Adjustable Bias Mod Kit First Impressions

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SRVYJM
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Adjustable Bias Mod Kit First Impressions

Post by SRVYJM » Sun Aug 29, 2010 4:25 pm

So I just installed my Mansfield Guitars adjustable bias kit. First I have to say it was a pretty easy install all things considered. I didn't worry too much about whether or not I could do it, I've been doing mods on vintage amps with point to point tag boards ('66 Super Reverb, etc) for a while both successfully and unsuccessfully so I'm a fairly competent solderer. The most difficult part was getting the old resistor out. It actually fell apart when I heated up the solder joint. Maybe I used to hot a setting on my iron (40watts) but that is what I've always used so...

Anyway, I'll attach pictures of the mod. I found a big square hole (can a hole be square?) right behind and between the middle power tubes and it was just begging me to be used to thread the wire through. I put two layers of shrink wrap around the wires for protection. I think in the future I'll put silicon seal or a grommet of some type in there to further protect it, but the two layers of shrink wrap should work fine for now. This location allowed me to use a very short section of wire to reach the PC board spot where the resistor was removed. The whoel project took a little less then an hour. Here are the pictures.

By the way, a quick note that I didn't notice on anywhere on the other threads on both mid mods and the bias kit. Everyone is deathly afraid of the shock hazard of the filter caps. You should be. They are deadly if not properly discharged completely. I show how I do this in the pictures. It's simple, I wired up a couple of alligator clips to a piece of wire from some standard romex I had laying in the house, and sealed the alligator clips to the wire. Then you can clip one end to the chassis for ground, and touch off the + side of every filter cap to insure there is no stored up and deadly electricity. Before I do this I usually shut the power of the amp off but leave the standby switch on and play a few chords until the sound dies down. That helps to drain the caps, but it doesn't always drain them completely. Then turn off the stand by switch (or you'll forget to later and turn your amp on with the standby engaged) unplug the amp and take the chassis out of the head. It's four screws on the top. See my pics below.
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Anyway, the amp is MUCH warmer now, and touch sensitive. I've only been on it for a few minutes, so I'll edit this post after playing for a few hours. May take her out to my local blues jam tonight and see how she feels at volume. It's my birthday, so I'll get to crank it up!

Alright. Post playing for several hours. I did take it to my local blues jam last night and it sounded awesome. Got a lot of compliments on my tone afterwards and there was a ton of interest in the amp itself. Turning up the bias in this amp definitely changes the amp for the better. It's warmer, much more articulate with chording, get's a nice distortion without being fuzzy and the bottom end is huge. Doesn't hurt that I have a 1X12 ported Mesa Roadster cabinet with V30's in it either. It actually has as much or more bottom end then my 4X10 Super Reverb, which is huge.

I played in Channel 1 last night, but channel 2 might very well become my new channel of choice. I biased the amp to 40ma in the 50 watt mode, and I can clean it up real nicely with the volume control on my guitar. It has that singing sustain and growl that I was looking for, but sounds as good as my Blackface Super Reverb when it's clean without the ice pick. A very good thing indeed. Suddenly this amp is not just versatile, but has the beautiful musical quality I felt it was lacking before. Much fuller sound. Much better distortion overall. In fact, absolutely beautiful distortion. Now it sounds like Andy Timmons amp, without crushing the neighbors with volume.

ja22y
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Re: Adjustable Bias Mod Kit First Impressions

Post by ja22y » Mon Aug 30, 2010 2:22 pm

Stock do you know what was it biased at?

SRVYJM
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Re: Adjustable Bias Mod Kit First Impressions

Post by SRVYJM » Wed Sep 01, 2010 7:59 am

ja22y wrote:Stock do you know what was it biased at?

22ma. REALLY cold.

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Re: Adjustable Bias Mod Kit First Impressions

Post by vanceen » Fri Sep 03, 2010 11:15 am

SRVYJM,

Thanks for the information about the bias mod.

I have a question about draining the capacitor voltage, though. I've read several times about the technique you describe, but there's something I don't understand.

If the amp is unplugged, then it doesn't have a path to ground, right? So connecting the capacitor lead to the chassis should distribute the capacitor's charge between the cap and the chassis, but not remove it from the amp. So in principle (at least), there's at least the potential to build up a significant potential difference to ground on the chassis, leaving a shock hazard.

What I've done is to put alligator clips on both of the leads to each capacitor, the clips being connected with a wire with a 1 kohm resistor in it. This way, the potential across the cap is converted to heat and removed.

I'm probably making some mistake in how I'm looking at it, but I'd welcome anyone's comments.

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Re: Adjustable Bias Mod Kit First Impressions

Post by ja22y » Fri Sep 03, 2010 12:08 pm

You might get a spark if you just shorting the cap + to chassis or ground. The safest way is to do what Vanceen stated. I rigged up 2 alligator clips/wire with a 2W resistor in between. The resistor value determines how fast/slow the cap is discharged. 1K sounds good.

SRVYJM
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Re: Adjustable Bias Mod Kit First Impressions

Post by SRVYJM » Fri Sep 03, 2010 3:05 pm

Yeah, you can get a spark when discharging the caps this way, but the chassis is ground. As long as you're insulated from the lead (it's insulation) you can safely discharge the caps this way. You can't get electrocuted by the chassis after you've discharged the voltage into it (assuming you're not holding onto the chassis with your other hand when you do it, of course) because the chassis won't "hold" the voltage the way the filter caps do. Once the amp has been discharged this way it's perfectly safe to work on it.

