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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Wed Jun 29, 2011 8:53 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
Posts: 124
photogold wrote:
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


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Dec 02, 2012 7:50 pm 
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Mark II

Joined: Tue Dec 21, 2010 3:44 pm
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Two years later amps working fine still. Being played moderately (few times a week)tubes arfe holding up fine. Bob


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Mon Dec 03, 2012 4:15 pm 
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Donating Member

Joined: Sun Jul 06, 2008 9:48 am
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Location: Central Texas
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:

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Tue May 21, 2013 9:24 pm 
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Mark III
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Joined: Fri Jul 29, 2011 12:52 pm
Posts: 357
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.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sat Sep 14, 2013 11:47 pm 
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Bottle Rocket

Joined: Mon Jun 03, 2013 9:38 pm
Posts: 17
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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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Sep 15, 2013 8:08 am 
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Mark III
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Posts: 357
The 50k pot is only if you want the bias to be adjustable. It is not necessary. The only needed change is to swap the 120K resistor out for a 100K.

Adding a fan helps a great deal with heat (and thus reliability). You might want to check that option out in addition to the resistor change.

The resistor change will take care of your power tube red plating (that is what is happening to you), and some of the heat. The fan will take care of all of the heat.

I have lugged my DC3 head everywhere. Practice, gigs, business trips, vacations, etc. It has been extremely reliable. I haven't had to change a tube in quite a while.

FYI, replace the power tubes. They are done. Once they have red-plated for any more than 5 minutes or so, they are basically done in my experience. If they don't fail right away after fixing the bias, they will soon after. Happened on one of my sets of Mullards. Took out the screen grid resistors and the fuse.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Sep 15, 2013 9:42 am 
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Bottle Rocket

Joined: Mon Jun 03, 2013 9:38 pm
Posts: 17
So I shouldn't worry about possibly shifting 2.3V PI voltage? (just making sure...)

Also, I now get why you said that output tubes could go in a hurry in the DC-3. Back on the other thread it didn't make sense to me, but now that I am aware that it runs soooo hot, that probably explains it. Yes, they glow bright red.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Sep 15, 2013 10:40 am 
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Mark III
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Posts: 357
jakep wrote:
So I shouldn't worry about possibly shifting 2.3V PI voltage? (just making sure...)

Also, I now get why you said that output tubes could go in a hurry in the DC-3. Back on the other thread it didn't make sense to me, but now that I am aware that it runs soooo hot, that probably explains it. Yes, they glow bright red.


The P/I voltage does jump a bit (mine went to 3.02V). I hardly doubt that is going to cause any issues, and many of us have been running our amps this way for over a year or more. I have had 0 issues. Just make sure you are using a good V6 P/I tube. I use JJ 12AX7s there and they have been solid.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Sep 15, 2013 1:54 pm 
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Bottle Rocket

Joined: Mon Jun 03, 2013 9:38 pm
Posts: 17
Thanks, I'll try that then.

Can you recommend good replacements for the output tubes? Mine are still the fairly new (but probably cooked) Mesa-branded EL84s. You probably mentioned good replacements before, but tell me what you use. I still mostly use the clean channel.

- Jake -


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Sep 15, 2013 6:04 pm 
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Mark III
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Joined: Fri Jul 29, 2011 12:52 pm
Posts: 357
Well, you don't want Mesa's. They are currently using re-branded Sovteks, and they don't survive.

JJ is what I have been using, and will continue to use. The sound great, and are very durable. You'll need a set rated around 20.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Thu Feb 20, 2014 12:30 pm 
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Single Recto
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Joined: Tue May 20, 2008 7:33 pm
Posts: 1034
Location: Star City - River City
I did this mod, changed the resistor to a 100k, but it had no effect on the red plating I was getting. I had to swap out 4 or 5 sets of tubes (Sovtek EL84's and EL84M's and JJ's and Tung Sol's) before I got a quad that had no red plating. I found one set of EL84M's that do not red plate.

The only tubes I've never seen red plate in my Mesa amps are the JJ's.

Did I have weak tubes? Or is this mod not enough to prevent red plating?

I have a DC3 on the way now that has bias points and pot installed. I will be very curious to see how that mod was done, and what affect it has on the tones.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Sun Feb 23, 2014 12:31 am 
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Single Recto
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Joined: Tue May 20, 2008 7:33 pm
Posts: 1034
Location: Star City - River City
BobR wrote:
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


Excellent info. I just acquired a DC3 with an adjustable bias mod installed, so I am wondering what the idle current should measure at. The sound cleans up quickly when it is at only 5-7ma. But your post suggests that I can still go quite a bit hotter without abusing the JJ EL84's that are in there now. It's interesting to hear the sound clean up as I turn the adjustment pot and the bias gets warmer. Too cold and the amp won't make any sound at all, then it's just a quiet crackle and buzz, then it suddenly becomes a guitar sound as the bias current goes up to about 5ma and up.

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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Mon May 30, 2016 4:03 am 
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Bottle Rocket

Joined: Mon May 30, 2016 3:43 am
Posts: 1
Hi, I'm new to this forum but have been an owner of a DC-3 since 1996, wow, 20 years. I read, with interest, the posts on the bias mod, having always felt the amp was a little too hot.I 've never had any major problems with the valves (tubes) red plating, having revalved a few years ago with JJ's for the output stage. I am very interested in applying the bias mod myself, am comfortable handling a soldering iron but am not experienced with electronics. My DC-3 circuit board is marked 1993 and is somewhat different to the photo Bob has posted for the simple 470k mod (much respect to all who contributed to this post, especially Bob). I cannot identify the 120k resistor close to the location in the photo, I can see some elsewhere. Has anybody applied this mod to a 1993 circuit board and can give some advice as where to find the 120k resistor to parallel with the new 470k. Any advice would be gratefully received.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Tue Aug 23, 2016 5:38 pm 
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Bottle Rocket

Joined: Tue Aug 23, 2016 4:47 pm
Posts: 2
Checked bias current on my dc-3 (DC23B/1993) recently, using o/p transformer resistance method, as I felt that it was running hot (no red plating tho').
One side (pair of el84s) was pulling 40.1mA (393Vdc on plate) & other side 48.0mA (392Vdc on plate); this is at a bias voltage of -10.7vdc.
I noticed that the piggy-back mod has been done by previous owner...........I now know why, so thanks to TBB/Bob for that.
Also noticed piggy-backed resistor (beneath) had suffered a burn-out (1/4W); will remove it & the mod, then replace with 100k half-watt the next time I replace output valves & re-check bias current.


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 Post subject: Re: PSA- DC-3 bias mod
PostPosted: Thu Mar 30, 2017 4:20 pm 
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Mark II

Joined: Tue Mar 12, 2013 7:49 pm
Posts: 53
My amp tech did the bias mod. He went with a 620k resistor.
No more red plating & the chassis is not nearly as hot.
No change in tone either. It still sounds amazing..

The DC-3 is slowly becoming my favorite Mesa.


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