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Thread: Head scratcher: 5150 combo bias mod screweyness

  1. #1
    Old Timer Gtr_tech's Avatar
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    Head scratcher: 5150 combo bias mod screweyness

    Had a 5150 2-12 come in for a simple retube and what I thought might happen did.....redplating with the replacement tubes. Given the arrangement they chose, there is no upward (voltage) adjustment. You can go down if need be. I decided to omit the stock setup and pull bias from the HV winding via 100k 1w and just tapped that into the existing circuit (omitted c65 and the 100k runs to cr22...cr23 was clipped out. Now I can get plenty of AC voltage at the diode, around 190v. But no DC where it should be.

    For those not familiar with this circuit, there's nothing to it. After the diode passes negative ac cycles, there's a 220u cap to gnd, series r (was 470r) which was changed to 15k, on to another 200u to gnd and the other divider R (which was changed to 22k fixed and 100k trimmer in series) to gnd.

    http://music-electronics-forum.com/a...2909-5150c.gif

    So ahh....what gives? Its just a variation of what they were doing to begin with and a pretty std circuit. Just for reference, the bias circiot worked as intended before the changes, so the diode is good.
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    Senior Member Enzo's Avatar
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    The original supply drew AC from a center tapped winding with grounded CT. The HV winding is not center tapped and has no direct reference to ground. I suspect that is your issue. You may have a lot of AC at the diode, but scope it on DC, is it offset positive? If so, it won't get through the diode.

    Maybe?
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    Old Timer Gtr_tech's Avatar
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    Hmm...that very well could be. I was inna hurry and didn't look at the secondaries. We'll get back onnit tamarra...i'm done for the nite. Spendin' some quality time with my MAME machine for the remainder of the evening....
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    Old Timer Gtr_tech's Avatar
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    Got curious and looked at the schemo. That is what's happenin' here. Well shit...there goes that idea. Dammit....PV shoulda done it right to begin with...
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    Senior Member Enzo's Avatar
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    Well, current isn't an issue. The stock circuit is a doubler. I imagine you could add a section and make it a tripler.


    Or...or...you could use a very small transformer. Say a little 12v transformer, but wire its secondary across the 6vAC. That ought to spit 60v out its primary. That would rectify and filter to 80 some volts. Work with that. Or hang some sort of tranny across the 35vCT low voltage supply winding and step that up.
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    Old Timer Gtr_tech's Avatar
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    Just a follow up on this. Today I built an outboard tripler on stripboard....3 1N4003s, 3 47u 100v caps. Worked great. Got -85vdc no load. Did exactly what I needed it to do once wired into the existing filter/divider network. Was easy to dial in the idle current I wanted. Thank you PV for making this "so easy". I dunno what they were thinkin', but I rarely install 6L6 tubes these days that will be happy with -51v on the grids. Oh well...next time I get onna these in, I know what I'm in for.
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    Quote Originally Posted by Enzo View Post
    Well, current isn't an issue. The stock circuit is a doubler. I imagine you could add a section and make it a tripler.


    Or...or...you could use a very small transformer. Say a little 12v transformer, but wire its secondary across the 6vAC. That ought to spit 60v out its primary. That would rectify and filter to 80 some volts. Work with that. Or hang some sort of tranny across the 35vCT low voltage supply winding and step that up.
    Enzo, I just bought a 5150 combo and when I opened it up I saw that it has been modded for adjustable bias as is common with this amp. However, I was surprised to see that this had been accomplished with an additional transformer, similar to your suggestion. In this case the primary is connected to the 120V mains and the secondary has a kind of impromptu point to point rectifier circuit (see photo below). Most of the 5150 bias mod info I've found talks about simply replacing R68 - 15K, (I believe in the combo the equivalent is R114 - 47K, per the schematic attached by the OP) with a pot. So I read through this thread and my takeaway is that if you simply swap that resistor with the pot you can only reduce the bias voltage and not increase it and if a given set of tubes is drawing too much current the bias voltage needs to be increased, hence the need for an add-on utility voltage supply like you suggested and the tech in the case of my amp installed. Do I have that right? I'm hoping I've solved the mystery after reading forum posts for a couple of hours and finally stumbling on this one.

    p.s. I should add that the amp was re-tubed with Mesa 6L6's which may or may not have been a factor.

    img_1452.jpg
    Last edited by bobloblaws; 12-03-2017 at 06:44 PM.