I'm not sure about the resistor in line, but it makes sense, it would still discharge without a spark, so that's probably a good idea. Never did that, never needed to. I don't mind a spark. Let's me know I really discharged something!

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jaquetapus
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Re: Adjustable Bias Mod Kit First Impressions

Post by jaquetapus » Fri Sep 03, 2010 3:20 pm

Every time before I open up an amp I play a loud chord and switch off the main power while leaving the standby switch on. When I get it open I check the caps and there's never more than like 2v in them...hasn't failed me yet.

I too am excited to try this mod on my LSC...I'm just finishing an Andy Timmons clone on a Strat body and can't wait to try them together!

-dave
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Re: Adjustable Bias Mod Kit First Impressions

Post by gregrjones » Fri Sep 03, 2010 4:42 pm

So am I understanding correctly that if I installed one of these kits on my LSC, that I'd have to also buy new power tubes? If so, what would I be looking for?
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Re: Adjustable Bias Mod Kit First Impressions

Post by plan-x » Fri Sep 03, 2010 7:14 pm

I don't think you need to do that. That's the beauty of it. It just cooks the tubes you aready have.
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Charles Reeder
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Re: Adjustable Bias Mod Kit First Impressions

Post by Charles Reeder » Tue Sep 07, 2010 12:47 pm

NO! Plan-X has it right! You DO NOT need to buy any new tubes...the bias-mod allows you to adjust the bias on ANY tubes to get them into the 'warm' or 'Hot' zone. I repeat...I REPEAT...the installation of the 'bias-kit-mod' allows you to use ANY 6L6 tubes you currently own. (and ever buy in the future)..and adjust the bias so as to get optimum performance from those tubes!

If you install the 'bias-mod' kit and happen to know the color or number code of the power tubes you just installed and biased...then it would be both advantageous and convenient to buy backup tubes of the same color or number code. That way; if you ever needed to replace tubes quickly (such as at a gig) you could do so without taking the time to re-bias right then. They would (or should) be close enough matches to allow you to 'slog-on' until the gig was over and 'fine-tune' your bias later.

Cheers: Charles

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Re: Adjustable Bias Mod Kit First Impressions

Post by shawnpurcell » Tue Sep 21, 2010 9:06 pm

Are the pictures above of a LSC 10/50/100?? That's the LSC I own, and just wanted to know if my bias resistor would be in the same spot as the pics.

Thanks!!

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Re: Adjustable Bias Mod Kit First Impressions

Post by ja22y » Wed Sep 22, 2010 10:21 am

Another way is to get a 40k cermet trim pot and solder a 10k resistor in series and replace the bias resistor with it. Less than $1 in parts and 10 minutes of labor. Of course you need to take the chassis out when biasing. Not a big deal if you only do this when changing to new tubes or when the tube is at its half-life. And if you're using the same Mesa tube grade you might not need to bias after the initial one. Just some food for thought.

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Re: Adjustable Bias Mod Kit First Impressions

Post by SRVYJM » Fri Sep 24, 2010 9:32 pm

shawnpurcell wrote:Are the pictures above of a LSC 10/50/100?? That's the LSC I own, and just wanted to know if my bias resistor would be in the same spot as the pics.

Thanks!!

Shawn
http://www.shawnpurcell.com
Shawn,
I PM'd you too, but to answer your question so anyone else knows, yes, my Lone Star is a 10/50/100 unit. Small box head.

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Re: Adjustable Bias Mod Kit First Impressions

Post by SRVYJM » Sat Oct 02, 2010 9:42 am

jaquetapus wrote:Every time before I open up an amp I play a loud chord and switch off the main power while leaving the standby switch on. When I get it open I check the caps and there's never more than like 2v in them...hasn't failed me yet.

I too am excited to try this mod on my LSC...I'm just finishing an Andy Timmons clone on a Strat body and can't wait to try them together!

-dave

Dave, I'd like to see your AT Clone when your done. Post something on it. I just finished (well, close anyway it plays now) a AT clone on my strat too. Really it's just a nice Road Worn body with an Yngwie neck (big fan of scallops) and a Tone Zone humbucker. It was getting good AT tone, but not perfect, with the stock single coils from Fender. Then yesterday morning I installed the DiMarzio Cruiser in the neck position (still waiting on the middle one from DiMarzio's dealer in town, it's taking forever, but not sure how much AT really uses the middle position anyway) and I can say that it truly makes a difference. A very noticeable difference too. DOn't have to spend north of $2k to get an Ibenhad guitar to get that tone, just an alder bodied maple neck guitar with the Cruiser. Wow what a sweet tone that is in my Lone Star. I find for lower volume playing I can get a sweet, sweet addictive tone in the tweed mode with the Output at just past 9:00 in the 50 watt mode. As far as I'm concerned I've found my tone. Well, actually I've nailed Andy Timmons' tone, but I'll make it my own!

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Re: Adjustable Bias Mod Kit First Impressions

Post by gonzald1 » Wed Oct 13, 2010 8:42 pm

It appears that you remove two resistors?

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