How to install a bias pot in a Rectifier

[mikey383]

How does that 15k resistor work? Would replacing it with a solid wire make the EL34 - 6L6 switch on the back of the amp effectively do nothing?

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.

 

[Anonymous]

How does that 15k resistor work? Would replacing it with a solid wire make the EL34 - 6L6 switch on the back of the amp effectively do nothing?

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.

Ahh, I see! In that case, why not replace the 15k resistor with the bias pot so in 6L6 mode nothing is changed, and in EL34 mode you can run it where you want? Maybe I'm getting a little ridiculous here, but going back to the other thread, it would be very nice to have the bias way up for low volume playing - which I do a lot of - and the normal 6L6 mode for loud playing. If I'm understanding correctly, turning the bias pot up would essentially cut down on resistance?

 

[skoora]

Thanks Mikey, appreciate the feedback.

 

[mikey383]

Ahh, I see! In that case, why not replace the 15k resistor with the bias pot so in 6L6 mode nothing is changed, and in EL34 mode you can run it where you want? Maybe I'm getting a little ridiculous here, but going back to the other thread, it would be very nice to have the bias way up for low volume playing - which I do a lot of - and the normal 6L6 mode for loud playing. If I'm understanding correctly, turning the bias pot up would essentially cut down on resistance?

You could do that, but it would be overkill IMO. I would simply install the pot and run with it. If you're playing at loud volumes and find that the tubes are breaking up too fast, you could always turn the bias down until the breakup disappears.

The way it works, when you turn the bias pot up, the resistance goes down, which brings the negative bias voltage closer to zero.

 

[Anonymous]

You could do that, but it would be overkill IMO. I would simply install the pot and run with it. If you're playing at loud volumes and find that the tubes are breaking up too fast, you could always turn the bias down until the breakup disappears.

The way it works, when you turn the bias pot up, the resistance goes down, which brings the negative bias voltage closer to zero.

Haha, I like overkill! I think it would be cool to note the differences between the two, and the advantage to me of putting it there instead of in the 6L6 spot is if it accidentally gets flipped into 6L6 mode, nothing happens.

If I'm understanding correctly though, in order to completely render the 6L6 - EL34 switch irrelevant, you'd need to remove the 15k resistor and replace it with nothing, breaking the circuit since they run in parallel?

 

[mikey383]

Haha, I like overkill! I think it would be cool to note the differences between the two, and the advantage to me of putting it there instead of in the 6L6 spot is if it accidentally gets flipped into 6L6 mode, nothing happens.

Alternatively, you could bias in the EL34 mode. That way if the switch gets bumped, the tubes run colder and not hotter. I have enough adjustment on mine to run 6L6s in the proper range. Now if my bias pot gets bumped, that's a different story.

If I'm understanding correctly though, in order to completely render the 6L6 - EL34 switch irrelevant, you'd need to remove the 15k resistor and replace it with nothing, breaking the circuit since they run in parallel?

Correct. You would just need to remove the 15k completely out of the circuit. The 22k would still be there, so flipping the switch would do nothing.

 

[Anonymous]

Haha, I like overkill! I think it would be cool to note the differences between the two, and the advantage to me of putting it there instead of in the 6L6 spot is if it accidentally gets flipped into 6L6 mode, nothing happens.

Alternatively, you could bias in the EL34 mode. That way if the switch gets bumped, the tubes run colder and not hotter. I have enough adjustment on mine to run 6L6s in the proper range. Now if my bias pot gets bumped, that's a different story.

If I'm understanding correctly though, in order to completely render the 6L6 - EL34 switch irrelevant, you'd need to remove the 15k resistor and replace it with nothing, breaking the circuit since they run in parallel?

Correct. You would just need to remove the 15k completely out of the circuit. The 22k would still be there, so flipping the switch would do nothing.

Thank you very much for your insight! I'm gonna have a blast playing around with all this stuff, not to mention a tone that will KILL!

 

[mikey383]

I just reread your post and saw that you said the same thing I said...if it gets bumped into 6L6 mode, nothing happens except the tubes run colder.

If it's any help, you can hear the tonal change when you turn the pot. It not only gets louder, but the tone gets fuller too.

 

[Anonymous]

I just reread your post and saw that you said the same thing I said...if it gets bumped into 6L6 mode, nothing happens except the tubes run colder.

