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Thread: 5g9 Tremolux, Need Bias mod Advice to bias each 6V6GT Separately

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    5g9 Tremolux, Need Bias mod Advice to bias each 6V6GT Separately

    Hi all,
    I am new here and I have scoured forums etcetera for an answer to adding independent bias for each 6V6GT in a tweed 1958 Fender Tremolux 5g9. The closest I've found is on robrobinette's mod site where he shows an independent bias mod for the 5f6a. The added challenge is how to add this mod and address the bias modulating tremolo?

    Any help is appreciated.


    Thank you

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    Supporting Member Jazz P Bass's Avatar
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    It's a push/ pull output stage.

    Why would you want to make each tube separate?

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    Lifetime Member Enzo's Avatar
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    Hi, welcome.

    Good etiquette would be to put up links to the other stuff you refer to. I don't know a robinette site for example. And schematics. I can go find them but we should be able to just open them from here.

    Making independent bias is generally just to duplicate the bias supply having one for each tube. But your amp has the trem in the formula.

    I know you want to do what you want to do, but precision independent bias for this little circuit seems like huge overkill, at least to me. Nothing else in the circuit is precision. Look at the note upper right on the 6G9 schematic. First one says all voltages plus or minus 20%. That is what Fender thought about precision here. Adding this to the trem makes it a lot more complex.

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    Hi,

    I have about ten pairs of various NOS 6V6GT tubes and a good many are way off, as much as 20 plus milliamps when measured in circuit for bias. So one tube will either run really cold or really hot with the other being okay. Independent control of bias to each tube would allow these to be brought to closer operating specs so the tubes wear more evenly.

    I understand Fender schematics show + or - 20% tolerances. This '58 Tremolux shows a really hot bias to start with, even at 110VAC. With that, plate voltage is 360VDC and I see over 40 milliamps on the tubes. All components check out and are within tolerances specified. So, I'd like to have more control over the bias setting for two reasons, one, for bringing the bias in line with more typical operating conditions for the tube type, and two, to be able to use two tubes of the same type and have them wear more evenly.

    Here is Rob Robinete's site for the Bassman bias mods: https://robrobinette.com/5F6A_Modifi...djustable_Bias

    5g9 schematics: http://ampwares.com/schematics/tremolux_5g9.pdf

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    Lifetime Member Enzo's Avatar
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    Assuming tube wear is based upon idle current. My own opinion is wear comes from cranking.

    Easy enough to make bias adjustable for the pair. The common ploy is if you have two tubes and they are 20ma apart, and you want 20ma as a target, set the bias in between, so one runs at 15 and the other at 25. I don't see a simple way to do that while keeping the trem.

    Someone may have a good idea.

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    I also found this posting showing a method to add adjustable bias to the 5g9 tweed Tremolux. How do I go one step further by making each tube independently adjustable for bias?

    Bias Mod | Telecaster Guitar Forum

    Thank you again for your help.

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    Quote Originally Posted by Enzo View Post
    Assuming tube wear is based upon idle current. My own opinion is wear comes from cranking.

    Easy enough to make bias adjustable for the pair. The common ploy is if you have two tubes and they are 20ma apart, and you want 20ma as a target, set the bias in between, so one runs at 15 and the other at 25. I don't see a simple way to do that while keeping the trem.

    Someone may have a good idea.
    Given the tubes have at least 20 ma difference it would be more like 10ma and 30ma. So again one cold and one hot.

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    Lifetime Member Enzo's Avatar
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    Yes, as said, it is simple to make the bias adjustable. The problem is the trem. The non-trem circuits reference the bias to ground then sent to the tubes. The bias voltage is determined by a voltage divider to ground first. The bias voltage is set first, then feeds through the trem pot and hte other end is yanked up and down by the trem circuit. The result is sampled off the pot to the tube grids. To make them individual, we'd need to make each tube grid referenced again to ground through voltage dividers. Not really a garden I want to weed through.

