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PostPosted: Tue Oct 26, 2010 8:07 am 
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Mark I

Joined: Fri Oct 22, 2010 11:37 pm
Posts: 21
stratele52 wrote:
shatteredsquare,
No you can't do that, you will damaged your 6L6 if you flip the bias switch from 6L6 to EL34.
You won't have a EL34 sound if that's what you hope
The only difference in this bias switch ( if you flip to EL34) is to give a hotter bias to power tubes. EL34 need a hot bias. I can't tell you if you wear your 6L6 in couple months, weeks or hours !!! Don't do that.
In worst case, tubes will glow red and you will damage your output transformer before fuse will blow to protect the amp.


I'm tracking what it will do, slam alot of voltage into the 6L6s. My question is how much is the difference with the flip of that switch? I've heard mesas are pretty robust as far as running improper cab hookups, and if you can run a marshall real hot, can't you run a mesa real hot? Is it enough differnce to cook something? I'm not going for the EL34 sound, just seeing what I could get by slaming 6L6s with EL34 volatge.


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PostPosted: Tue Oct 26, 2010 9:06 am 
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Bottle Rocket

Joined: Thu Oct 14, 2010 7:45 am
Posts: 15
Location: Quebec Province , Canada
By switching to EL34 bias you won't use EL34 voltage for your 6L6, but you change current flow in tubes, by changing negative apply to 6L6's grids. This is more dangerous.
More current will flow in tubes and could destroy it.

If you want more voltage on your 6L6's, Use the Bold/Sponge swith to Bold, and if you want more, switch Silicon diodes/ Tubes to Silicon diodes.
I don't know if you can run a Mesa hot. In my opinion, power transformer looks smalls for their power.

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PostPosted: Sun Jan 02, 2011 11:05 pm 
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Mark III
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Joined: Fri Nov 24, 2006 1:14 am
Posts: 290
Location: San Benito, Texas
I would love to do this to my roadster head. Would it be the same, im assuming not since its a different circuit but you never know. Is there a roadster how to thread???

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Mesa Tremoverb Head
Mesa Rectifier 4x12 Standard Cab


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PostPosted: Sat Jan 22, 2011 9:25 pm 
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Mark III

Joined: Sat Jul 01, 2006 1:23 pm
Posts: 263
fluff191 wrote:
I just ordered a 10 piece lot of small trimmers for like $3....... The external pot you used is almost $20 a piece!!! Wow!

I never thought just replacing the one resister with a pot would be so easy. For insurance I am having a co-worker who has modded his Recto come over for guidance.


Is it better to use the more expensive multi-turn pot or the cheaper single turn pot for this? When I get this figured out, I'm going three of the pots needed and do all my Mesa amp at the same time. I'm tired of always having to 'hope' that the Mesa tubes will be in the right range. 2 of my amps aren't in warranty and I'll just wait on the third one until it's out too. Thanks, btw, for a great Post!!!


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PostPosted: Sun Jan 23, 2011 4:19 am 
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Bottle Rocket

Joined: Thu Oct 14, 2010 7:45 am
Posts: 15
Location: Quebec Province , Canada
I don't know where you see cernet multitour pots are expensive ? My local electronic store sell them $1.99 Canadian.

And I do lot of bias mod with them.
Multitour are easyer to adjust, to fine tune and won't move by itself. And I read they are more DC volt "resistant"

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I'm french speaking, so I could make some mistake, please excuse me


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PostPosted: Sun Jan 23, 2011 9:28 am 
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Mark III

Joined: Sat Jul 01, 2006 1:23 pm
Posts: 263
stratele52 wrote:
I don't know where you see cernet multitour pots are expensive ? My local electronic store sell them $1.99 Canadian.

And I do lot of bias mod with them.
Multitour are easyer to adjust, to fine tune and won't move by itself. And I read they are more DC volt "resistant"


Thanks for the reply. Here is where I found them: http://www.mouser.com/Passive-Component ... ns&FS=True

There are others on the webpage that are cheaper, these were just the most expensive. Typically more expensive suggests some additional process to manufacture or better material, but maybe not. I know that a few milliamps warmer and my amp is smoother, a little warmer and just has a better tone for both the clean and gain sounds. I cannot tell you how many times I've purchased power amp tubes that were supposed to be in the right range and after I checked them the read 10-20ma lower than the recommended 60-70% of plate dissipation I was expecting. If there is no difference in the pots I'll order up a few of the cheaper multi-turn version and give it a shot.

thanks again,
Stevee


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PostPosted: Sun Jan 23, 2011 3:02 pm 
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Bottle Rocket

Joined: Thu Oct 14, 2010 7:45 am
Posts: 15
Location: Quebec Province , Canada
I look at Mouser:

Bourns have some at $2 - $3 each too, I don't take time to check if they made 24 K ohms for those model

The name on mine are Elecsound, Trimmer, BI and I have some Bourns and some Alen Bradley buy at a surplus store.