If it's any help, you can hear the tonal change when you turn the pot. It not only gets louder, but the tone gets fuller too.

Haha, yeah I wasn't going to say anything.

Actually, that's how I bias my amps nowadays: by ear. My process is basically to set up the amp at the volume I plan on recording or playing, set the tone controls best I can, and listen to what sounds best. I'll of course check to make sure the tubes aren't running too hot, but setting bias my ear is one of the best ways to ensure the best tone possible!

 

[mikey383]

Haha, yeah I wasn't going to say anything.

I misread it and thought you meant if it got bumped into EL34 mode

Actually, that's how I bias my amps nowadays: by ear. My process is basically to set up the amp at the volume I plan on recording or playing, set the tone controls best I can, and listen to what sounds best. I'll of course check to make sure the tubes aren't running too hot, but setting bias my ear is one of the best ways to ensure the best tone possible!

That's the best method. It doesn't matter where the tubes are set, as long as they sound good to you and aren't going to cause a fire. It doesn't matter if that sound is at 11mA or 50mA. As long as the tubes aren't redplating.

 

[skoora]

Haha, yeah I wasn't going to say anything.

I misread it and thought you meant if it got bumped into EL34 mode

Actually, that's how I bias my amps nowadays: by ear. My process is basically to set up the amp at the volume I plan on recording or playing, set the tone controls best I can, and listen to what sounds best. I'll of course check to make sure the tubes aren't running too hot, but setting bias my ear is one of the best ways to ensure the best tone possible!

That's the best method. It doesn't matter where the tubes are set, as long as they sound good to you and aren't going to cause a fire. It doesn't matter if that sound is at 11mA or 50mA. As long as the tubes aren't redplating.

I used to do that with a 78' 2203 that I used to have. It worked great. I'd set the controls, using an attenuator as well, so I could put the master up to gig volume but not get blasted. Funny It was great to listen to when you could hear it get out of the sweet spot from being too cold or hot. When I was done invariably the bias would usually be in the low/mid 30's on EL34's which was about right for the B+ on that amp. I would set the bias at 25mA to start and go from there.

 

[Anonymous]

Got the part from Mouser tonight and did the mod. The way I rigged it was to replace the 15k resistor with the 50k pot, and I'm very happy with the results! Now, not only can I set it up so that I can go back and forth between stock 6L6 mode and proper bias range, but I can get more out of the different power options!

I can use the stock 6L6 mode with diodes and bold, then switch over to EL34 and set the bias for use with rectifier tubes and spongy! Now, I just got an Uberschall Twin Jet and I'm honestly not sure if I'll keep my Rectifiers; the Twin Jet is ******* unbelievable! But, if I'm able to get the Rectifier to sound good enough with this bias mod - and I'm pretty sure I'll be successful - then I would like to switch out the 22k resistor as well for another bias pot. The reason for this is so I could set up the best tone for both diodes and rectifier tubes! Overkill? No, I don't think so. I see it more as taking advantage of the options the head has to offer without making compromises. If it costs $10 and about 5 minutes of extra work in biasing for a big improvement in tone and feel, it's more than worth it for me!

If you're reading this thread and haven't yet done the mod but feel you would be competent enough to, don't wait! The bias mod and JJ tubes will own a stock Recto with the best NOS tubes you can find, so spend the money and the 20 minutes it takes because it's worth it!

 

[mikey383]

That's quite interesting. I never thought of doing it that way.

 

[94Tremoverb]

I modded my Tremoverb when I found out just how cold the tubes run in the Spongy modes with the lower voltages - they're barely on (under 5W at idle in Spongy/Tube Rectifier), and it helps explain why I never liked the Spongy modes. I can see why it's like that because in Bold/Diode mode it gets pretty hot (at least by Mesa standards!) but it really needs some compensation when it's running with something like 100V less on the plates.

What I did was to use the stock Slave Level pot - removing the resistor and connections to it and the jack apart from the ground, since I never use the Slave function, and it could be derived from one of the speaker jacks with an external pad box anyway, should I ever need to. The pot is 10K, so I simply replaced the 22K resistor with a 12K soldered to the board in the same place at one end, and a wire to the 10K pot in series with it. So now the coldest setting is the factory bias (easy to return it to if I want, just dime the pot), and it goes usefully hotter but not over the top - the maximum on Bold/Diode is around 50mA, which is a bit too hot but not instant tube-melt territory - and I rarely use that anyway, I prefer Bold/Tube, which maxes at somewhere in the high 30s. So I can just dial it by ear in use and not worry about it.

