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Webcor Reel to Reel conversion thread

Discussion in 'Shock Brother's DIY Amps' started by ArcticWhite, May 17, 2018.

  1. Platefire

    Platefire Friend of Leo's

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    ArticWhite

    Wow! You have got some amps. Where did you come by all those?

    Regarding the Webcor, that's your call. There have been amps I have got rid of because once I studied
    it a while, figured out it just wasn't worth the effort required to make something out of it. There are others that I probably should have scrapped/parted out but stubborn me pushed on through it. When you demo all the unusable parts in an amp like this, it's quite a chore! You sure hone you De-soldering skills:>)

    I finally decided in a lot of cases in converting these old amps, it would have been a lot easier just to build something new from scratch than going through all the demolition and re-purposing. So it makes sense to chose one that is easier to bring around instead of the hard cases. Platefire
     
  2. ArcticWhite

    ArcticWhite Tele-Afflicted

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    I bought most of the inventory of a retiring Hammond repairman last year. 40 amps with tubes , about 20 speakers, all for 200 bucks. An embarrassment of riches.(I'm in Portland if anyone wants a good deal on anything )
     
  3. bigguy12321

    bigguy12321 Tele-Meister

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    ...umm... no... but isn't that the point? Just fun. Projects for giveaways. Teachable moments. Fighting boredom.

    Your Hammonds are superior and there is really no good reason dealing with lesser transformers or parts. I use most of these style amps for giving to friends. I bend up some 18-20 gauge sheet steel into a c-channel and build them up 5F2A Tweed Princeton style. Wired point to point and usually solid state rectified. Throw them in a box and use as much hardware from the recorder to keep cost down. I consider these builds my tuition and write off the cost as hobby fun.

    Enjoy the stuff. Don't let it bog you down. Only build the ones you want to for whatever reason you want to. Pass on the parts you don't use to someone else who wants to get started building but doesn't know where to begin.

    It really is about having fun innit?

    a
     
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  4. Platefire

    Platefire Friend of Leo's

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    Amen! It is a joy to build these things and give these discarded amps because
    technology has passed them by a new life. Plus this old vintage stuff were built like a tank military grade heavy duty so if you can repurpose them, they just keep going and going. Platefire
     
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  5. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Good point, guys . And I guess you can't really build a cool little amp like a Silvertone or a Tremolux with a big PT anyway.
    Mkay. Proceeding as planned..
     
  6. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I've been a rubbish contributor to Shock Brothers lately, but I mean to turn things around. Today I picked up a Webcor Viscount from 1961 with what appears to be all original parts.
    IMG_2118.JPG
    All the Webcor chassis look mostly the same, so here's the distilled goodness after a quick strip down. The Viscount had a 12X4 rectifier, single-ended 12AB5 power section, two 12AU7s doing various duties in the amp and tape recorder, and a 12AX7/7025 in the preamp. Tiny built in speaker.

    12X4 and 12AB5 are Webcor USA, 12AU7s and 7025 are Made in Britain (Mullard?). OT is tiny! 12AB5s are supposed to be good for about as much output as a 6V6GT, but that doesn't look like other 5W OTs I've seen. PT looks pretty Champ-ish, but with only one heater winding. I think this will go into the general parts box rather than stay together as a set, but I'm going to mull it over.

    Picked up my very first curbside Hammond last week -- unfortunately it's one of the challenging ones with an oversized PT meant to run a field-coil speaker. That too is getting slowly considered, since it's not a candidate for an obvious conversion. Cheers
     
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  7. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Here is an example, though of course there have been others posted to TDPRI and elsewhere. The new-to-me Viscount OT is in the top left. The other three transformers were recently pulled from an old Hammond AO-29. Based on the power tube(s) and load, and with help from the schematic, I could have simply guessed at the primary impedances of the Webcor OT (single 12AB5) and the Hammond's main OT (pair of 6V6GTs), but the other two Hammond transformers from the percussion system were a mystery. So why not take a minute to measure how they all really work.

    Setup: I have a known-good amp chassis on my workbench today as my voltage supply. Reverb tank and speaker installed, plugged into lightbulb limiter like it would be to do a diagnostic. Pair of jumpers from any two convenient places in the heater supply to get an AC voltage across the two jumpers.

