champ without board....

[Carlin Mitchell]

Is it possible(without harming anything) to wire up the power portion of my Champ without the board ? I understand that the red and yellow wires will still be hot so they'll need to be capped but should there be a load on them ? I guess I just want to confirm that the heater circuit is working. Does that make sense ?

[Zippy]

Can you not trust a continuity check?

[Carlin Mitchell]

Everything checks out with the meter but I am still blowing fuses... hmm.

[Zippy]

"Everything" including isolation from ground?

When did it start blowing fuses?

[Carlin Mitchell]

Keep in mind this is my first attempt at a tube amp. I had it all together and plugged in, all the tubes were glowing.... the volume knob seemed to have no effect on the output.... I got a little sound out of the speaker if I touched the end of the guitar cable. . I had no sound when I actually plugged the guitar in. I rechecked everything and decided to leave it on for a few minutes ... after about a minute ?? the fuse blew. I have not been able to get it to turn on since. I talked to a few people on the garage and decided to go back and start from the power source. I have the chassis with transformers.... fuse, switch, and all sockets wired with heaters only. The rectifier tube if fully wired. I've capped the yellows and reds. With the red and yellow center tap grounded I get about 37 ohms from one red wire to ground and about 64-ish from the other red to ground. Red to red I get about 100 ohms. Is that right ?[/i]

[karczochrulez]

In my opinion - strip, and build once again

[Carlin Mitchell]

Yes..... That's a good idea. Since it was my first I guess I shouldn't be too discouraged. I know the reward of flipping the switch and getting that thing to sing will be worth it. It seemed liked a long process to get this far... but looking at it now... it seems a lot easier after just going through the motions once. Wish me luck.

[Phil_S]

...With the red and yellow center tap grounded I get about 37 ohms from one red wire to ground and about 64-ish from the other red to ground. Red to red I get about 100 ohms. Is that right ?

I would expect the two halves of the high voltage secondary to be more symmetrical. They are rarely identical. The chassis ground may have an unaccounted for affect, so lift that and measure again.

Pull all the tubes so you don't need to lift the filament supplies. Lift the center tap from ground. Then, put your meter across the two red leads set to measure ACV and turn the power on. Then, measure from each red to the red-yellow. The red to red-yellow should be half of the red to red reading on each side.

A few volts difference is insignificant. Let's say you've got 500VAC across the red to red. Then you'd expect 250VAC across each red to red-yellow. If it turns out to be 245 and 255, that's not a big deal. If you get 300 and 200, you've got a problem with the transformer.

With no load, the output will be higher than the transformer rating, so don't be alarmed that it is too high.

Post results. Provided this is OK, we can go to the next step. From your description, I think there is more than one wiring error or omission.

[Carlin Mitchell]

I know I need to do more research on star grounding but can somebody explain the purpose ? I just watched the youtube video somebody linked about noise reduction, shielding, and grounding. It was very informative. I'm glad I'm re-doing this... It's so much clearer the second time around. The colors of the wires... the pins on the tubes... it was so intimidating. It's getting easier. Thank you all for the help you've given me. I will post more tomorrow.

[Bob-I]

I know I need to do more research on star grounding but can somebody explain the purpose ? I just watched the youtube video somebody linked about noise reduction, shielding, and grounding. It was very informative. I'm glad I'm re-doing this... It's so much clearer the second time around. The colors of the wires... the pins on the tubes... it was so intimidating. It's getting easier. Thank you all for the help you've given me. I will post more tomorrow.

Forget star grounding. The purpose is supposed to be preventing ground loops, meaning more than one path to the same ground point. In reality is creates more problems with the current running though all the ground wires.

How about you post some more information about your build? Pictures, layout, schematic etc. Also, take voltage readings from the pins on the tubes. On the 12AX7 post the voltage on pins 1, 3, 6, 8, on the 6V6 post pins 3, 6 and 8.

I'm sure you can get this sorted out without a complete rebuild.

[Carlin Mitchell]

With the yellows and greens capped I measured AC voltage across the two reds with the red/yellow GROUNDED... and got about 360V for about 10 seconds and then blew a fuse. I'm going to try to get a Ground fault checker and check my outlets. This is with only the switch, fuse and transformer in the chassis. I have a 2amp slo-blo fuse in it and ike I said it gave me about 360-365 for a bout 10 seconds before the fuse went. I'm on my lunch break , I will try to read it with out the red/yellow grounded after work.

[Carlin Mitchell]

I pulled the tranny out completely and ohm-ed out all the windings.
I got ...

4.6 ohms black to black
1.2 ohms green to green
1.0 ohms yellow to yellow
101 ohms red to red
64.7 ohms red1 to red/yellow
38.5 ohms red2 to red/yellow

What gives ?

[Phil_S]

Either one side of the high voltage winding is partially shorted or your center tap isn't in the center. It's possible the center tap isn't in the center, or it isn't really a center tap, but an alternative to using the full winding for reduced voltage. This is why you need to power up the transformer and measure red to red, and then each red to the red-yellow.

Do you know for sure if the red-yellow is meant to be a center tap? I don't recall if you said what brand and model of transformer it is. What did the seller or manufacturer say? Or is this used iron? (I have a number of amps with used iron; really, it is generally a good deal to recycle.)

Now that you have it out of circuit, the matter is simple. Use an ordinary lamp cord with a plug on one end and stripped pigtails on the other ("suicide cord"). Use wire nuts to connect the pigtails to the primary (black and black). Make sure all the secondaries are safe. You can cap them with a bit of heat shrink or just make sure they don't contact anything when the transformer is hot.

Now, find a power strip with an off switch. This allows hands-off work with your tranny. Plug your suicide cord into the power strip with the switch off. Set your meter to measure AC voltage and clip the meter leads to the two reds. Flick the switch and check the voltage. Write that down and turn the switch off. Then do the same, checking each red + red-yellow combination.

For example, if the red to red voltage is 500v, you are looking for each red to red-yellow to be close to 250v. Based on the ohm readings, I am expecting you will get 333v on one side and 167v on the other. (Adjust the numbers as appropriate to your transformer.)

Keep your hand on the off switch. If you smell any smoke or see any smoke, turn it off and leave it off. You are likely to smell it before you see it, so trust your nose. And you don't need to ask me how I know this, but let's say I am in a position to speak with authority. It is a good idea to have a fire extinguisher handy, but I seriously doubt you will need to use one. This is just an appropriate safety measure for your work bench and more important than getting a GFI detector.

If this lets the smoke out of your tranny, please don't feel bad about it. It just means the tranny was already bad and you have, at this stage, lost nothing, as it was lost before you started the confirmation process. What I'm asking you to do won't damage a good transformer.

If there is a dead short (remote possiblity) on the primary, the household circuit breaker will blow or the power strip switch will trip off.

Post your results and then we can see what's next.

[gahult]

Is this what you have?

this may help.

[Carlin Mitchell]

Yeah... I think. It's a hammod 25?-x I think. I'm not sure off the top of my head. I bought it from RJ. I'm going to try the hardwire thing tonight. wish me luck. I will be safe.