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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 8:17 am
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BMW2002Ti wrote:
7chord,

I think you need to lift the grounded heater line and have two twisted pairs of heater leads, to each tube.

l


Only this will fix 90 % of the hum from the heater, other hum is because it is a single end amp.


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 6:36 pm
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BMW2002Ti wrote:
7chord,

I think you need to lift the grounded heater line and have two twisted pairs of heater leads, to each tube. You can use the cathode voltage off the 6V6GT to supply the needed DC. Valve Wizard discuss this, in his website.

:)

http://www.valvewizard.co.uk/heater.html


What I'm talking about is what Valve Wizard refers to as heater elevation. Interesting link though!


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 7:53 pm
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Right. I have done this maybe once, in the last 25 years.

"Another convenient source of elevation voltage is the cathode of a cathode-biased power valve. No current flows ‘into’ the heater supply from here, so the power valve bias is not affected."


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Post subject: Re: Champ Hum Mitigation
Posted: Thu Jul 27, 2017 6:43 pm
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I finally had time this evening to do the virtual center tap mod. When I powered up the amp afterwards, for a moment I thought something was wrong. It was THAT quiet. No need to mess with anything else.

There is no discernible hum with nothing plugged into the input. Any noise at this point is from my pedals, and that is minimal.

Excellent result!

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Post subject: Re: Champ Hum Mitigation
Posted: Thu Jul 27, 2017 9:56 pm
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Nice job! What little noise is left, is prolly due to the single-ended nature of the amp.

:D


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Post subject: Re: Champ Hum Mitigation
Posted: Fri Jul 28, 2017 10:51 am
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Congrats on your dedication to getting it as close to possibly humless as is possible. Does look much better w/ tightly wrapped doubles as well.


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Post subject: Re: Champ Hum Mitigation
Posted: Fri Jul 28, 2017 11:38 am
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I stick a pair of wires into a drill motor and wind 'em up. Been making twisted pair that way for decades. Once had to make several hundred feet for a project... :shock:


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Post subject: Re: Champ Hum Mitigation
Posted: Fri Jul 28, 2017 6:03 pm
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7chord wrote:
I stick a pair of wires into a drill motor and wind 'em up. Been making twisted pair that way for decades. Once had to make several hundred feet for a project... :shock:


Yup, this is how I do it, too! Put one end of pair into vise. The other into drill chuck. I turn clockwise at slow rate. Make the length longer than what you need. Cut the wire at the points by each socket tang, as needed. Leave plenty of length, so you can lift the soldered twisted pair, up & away from signal and B+ lines going into the sockets.

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Post subject: Re: Champ Hum Mitigation
Posted: Fri Jul 28, 2017 9:13 pm
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Location: Coastal Bend, Tx.
Admiring your chassis jig Steve, did you make that?
So far I've been making them as I go and fortunately the dlx and bronco worked on the one I have made.
The super is another story but has been ok for now sitting on the pt/ot.

I've been hand tying bimini twists for years, a drill is handy though for sure and looks darn good.


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Post subject: Re: Champ Hum Mitigation
Posted: Sat Jul 29, 2017 7:45 am
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Thanks! It's a Weber unit. Adjustable width and tilt angle.

https://reverb.com/item/2914717-weber-amp-cradle

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Post subject: Re: Champ Hum Mitigation
Posted: Sat Jul 29, 2017 9:41 am
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Mojotone sells a stand also, and it's probably an easier build if you want to make one - http://www.mojotone.com/amp-parts/AmpBuildingSupplies_1/Chassis-Stand Image


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Aug 02, 2017 6:25 pm
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Here's a sample schematic of a DIY pre-amp with rectified DC heater line and added DC to raise line above 6.3 volts.

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