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Thread: Adding a bit more treble to Silvertone amp

  1. #1
    Senior Member SpareRibs's Avatar
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    Adding a bit more treble to Silvertone amp

    Hello,
    Are there any of the capacitors that can be changed to add a bit more treble to this amp?
    I removed the guts, turned them upside down and, added an 12" speaker. It is fully functional but is a bit dark sounding.
    I googled the Silvertone page and it said it had a dark and creamy sound.
    I was wondering if there are any slight changes that could brighten it up a bit. As I looked through it has been recently recapped.
    Thanks in advance for any help.
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    Last edited by SpareRibs; 11-28-2017 at 06:06 AM.

  2. #2
    Supporting Member The Dude's Avatar
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    Maybe try adding a "bright cap" across the volume controls? Input side to wiper about 100pf. Tweak to taste.
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  3. #3
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    +1, a bright cap might be a good starting point and is easy. You could also reduce the mixing resistors (r11, r12) to 470K or even 220K. The series resistance of the volume pots + the mixing resistors causes a drastic rolloff of the highs due to the Miller capacitance of the following 6SN7. You could also stick a small cap in parallel with one mixing resistor to give yourself a "bright" channel, jumper the two channels together and blend to taste, if that's not overly "Marshally."

  4. #4
    Better Tone thru Mathematics bob p's Avatar
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    I've always thought my Silvertone amps were too dark sounding. I never really bonded with them because of it. As a result they've been sitting on a shelf for a few years. A week or two ago I decided to pull them out and dust them off but I haven't powered them up yet. I'm in a slow motion effort to bring them off of the back burner.

    On the subject of the bright cap -- it's worth a try, but I'm not sure that the bright cap will be enough, considering that the low pass filter is just going to roll everything away. I think octal makes a good point about R11 and R12. If I were doing this I would try using alligator clips to strap a resistor/jumper wire combination across R11 (in parallel to lower the net resistance of R11) to see what kind of grid value gives a decent result. You could perform multiple resistor swaps that way to assess the changes in the amp before you have to do any soldering. In all likelihood you might end up adding a bright cap and changing the value of C3 or R11.

    It might also be worth looking at the value of C5 in the tone circuit to tune that control into a more useful range.

    Wow, those 6SC7 used huge grid bias resistors. 15M!
    "Stand back, I'm holding a calculator." - chinrest

  5. #5
    Senior Member SpareRibs's Avatar
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    Quote Originally Posted by bob p View Post
    It might also be worth looking at the value of C5 in the tone circuit to tune that control into a more useful range.
    Wow, those 6SC7 used huge grid bias resistors. 15M!
    Hello,
    Where would I start, finding a more useful range? .00068 seems very small. It seems it would be letting any treble through that was available.

  6. #6
    Old Timer J M Fahey's Avatar
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    *Start* by doing what the dude suggests in post #2

    And of course set tone control to the "treble" end, no sense in getting a little extra brightness and then pouring it down the drain.
    Unfortunately in the old days "dark/creamy/bassy" was considered "good" ; only revolutionary who did the opposite was Leo Fender who gradually searched for a brighter and clearer sound and was very successful at that.

    But most others? ... including arch-rival Gibson? ... mud city.

    Personally if I wanted to go further, I´d rebuild the preamp as some Tweed Fender one, where tone control can actually cut (like this one) and boost highs (in a passive way of course).

    As is, notice those 6SC7 currently have meager 75V on plates and huge plate resistors, they are being operated in "starved" mode which gives them *a little* more gain but murders highs.

    Download the 6SC7 datasheet http://www.r-type.org/pdfs/6sc7.pdf , you have 240V available for it, so use 180V suggested values.
    Why?
    Because voltage can only go down, not up, and given the slightly larger tube current, those 240V will drop somewhat extra.

    It´s described as a "high Mu dual triode", so in the 12AX7 "family" with the only weirdness that both cathodes are internally joined so you can´t do EQ tricks with them (brightening cathode cap), just stick what the datasheet requires from pin 6 to ground (1800 to 2200 ohms) , bypass it with, say, 25uF, and use 220k platev resistors.
    Oh, and replace those 15M grid leak bias resistors with standard 1M .

