Express build 1st startup, B+ voltages too high?

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shipwreck
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Express build 1st startup, B+ voltages too high?

Post by shipwreck »

The white bias wire (disconnected from the bias board) also has continuity to ground, as does the junction of R16/R17 that the bias wire connects to.
sluckey
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Re: Express build 1st startup, B+ voltages too high?

Post by sluckey »

shipwreck wrote: Fri Jun 23, 2017 2:35 pm The white bias wire (disconnected from the bias board) also has continuity to ground, as does the junction of R16/R17 that the bias wire connects to.
Ah. Finally. That's the problem. That white wire should have no (infinite) resistance reading to ground. Check the board closely top and bottom. Should be able to see the problem.
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Re: Express build 1st startup, B+ voltages too high?

Post by pompeiisneaks »

Yup and I was going to say that the main winding will usually have pretty low resistance so you should get continuity through them to ground, its just a lot of copper wire in a loop, so maybe 1 to 2 ohms max, which usually falls under the continuity range of most meters.

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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Fri Jun 23, 2017 2:23 pm Haven't tried to power it up yet.

I also switched the location of the blue wire from C15.
Yes! DO NOT START UP THE AMP with this re-done board! The main rectifier is now shorted directly to ground! Re-do that blue wire which connects to what looks like two new turrets. That new two turret thing is directly connecting your four main rectifier diodes to the chassis lug. If you are lucky, the worst that will happen is loosing the main fuse.
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Fri Jun 23, 2017 2:35 pm The white bias wire (disconnected from the bias board) also has continuity to ground, as does the junction of R16/R17 that the bias wire connects to.
I agree with sluckey. This is where the main bias problem lies. The white wire should connect to the junction of two 100-220k or so resistors and then each of those should connect to a 5kish or so grid stopper and then the power tube grids. The white wire should have no connection to ground. Since it does there must be something wrong with the main board connections or something is shorting this out under the main board.

You should be able to test the bias board. First fix your main rectifier short. Keep the white wire disconnected, no tubes installed and use a light bulb current limiter. You can then measure the voltage at the point were the white wire would connect to the rectifier board. My guess is that your bias voltage will now look ok there. Again, don't power it up without addressing the short on the main rectifier.

Finding where the short is on the main board is going to be more painful. I'd first try lifting the two 100k or 220k bias feed resistors from the point where they join. See if the white wire still connects to ground.
shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Image

As I read the schematic, the two 100k bleeder resistors comes before the capacitors, but does it matter if I have them like this?
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

Those bleeders look good. Thanks for posting that new picture. The new bleeder resistor mounting locations confused me as I thought they were turrets with a solid wire jumper between them :). Disregard my previous warning. Your main rectifier now looks good to me.

Given that I think a no tube test (using a current limiter) of the bias supply voltages (white wire disconnected) would be what I'd do next. I bet you should see reasonable voltages on the bias board now.
shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Image

All speaker inputs have continuity to ground even when I insert a jack. Did I use the wrong speaker jacks?
shipwreck
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Express build 1st startup, B+ voltages too high?

Post by shipwreck »

I wondered if I had connected the speaker wires to the wrong side of the shorting jack, but after changing them over to the right, I still have continuity to ground from the bias wire. I could not see anything obvious on the main board in the R16/R17 area either.
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Fri Jun 23, 2017 5:27 pm All speaker inputs have continuity to ground even when I insert a jack. Did I use the wrong speaker jacks?
One side of the OT secondary is connected to ground. So, yes, it is normal to see continuity on the other lead. As was mentioned by someone else earlier a transformer winding is just a long length of wire.

If you have connected the speaker leads of the OT to the wrong side of the shorting jack then this needs to be fixed as the amp will have no load when a speaker is plugged in. Do not run the amp like this or you can fry the power tubes and or OT.
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Thanks for all the continued advice and help. Do I need to connect a speaker load for test startups witout tubes as well?
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Re: Express build 1st startup, B+ voltages too high?

Post by sluckey »

Stay focused on the bias white wire issue. That's the most important problem you have at the moment. Plenty of time to sort speaker jacks and other stuff later.
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Fri Jun 23, 2017 6:01 pm Thanks for all the continued advice and help. Do I need to connect a speaker load for test startups witout tubes as well?
No. If there are no tubes then you don't need to worry about a load. As sluckey says concentrate on one thing at a time. On new amps I think it is a good idea to keep all the tubes out and first make sure all your power supplies are working first. So, you should first do that.

Then, after you have reasonable voltages at all relevant spots, I'd take some time and double check the rest of the circuit against the schematic. A little time spent with a highlighter pen, your schematic and amp can often save you tons of time (and money) later. But first, sort out your power supply.

Are you 100% sure that spot on the main board you have the white wire connected to is for the bias voltage? Since it is shorted to ground, are the two bias feed resistors as well?
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

I've marked up one of your images with some labels. Point A is where your bias voltage is connected to the main board. It needs to connect to the two grid leak resistors at point B. I'm assuming there is a trace under the board that does this. Did you take any pictures of the under side of this board before you installed it? Can you get pictures from somewhere? Perhaps the manufacturer?

Assuming the bias voltage makes it to point B, then these two resistors connect to two caps (which will block DC to ground) and then points C and D. Points C and D simply connect wires to grid leak resistors and then to the power tube grids. I can see the traces to points C and D. You can do a quick check for shorts to ground from C and D onward by disconnecting the blue and yellow wire at C and D. These disconnected wires should have no continuity to ground. If they do, examine your wiring around the power tubes.

If disconnecting the wires at C and D still leaves the white wire connected to ground then you must have a short under the board or point A is not where the bias voltage is suppose to connect.

Also, it would be helpful to provide a link to the schematic and layout (if any) you are working from. That way everyone can be literally be "on the same page" :)

Mike
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shipwreck
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Express build 1st startup, B+ voltages too high?

Post by shipwreck »

IMG_2168.JPG
IMG_2223.JPG

Good points, I have attached the schematic and the layout I have used. Sorry for not doing that to begin with.

But now I think I have located the problem. I had used a connection on the main board for the NFB resistor (under romberg's B label), because I didn't want it suspended in the air from the precence pot. So that was a deviation from the layout, and when I redid the NFB wiring to conform with the layout, the bias wire does not short to ground anymore.

So I am ready to do a new startup without tubes, but I will wait until tomorrow when I am rested. In the meantime, I am posting a new picture of my build for review.

Image
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