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PostPosted: Sat Mar 05, 2011 11:12 am 
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Bottle Rocket

Joined: Tue Mar 09, 2010 9:14 pm
Posts: 18
Hi,

I just finished installing the Mansfield Guitar variable bias mod kit in my Mesa Roadster 212 combo. I can't say enough positive things about the kit. John's parts and instructions make the mod incredibly easy -- remove the amp chassis from the cab, DISCHARGE THE CAPS!!!, locate the 22K resistor, remove it, decide on a location for the potentiometer circuit board (comes with 3M self-adhesive, simply peel and stick), route the lead wires, and solder them to the board where the resistor was. That's it. Takes about an hour max if you take your time. I mounted my pot on the outside of the chassis enclosure, tucked it up under the enclosure under the foot switch jack on the left rear of the amp so that I can adjust the bias without opening the amp up.

Now the really cool part of my experience with John. I wanted to use his bias probe to allow measuring the plate voltage and bias without pulling the amp out of the cabinet. Problem is that his bias probe can't be installed in a 212 combo because there is not enough clearance for the probe and tube to fit. I shot John an e-mail explaining the problem, and he made up a new prototype bias probe that solved the problem. The new probe allows you to remove a power tube, and plug in a blank male tube base into the amp tube socket. The blank tube base has a 5-6" lead wire that connects to a small box that has another tube socket in it and also has sockets for your multi-meter probes. You set the box on top of the amp, plug your tube into the socket on the box, plug your multimeter probes into the sockets, fire up the amp, and off you go. To measure the plate voltage, you simply touch the negative meter probe to the chrome metal screws on the top of the amp that hold the chassis in place. Biasing the amp is child's play with this setup. I highly recommend contacting John to inquire about this new probe kit if you are interested.

The only thing that you need to be careful of when using it (and this is the same for any method of doing a bias adjustment -- John's probe is WAY safer than opening up the amp and taking measurements directly from the circuitry inside) is to remember that you are playing with 400+ volts of DC current at an amperage that's twice what it takes to stop your heart, so NEVER TOUCH THE MULTIMETER PROBE TIPS, and think about how you handle the meter probes during measurement. ALWAYS REMOVE THE POSITIVE (RED) PROBE FROM THE BOX BEFORE REMOVING THE NEGATIVE ONE. When measuring the voltage especially, remove the red probe, then the black one, and set the meter to VDC. Then hold the black probe very carefully with the tip in a safe place, plug in the red probe, touch the tip of the black one to the chassis screw head, read the voltage, and then pull the red probe. This minimizes the time that you have the high voltage circuit open with the ground available to cause harm.

:)

Craig


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PostPosted: Sat Mar 05, 2011 5:20 pm 
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Bottle Rocket

Joined: Thu Oct 14, 2010 7:45 am
Posts: 15
Location: Quebec Province , Canada
It's a nice probe but you can only test the Kathode current ( milliamps or millivolts) . You should be able to mesure Plate voltage ( 400 volts) to know if your bias is ok ( Kathode current).

With a probe made by Amp-Head.com. you can do those two mesurements safely

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PostPosted: Tue Mar 08, 2011 10:53 am 
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Single Recto
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Joined: Tue Jan 25, 2005 12:14 am
Posts: 1391
Location: North of Dayton, Ohio
:roll:

More Mansfield alt spam?

Funny how all these guys who rant and rave and go on with lengthy posts like this all have like 10 posts.

