PSA- DC-3 bias mod

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BobR

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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!
qnskf8.jpg
 
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
 
Yeah it dawned on me afterwards. :roll: Still the bias voltage is close to -14 and way better so Im happy. Bob
 
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'
 
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
 
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.
 
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
 
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?
 
cardinal said:
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
 
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.
 
BobR bias mod update:Did the mod months ago.Several gigs at loud volumes and still running great!! -14v bias and going strong.
 
BobR said:
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?
 
photogold said:
BobR said:
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..
 
bhsbhswarp said:
photogold said:
BobR said:
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?
 
photogold said:
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.

A fan might help but the REAL issue is running the tubes at WAY above 70% dissapation. Stock can be anywhere from 110-150%! One 50 cent resistor will get it down to reasonable levels. Again the stock bias setting is the real issue. A fan is nice but really its only nice with an amp thats biased correctly. Bob
 
Two years later amps working fine still. Being played moderately (few times a week)tubes arfe holding up fine. Bob
 
Even with my bias mod and running the tubes at 70% - 80%, my DC-3 runs very warm! But yeah, before the bias mod, things were scorching. :evil:
 
I don't like piggy backing resistors, so I just changed out the 120K for a 100K (Actually around 95K). I see -bias voltage of -19.5. The amp runs SUPER cool now, and I have the classic blue glow now that I have gotten from every other EL84 amp I have owned. Before the bais mod, the brand new Mullard's had no blue glow, and were on the verge of red-plating (one actually started to just slightly so I changed the PI tube, and that went away). Now, they have a nice blue glow when off standby, and they are running much cooler. And, NO RED PLATING!! However, the Mullards went Microphonic and I am now switching back to JJs. They still sound great, and I can get them in a -20 current draw.


On top of that, My blue power light is now brighter as well. This was done on a RevB (May 1996) DC3 with an attached power cord.
 
Bendo:

Some time ago you answered my post on resurrecting a neglected DC-3. You may not recall, but I eventually took the amp to a good tech, and it sounds great. You were convinced that output tubes were a problem. Well, you may have been right in a sense, because despite sounding good initially, the thing is really running way too hot. I believe it is causing some problems with the output tubes; they just glow red red red. In fact I am a bit uneasy running the amp in my bedroom.

My question is whether the only thing necessary to complete the mod on this thread is to swap out the 120k bias supply resistor for 100K. What is the meaning of the comment (elsewhere on this thread) about a 50k pot? And how does this all relate to the PI voltage (which is also mentioned)?

If the only change is the bias supply resistor I will do this myself.

Thanks a million.

- Jake -
 
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