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Silvertone Widowmaker - Critique My Update

Discussion in 'Amp Input - Normal or Bright' started by GunMonkeyINTL, Jan 31, 2016.

  1. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

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    I got this Silvertone 1448 Guitar/Case/Amp combo for a good price at the pawn shop. My wife wanted it as an antique-decoration-thing for the library. Who was I to argue about the wife wanting me to buy another guitar and amp?

    My first internet searching, just to find out what it was worth and what to offer, pointed out the well-known safety issues with it. It was designed with a tube-set used in old budget radios, with a voltage that roughly added up to 120v, and did not have a power transformer. It also had a non-polarized two-prong plug, so, depending on how you plugged it in, the chassis/ground was either direct wired to hot or to neutral.

    I tested it when we first got it home, and, after rearranging the tubes to their proper slots (and replacing the 12v preamp tube that burned out, I'm guessing, from being in the 35v rectifier tube slot while powered on), and cleaning the volume pot, it worked.

    I'm guessing most folks either put up with the safety issue and left them alone, or followed the common advice to take them to an experienced amp tech, so the update schematic wasn't available anywhere. I have very little experience with electricity beyond some wiring in my house/machinery and replacing a couple broken components with exact matches on other electronics in the past, but I really wanted to use this as a toe-in-the-water for working on amps, and figure out how to do it myself.

    There was plenty of mention on the interwebs that it needs an isolation transformer and 3-prong plug added, but I couldn't find a schematic for performing the update that matched the schematic of this specific amp (or any of the many models that exactly match it). There was also mention that this sort of amp would have a "death cap", but none of the schematics that pointed out where a "death cap" could be found matched what I saw on this amp's schematic.

    I didn't want to start with a "show me how to fix this"-type thread anywhere, since most of those I see end up with "if you have to ask, you don't need to know"-type responses. Also, I figured I'd learn more if I had to do some leg-work and figure out how to adapt the tribal knowledge to my specific application.

    I figured out what I could regarding sizing and selecting the isolation transformer, and got one ordered. When I got it, I didn't like any of the mounting possibilities inside the case, so I decided to mount it remotely in a power cord brick. I gathered that having the primary of a transformer before the on/off switch was poor form, because it would continue to draw power when plugged in and 'off'. So, I put a switch on the power brick, and added another fuse to get it on the hot line as early in the circuit as possible.

    I also gained an understanding of the problem with a "death cap", why they were used, and why they could be removed once an amp was properly grounded. This amp did not have a ground switch (you just had to reverse the plug if it was buzzy, I guess), but I did find one place (C3) where the only thing between hot line and ground was a capacitor (0.05uf / 600v). After I had everything wired up and proven still-working, I clipped out C3, and it still worked, so I think it was the correct one and no longer served a purpose with the proper grounding installed.

    I've got about an hour play-time on it, and have had it plugged in, but powered off, for as long as 6 hours. The transformer is not any warmer than room temperature, and the amp sounds surprisingly good for its size and speaker. The 50c5 power tube is uncomfortably hot to the touch, but I gather this is normal for this tube/circuit.

    I'd appreciate it if someone who actually understands amps/electronics would compare the before/after schematics and tell me if they see anything I did wrong, or still need to do. I think it's right, but I'd like someone with experience to critique it.

    The first picture is the original schematic. The second is my revised schematic. The third is the actual wiring. I took it before I removed C3 (the one directly above the red tube electrolytic), but with the new filter cap in place.

    Thanks in advance.
    silvertone_1448_sch_pdf_1.jpg

    ST-1448-Corrected.jpg

    IMG_1441.JPG

    IMG_1446.JPG

    IMG_1447.JPG
     
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  2. Stevem

    Stevem Senior Stratmaster

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    You have it wired right!
    That amp is what's called a class A type output stage , and that means that that 50C5 tube is running near full bore all the time and hence all that heat!

