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Thread: bias pot in a 5E3

  1. #1
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    bias pot in a 5E3

    Hello everyone, I would like to put a bias pot in my homebrew tweed deluxe. I have done the complete 6L6 mod. I run a 5u4 2x 6L6 12ax7 and 12au7.
    I am not 100% sure where to put the bias pot or which size of pot I should use. I have seen a bias pot put in the bigger tweeds but it seems they have a slightly different set up compared to the deluxe. My amp ain't really a deluxe any more because of all of my mods but the bias part of the layout is still deluxe. please advise with detailed directions. Thanks again for everyones help last year with building this amp. jamie

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    Senior Member TD_Madden's Avatar
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    what was your old account name......all I see is two posts from you, including this one....?

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    Supporting Member loudthud's Avatar
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    The traditional 5E3 has cathode bias while most of the bigger tweed amps using 6L6's have fixed bias. Installing a bias pot usually means converting your amp to fixed bias. It is possible to make cathode bias "adjustable" but this is almost never done. You would need a 5W rheostat or a 15W adjustable resistor. Not impossible, but we would have to find the right part.

    An easier way is to install a slightly larger than required resistor in the amp, maybe a 330 ohm 10W and then add one or more 2W resistors in parallel until the tubes are idleing where we want them.

    The 250 ohm 5W resistor at the end of the board nearest the pilot light is the cathode resistor for the output stage. Most people find it necessary to increase this resistor to 10W when using 6L6's. Have you done this?

    Have the transformers on you amp been upgraded to larger ones? Are there any unused wires coming from the power transformer (a bias tap, usually red with blue stripe)? Is there room to drill a 3/8 inch hole between the power and output transformers and mount a 1 inch diameter pot?

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    jamie was my user name on the old forum, which I find alot easier than this one.
    thanks for asking.

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    Hello, I am running the Merc Mag Tweed deluxe power trans and the Merc Mag tweed super output tranny. I am thinking about getting the double deluxe power tranny from Merc Mag but haven't done it yet. Yes I have the 10w 250ohm resistor. My power tranny doesn't have the red and blue wire. It has the red and yellow stripe which I have going to ground. Also I do have room for the bias pot. I actually have three full size bias pots from 1962 that I've had for a long time but I haven't checked them to see what ohms they are. Thanks again jamie

  6. #6
    Senior Member hasserl's Avatar
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    Jamie, there's really no reason to add a bias pot unless you're going to add fixed bias. The amp is more or less self biasing as it is.

  7. #7
    Supporting Member loudthud's Avatar
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    I looked through some parts suppliers websites and catalogs. A new rheostat is going to cost you something like $20.00 or more. Even a tubular adjustable resistor is $9 or $10 USD. More expensive than I thought. If you can't find the part you need at a surplus shop, I'd go the parallel resistor route. Change that 250 ohm to something like a 330 ohm and get some 2W 2K resistors and add them across the 330 one at a time. All the normal biasing rules apply except you have to power the amp down, solder a resistor and then warm up the amp again and measure voltage and current.

    Creating a bias supply without the bias tap on the transformer is sort of a pain. It takes a diode, a 1 or 2W resistor, one or two electrolytic caps, 1 or 2 half W resistors and the pot. Search for a schematic of a Marshall 1987 for a schematic. Try to figure out where you are going to put the parts.

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    Thanks guys for all the help. I plan on putting in a bias pot one way or the other. I am what you would call a tone freak and I am always changing power and preamp tubes. Like I said before I am running NOS Tungsol 5881's which I love but know that I will have to replace eventually. I use Webers BIAS RITE meter which checks voltage and milliamps and is one of the best things nonmusic wise that I have purchased in the last 3 or 4 years or so. Also biasing power tubes for the amp to run affectively and to adjust the "sweet spot" ie.. (running on the cold side or hot side) is vital for a guy like me who gigs on a regular basis and needing the amp to perform at its full capacity. Thanks again for everyone's input, it is much appreciated. jamie

  9. #9
    Old Timer
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    jaime,like you I like to tweak my cathode biased amps to get that "sweet spot".I had a bunch of Ohmite 500ohm 10watt adjustable wirewounds.I got them from a supplier here in NYC,but he went out of business a few years ago.I also have a 25w 500ohm rheostat I use to find the bias point I like when I change tubes,measure the resistance and then install a matching resistor.I got the rheostat from Ted Weber.Check his site,it was only a couple of bucks.It is too large to install in most chassis,but is good to use as I described.

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    Hello, I just put in a 10w 500ohm bias resistor in and it brought it down from 50ma to 35ma which is about where I want it and it sounds good. Eventually I will put in the bias pot and what ever else goes along with it but for know I have a gig tomorrow night so I had to put in the resistor to get me going. Thanks again for everyones help so far. You guys are the best.

    jamie

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    "Hello, I just put in a 10w 500ohm bias resistor in and it brought it down from 50ma to 35ma which is about where I want it and it sounds good. "

    Well if it sounds good that's the acid test, however 50mA would be the more usual kind of current for this kind of amp. What's your plate voltage?

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    hello, my plate voltage is about 368 - 370. thanks again, jamie

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    If you like the sound then by all means stick with it, but at those kinds of voltages 250ohms would be the more usual value, 50mA = 18.5W plate dissipation. at the moment you're only dissipating about 13W. Which is fine if you're after a less grindy tone, but you won't be damaging the amp when running at 50mA, if that was a concern.

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    Yes, I do like the less grind that I get right now so I'll keep it the way it is until I change my mind AGAIN. thanks for all the replies. jamie

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    Senior Member Mandopicker's Avatar
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    I installed an adjustable bias on my 5E3 w/ 6L6's

    I was able to install an adjustable bias in line with my cathode resistor. It was simple and is very effective. The amp still retains the same tone and feel as running a resistor/cathode bias cap only, in my opinion.

    I used 200 ohm bias pot (Same as I used for adjustable bias on BF/SF Champs, BF/SF Princetons) with the resistor wired directly to one wiper, then grounding the remaining wipers to the pot with a 1 ohm, 1 or 2 watt resistor. From here I run a wire from the ground side of the resistor to the "-" bias probe point and another wire from the hot side of the resistor to the "+" bias probe point.

    The photo shows 470 ohm resistor in line with the 200 ohm pot. Total resistance range in photo is 470 ohm + 200 ohm pot =670 ohm.) I believe that it now has a 330 or 250 ohm 5 watt + 200ohm pot resistor installed now.

    Tha amp also now has a P12Q.

    The big switch is a pentode/triode switch which adds to the amps versitility.

    I really enjoy playing it.

    Hopefully this was useful/interesting.

    Mandopicker

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    Member tele-tubby's Avatar
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    cool. what tubes do you have in there?

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    Senior Member Mandopicker's Avatar
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    I have Tung Sol 6L6's EH12ax7 EH12ay7.

    I decided to make a more powerful deluxe. It also runs 6V6, EL84, and a numer of other tubes. Just adjust bias and go.

    There is also a triode/pentode switch to cut the power down for a more brown tone at a lower volumes.

    And...topped it off with a P12 Jenson Alnico (not pictured)and it really is versitile and sounds amazing.
    Mandopicker

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