But just in case I do want to know, I fitted a 1-ohm resistor in the cathode connection of one of the tubes (not the one immediately behind the jack since the reverb transformer ground connection goes there, but the next one), and hooked up the tube end to the Slave jack - so now all I have to do is plug a test lead with a 1/4" plug on one end and meter plugs on the other into the jack, and I can read the current directly - OK, only on that one tube, but if I'm using a matched set it doesn't really matter.

I would have to agree that making it variable (within limits) really is a big improvement on this amp and opens up tones it just wasn't capable of before, especially in the Spongy modes. Sorry Randall!

 

[theDogger]

Just reading through the thread and have a few questions?

1-What are you BIAS'ing the tubes at?
E34L @ 25watts dissipation
6L6GC @ 25watts dissipation

The formula for calculating the bias is the dissipation of the tube divided by the plate voltage which gives you 100% and then you can bias anywhere between 65% up to 85% of this. Most amps will just start to come out of crossover distortion at 65% and so we recommend using 70%. You can of course go up to 85% but you will wear out your tubes quicker and brown out the tone a little.
(25 watts dissipation can be used for just about any new production EL34, 6L6, KT77 or KT66 tube and most amps)

What are you getting for Plate Voltage at the 3rd pin?

I noticed a lot of cold settings. Isn't the formula for BIAS'ing as follows

70% times 25 watts dissipation divided by the plate voltage measured at the 3rd pin.
(.7 * 25watts/480volt) = .036mA

I know that with the fixed BIAS the Mesa's are BIAS'ed cold.

Also I am assuming that with the different settings that are available w/ the the Mesa that the BIAS will change according to the setting?

Reason for asking is that I am in the market for a Single or Dual Rec. and I am doing my research!

theDogger

 

[mikey383]

Just reading through the thread and have a few questions?

1-What are you BIAS'ing the tubes at?
E34L @ 25watts dissipation
6L6GC @ 25watts dissipation

That's standard dissipation for those tubes, so that would be what you want to shoot for.

What are you getting for Plate Voltage at the 3rd pin?

On my 2 channel Dual, I'm getting 486 volts in the Bold/Diodes setting. That number will change depending on the settings. It will be much lower in Spongy/Tube rectifier mode.

I noticed a lot of cold settings. Isn't the formula for BIAS'ing as follows

70% times 25 watts dissipation divided by the plate voltage measured at the 3rd pin.
(.7 * 25watts/480volt) = .036mA

I know that with the fixed BIAS the Mesa's are BIAS'ed cold.

Also I am assuming that with the different settings that are available w/ the the Mesa that the BIAS will change according to the setting?

Reason for asking is that I am in the market for a Single or Dual Rec. and I am doing my research!

theDogger

You are correct. The bias will change depending on the settings, because the plate voltage changes. Mesas are biased cold from the factory because they are mostly preamp gain and need a clean power section, so with the power section biased cold, there is much less of a chance of driving the power tubes into distortion. Plus, on Rectifiers, power amp distortion does not sound good, no matter what tubes you are using. On the other hand, amps like Marshalls and EL34 based amps with medium gain tend to sound better with power amp distortion.

 

[94Tremoverb]

I don't agree that you need the tubes biased as hot as 65-85%, that's too hot in my opinion. 70% is the normally accepted safe upper limit in a fixed-bias Class AB amp, and you can be much lower than this and still have good (often better) tone. For example vintage EL34 Marshalls sound best and are much more reliable at only about 55% in my experience. Mesa bias cool too, below 50% in a lot of their amps with some settings, and they don't sound bad or with audible crossover distortion (which is a function of the amp design, not the tube rating). Running too hot doesn't guarantee better tone, but it does wear the tubes out much faster and increases the chance of sudden failure.

