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Post subject: Bias setting
Posted: Fri Dec 21, 2012 1:15 pm
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I have a 64 Tremolux AB763 putting in new caps and tubes. What is the correct bias voltage?

Also, same repair and question for a 67 Dual Showman AB763.

Thanks your replies.


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Post subject: Re: Bias setting
Posted: Fri Dec 21, 2012 2:21 pm
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For your Tremolux, you should measure -45 VDC at the wiper of the bias pot -- the Showman's should be -52 VDC. The actual current draw of the output tubes will be contingent upon a number of factors, not the least of which is the brand and type of glass used. Another factor which will determine the amp's idle bias is the age of the components and how much they've drifted in value over the years. For NOS RCA blackplates or Fender-branded Sylvanias, 37.5 or 38 mA might be optimum. Newer glass like Winged C's, Sov-Tek, or EHX might be more comfortable at 33 or 34 mA.

HTH

Arjay

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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 2:38 am
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Bias voltage is only

The bias voltage is only a relative reference. The cathode current reading is the only consideration.
For the same bias voltage some tube can have different cathode's current ( milliamp)


Last edited by stratele52 on Mon Dec 31, 2012 4:41 pm, edited 1 time in total.

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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 1:45 pm
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Set the bias to 33mV. The sound is great. However, in the basement the hum was gone. Moved it upstairs and the hum is back. Here is a list of the work.

New:
SED SV6L6GC (2)
Sovtek 5751 (2)
12AX7LPS
12AT7WC JAN Philips
Filter caps, F&T Germany (3) 16µF / 475V & (2) 22µF / 500V (5) and Sprague Atom (5) 25µF / 25V
(2) 10K 1/2 Watt Metal Film Resistors by the filter caps
New ground cord and removed the death cap

It is amplified by turning up the treble.

Any ideas?

Thanks!


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 1:54 pm
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In a basement, house wiring is usually installed into steel conduit. Upstairs, it's likely romex stapled directly to wall studs behind the sheetrock. I don't think it's your home's wiring per se but the amp seems to be susceptible to 60-cycle AC hum -- possible shielding issue somewhere. Does the head cabinet still have its steel screen stapled to the underside of the top piece?

Arjay

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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 2:11 pm
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Or you have neon light ,or halogen with transformer , this hum. Or cell phone close to the amp ? PC ?

New three prong power cord ; well polarized ? Black wire on the "hot" side of wall outlet, white on the "cold" side ?

Wall outlet ; black ( wall ) wire on the "hot" side , white wire .....?


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 3:34 pm
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stratele52 wrote:
Or you have neon light ,or halogen with transformer , this hum. Or cell phone close to the amp ? PC ?

New three prong power cord ; well polarized ? Black wire on the "hot" side of wall outlet, white on the "cold" side ?

Wall outlet ; black ( wall ) wire on the "hot" side , white wire .....?


+1

Use a phase checker to confirm the upper-floor wall outlets are wired correctly. And proximity to any of those devices mentioned could also be the culprit. Good call, Stratele!

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 4:36 pm
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Thank's Retroverbial.


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 6:24 pm
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You bring a lot of technical expertise and a well-tempered perspective to this forum, Stratele. I can't speak for the others here but I'm certain that I've not thanked you enough for your innumerable contributions.

Rawk on, mon amí!

8)

Arjay

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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 6:43 pm
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It was a missing ground in the upstairs outlet. Thanks!!!


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 6:43 pm
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Glad you caught it, Matt.

Congrats all around!

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bias setting
Posted: Sat Dec 22, 2012 11:48 pm
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Glad you found the offender, Matt!! Yeah Arjay, you hit on a great subject!! My Canadian brother has really been there, time and time again for the folks around here. Just like Beemer, Bill, Reb, Shimmy, yourself, and all the other folks, that I failed to mention. We are a strong part of the community. And I count myself lucky enough to be a minor brother in this village! Best Season's Greetings and Well Being regards to all you folks!!!! Art

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Post subject: Re: Bias setting
Posted: Sun Dec 23, 2012 2:20 am
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Retroverbial , aclempoppi, MattGutarKy,
It is a pleasure to share knowledge with you all on the forum

I wish you a merry Christmas


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Post subject: Re: Bias setting
Posted: Sun Dec 23, 2012 8:37 am
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I second the appreciations! All you guys have been great. I pulled my stuff out of the closet in 2010. And you guys have help tremendously. I now have a recovered and very sweet sounding 64 Tremolux. I built a new 2x12 cab for my 67 Dual Showman, it's recovered, discovered two opens connections, and will soon have new caps and tubes in it. I can't says thanks enough.

Everyone have a great Christmas!


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Post subject: Re: Bias setting
Posted: Sun Dec 23, 2012 8:27 pm
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1964 Tremolux! That amp is crying for some "Texas Flood." :D


http://www.youtube.com/watch?v=tWLw7nozO_U


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