How to install a bias pot in a Rectifier

[kramerxxx]

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-Components/_/N-5g73?Keyword=50K+Multi-turn+potentiometer+Bourns&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

 

[stratele52]

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.

 

[Rocker]

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

 

[stratele52]

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

 

[mikey383]

: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.

 

[Rocker]

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

 

[Rocker]

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

 

[mikey383]

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!

 

[kramerxxx]

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

 

[RussB]

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

AFTER

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

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

 

[Rocker]

[/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]

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

 

[Raggi73]

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.

 

[RussB]

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?

 

[ss22]

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.

 

[RussB]

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

 

[motormenace]

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.

 

[darthpoodle]

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

 

[R_ADKINS80]

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.

 

[taylor51]

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.