 

[mansfieldguitars]

First off I'd like to state that this is a very informative post. Just to add my opinion, this method works very well but there is absolutly no safety built in in the case of a pot failure. There seems to be alot of controversy over adding a series resistance to the trim pot. I would never do the mod without one! Yes, the resistor does reduce the sweep of the pot a little but by adding a series resistance it prevents you from 1. turning the pot all the way down which would give you 0 resistance and 2. should the pot ever short out you still have the series resistance as some protection. In either case the lack of resistance would would surely damage something. Most likely your output tubes. Also the pot poses an issue by only using 2 of the leads. I use all three by connecting the middle lead to one of the outside leads. By wiring the pot this way you eliminate the hazard of possible losing the wiper connection again resulting in no resistance. Should the wiper connection lift and it does happen, the circuit will see the full resistance of the pot. That will shift you bias to the cold side saving your tubes.These are definatly not necesary but add safety features to the circuit. When you add up the cost of burnt output tubes vs. the cost of adding a series resistor to me it's a no brainer and the pot wiring does not cost a thing. I have personally repaired amps with bias pots shorted. Although this is not common, It can and does happen. The problem I had was to fit everything inside the amp neatly without the risk of exposed leads to short out. That is why I designed my bias mod. The circuit board allows me to house the trim pot and series resistance and the board allows the safety wiring of the trim pot. Again, this is my opinion and bothe ways work equally well. I just feel better knowing mine and others $2000.00 investment has whatever safety features it can.

 

[jonamojo]

First off I'd like to state that this is a very informative post. Just to add my opinion, this method works very well but there is absolutly no safety built in in the case of a pot failure. There seems to be alot of controversy over adding a series resistance to the trim pot. I would never do the mod without one! Yes, the resistor does reduce the sweep of the pot a little but by adding a series resistance it prevents you from 1. turning the pot all the way down which would give you 0 resistance and 2. should the pot ever short out you still have the series resistance as some protection. In either case the lack of resistance would would surely damage something. Most likely your output tubes. Also the pot poses an issue by only using 2 of the leads. I use all three by connecting the middle lead to one of the outside leads. By wiring the pot this way you eliminate the hazard of possible losing the wiper connection again resulting in no resistance. Should the wiper connection lift and it does happen, the circuit will see the full resistance of the pot. That will shift you bias to the cold side saving your tubes.These are definatly not necesary but add safety features to the circuit. When you add up the cost of burnt output tubes vs. the cost of adding a series resistor to me it's a no brainer and the pot wiring does not cost a thing. I have personally repaired amps with bias pots shorted. Although this is not common, It can and does happen. The problem I had was to fit everything inside the amp neatly without the risk of exposed leads to short out. That is why I designed my bias mod. The circuit board allows me to house the trim pot and series resistance and the board allows the safety wiring of the trim pot. Again, this is my opinion and bothe ways work equally well. I just feel better knowing mine and others $2000.00 investment has whatever safety features it can.

What value resistor would you recommend when using a 50k pot

 

[mikey383]

First off I'd like to state that this is a very informative post. Just to add my opinion, this method works very well but there is absolutly no safety built in in the case of a pot failure. There seems to be alot of controversy over adding a series resistance to the trim pot. I would never do the mod without one! Yes, the resistor does reduce the sweep of the pot a little but by adding a series resistance it prevents you from 1. turning the pot all the way down which would give you 0 resistance and 2. should the pot ever short out you still have the series resistance as some protection. In either case the lack of resistance would would surely damage something. Most likely your output tubes. Also the pot poses an issue by only using 2 of the leads. I use all three by connecting the middle lead to one of the outside leads. By wiring the pot this way you eliminate the hazard of possible losing the wiper connection again resulting in no resistance. Should the wiper connection lift and it does happen, the circuit will see the full resistance of the pot. That will shift you bias to the cold side saving your tubes.These are definatly not necesary but add safety features to the circuit. When you add up the cost of burnt output tubes vs. the cost of adding a series resistor to me it's a no brainer and the pot wiring does not cost a thing. I have personally repaired amps with bias pots shorted. Although this is not common, It can and does happen. The problem I had was to fit everything inside the amp neatly without the risk of exposed leads to short out. That is why I designed my bias mod. The circuit board allows me to house the trim pot and series resistance and the board allows the safety wiring of the trim pot. Again, this is my opinion and bothe ways work equally well. I just feel better knowing mine and others $2000.00 investment has whatever safety features it can.

True, it is good and cheap insurance. I debated for a while on whether or not to install one, and though I probably should, I didn't.

Although I've yet to see a pot go bad on it's own.