    An alternative is to duplicate the trem control. Instead of one trem control wiper splitting to two grids, we have a dual trem control, and the wiper of each goes to just one grid. On one end the two parts of the pot are at individual bias supplies, the other end of the two pot parts would join and go to the trem yanker. And that is a long row to hoe.

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    Just a thought, and I understand that it may not be for everyone, but if you have 20 6V6's surely you could pair them up in different ways to match the currents more closely than that. The amp doesn't care if you pair an RCA with a Sylvania or Tung-Sol or whatever.

    Andy

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    Supporting Member Jazz P Bass's Avatar
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    The bias reading of the tube is simply that.
    At idle, with x B+ & x bias voltage, the tube draws X.

    Now put the tube in an amp & see what kind of output voltage it can produce.
    That is the measure of the tube.

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    Can I make a 'global' adjustable bias to feed the Depth pot, which I know how to do, then add a resistor/pot to tweak the voltage going to each control grid of the tubes at the tube sockets?

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    Lifetime Member Enzo's Avatar
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    But that is the heart of the problem. You can make those 220k resistors variable, but it won't change the voltage. To change the voltage you have to introduce a voltage divider. Where you going to lever that divider from?

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    Quote Originally Posted by Enzo View Post
    But that is the heart of the problem. You can make those 220k resistors variable, but it won't change the voltage. To change the voltage you have to introduce a voltage divider. Where you going to lever that divider from?
    Could a...


    I'm just thinking out loud. Flawed?

    Peeler, please don't take this idea as anything to try unless others confirm it. I expect if this would work Enzo would have suggested something like it. I'm just asking.

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    Last edited by ric; 06-01-2018 at 10:30 AM. Reason: deleted idea in case it's wrong

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    Lifetime Member Enzo's Avatar
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    Draw the actual circuit you propose, then show how it changes the bias to each tube.

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    Quote Originally Posted by Enzo View Post
    Draw the actual circuit you propose, then show how it changes the bias to each tube.
    Working on a phone I can't post a drawing, sorry. Will try to give brief accurate description.

    That was my idea.

    Would it work? Is it safe? Did I miss completely?

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    Last edited by ric; 06-01-2018 at 10:28 AM. Reason: deleted idea in case it's wrong

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    I you have such a wide spread in your spare tubes, then it's quite likely that the transconductance has a large variation too, probably due to wear. So, you can fix the the bias but they are still unbalanced because of the gain difference. This is a guitar amp so I suggest you chose a pair that are not so far apart and just enjoy the result.

    I promise the world will not come to an end.

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    I have tested all the tubes I have on a Mighty Might and yes, it does not test for transconductance by giving values and is not providing much in the way of deeper tube health information but they all test well in that device. I also understand you can have perfectly fine NOS tubes that are very different in bias reading. I am shooting for 3ma-8ma range for a tube set as I've read it gives great results for guitar amplifier sound. And, I don't have enough of each tube type by the same manufacturer, materials and construction, to match them up as close enough. I do hear a difference in tubes of different types, enough so that I'd like to use two of the same tube.

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    Schematic found online for an adjustable bias pot mod for the 5G9 tweed Tremolux:

    Bias Mod | Telecaster Guitar Forum

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    Lifetime Member Enzo's Avatar
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    Making it adjustable is simple, as we have already discussed. The problem is when you want to make each tube adjustable on its own.

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    I think that grossly mismatched tubes is bad news with a bias vary trem; you're going to get thumping/subsonics. Having said that, if you really want to do this, you could break the cathode to ground connection of the hotter tube of the pair and stick a low voltage zener diode between the cathode and ground. You'll have to experiment with the voltage of the diode till you get the bias of the hotter tube equal to the colder tube. Remember, zeners are used backward compared to regular diodes, so in this case you'll connect the cathode of the output tube to the cathode of the zener and the anode of the zener to ground. Once you have the current matched, you can then use the regular bias adjustment tricks to get the actual current you want. (adjusting the negative bias voltage.)

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    What Bloomfield and nickb said

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by tubeswell View Post
    What Bloomfield and nickb said
    Second that^^^^^^^^^^^^^^

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