How to install a bias pot in a Rectifier

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Rocker
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Re: How to install a bias pot in a Rectifier

Post by Rocker » Fri May 27, 2011 5:26 pm

[/quote]

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[/quote]

I can try. The amp had great punch and low end before the modification, but always seemed to lack crispness -- the highs were somewhat muted -- and I couldn't seem to dial in the gain and treble to get the balanced distorted tone that I was looking for. I went though a couple of years of experimenting with preamp tubes, which really improved things. Went through a few sets of 6L6s and decided to look into EL34s. I was able to pick up a quad of vintage Mullard xf2 EL34s at a good price. Because they weren't Mesa tubes, I was hesitant about using them without adjusting the bias. So after a bit of research, I learned that there was little risk to this, so I plugged them in and was impressed, but not blown away in comparison to the 6L6s. After a little more research, I ran across the Mesa bias discussion, and the mod kit and probe, ordered them, and installed the mod. I first tried the 6L6s. After proper bias, there was a very noticeable change in the tone. The highs were much more defined and bold. The distortion became more pronounced and focused, coming on with more intensity at a lower gain knob position. Low end remained strong with lots of depth and character. I liked the tone so much that I didn't have any desire to try the EL34s. The 6L6s were actually at the end of their life, and died within a few days (and the higher current that resulted from proper biasing the amp probably didn't help prolong their life). So, in went the EL34s, re-biased the amp. Now I was blown away by the EL34s. Like a different amp. Absolutely perfectly balanced distorted tone with finely detailed highs, rich mids, and that unmistakable Rectifier low end. Guitar harmonics became present that weren't significant before. Producing sounds from the guitar as a result of pick technique variations is easier. Clean tones are also wonderfully detailed, but I find that to keep the purity in the cleans, I have to roll off the gain more to avoid unintended breakup. Somehow the power section responds differently from the preamp signal. I think that overall a small amount of volume at the upper end of the range is sacrificed on the clean channels as a result. The opposite is true for the distorted channels -- once you go there the volume potential is incredible, as it should be with a Roadster. But I'm a classic metal type, so I'm more than delighted with a hotter overall amp setup. I'm looking forward to experimenting with non-mesa 6L6s one day soon (winged C's probably) but I'm really having a good time with the Mullards, and they seem to be very strong and stable for now. That's about the best that I can do to describe with words the sounds that we all seek, but that we know are impossible to describe with words. (lol)

Craig

kramerxxx
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Re: How to install a bias pot in a Rectifier

Post by kramerxxx » Sat May 28, 2011 9:10 am

Craig,
Thanks for posting this, I appreciate it.

I'm going to give it a try as what you are describing is exactly what my Dual Rec needs.

Steve

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Re: How to install a bias pot in a Rectifier

Post by Raggi73 » Wed Jun 08, 2011 10:11 am

I got the BIAS mod done with the kit from Mansfild guitars and I am really happy with it.
From what I can hear I belive there is a difference and a tighter sound.
My setup:
Mesa Boogie Dual Rectifier 3ch (2007 modell)
4x12 Mesa traditional cabinet
4x12 Randall cabinet.
Korg DT2000 tuner
Furman Power conditioner
Axess CF4 midi switch
T.C. electronic G-major effect ++++

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Re: How to install a bias pot in a Rectifier

Post by RussB » Wed Jun 08, 2011 6:50 pm

Raggi73 wrote:I got the BIAS mod done with the kit from Mansfild guitars and I am really happy with it.
From what I can hear I belive there is a difference and a tighter sound.

What tubes to you have in your amp, and what did you set the bias to?

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Re: How to install a bias pot in a Rectifier

Post by ss22 » Mon Jun 18, 2012 6:35 pm

Sorry to necrobump but I'm just curious about something. I followed your brilliant instructions and the bias mod went off without a hitch.
However I did notice a voltage drop due to the length of wire from the slave knob and back. Prior to the mod my plate voltage was about 470, now it's dropped to 450. Aside from affecting the ideal bias current, does this voltage drop have any other effects on the amp?
Cheers.

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Re: How to install a bias pot in a Rectifier

Post by RussB » Mon Jun 18, 2012 7:04 pm

Measure the ac input voltage. 1 volt there can make 4 or more volt variation in plate voltage.

Also, as you increase the current through the tube (turn up the bias) the pv will drop

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Re: How to install a bias pot in a Rectifier

Post by motormenace » Thu Aug 02, 2012 3:31 pm

I've been thinking of doing this mod to my Blue Angel amp. I was wondering if anyone has done this mod to their amp? I looked at the schematic, and it looks like there is only one bias for the 6V6's and EL84's.

