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 Post subject: PSA- DC-3 bias mod
PostPosted: Tue Jan 25, 2011 4:44 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
OK so I believe Ive gotten it squared away. Used a Mansfield kit(50 k pot with back up resistor) but you could just get the pot and resistor. I didnt want to source them. Installed it in place of the 33k resistor. Then I piggiedbacked a 470K on the 120K resistor effectively making it 100K. The PI raised a tiny bit so Im not going to bother with the 15K resistor just yet. The pic will show better.

Disclaimer- Leathal voltages inside! Work on your amp at your own risk!

Quick fix mod? Simply piggyback the 470K (1/2 watt) on the 120K in the bias supply. Done! Image


Last edited by BobR on Sun Mar 27, 2011 6:58 am, edited 5 times in total.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Tue Jan 25, 2011 8:46 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
They say hindsight is 20/20. I imagine a better way to do this mod would be to replace the 120k with a 76k and 50k pot. Watch the PI voltage and adjust resistor as needed. Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Wed Jan 26, 2011 7:29 am 
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Mark II
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Joined: Thu Nov 25, 2010 7:49 am
Posts: 114
Location: København
^it sounds right

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Jan 30, 2011 6:24 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
Yeah it dawned on me afterwards. :roll: Still the bias voltage is close to -14 and way better so Im happy. Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Feb 27, 2011 2:53 pm 
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Mark II

Joined: Sat Nov 06, 2010 11:31 am
Posts: 103
after doing Bobr's bias mod I've done a few gigs with the dc 3 without any problems at all.No heat issues to worry about no red plated tube problems.3 gigs running here real hard without a hitch.Now after first startup I never look back 8) RAWKIN'


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Feb 27, 2011 3:26 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
The " quick fix mod " that is,correct? Simple ,extremely inexpensive and effective. For those that are looking-Piggyback a 1/2 watt 470K on the 120K in the bias supply. Done! Glad its working well for you . I cant imagine it not being better than the stock -11vdc! Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Mon Feb 28, 2011 4:56 pm 
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Mark I

Joined: Fri Nov 20, 2009 2:10 pm
Posts: 22
Please help me understand this mod. What exactly is the problem with the original design. Why is a -11VDC bias "bad"? Is this strictly a high temperature issue, or are there other problems? Why does the original design cause this problem? How does the mod fix the problem?

I don't have any experience with adjusting the bias of tube amps, but from what I understand, it can affect the gain or output of certain stages. I'm an electrical engineer, I just don't have experience in this area.

Does this mod affect the sound or the output level? If I have a fan installed on my cabinet, will that basically take care of the problem just as well as changing the bias? This amp is known as a "fixed bias" amp, so when replacing the tubes, you buy tubes already speced to your amp and just plug them in. Will doing this mod still allow you to "plug and play" new tubes speced to the original amp? Or should new tubes have different specs?

From what I understand, after the DC-3, Mesa has always designed a cooling fan into every amp that has four or more power tubes.

I have many questions, but I'm just trying to understand this issue.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Mon Feb 28, 2011 6:03 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
No tubes you can buy from Mesa(or anywhere else) will bias cool enough. Will 400 vdc on the plates and only -11 grid bias any tube will draw way more current than is should. Its what I consider a desgn flaw on Mesa's part.

-11 isnt inherently bad,per say,in and of itself. The amount of -bias voltage determines how much current the tube will draw. If the plate voltage was ,say 250-300 volts it might be fine. To determine an optimal bias setting you need to know the current draw and plate voltage.
Divide the max dissapation of the tube(12 watts for EL84's) by the plate voltage and you get your maximum dissapation. The shoot for 70% of that for a maximum. At 400 volts on the plates your maximun current would be roughly 21 ma. Unfortunately if you measure current draw of the stock DC-3 with its -11 bias voltage it will range from 35-50 ma with almost any tube you try. Way too hot and well beyond the disign specs of an EL84. Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sat Mar 26, 2011 4:18 pm 
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Mark I

Joined: Thu Sep 03, 2009 4:01 pm
Posts: 34
lol that was confusing for me. So, in basic terms, the DC-3 in stock form is running the EL-84's too hot? And this mod reduces the level at which the tubes are run to be more appropriate?

I've never had reliability issues with my DC-3, but I'm curious: what effect does this have on the sound of the amp?


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sat Mar 26, 2011 4:55 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
cardinal wrote:
lol that was confusing for me. So, in basic terms, the DC-3 in stock form is running the EL-84's too hot? And this mod reduces the level at which the tubes are run to be more appropriate?

I've never had reliability issues with my DC-3, but I'm curious: what effect does this have on the sound of the amp?



Stock the amp runs any tube WAY to hot. This gets them down into reasonable levels. Tone still sounds great. I cant imagine any tube(regardless of its rating) lasting more than a few months stock.They may not blow up but the life is being sucked out of them quickly. Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Mar 27, 2011 12:35 am 
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Mark I

Joined: Thu Sep 03, 2009 4:01 pm
Posts: 34
Got it. Thanks man. I've had the amp since it was new (1999? 2000?), and I think I've swapped the tubes twice. It's not my primary amp, but now I think maybe I should swap the tubes more often and see if I notice them getting chewed up.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Mon Jun 27, 2011 7:25 pm 
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Mark II

Joined: Sat Nov 06, 2010 11:31 am
Posts: 103
BobR bias mod update:Did the mod months ago.Several gigs at loud volumes and still running great!! -14v bias and going strong.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Wed Jun 29, 2011 4:12 pm 
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Mark I

Joined: Fri Nov 20, 2009 2:10 pm
Posts: 22
BobR wrote:
Stock the amp runs any tube WAY to hot. This gets them down into reasonable levels. Tone still sounds great. I cant imagine any tube(regardless of its rating) lasting more than a few months stock.They may not blow up but the life is being sucked out of them quickly. Bob


But if I have a cooling fan installed (with no bias mod), will that also extend the life of my tubes?


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Wed Jun 29, 2011 5:51 pm 
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Mark II

Joined: Sat Nov 06, 2010 11:31 am
Posts: 103
photogold wrote:
BobR wrote:
Stock the amp runs any tube WAY to hot. This gets them down into reasonable levels. Tone still sounds great. I cant imagine any tube(regardless of its rating) lasting more than a few months stock.They may not blow up but the life is being sucked out of them quickly. Bob


But if I have a cooling fan installed (with no bias mod), will that also extend the life of my tubes?

only to a point.Way too hot for today's tubes,I've tried..


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Wed Jun 29, 2011 6:44 pm 
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Mark I

Joined: Fri Nov 20, 2009 2:10 pm
Posts: 22
bhsbhswarp wrote:
photogold wrote:
BobR wrote:
Stock the amp runs any tube WAY to hot. This gets them down into reasonable levels. Tone still sounds great. I cant imagine any tube(regardless of its rating) lasting more than a few months stock.They may not blow up but the life is being sucked out of them quickly. Bob


But if I have a cooling fan installed (with no bias mod), will that also extend the life of my tubes?


only to a point.Way too hot for today's tubes,I've tried..


But what I'm trying to say, is heat buildup the "only" issue here with regard to tube life? Is there something else about the bias mod that extends tube life? Because the way that my fan is setup, there isn't much heat buildup. It's way cooler than it was.

Is it possible that even with good cooling, there is so much current through the tubes, that this extra current reduces the tube life even if they are cooled? Or is heat buildup the only issue, and if that is solved one way or another, then the tube life will be OK?


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