    That being said you might want to check that tubes cathode resistor ( R7) and make sure that it does not check in at less than 150 ohms, if it does get a new 5 watt 150 ohm 1% tolerance resistor and replace it.
     
  3. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

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    Thanks.
    Once I had it running and known-reliable, my next step was going to be working out how to set the bias and tame it down a little.

    It's plenty loud enough for what I expect it to do, and I'm OK with giving up a little power for long-term reliability.

    I ordered a full set of NOS tubes right away, and, thankfully, the 12au6 that was blown was one of the cheap ones. It and the 35w4 were less than $5 each, but the 50c5 was almost $25.

    I've now got a spare, and the original in it has been going strong for 53 years, but, with the little I've learned so far, I believe biasing it a little cooler would extend the life.

    I think (think) the fixed bias is adjusted by using different cathode resistors. I've got some more reading to do, but if that understanding is correct, I may put a pot in place of that fixed resistor.

    I will check the original component tonight, though, and make sure it's in spec, before I move on. Thanks for the tip.
     
  4. pstama2

    pstama2 New Member!

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    Hi there,

    I've been reading this post with great interest as I recently picked up a 1448 Silvertone gtr/amp case.

    I bought a one of these setups many years ago, but the amp wasn't included in the case. I purchased this one so I'd have a schematic and working version to help me with my task of building a 2nd amp from scratch. I'll be adding the isolation Xfmr since one of these setups will be played by my daugher. So, your post has been extreemly helpful.

    I'm curious, what did you learn from your experiments with the tube bias?
     
  5. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

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    Wow, that's a blast from the past.

    Honestly, I never did adjust the bias in that amp. It's in my office, and gets 30 min or so of play time 3-4 times per month. I like the way it sounds with a drive, fuzz and/or reverb pedal in it, and it never did blow the power tube, so I've just left it alone.

    I did build another amp from scratch, in the meantime, and I found this bias calculator to be really valuable. You can run the equations manually, but this calculator can let you look at different hypothetical values pretty quickly:

    http://www.tedweber.com/webervst/tubes1/calcbias.htm
     
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  6. pstama2

    pstama2 New Member!

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    Haha, yeah...I'm surprised that you saw the message on this since it's an older post. Thanks for responding.

    I appreciate the bias calculator! Very handy.

    When I unpacked the 1448 and tested it, found out the speaker was shot.
    I used some alligator test clips to temporarily hook up an 8", 4 ohm Eminence speaker I had laying around. The amp works!!

    It sounds pretty sweet—even though it sounds like the speaker is naked and needs a cabinet. ; )

    Also, all 3 tubes are original Silvertone tubes—still kicking. I'm thinking about replacing them and storing them with the original speaker because I want to really use this thing for recording. It has such a unique tone compared to my other Fender amps.

    I'll order a new 6" speaker and try to figure out how to move that transformer that is riveted to it. I almost want to leave it intact and find a new transformer for that new speaker. I don't know how to spec it though. Any suggestions on how to determine what the primary to secondary ratio is for that output transformer?
     
  7. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

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    Great job! again.
    I told you it would not be the last one.
    Those sweet little amps form the past when life was hard but more simple in a way.

    All the best,
    Robin
     
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  8. Triple Jim

    Triple Jim Senior Stratmaster

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    More simple in many ways!
     
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  9. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

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    Thanks, but I actually did this repair before the scratch-build you helped me through. This is an older thread.
     
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  10. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

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    If you unhook it and apply a known ac voltage to the primary, you can then measure the voltage at the secondary. I think that’s how you would do it, but I will defer to @Robins and @Triple Jim
     
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  11. Triple Jim

    Triple Jim Senior Stratmaster

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    If you do that you'll get the turns ratio. Square that and you'll have the impedance ratio. Since it's a 4 ohm speaker, you can then multiply the numerator of the ratio by 4 to get the primary impedance, and you may be able to find a transformer to match that.

