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Thread: Fender DRRI bias range mod

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    Supporting Member Randall's Avatar
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    Fender DRRI bias range mod

    I don't have the amp yet, but I just got a call from a guy who likes modern TungSol 6V6s in his amps, but says they won't bias more than ~ 20mA in both of his DRRI amps. He wants me to get him to 25mA. I told him he might accomplish this by trying a hotter matched set, but he'd rather do the mod. Looking at the schematic, I wonder if the quickest path to this would be to lower the value of the 10K R59 between the adjustment pot and ground? Would tacking in a 10K across it be going in the right direction?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    That would lower it alright.

    But really, ten seconds with a resistor and clip wire and then you tell us.

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    Supporting Member Randall's Avatar
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    As I look at a Allessandro pulled DRRI board stamped 2010 that I have, there is a 22K in that spot, so I will be interested to see what is in his amp.

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    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    I don't have the amp yet, but I just got a call from a guy who likes modern TungSol 6V6s in his amps, but says they won't bias more than ~ 20mA in both of his DRRI amps. He wants me to get him to 25mA.
    Twenty is plenty. Crustomer must have read somewhere on the interwebs that more is merrier. He must love to buy new TungSol 6V6's. If he insists, let him. Make sure he won't come whining back to you when his outputs wear out early. I could see going 25 mA on JJ's, they can take it, but really not necessary with them either.

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    Lifetime Member Enzo's Avatar
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    Revision A - 10k
    Revision B - 10k
    Revision C - 10k
    Revision E - 22k


    Looks like the later revision they were going for even cooler.

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    Supporting Member Tone Meister's Avatar
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    Well a mA figure is meaningless without plate voltage. The Tung Sol 6V6 is a 12 watt tube, so with typical DR voltages 25 mA would be awfully hot. Even with 420 Vp they'd draw 10.5 watts each. Good luck with that lasting very long.

    18-20 mA is typical for most DR and DRRI and again, you need the Vp to calculate an accurate bias setting.

    Of course, we all know that but no one spoke up. I feel important now.

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    Quote Originally Posted by km6xz View Post
    "I have come to the conclusion that the biggest risk to amp performance/reliability and "tone" is players reading the internet, not bad tube brands, and certainly not the often argued over capacitors."

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    Twenty is plenty. Crustomer must have read somewhere on the interwebs that more is merrier. He must love to buy new TungSol 6V6's. If he insists, let him. Make sure he won't come whining back to you when his outputs wear out early. I could see going 25 mA on JJ's, they can take it, but really not necessary with them either.

    Around here we treat the JJ 6V6 as a 14-watt tube. https://www.jj-electronic.com/en/6v6s

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    Quote Originally Posted by km6xz View Post
    "I have come to the conclusion that the biggest risk to amp performance/reliability and "tone" is players reading the internet, not bad tube brands, and certainly not the often argued over capacitors."

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    Supporting Member Randall's Avatar
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    The first thing I told him is that sounded pretty hot to me, so I am covered here. Actually, he asked me if I could make it go as high as 25 - 30mA.

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    It's weird, because it WAS working fine.....

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by Randall View Post
    The first thing I told him is that sounded pretty hot to me, so I am covered here. Actually, he asked me if I could make it go as high as 25 - 30mA.
    The guy is blowing smoke out his a@@.
    I would tell him no, I will not do it.

    In the end he may end up respecting you more than if you crippled his amp.

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    Bent Member Chuck H's Avatar
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    Bias the amp in the safe zone and tell him:

    "I made an adjustment so that the tubes will perform at the higher current you want when conducting signal, but still give the tubes a break at idle."

    Then ask him:

    "How does it sound?"

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    "I made an adjustment so that the tubes will perform at the higher current you want when conducting signal, but still give the tubes a break at idle."
    I can see crustomer asking "What exactly did you do?" ("So I can boast about it on the interwebs page where I learned more is better.")

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    Bent Member Chuck H's Avatar
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    I don't have a plan for the 'second question' contingency

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  13. #13
    Lest We Forget g1's Avatar
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    "the re-issues don't use the secret bias numbers that the originals did, I set it up like a real Blackface".

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Supporting Member Randall's Avatar
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    He has a bias probe, and sets and plays around with it. He says the Russian Tung-Sols sound best to him, so already I think he is full of Internet. He is convinced his amp running at 20mA (don't know what the B+ is) sounds cold to him. I don't think he is aiming towards the 30mA bias point, but just thinks he needs the range. I think.

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    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    He is convinced his amp running at 20mA (don't know what the B+ is) sounds cold to him.
    Let him try playing his Russian tubes in Russia, he'll find out what cold is right quick.

