How to install a bias pot in a Rectifier

[mansfieldguitars]

The exact value of the series resistance can vary with different amps. The idea here is to have enough resistance built in in case if a pot failure but you still want enought range to dial in your bias.
I would say the general range would be somewhere between 3.9K to 10K. My roadster has a 68K in parallel with the 22k which gives you approx -51 volts. On I used a 3.9k in series with a 50k pot.

 

[jonamojo]

thanks mansfieldguitars for sharing your tips I appreciate it

 

[mansfieldguitars]

No problem. As I get time I will keep posted anything I think is useful and if you have something specific you would like to know feel free to contact me. I'll help in any way I can. FYI I'm working on a section for my website that will have this free information to frequent problems and questions. It is in the works but feel free to stop by from time to time and check.

Thanks
John

 

[mikey383]

I just edited the original post. I went back and reworked my bias mod. (I should have done this a while back, but since my head was packed in a road case, I just got lazy :lol: )

This time, I took a 50K pot, soldered a 56K 1/2 watt resistor across the outside lugs to bring it down to a total of 26K, which will run a little colder than stock. I also tied the wiper to the outside leg so that if the wiper loses contact with the pot, there's still the 26K resistance.

However, I didn't use a safety resistor to prohibit accidental adjustment.

Thanks for the tips John!

 

[sbalderrama]

Replacing that 15K resistor with a solid wire would put a really high bias voltage to the tubes. The bias switch brings that 15k in parallel with the 22k bias resistor. When you parallel resistors, you take the total resistance divided by the number of resistors (22k + 15K = 37k/2 = 18.5k) If you replace the 15k with solid wire (that wire will have negligible resistance...IE zero ohms) , you end up with 22k + 0 ohms /2 = 11. So, effectively you would make the 22k resistor become an 11k when you flipped the switch. That will run the tubes very hot.

Just wanted to add for the sake of completeness that this calculation is wrong. If you put a solid wire in parallel with a resistor, the end resistance is 0 ohms, it's a short. The actual calculation for two resistors in parallel is (R1*R2)/(R1+R2). Since one of the values in this case is 0, the result is 0.

 

[mikey383]

Replacing that 15K resistor with a solid wire would put a really high bias voltage to the tubes. The bias switch brings that 15k in parallel with the 22k bias resistor. When you parallel resistors, you take the total resistance divided by the number of resistors (22k + 15K = 37k/2 = 18.5k) If you replace the 15k with solid wire (that wire will have negligible resistance...IE zero ohms) , you end up with 22k + 0 ohms /2 = 11. So, effectively you would make the 22k resistor become an 11k when you flipped the switch. That will run the tubes very hot.

Just wanted to add for the sake of completeness that this calculation is wrong. If you put a solid wire in parallel with a resistor, the end resistance is 0 ohms, it's a short. The actual calculation for two resistors in parallel is (R1*R2)/(R1+R2). Since one of the values in this case is 0, the result is 0.

DOH!

Where was my head in that post?

 

[stratele52]

I'm new to this forum and I read the first page of this very interesting post. I'm working now to built a 2 adjustment pots at Triple Rectifier. One for the 6L6 and one for EL34.
I did not read other pages, maybe somebody write about that :

I noticed on shematic of the Two rectifier, it's write about R 333 : Unit Domestic. I'm in Canada so I think domestic is for USA and Canada. This resistor is missing in the amp I'm working now and same I see on your picture.
This missing resistor , if there, give more negative bias.

What do you think about that ?

 

[mikey383]

I'm new to this forum and I read the first page of this very interesting post. I'm working now to built a 2 adjustment pots at Triple Rectifier. One for the 6L6 and one for EL34.
I did not read other pages, maybe somebody write about that :

I noticed on shematic of the Two rectifier, it's write about R 333 : Unit Domestic. I'm in Canada so I think domestic is for USA and Canada. This resistor is missing in the amp I'm working now and same I see on your picture.
This missing resistor , if there, give more negative bias.

What do you think about that ?

Welcome to the Board!

I believe what the schematic is supposed to say is "omit domestic", meaning it's not there for 117V 60 HZ models (which would be US and Canada), but is present on the export models.

What are you wanting to do...run both 6L6s and EL34s at the same time?

 

[stratele52]

Thank's mickey383, I Didn't receive your answer on my e-mail adress. I have to check the forum set up on my profil.

My bias circuit are working well, the 2 pots. Yes I want to put 6L6 or EL34 and have bias with easy adjust. This job is for a customers.
About the resistors. You are right; omit domestic could be the good word.

Many things about bias is strange. It's my first Mesa to fix. I do lots of fender Marshall, Hi Watt etc.

I set bias hot (70% of power dissipation on EL34) WITH these switches at max voltages : Power switch to Bold and Rectifier switch to Silicon Diodes. My bias is set to 39 milliamp at 448 volts DC, each tube.

My problem is if I change any position of the Power switch OR the rectifier switch, the voltage DC drop ( this is normal) and my bias is to cold :
Spongy and Tubes; 22 milliamps at 384 volts = very very cold
Bold and tubes ; 25 milliamp at 422 volts = cold

What is the proper way to bias a tripple rectifier ?

 

[mikey383]

That is typical of these amps. When you flip the switch to either spongy or tube rectifier, the DC voltage will drop, and therefore, the bias will go down. Bias is directly related to voltage, so that's a normal occurrence. When the voltage decreases, so does the bias on these amps.

In this situation, the proper thing to do would be to set the bias with the switches in the preferred position. If you're using (say for example) spongy and tube, check the voltage (in this case 384 volts), and bias appropriately (45.5 mA with EL34s in this case).

The bias WILL change depending on where the switches are set, simply because the voltage supplied to the tubes is different on each setting.

There really isn't a way to bias the tubes to be at peak performance in each setting, since each setting differs the voltage. The only thing to do is to bias the tubes in that particular setting, and the bias will change if a different setting is used.

I would bias them at 70% on bold/solid state rectifiers since that will produce the highest voltages, and protect against accidental flipping of switches which would raise the bias to unsafe levels.

If your customer wants the tubes biased with a certain setting, I would make sure they know that changes will occur when the switches are flipped, and make sure you're not responsible for accidental adjustment.

 

[stratele52]

Thank you very much for the quick response mickey383. It is exactly what I believe and this is the way to the bias that I did.
I am very disappointed with the way Mesa Boogie built this amp . There are too many compromises with regard to the bias.

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I' m french speaking so excuse me if my english is bad

 

[mikey383]

With the different settings, each one does something different. To use it the way it was intended (but with adjustable bias), it would be proper to set the bias with the power switch on bold and the normally used rectifier setting. The power switch is designed to decrease the voltage as well as the bias, and biasing to the proper range with the power switch on spongy would defeat the purpose of the switch.

All in all, that's a nice idea you have there.

 

[stratele52]

You are right about bias and that is I have done, but in all other switches position (lower voltage) the bias are very cold. Too bad, this is Mesa Boogie with all their gadgets !!!
Thank's for everything. For your information mickey this is the sketch of my new two pots bias circuit in the low half of the sheet.
In upper half, original Mesa circuit.

 

[shatteredsquare]

say i wanted to run 6L6s in stock EL34 mode, without this pot or any kind of bias adjustment installed. Would it do anything besides roast my 6L6s? Would it, or could it damage the amp? I wouldn't care about tube wear if making 6L6s glow made it sound like EVH with an almost-on-fire marshall.

 

[stratele52]

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.

 

[shatteredsquare]

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.

 

[stratele52]

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.

 

[RoadKinger483]

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???

 

[kramerxxx]

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!!!

 

[stratele52]

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"