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Guitars:Schecter C-1 Classic, Ibanez RG7321, Epiphone LP Supreme
Amps:70's Earth head
Cabs:Recto 4x12, 5150 4x12
Effects:Digitech GSP2101, Budwah
Past amps '95 Dual Recto, '96 Dual Recto, Stiletto Trident, 3 ch. Triple Rec


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PostPosted: Tue Mar 08, 2011 9:29 pm 
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Bottle Rocket

Joined: Tue Mar 09, 2010 9:14 pm
Posts: 18
Mikey-Boy,

Well, that's an intelligent and insightful reply to my post! Makes me feel really welcome here on the board. The fact is that my Roadster is sounding really great, and John Mansfield really went the extra mile to fix me up with a bias probe that works well and allows me to do the adjustment without taking my amp apart. Just wanted to share my experience with others who may benefit from it. That's what discussion boards like this are all about, I thought. Maybe you'll have an idea to share with others one day too!

Craig


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PostPosted: Tue Mar 08, 2011 9:33 pm 
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Bottle Rocket

Joined: Tue Mar 09, 2010 9:14 pm
Posts: 18
By the way, your posts with detailed instructions about one way to do the bias mod are nothing short of awesome -- I really admire your expertise. Guys like you are what makes these boards so valuable to others!

Craig


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PostPosted: Wed Mar 09, 2011 5:04 pm 
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Single Recto
User avatar

Joined: Tue Jan 25, 2005 12:14 am
Posts: 1391
Location: North of Dayton, Ohio
Rocker wrote:
Mikey-Boy,

Well, that's an intelligent and insightful reply to my post! Makes me feel really welcome here on the board. The fact is that my Roadster is sounding really great, and John Mansfield really went the extra mile to fix me up with a bias probe that works well and allows me to do the adjustment without taking my amp apart. Just wanted to share my experience with others who may benefit from it. That's what discussion boards like this are all about, I thought. Maybe you'll have an idea to share with others one day too!

Craig



No offense man.

I just find it to be an odd coincidence that I've seen pretty much the exact same post from several other members who all have low post counts.

Although I do think that John's mod is flat out ingenious.

And welcome!

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Guitars:Schecter C-1 Classic, Ibanez RG7321, Epiphone LP Supreme
Amps:70's Earth head
Cabs:Recto 4x12, 5150 4x12
Effects:Digitech GSP2101, Budwah
Past amps '95 Dual Recto, '96 Dual Recto, Stiletto Trident, 3 ch. Triple Rec


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PostPosted: Sun May 22, 2011 10:27 am 
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Mark III

Joined: Sat Jul 01, 2006 1:23 pm
Posts: 263
Rocker wrote:
Hi,

I just finished installing the Mansfield Guitar variable bias mod kit in my Mesa Roadster 212 combo. I can't say enough positive things about the kit. John's parts and instructions make the mod incredibly easy -- remove the amp chassis from the cab, DISCHARGE THE CAPS!!!, locate the 22K resistor, remove it, decide on a location for the potentiometer circuit board (comes with 3M self-adhesive, simply peel and stick), route the lead wires, and solder them to the board where the resistor was. That's it. Takes about an hour max if you take your time. I mounted my pot on the outside of the chassis enclosure, tucked it up under the enclosure under the foot switch jack on the left rear of the amp so that I can adjust the bias without opening the amp up.

Craig


Craig,
Would you mind providing how you thought that the bias kit added to your sound/tone or what you thought it was lacking before and you have now? The reason I ask is I have a Dual Rec I've been interested in doing this to but for $45 I'd like to hear what the difference is before I plunk down the cash. Better Cleans? A little brighter tone, more mids, more harmonic content to the notes, etc?

thanks,
Steve


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PostPosted: Tue May 24, 2011 10:41 pm 
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Mark III

Joined: Sun Jan 30, 2005 6:53 pm
Posts: 369
Location: Connecticut
I installed a bias pot in my Single Recto today. I used a 50k pot with a 1/2 watt 47k resistor in parallel with the two outside legs. Leg 1 and 2 of the pot are tied together. I used the legs of the resistor to attach the pot to.

I removed the factory 22k resistor (to the right of the 220uf/63v bias cap, just above the 30uf/500v blue cap for the screen supply) and installed the pot assembly in the next "spot" over because it's easier to solder there... :lol: Electrically, it's the same connection

The amp is used and came with some tired STR-440's in it. I installed a new set of Chinese Ruby (Shunguang) 6L6GCMSTR's which look to me to be the same tube Mesa re-labels as STR-440.


BEFORE

Image

AFTER

Image


I set the bias at a modest 31mA, for now 8)

Image


Thanks and Kudo's to those who went before me for the inspiration


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