    So 5 resistors and 1 cap will mimic what a Fender gain stage does; that plus modding tone control to a Fendery one should give you as close as possible sound with minimum investment, I mean time and effort rather than $$$ which are nil. .

    The power amp is fine.
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    Juan Manuel Fahey

  7. #7
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    I was wondering about those crazy low preamp voltages... then again, I see the same in a lot of EF86 preamp designs, too - below 100Va.

    Justin
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    "Are you practicing in the lobby of the municipal library? It's still a guitar amp and it SHOULD make some noise (!!!)" - Chuck H. -
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  8. #8
    Supporting Member mozz's Avatar
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    You crank those plate voltages up too high or try to get to much gain, 6Sc7 will be microphonic.
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  9. #9
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    That treble control is a cut only type, whereas many guitar tones need some treble boost (additional to that which normally occurs with the interaction between open loop power amps and the inductive load presented by speakers).
    As noted, there will already be plenty of treble cut due to miller capacitance and those 1M mixers.
    I suggest to change over to the 6G3 vol/tone/mixer arrangement and tweak from there http://schems.com/schematicheaven.ne..._6g3_schem.pdf
    That may mean only one channel has a tone control, but if that's a problem, then a dual pot could be used.
    The 6G3 arrangement allows for treble cut and boost (at volume settings below max), and the 220k mixers mean that the miller capacitance treble cut will be reduced.

  10. #10
    Old Timer J M Fahey's Avatar
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    That´s about it, and a good portion can be copied verbatim:
    * shared cathode resistor (you are forced to do it anyway so .... ) 1k8 is perfect.
    * 220k plate resistors ... check.
    Just lose that .001 treble killing cap in parallel withy 220k on Normal channel, not what we want here.
    * use this tone control on just the treble channel ... no big deal, better to have at least one good channel than two so-so ones.
    Leave the darker one to your friend playing with an 1954 Precision and flatwound strings or reserve it for your buzzy pedals, it will definitely be more "pedal friendly" .... or plug your digital multieffect simulator there, *it* will need a "warm" channel for certain.
    * you can also accentuate the difference by adding 470pF in parallel with the bright channel 220k mixing resistor, an old Marshall trick.
    * although yes, it´s possible the 6SC7 may be more microphonic than other tubes, this amp is relatively low gain (it has 1 full triode less than Fender 6G3) so it should not be a big deal.
    And it was usable enough that hundreds (at least) of these amps were made and sold.

    I bet that with these mods which are not complicated the amp will change big time.

    Last but not least: change the old worn speaker to a modern bright one: Italian Jensens come to mind.
    They are way brighter than the 60 y.o. worn and tired original Jensens, I bet a C12Q will be killer there.
    Just curious: what speaker did you fit there?
    Juan Manuel Fahey

  11. #11
    Better Tone thru Mathematics bob p's Avatar
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    Wearing the Modder's Hat

    There's a difference between what's practical when hypothetically modding a Silvertone schematic on paper and what's practical when actually modding a real Silvertone amp. The chassis are small, the PTP components are packed tight, and they're typically stacked in vertical layers. Simply performing a "simple resistor swap" can turn into a major adventure. Maybe it would be a good idea to post a pic of your chassis.

    In terms of where to start -- I'd start of performing both Dude's and Octal's recommendations. To do that I would recommend that you avoid soldering-in each and every component as you tweak values. If you don't already have a capacitor substitution box and a resistor substitution box then now would be a good time to build them. By clipping the cap load box into place on the volume pot as Dude recommended you can test multiple values to see what gets you into the zone before you have to commit to soldering. Same recommendation for the resistor load box for Octal's recommendation re: R11 and R12. Or if you don't want to build a load box you can to the alligator clip substitution method. I recommended previously using a resistive probe draped in parallel across the 1M mixer resistors to simulate the effect of replacing the resistor with one of lower value. Using a resistive lead with alligator clips to grab onto the leads of that 1M (R11) resistor allows you to perform multiple virtual resistor swaps without doing any soldering. Repetitive soldering can be a real asspain on a Silvertone.

    I'd try those simple steps before committing to gutting the preamp and rebuilding it as a 6G3. There isn't a lot of room to work in most Silvertone amps and the horizontal footprint typically is extremely limited. I can't imagine trying to use an eyelet board, which means that you're going to have to determine how to rewire the circuit as a 6G3 using PTP wiring.

