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Thread: Beginner, want to build a TAD 5F1 kit

  1. #36
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    Did you take the resistance measurement immediately after soldering? Maybe there's a temporary increase from the soldering heat?

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    Quote Originally Posted by JoeM View Post
    Did you take the resistance measurement immediately after soldering? Maybe there's a temporary increase from the soldering heat?
    No, just took another one now (at least a couple of hours later) and it's still 45k. It seems permanent and irreversible.

    I guess if they test their resistors there and find there's truth to my claim they will send out replacements, or I could just order new ones, they're not all that expensive and a Champ kit doesn't use so many of them. In your experience, is this something that's likely to happen again with new ones? Maybe I should switch to carbon film, or metal film resistors?

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  3. #38
    Senior Member JHow's Avatar
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    You asked for "thoughts", s here goes:

    My initial thought was, does the resistance go back to spec when it cools down? Also, are these carbon composition resistors? Maybe NOS? You could try to use a clip-on heat sink if you are worried about solder heat damaging a part, but resistors are pretty robust, compared to ICs or transistors.

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  4. #39
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    Quote Originally Posted by motzu View Post
    No, just took another one now (at least a couple of hours later) and it's still 45k. It seems permanent and irreversible.
    Oops. Sorry, you posted while I was posting, I guess. I see they are CC. If the value has drifted that much, you probably should go with a new resistor. CF or MF will work fine.

    If this is this the 1/2 watt Neg. feedback resistor we are talking about, the value isn't overly critical to function, so the amp would probably work with that part out of spec, but if this is the larger 22K in the power supply, then you will definitely want something close to spec.

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  5. #40
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    Quote Originally Posted by JHow View Post
    Oops. Sorry, you posted while I was posting, I guess. I see they are CC. If the value has drifted that much, you probably should go with a new resistor. CF or MF will work fine.

    If this is this the 1/2 watt Neg. feedback resistor we are talking about, the value isn't overly critical to function, so the amp would probably work with that part out of spec, but if this is the larger 22K in the power supply, then you will definitely want something close to spec.
    Yes, it's an 1/2W resistor. It'll probably work like that too, but I think I'll stay safe and get new ones for all the resistors that have drifted further than 10% off spec.
    Thanks!

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  6. #41
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    I haven't used CCs for a real long time. I'd just use CF for general purpose use.

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  7. #42
    Senior Member JHow's Avatar
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    In case you haven't seen it yet, I found this article interesting on the subject of carbon comp resistors and their "mojo" factor:

    http://www.geofex.com/article_folder...carboncomp.htm

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    Last edited by JHow; 06-04-2010 at 06:35 PM. Reason: typo

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    Quote Originally Posted by motzu View Post
    OK, the kit is here, I've drilled all the relevant holes in the cabinet, chassis and eyelet board, populated the eyelet board and started soldering some components.

    No problems so far understading the schematic or layout, but I do have an issue that I didn't really take into account at all, what with all the making sure I won't get electrocuted and trying to figure out how vacuum tubes work. That is, I'm baffled by how the carbon composition resistors are behaving.

    To the point: I've had this bright idea of measuring all the resistors with my digital multimeter, just to make sure that if I need to troubleshoot later I'll know that it wasn't the resistors. So I look at the schematic, the resistor is 22kOhm, I measure it - it's really 23kOhm, within tolerance so OK - and I solder just one leg to the eyelet. I measure again, and now it's 45kOhm.

    By now very careful, I solder only one leg of a 68k resistor on the schematic, in reality 70k. It becomes 85k after this.

    Now, I will openly admit all of my shortcomings as a tube amp apprentice, but soldering is not one of them. I didn't take longer than 2 seconds in contact with the joint, and I've used the 40W Weller that's previously been approved by the nice people of the forum.

    Is this something to be expected of carbon comp resistors? 10%, or within tolerance, is OK, but from 23k to 45k I do believe there's a problem with the resistors.

    Your thoughts on the subject? Thanks!
    You have to measure resistor out of circuit for a true reading.

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  9. #44
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    Quote Originally Posted by bigsmitty View Post
    You have to measure resistor out of circuit for a true reading.
    You're right, and I did. The first thing I did was to pull out the resistor and measure it by itself. It's 45k.

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    Quote Originally Posted by motzu View Post
    You're right, and I did. The first thing I did was to pull out the resistor and measure it by itself. It's 45k.
    Cool! just making sure, wasn't clear from your post. I guess you need a new resistor then. unless its the NFB you might like it that way.

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    Quote Originally Posted by motzu View Post
    You're right, and I did. The first thing I did was to pull out the resistor and measure it by itself. It's 45k.
    Just making sure you knew.

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    Quote Originally Posted by bigsmitty View Post
    Just making sure you knew.
    No problem, thank you for posting!

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    Quote Originally Posted by JHow View Post
    In case you haven't seen it yet, I found this article interesting on the subject of carbon comp resistors and their "mojo" factor:

    http://www.geofex.com/article_folder...carboncomp.htm
    Yep, I did, but being at the stage that I'm at, all I got from it was that if you get carbon comp resistors with your tube amp kit, use them, but they're worthless in pedals .

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  14. #49
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    Quote Originally Posted by motzu View Post
    ...if you get carbon comp resistors with your tube amp kit, use them...
    Yes. He suggested they might be of most use in place like your plate resistors (the two 100KR 1/2W resistors in your 5F1) where there is high voltage and large signal swings, and to avoid them in places like your cathodes (1500R 1/2W in your circuit), grid and input circuits.

    From a practical standpoint, use whatever you have, as long as it is the right power and value. Many ancient CC resistors are out there in the world, doing their job every day.

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    Thanks!

    In the meanwhile, I've started wiring the rectifier tube socket to the input transformer. Here's a TAD picture of how it's supposed to look:



    What I'm not clear on is this: the layout says I should wire the two yellow wires to pins 2 and 8, and the two red wires to pins 4 and 6. But they don't say which of the pair goes where. I'm assuming that it doesn't matter? Any of the yellow wires can go to either pin 2 or 8, and any of the red wires to 4 or 6?

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  16. #51
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    Nevermind, I figured out that that's AC, so it doesn't matter if the yellow or red wires get "reversed". Sorry for the dumb question.

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    Quote Originally Posted by motzu View Post
    ...Sorry for the dumb question.
    He he, if you're nervous now, wait until the here-goes-nothing moment. If you haven't triple checked everything it can be some what dishearting.

    I see that you've got some fingers pointed at you in this thread. You'll probably be zapped at some point. Just make sure it only one finger tasting the Bzzzzt. Always measure with one hand on your back.

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    In this forum everyone is entitled to my opinion.

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    Well, I've built it and it works. Thankfully no debugging was necessary. I've measured the DC voltages inside and they're about 15-20% less than what the schematic says they would be, but the TAD schematic says that a +-10% difference is fine, and the original Fender schematic allows for +-20% so that should be OK.

    Thank you to everyone who answered my questions!

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