    A customer with a bias probe is a dangerous thing... You could show him where the adjustment pot is, let him set it anywhere he wants with the proviso you are out of the blame loop when things go wally. OR ... stick a resistor in series with Fender's current sense resistor so it looks like 25 mA when it's really 20. There's your secret mod!

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    Bent Member Chuck H's Avatar
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    Well if the customer has a "bias probe" it's probably a plug in jobbie that he bought so he could play "pseudo tech tone master". Which pretty much puts the matter out of Randall's hands. So Randall's quandary is whether to give this guy the ability to blow up his gear by granting a request to change an adjustable parameter from *cold <> hot* to *cold <> melt*

    I think I would try to explain to the guy about how the circuit has to balance voltage with current to keep the tubes in a reliable operating condition. Basically, make sure he understands that current is only half of the equation. Voltage may be the half he is ignoring. Then I would refuse the modification on those grounds. as in: "As your repair tech I can't, in good conscience, modify your amp so that it can be adjusted into an unreliable operating condition."

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Lifetime Member Enzo's Avatar
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    1. Advise them what is proper.

    2. Do what they want.

    3. You can always just decline the repair.

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    Old Timer J M Fahey's Avatar
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    FULLY agree withb Enzo.
    Just advise him on good procedure, do what he wants, cas $$$ and forget about him.
    Who cares? ... you´re not his Mommy (I guess )
    You can´t cure stupid so Plan B is just follow his folly.

    IF it were something really dangerous, such as re-attaching a death cap, bypass a fuse or downgrade to 2 wire ungrounded cord, I´s refuse based on liability issues, but ... letting him *need* a new pair tubes a year? orr even every 6 Months? who cares?

    I lost **MANY** jobs by being honest beyond the call of duty ... ONLY result being they went to somebody else who did what they wanted and got the cash

    To boot, a few bad mouthed me calling me an old fool who lived in the past or too callous to follow customer´s orders so I didn´t even get good Karma for my good deeds.

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    Juan Manuel Fahey

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    FULLY agree withb Enzo.
    Just advise him on good procedure, do what he wants, cas $$$ and forget about him.
    Who cares? ... you´re not his Mommy (I guess )
    You can´t cure stupid so Plan B is just follow his folly.

    IF it were something really dangerous, such as re-attaching a death cap, bypass a fuse or downgrade to 2 wire ungrounded cord, I´s refuse based on liability issues,
    ^^^^^ I absolutely agree with Juan on this point and follow the same policy. The liability issues alone make this an important and wise practice. I look at it on a personal level as a matter of character as well. Bottom line is I love what I do, and I care about my customers.
    Juan's right in that you can't cure stupid. If someone wanted to go home and install lead water pipes because they're trying to recapture some of the some of those good times they had biting the window sills at grandma's house, there's not much I can do about that.
    But, if a customer was insistent that I do something I know to be dangerous I won't even touch the equipment, and respectfully explain why. While that kind of dumb can be really frustrating to deal with, it certainly doesn't call for a potential death sentence. I certainly look back at some of the stupid shit I've done, and managed to escape with my life . Plus, what if someone else happened to play his gear at the wrong time and something tragic happened. I want no part of that.

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    Supporting Member Randall's Avatar
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    Update. Customer came in with his amp today, nice guy. He brought his bias probe, so I hooked it up and measured the plate voltage. He was running the hotter tube at 25mA @ 416v = 80%. I explained all the bits about how plate voltage plays into it, and how to determine wattage with those readings. Also, how Fender sets them cool, and the 50 -70% thing. His amp had the 22K R59, and since he still wanted to play around with it, I tacked another 22K across it, making it easy to remove if ever decided. Now he can get as high as 40mA. I told him again and again that 30mA was way high and 40mA was probably going to explode high. It's in his hands now. If it does go boom, I'll get another paying visit.

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    Supporting Member Jazz P Bass's Avatar
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    I had a jazz guy with a Fender something and he insisted the bias was too cold.

    Cranked it up.

    He came back the next week and asked me to put it back where it was.

    I guess it depends on the playing style/ type.

    40 ma's on a 6L6 is suicide.
    I don't care what the plate voltage is.

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    Supporting Member Tone Meister's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    I had a jazz guy with a Fender something and he insisted the bias was too cold.

    Cranked it up.

    He came back the next week and asked me to put it back where it was.

    I guess it depends on the playing style/ type.

    40 ma's on a 6L6 is suicide.
    I don't care what the plate voltage is.
    Problem is, these are 6V6 which dissipate less than half the wattage of a 6L6. (I realize that was probably a typo)

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    Quote Originally Posted by km6xz View Post
    "I have come to the conclusion that the biggest risk to amp performance/reliability and "tone" is players reading the internet, not bad tube brands, and certainly not the often argued over capacitors."

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