    If you want to calculate the filter frequencies for the tone controls as you rewire them, you can calculate it using the equation f=1/(2PiRC). The filter frequency chosen on the Silvertones is low compared to later amps, reflecting those changes in tonal preference with the times that Juan was talking about. So re-scaling the tone control values might be helpful once the circuit produces a bright enough tone.

    Juan makes a very good point about the anode voltages on the tubes. Low voltages result in darker tone and high voltages result in brighter tone. Looking at the schematic the preamp tubes aren't the only ones being voltage starved. The 6V6 are being run at 270V, which is about 100V below where Fender ran them. At 270V a 6V6 can produce a "smokey" tone that some people may not like. It's certainly not going to be Fender-bright. There are some power supply mods that could be used to bring up the voltage a bit in the power amp, moreso in the preamp stages, but I would hold off on doing those until you've had a chance to assess the basic preamp mods.
    Last edited by bob p; 11-28-2017 at 08:04 PM.
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  12. #12
    Better Tone thru Mathematics bob p's Avatar
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    Wearing the Vintage Gear Hat

    Now for a few words of caution to temper our enthusiasm toward gutting an amp from the first batch of 1950s Silvertones:






    The Silvertone 13oo series amplifiers were the first line of amps built by Danelectro for the Sears catalog. The 1333 was Fender Deluxe sized amplifier with tremolo and footswitch. The amp made about 12 watts of output through a 12″ speaker. These amps have that great thick tube drive and distortion that old amps are known for. It’s all hand wired and features USA tubes.
    http://truevintageguitar.com/invento...333-amplifier/

    Some of the recommendations that have been made assume that you're willing to gut a classic Silvertone amp and rewire it as a Fender, change it's speaker, etc. For decades the 1960s Silvertones were popular mod fodder because they got no respect. They were cheap disposable amps that nobody wanted.

    That's changed in recent years, with the value on 50s and 60s Silvertone amps jumping to ridiculously high levels here in the USA. If you're only interested in adding a Dude Cap or an Octal resistor, that won't degrade the value of the amp much. OTOH, gutting the preamp circuit and rebuilding it as a 6G3 is going to do serious damage to any value that the amp has on the used market.

    Your 1333 isn't a 60s amp. It's a 50s amp and it's more collectible than most Silvertones:

    I'm not sure I'd proceed with any of these mods. In all likelihood you can sell this amp for enough to buy a used Fender amp and go to town modding that.


    1956-silvertone-1333-001.jpg
    1956-silvertone-1333-012.jpg
    Last edited by bob p; 11-28-2017 at 08:19 PM.
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  13. #13
    Better Tone thru Mathematics bob p's Avatar
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    I see that a 1333 recently sold for $589 on ebay. That's enough to buy a recent production fender on CL.

    https://www.ebay.com/itm/Silvertone-...-/272826718921
    "Stand back, I'm holding a calculator." - chinrest

  14. #14
    Senior Member SpareRibs's Avatar
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    Hello,
    Rest assured I am not going to to do any mods that cannot be undone. I have only removed the guts and flipped them to make room for a speaker with a deeper basket. A Hammond 12" speaker with the yoke type magnet. That is in keeping with originality. I kept the original speaker in case someone would care to have it re coned.
    I will only be changing resistors and capacitors, keeping the originals. I am sure I am not the only one that has ever tried to make improvements on that particular amp. The replacement speaker was out of necessity, as it had get wet at some point, dried and cracked.
    I realize the value of the amp, and only want to get it functional and sounding a little better.Thank you for your concern.

    Some of the recommendations that have been made assume that you're willing to gut a classic Silvertone amp and rewire it as a Fender, change it's speaker, etc. For decades the 1960s Silvertones were popular mod fodder because they got no respect. They were cheap disposable amps that nobody wanted.
    That's changed in recent years, with the value on 50s and 60s Silvertone amps jumping to ridiculously high levels here in the USA. If you're only interested in adding a Dude Cap or an Octal resistor, that won't degrade the value of the amp much. OTOH, gutting the preamp circuit and rebuilding it as a 6G3 is going to do serious damage to any value

  15. #15
    Supporting Member The Dude's Avatar
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    That's why I made the suggestion I did. The cap(s) is/are easily removed. Well, that and it's easy. I'd recommend you do one mod at a time and test so you don't end up going overboard. If you need more, move on to the next. The bright cap is an easy start.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  16. #16
    Senior Member SpareRibs's Avatar
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    Quote Originally Posted by The Dude View Post
    Maybe try adding a "bright cap" across the volume controls? Input side to wiper about 100pf. Tweak to taste.
    Hello,
    I am going with the bright cap however, I don't have a cap that small on hand. Also does there need to be a cap on both volumes and the tone potentiometers?