    Example: 10 vac into the output tube side of the original output transformer gives 0.20 volt on the speaker side. 10/.20= 50, so that's a 50:1 turns ratio. Square that and you get a 2,500:1 impedance ratio. Multiply 2,500 x 4 ohms (original speaker) and you get a 10,000 ohm primary. You know the speaker side is 4 ohms, so you shop for an output transformer with a 10k ohm primary and a 4 ohm secondary. Or an 8 ohm secondary if you want to use an 8 ohm speaker.
     
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  12. pstama2

    pstama2 New Member!

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    Fantastic! Thank you both so much for the assistance. This is extremely helpful info and I greatly appreciate it!
     
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  13. pstama2

    pstama2 New Member!

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    Would I need something like a Variable AC transformer (Variac) to supply the 10 vac to the transformer primary?
     
  14. Triple Jim

    Triple Jim Senior Stratmaster

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    Or a small wall cube that puts out AC as many do. I don't mean this in a bad way, but your asking that makes me think you might be better off getting help from someone with experience.
     
  15. pstama2

    pstama2 New Member!

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    Thanks. Yes, it's true, I am new to guitar amps and power supplies. But, I've built a ribbon mic and many tube mics that work and I am trying to gain experience in this area—hence the simple amp for a starter project. The only way for me to gain experience is for me to reach beyond what I already know.

    I could have posted the schematic and merely asked for somebody to tell where to by a replacement transformer and provide a link to it. Instead, I'm trying to understand how this works.

    I ask this last question sans attitude or snarkiness—Is this a place for seasoned pros only?
    If newbies should NOT post questions here, then, I truly apologize. I'll find another way/place to learn.
     
  16. pstama2

    pstama2 New Member!

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    I supplied 27.6 VAC to the primary and it measured 1.125 V on the secondary. Based on the formulas you provided:

    Turns Ratio is 25:1
    Impedance Ratio is 602:1
    Multiply Numerator, 602, by 4 for the 4-ohm speaker = 2,408 : 4
    so I'm looking for an output transformer a ~2.5k ohm on the primary and 4 ohm on the secondary.

    The amp has a 3-watt output.
    So, would this suffice?
    P-T125ASE 3 W 25 mA
    https://www.tubesandmore.com/products/transformer-hammond-universal-single-ended
     
  17. Triple Jim

    Triple Jim Senior Stratmaster

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    It looks OK. I assume the bias current going through it will be well under its rated 25 mA. You may find a better price if you shop around. For example, you could connect to the 5k ohm tap of this one, and put your 4 ohm speaker on its 8 ohm tap. That would make the output tube see 2,500 ohms. It's $30.

    http://www.classictone.net/40-18030.pdf

    The speaker you plan to use has a 4 ohm impedance, correct?
     
    Last edited: Aug 20, 2019 at 10:50 AM
  18. pstama2

    pstama2 New Member!

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    Thanks for checking it over and getting back to me. I appreciate it.

    Yes, the speaker has a 4 ohm impedance.

    Wow, ok. I didn't realize the impedance ratio worked like that. I see that 5K:8 is the same ratio as 2.5K:4
    But, I thought the output tube would see 5,000 ohms on this transformer?

    So, by connecting the speaker with the 4 ohm load to an 8 ohm tap, that actually "converts" (for lack of a better word) the 5K tap to 2.5k?
     
  19. pstama2

    pstama2 New Member!

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    I was in the process of checking the max plate current from a single 50c5 tube. It shows 50 mA. The transformer you suggested shows 5w and 50 mA, so, that will work or I'd have to go with the 60 mA version from the one I found, since the 25 mA is not gonna cut it.
     
  20. Triple Jim

    Triple Jim Senior Stratmaster

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    Edited: I forgot we're talking about class A. Yes, it looks like that tube can run up to 50 mA plate current. Check what it's actually running at with the stock transformer and you'll know what the new one needs to be able to handle.
     
    Last edited: Aug 20, 2019 at 10:06 PM
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