5VAC is measuring about the same. I wonder if I don't have a short in the secondary.

I hear no sizzling or popping. No smoke, no lights.

If I leave everything floating, should I still be able to measure 5V on the 5V tap? I think I should start there, maybe.

What about the 6.3V? Can I just plug the primary into the wall and check the 6.3VAC tap?

Kofi
 
#216
Time to go to bed. If you didn't plug in the rectifier, you didn't cook the HV secondary. At least not from what you've described. As mentioned earlier. When you do plug in the rectifier, leave the B+ unconnected to the tube at first. Fuse the primary with a low value and recheck the voltages across the secondaries. Make sure your meter is set properly. But before you do any of that, check the DCR of all the primaries and secondaries. Resistance in the filament secondaries will be very low, so unless you have a meter accurate in the low ohm range, this part may not be too useful. Compare with spec. or post here - in the morning.

One more thing: If you have a 115/230VAC unit, you must make sure that you have the primaries connected properly our it will be out of phase and that will short it out.

Sheldon
 
#217
Wow. I needed some sleep.

I know I'm screwing something up here-- classic Kofi. I'm about to head back out now.

I have a 1A slo-blo fuse in series with the "hot" AC, so at least I did that right. The only tubes that were ever in the amp were one EL84 and one E88CC one at a time. I wired three sockets (daisy chain) but only used one tube at a time to check the filaments.

It still bugs me that the AC voltage is so small, but I'll start testing the rectifier heater voltage and the DCR of the transformer secondaries and post what I get.

The transformer is not a universal primary-- its 115VAC only, so at least I can't screw that up.

More to follow.

Kofi
 
#218
Wow. What a good night's sleep will do for you...

It was the fuse. Once I replaced the fuse, I measure 5V across the rectifier. Perfect. Only problem is...

I keep blowing fuses. I'm using a 1A slo-blo, but I wonder if that's too small. Also, these are the little mini-fuses, which I didn't know until I received them. I wonder if I should replace the fuse section with one that can handle larger fuses and fit this with a 2A slo-blo.

Advice?

Kofi
 
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