  17. #17
    Senior Member Enzo's Avatar
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    Just volume.
    Education is what you're left with after you have forgotten what you have learned.

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    Of the vol control for the channel you want to make brighter.
    Consider experimenting with different values on each.
    If the vol control tends to be set low, then smaller bright cap values are better, eg <150pF.
    If set higher, then higher values may be preferred, eg 220, 470pF.
    Above example values are in regard of a 1M audio taper vol control fed from a regular 12AX7 CC stage.

  19. #19
    Senior Member SpareRibs's Avatar
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    Hello,
    Thank you all, for all the the input in solving this problem. I took the first two suggestions as they would do no irreparable damage to the amp.
    I installed two .0001 capacitors across the volume controls. I then paralleled R11 and R12 with 1MEG resistors, apparently decreasing their value by half. I didn't know how to get it down to 220k, as it involves a long mathematical equation.
    The amp didn't lose all of the dark sound but, it is is crisper rather than muddy.
    So it turned out really well, it should function properly for many years to come.
    Thanks again to all involved.
    Last edited by SpareRibs; 12-06-2017 at 12:21 AM.
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  20. #20
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by SpareRibs View Post
    I then paralleled R11 and R12 with 1MEG resistors, apparently decreasing their value by half. I didn't know how to get it down to 220k, as it involves a long mathematical equation.
    The mathematical equation is not that long and complicated.

    To compute the net resistance (Rt) of two resistors in parallel you take the reciprocal of the sum of the two reciprocals.
    Rt= 1/(1/R1 + 1/R2)

    To determine what value resistor (Radd) to add in parallel to an existing resistor (R1) to produce a specific net resistance (Rt) you take the reciprocal of the difference between the reciprocals of R1 and Rt.
    Radd=1/(1/Rt - 1/R1)

    Example: To determine what value resistor to add to 1M to bring the net resistance down to 220k.

    To simplify matters first divide both of the given resistances by 1000 (the resulting answer will be multiplied by 1000.)

    Radd=1/(1/220 - 1/1000) = 1/( 0.0045 - .001) = 1/(0.0035) = 285.7

    Therefore to approximate a resistance of 220k you can add a 270k or 300k resistor in parallel to the 1M resistance of the pot.

    These computations are easily done if you have a calculator (or calculator app) which computes reciprocals.

    Steve A.
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  21. #21
    ric
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    Steve, I've found the "ElectroDroid" phone app to work well for those calculations (as well as a pile of other things).

    This is from someone who learned to actually count back the difference from the amount owed to the amount paid, remember counting change Without a computer?

    Of course this goes back to when we were all surprised that the oil company sent someone out to put a THIRD digit on the price sign. Gas at over a dollar? What next.
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  22. #22
    Supporting Member The Dude's Avatar
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    Or, if you don't want to learn all of those pesky formulas, just plug in the values:

    https://www.allaboutcircuits.com/too...ce-calculator/
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  23. #23
    Senior Member SpareRibs's Avatar
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    Hello,
    For another modification. Would stepping down (C6) from 25MFD to 10MFD further decrease bass a bit more ? I hate to get to extreme however in one of Gerald Weber's books he says decreasing the value of the bypass capacitor will add more highs. Looking for thoughts or opinions on that.

  24. #24
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    Partial cathode bypass is a shelf filter, rather than a roll off, ie a limited effect.
    With the values suggested, only the super sub bass would be affected.
    The value would need to be brought down a lot more, eg 0.1uF, before it did what you want.
    See nickb's calculator Cathode Bypass Capacitor Calulator

    To roll off some low end, reduce coupling caps eg C3 or 4, to 1/10 or less of their current values.

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