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Thread: Sound City 120 Bias Doubler

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    Sound City 120 Bias Doubler

    Hello

    I am working on a Sound City 120 that could use the bias voltage doubler mod done to it. Unfortunately, all of the links that I have found that show photos of the installation have had their links go south as most of them are old. I have found a couple of schematics that show the parts but I'll need some help in performing the physical construction and installation of the mod.

    Can anyone help? Thanks In Advance

    Eric

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    Supporting Member Jazz P Bass's Avatar
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    Can you post what you have?

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    What I have for info is within this old post from this very forum

    http://music-electronics-forum.com/t37353/

    I guess what I am looking for is how to implement what is described in the post. I understand much better from photos. If someone has pix of their previous implementation it would help greatly.

    I can also provide photos of the amp I have if it helps.

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    Schematic

    Click image for larger version. 

Name:	sc_120_1.jpg 
Views:	312 
Size:	155.2 KB 
ID:	37027

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    Try googling it, a schematic or wiring diagram might pop up.
    There's one in the old Aspen Pittman tube amp book from before the internet

    The SC amp I had didn't need it, but it's not hard to do.
    I've had to do it for some Hiwatts.

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    I posted the schematic that I have in a previous entry.

    This is going to be my approach - I am going to try to make and install the voltage doubler, and I will post photos of my progress along with my questions. I will do this with the hope that people who have done this mod before (or at least are famliar with it) will chime in with any corrections and comments.

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    If you have old links you could try 'Wayback Machine' to search for them. It can sometimes turn up long-dead material.

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    Senior Member mhuss's Avatar
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    The DR103 doubler should work fine.
    http://hiwatt.org/Schematics/BiasCkts2.gif

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    Mick Bailey- thanks for the Wayback Machine idea. Not sure how it will work on old photo bucket links but it's worth a shot.

    MHuss - thanks! I will be putting that assembly together on some perfboard and will post the results.

    Once complete I may need advice on mounting and insertion into the circuit.

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    Senior Member mhuss's Avatar
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    Mind the bias winding connection. With the doubler (unlike the stock circuit) neither end is connected to ground.

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    I am working on a SC 50 plus. It has a similar anemic bias winding. I have been looking at some other work by a Lickliter amps called a Tage Mod on a SC 50 plus. It appears that he has the voltage doubler, plus a typical Marshall bias circuit. Anybody know how that would interact with the doubler like the one for the dr103 one above? Here is link
    http://www.rig-talk.com/forum/downlo...4282&mode=view

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by vmazz View Post
    I am working on a SC 50 plus. It has a similar anemic bias winding.
    I could not find any relevant info in the links you provided. I'm curious though. The schematic shows -36V for 420V on the plates. Those seem like reasonable numbers. Exactly what plate current and voltage do you get?

    If you just need a few more volts, which I suspect is the case, then you could disconnect the ground side of the bias winding and connect it to the heater supply. That will give you about 5V extra. Note you have to pick the correct phase of the heater. Pick the wrong one and you get 5V less instead. Best check that C25 has a high enough voltage rating before you do this.

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    Last edited by nickb; 03-10-2018 at 10:58 PM.
    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by nickb View Post
    I could not find any relevant info in the links you provided. I'm curious though. The schematic shows -36V for 420V on the plates. Those seem like reasonable numbers. Exactly what plate current and voltage do you get?

    If you just need a few more volts, which I suspect is the case, then you could disconnect the ground side of the bias winding and connect it to the heater supply. That will give you about 5V extra. Note you have to pick the correct phase of the heater. Pick the wrong one and you get 5V less instead. Best check that C25 has a high enough voltage rating before you do this.
    Interesting. Never heard that solution. I havent set the rework of this amp. It is a complete rebuild into a more super lead type circuit. What I Have heard typically with Sound City amps is the bias doesnt give enough range to bias safely for modern EL34s. So the doubler is used to get more negative voltage. In the link there is a picture of the rework Lickliter did and it appears on the right he used some kind of doubler combined with a typical marshall bias circuit, although there are 2 trimpots for either tube. That is what I was wondering about. Thanks.

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    Old Timer Leo_Gnardo's Avatar
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    Couple years ago I encountered an unusually powerful Sound City, I think it was claimed to be 200W. Individual bias pots for each output tube, and iirc they were KT88's. Bias was derived from a separate winding, pair of skinny brown wires, and the voltage was hopelessly inadequate. I went voltage doubler, adjusted the supply for a useful range by changing filter cap & a resistor or 2, then everything went well after that. So - as long as your bias winding is separate from the other PT secondaries, I can give that solution my blessing.

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    Quote Originally Posted by vmazz View Post
    Your link is not working, so I don't think anyone can see what the circuit looks like.

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    Ok Try this one. Look at the completed mod pic of able to. Rig-Talk ? View topic - Lickliter "Rage mod" Sound City 50+ re-work!

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    Well there's no schematic so not much to go on unless someone wants to draw it out.
    I only see 2 trimpots total, so I would guess one is bias and the other is balance.

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    Quote Originally Posted by g1 View Post
    Well there's no schematic so not much to go on unless someone wants to draw it out.
    It looks like it's the (Two controls) DR103 Doubler circuit in post #9 minus the 220k resistors but with 10u caps added from pot wipers to ground.

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    Last edited by Dave H; 03-11-2018 at 11:12 AM.

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    That is what I was seeing. I was just wondering if anyone would know of the advantage of doing it this way as opposed to the exact dr103 doubler circuit. It looks like the doubler attaches to a stock Marshall bias circuit of some sort. Of course with dual bias pots.

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    Mhuss quick question. Is there an error in the dr103 two Control schematic? There is a 25kwritten after the diode junction but no resistor symbol. Is that omitted?

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    Quote Originally Posted by vmazz View Post
    Is there an error in the dr103 two Control schematic? There is a 25kwritten after the diode junction but no resistor symbol. Is that omitted?
    It looks OK to me. I'd delete the 25k. I don't think anything is missing.

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    don't forget the joker g1's Avatar
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    I don't think there is any 25K missing there either. Only pots are 25K there and none are missing. I think it was just a copy & paste error.

    Quote Originally Posted by vmazz View Post
    I was just wondering if anyone would know of the advantage of doing it this way as opposed to the exact dr103 doubler circuit.
    To me, the circuit without the 220K 'safety' resistors is inferior. Without the resistors, any poor connection between the wiper and the carbon track of the pot will result in loss of bias and tube meltdown.

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