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Re: How to install a bias pot in a Rectifier

Post by darthpoodle » Sat Nov 10, 2012 4:02 pm

I know this is an old post but I was hoping someone could help me with the bias mod for a single rectifier version 1. Can't seem to find any information on it or a schematic. Pictures of the right resistor would be great too if you don't have the schematic.

thanks

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Re: How to install a bias pot in a Rectifier

Post by R_ADKINS80 » Mon Dec 17, 2012 10:35 pm

Just for everyone's info, if you mod the bias to be adjustable in the 6L6 setting, there is FULL range to adjust for any power tube up to 6550 and KT88. I currently have a sextet of Electro Harmonix 6550 in my Triple and it smokes EVERYTHING I have ever plugged into. Then again, I do have the "Pre-500" mods done to my 3ch head as discussed in the Pre-500 thread down just a ways.
2008 Triple Rec
Dean '79 V (Dean Tyrant/dimarzio mystery neck)
LTD Ninja 600 (SD Black Winter/59)
Epiphone Les Paul Standard (EMG 81/85
Ibanez GAX70 (EMG 81)
Hamer Standard (Golden Age Hot PAF w/ ceramic magnets)
BC Rich Warlock (L500-XL's)

taylor51
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Re: How to install a bias pot in a Rectifier

Post by taylor51 » Sat Dec 29, 2012 9:34 pm

Hi,I don't know if the guy who wrote this checks this every now and then,but I just wanted to say that it's an excellent post and L'm glad I found it.I especially like the suggestion about using the slave jack as a voltage test.Thanks for simplifying what seemed like a complicated job.

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Re: How to install a bias pot in a Rectifier

Post by Gnarkill3k6 » Fri May 03, 2013 5:02 am

@ mikey383

What kind of 50k pot did you use? I question this because of the price... here (Netherlands) a 50k pot costs like 0,36 cents.

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Re: How to install a bias pot in a Rectifier

Post by deeman » Wed Aug 28, 2013 3:12 pm

Instead of starting a new topic I figured I'd bump this old one.

I have my 3 channel dual rec at a shop right now for a cleaning and check up. The tech offered to perform the bias mod for $40 bucks. Price is not a problem for me but i'm just not sure if it is worth it to me to get this mod. I play mainly rhythm guitar on raw mode in channel 2 and have become quite fond of the tone i get out of the amp. I am feeling like if it ain't broke don't fix it, plus i'd like to keep the amp original if at all possible so I'm not going to do this mod if it really won't make a difference for me.

Any thoughts from those of you who have done this? I saw there were some people who commented earlier in this thread and said it improved the sound but it wasn't very clear what the changes were (except for the one post by someone that seemed to be advertising for a particular business).

Thanks!
Mesa Boogie 2001 3 Channel Dual Rectifier
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Re: How to install a bias pot in a Rectifier

Post by Gnarkill3k6 » Thu Aug 29, 2013 1:33 am

@deeman

In theory (and in practice I think) you can set the adjustable bias exactly the same as you amp is now.
The only thing that changes is that your bias goes from fixed to adjustable. People say Mesa amps are biased 'cold' and why people mod their amps is to run the tubes a bit hotter and to use different tubes then the Mesa-specced ones.

If you do the mod, measure the fixed bias first and adjust to that value after the mod. I think you get the exact same sound as you have now plus the option to run hotter plus the option of different tubes then Mesa's.

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Re: How to install a bias pot in a Rectifier

Post by RussB » Thu Aug 29, 2013 3:01 pm

Gnarkill3k6 wrote:@deeman

In theory (and in practice I think) you can set the adjustable bias exactly the same as you amp is now.
The only thing that changes is that your bias goes from fixed to adjustable. People say Mesa amps are biased 'cold' and why people mod their amps is to run the tubes a bit hotter and to use different tubes then the Mesa-specced ones.

If you do the mod, measure the fixed bias first and adjust to that value after the mod. I think you get the exact same sound as you have now plus the option to run hotter plus the option of different tubes then Mesa's.

The amp is fixed bias whether there's an adjustment pot or not.


Having the ability to adjust the (fixed) bias circuit allows you to use a wide variety of tubes that would otherwise run too hot or too cold.

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Re: How to install a bias pot in a Rectifier

Post by Gnarkill3k6 » Fri Aug 30, 2013 1:24 am

When you're able to change something that was fixed... that means it's variable/adjustable and not fixed anymore, right?

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