  8. #8
    Senior Member Enzo's Avatar
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    Most of the 5150 bias mod info I've found talks about simply replacing R68 - 15K,
    Yes, but that is 5150, not 5150 Combo. The two are different amps.

    Also the original premise here was he wanted to bias the amp colder, which meant raising the bias voltage. Most guys want to heat it up, which is a lot easier to modify for.

    But the combo also has a major difference from a 5150. In the bias supply, the two resistors that might form a voltage divider have the issue. The upper one is only 470 ohms, so making the 47k variable doesn't do much of anything.
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    I've installed a bias pot/ circuit in several of these and never had a problem with not enough voltage for colder bias.

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    Senior Member Enzo's Avatar
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    But the OP has, and that was what we were addressing,
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    I find that hard to believe as there is usually quite a large negative voltage to work with, unless the tubes are drawing a shit ton of current.

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    g1
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    I don't understand why in the OP, the 470R was increased to a 15K.
    Decreasing the 470R value (and increasing the 47K to ground) may have got him the cooler bias he was after.
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    Senior Member Enzo's Avatar
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    I find that hard to believe as there is usually quite a large negative voltage to work with, unless the tubes are drawing a shit ton of current.
    But there isn't. Look at the schematic. From the source of the bias supply, ther is only a 470 ohm resistor. Considering the tiny current involved, there is virtually NO voltage drop across it. So the bias voltage is essentially at max to start with I don't care if a lot of amps have 80 volts or something to work with, look at THIS amp circuit.

    And when you say you installed pots in a number of these, were the "these" the COMBO version? 5150 is different from the 5150 COmbo.

    I don't understand why in the OP, the 470R was increased to a 15K.
    Decreasing the 470R value (and increasing the 47K to ground) may have got him the cooler bias he was after.
    Because you are not looking at it in context. He needed more voltage than he had, so he said he redesigned the source circuit, thus having all the voltage in the world. He then altered the existing bias supply parts to make a reasonable voltage divider. To do that he had to get rid of the 470 ohm and make it big.

    Compare to the original 5150 bias circuit. That has 3.7k on top and only 15k on the bottom. That is only a 4 to 1 voltage divider, which leaves you tons of room to fiddle with values.


    In the existing circuit, it is a 470 ohms over 47k. A 1 to 100 voltage divider. That means the stock output is already 99% of the supply. You can reduce the 470 ohm to zero and all you get is that last 1%. Same with increasing the 47k, all you can get is that last 1%. What is that, about half a volt.

    When you have tons of voltage to work with , then yes, increasing the bottom and reducing the top will raise the voltage.
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    Senior Member nevetslab's Avatar
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    I like the idea of using a small heater transformer in reverse, powering a new bias supply off the heater winding with the new small transformer. Just what I need for my Traynor YBA-1A MK Ii, as I'm about to get back to completing that project.
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    g1
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    Quote Originally Posted by Enzo View Post
    He needed more voltage than he had, so he said he redesigned the source circuit, thus having all the voltage in the world. He then altered the existing bias supply parts to make a reasonable voltage divider. To do that he had to get rid of the 470 ohm and make it big.
    Ok. I just read that he changed the 470 before he had a working alternate source and figured it could only get worse from there. Crunching the voltage divider numbers, there were no gains to be had, as you say.
    Looking now, I see this is a doubler.
    For me, rather than a separate transformer, or changing to a tripler, I would probably remove the CR23 diode, change the C65 value as required, and go to a capacitor coupled bias circuit ala Ampeg V4 or Marshall JCM900.
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    Senior Member Enzo's Avatar
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    I think back when I offered the transformer method just as another alternative. Myself, I'd probably turn it into a tripler.
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    g1
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    No problem with the transformer or the tripler. The only reason I mentioned the cap-coupled is that transformer or tripler require additional components (the OP mentioned adding a board).
    Cap-coupled requires no extra components, actually reduces parts count by 1 (CR23). However, you would have to figure out the best value to replace C65 with. And maybe even change the AC feed to be from the HV bridge instead of from LV bridge.
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    Hmmm, I stand corrected. The combo is different from the head.

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