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PostPosted: 20 May 2018, 11:10 
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Joined: 07 Jan 2018, 07:13
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I would say it has to be. I just finished swapping out every single component in the right channels active circuitry including the tube, inputs and outputs both inside and outside the chassis, and power from the power supply, the right channel is still noisy.... The right channel is the tube in the back so it has the longest length of the shielded Mogami wire before it gets to the socket. The selector is in the back, and has short wires from the jacks, then the shielded wire runs to the pot on the front face then down to the sockets, so the right channel has roughly 4 inches more of length. I have copper braid shielding. Would it be worth double shielding the Mogami or is it a waste of time. Also note, the Mogami is grounded only at one end.

I have no idea what to try next.

Thanks guys


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PostPosted: 20 May 2018, 15:55 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I use the same cable all the time and up to several feet it works just fine. It is my preferred shielded cable. No purpose in double shielding. The problems have to be elsewhere.

Good listening
Bruce

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PostPosted: 21 May 2018, 18:46 
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Hello,

I attached a couple pics of the changes I made. I shortened the shielded cable from the selector switch to the pot and from the pot to the tubes as much as possible. I connected the shields from the sel. switch to the pot directly to the buss at the selector switch end only. I ran a separate ground wire to the buss for the shields from the wipers to the tube sockets. I also ran a separate ground run to buss for the pot ground pins. And I grounded the pot case to the chassis. Despite all this the buzz is still there. It may have even gotten slightly louder but not by much.

To note, other things I have done are is:

1. Completely replace all components in the right channel.

2. Swap all imputs and outputs both inside the chassis and out. (the buzz still follows the right channel circuitry)

3. Swap incoming B+ leads from the power supply.

Nothing seems to have an effect.

Not sure what else to do.

Thanks


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PostPosted: 22 May 2018, 03:48 
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With regards to hum: can you try to exclude the circuit breaker, which is installed at the rear wall? It can give interference to nearby circuits.


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PostPosted: 22 May 2018, 17:59 
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Thanks Poty!!! That was the issue. Dead silent now.

That is a time delay relay for the tube preheat. I’m going to try to relocate it and shield it.

Thanks again.


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PostPosted: 22 May 2018, 20:40 
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Hi, Good call, I wasn't thinking about how that could cause the problem since I don't use them. I aways use an IEC power entry with built in fuse. Very compact and shielded. One more thing to add to the knowledge base.

Good listening
Bruce

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PostPosted: 22 May 2018, 21:32 
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I’m going to test the time delay in a different area. I’d still like to use it for tube preheat. I’ll let you know what I find.

Another question I have is I initially put a 47R and an X2 cap across the power switch to reduce arcing. I saw this on the KT 120 schematic. But in my amp it was letting enough current through and the transformer was reading nearly full voltage with the switch off. Would I just increase the resistor value?

Thanks


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PostPosted: 23 May 2018, 06:43 
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Hi there. Note that the 47R and 100nf X2 capacitor are to be connected in series. Make sure you don't connect them in parallel by mistake or it will give the effects you are describing.

Best regards.

Diyengineer wrote:
I’m going to test the time delay in a different area. I’d still like to use it for tube preheat. I’ll let you know what I find.

Another question I have is I initially put a 47R and an X2 cap across the power switch to reduce arcing. I saw this on the KT 120 schematic. But in my amp it was letting enough current through and the transformer was reading nearly full voltage with the switch off. Would I just increase the resistor value?

Thanks


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PostPosted: 23 May 2018, 06:58 
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They were most definitely in series across the switch. But was still happening. Not sure why though.

Thanks


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PostPosted: 23 May 2018, 10:51 
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Hi, with the switch off and if you use a sensitive meter (most now are) and there is effectively no load on the AC, you will indeed measure much of the voltage....the current is rather small. Depending on your actual circuit and what is actually attached to the cold side of the switch several voltages are possible. If it is the transformer (no delay in place or any other sort of interruption) then the voltage will be small. If there is something that will disconnect the transformer then you can see up to the AC line voltage. If I can manage math this early in the AM the capacitor will have a reactance (think resistance in this case) of about 26K ohms. Around 4 ma max current worst case. In this preamp you need neither the arc protection nor a tube delay. The tubes will last many years without it. IMO Adding things to circuits that serve no real purpose or benefit is generally not a good idea as the possibilities for hum, noise and unusual behavior will increase. This preamp was intentionally designed with as few such things as possible in order to maximize the performance.

Good listening
Bruce

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