    Measure the supply voltage across the jumpers. Mine is 6.1VAC

    Use the resistance function of a multimeter to draw your primary and secondary connections, as shown in the photo.

    Apply the known voltage across a pair of secondaries.

    Measure the transformed voltage across the primaries.

    Divide both sides by the known (secondary) voltage to reduce the ratio to X : 1. This is the turns ratio.

    Square both sides X^2 : 1^2. This is the impedance ratio.

    Choose reasonable secondary impedances (for example 2R, 4R, 8R, 16R) you might run as loads. Multiply both sides by those loads. Now you see the reflected primary impedances on the left side of the ratio.
    IMG_2125.JPG
    Of the four, the transformer that looked least promising at first glance (Hammond percussion input transformer), actually looks like it will perform as a 5W-ish(?) push-pull OT with impedances I can probably find a tube type suited to. Saved from the landfill.
     
  8. ArcticWhite

    ArcticWhite Tele-Afflicted

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    That Webcor sounds like it might make a decent bedroom amp with 8"speaker. I used to have a tiny Kay amp that had great grind at conversational volume. But I got rid of it once I learned that it didn't use a power transformer.
     
  9. Platefire

    Platefire Friend of Leo's

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    Has any of you Webcor collectors ever picked up a Webcor Guitar amp?? I've seen them on e-bay but never went for one. Platefire
     
  10. ArcticWhite

    ArcticWhite Tele-Afflicted

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    OP here.
    I've decided to build a Silvertone 1482 out of the Webcor I started this thread with.
    Question:
    1. It calls for 1meg pots for volume, tone, tremolo speed, tremolo strength.
    Which ones should be audio taper?
    2. Are there any mods I should consider in my design?
    3. There's a microphone input. Any reason to include it?

    BTW, there's so much non amplifier stuff in here. Taking forever to strip it down. 20180805_182605.jpg
     

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    Last edited: Aug 5, 2018
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  11. Platefire

    Platefire Friend of Leo's

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    Hay, Glad you decided on a 1482. Don't know if I have any answers for you but here is the Original Silvertone 1482 manual including parts list and schematic. If you look closely, Resistor R-9 and R10 is listed as volume controls which I'm pretty sure are 1M Audio Taper. However the R-11, R12, R25, R30 are listed under a different part number. Those are the tone and trem controls. I can't help but wonder if they are Linear since they are listed under another part number than the volume pots.
    1482 Manual 3.jpg
    1482 Manul 4.jpg
    On the mic input, you have two gain stages on your 12AX7 V1, so you can build it like the 1482 with the mic and inst inputs or just have a single input and use the other gain stage as a hot switch to add more distortion. If it breaks up like most 1482's or 1472's, you might not need any more distortion.

    Mods? if it were me I would build the mic channel with a 5E3 tone control because a 1482 can be pretty dark and this would give you like a bright channel. Then you can make your inst channel classic 1482 just like the schematic and have the best of both worlds. Also you could add a switchable 22/25 cathode bypass cap to both channels which would give is a slight gain/treble/compression boost at the flip of a switch. Hope this helps! Platefire

    BTW-Here are the other two pages of the manual since you got the last two:>)
    1482 Manual 1.jpg
    1482 Manual 2.jpg
     
    Last edited: Aug 6, 2018
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  12. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Excellent information, thanks.
    Now that I've decided to go with the 1482, I'm wondering if this is the best platform to begin with. As I mentioned before, I have a garage full of Hammond amps, including this old AO-29.
    The Hammond uses a pair of 6V6s driven by a series of 12ax7s and 12AU7s. The voltages at the OT and PT seem similar to the Silvertone. and there's a socket for the 6AU6. It uses a 5U4 rectifier, and that might be better (I don't know) or I could sub in a 6X4 as there are plenty spaces for one.
    The Hammond almost certainly has better iron, the sockets are better quality, and the chassis is a more typical shape for a guitar amp conversion. And best of all the turret boards are much easier to strip and reuse than the Webcor.
    I'm thinking that the Hammond makes more sense, but I'm not sure. What do you say?

    20180807_024208.jpg 20180807_024112.jpg
     

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  13. Platefire

    Platefire Friend of Leo's

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    Well looking at your high end B+ on both amp schematics looks like the Webcor is around 210VDC more in line with a 1472. The Hammond is at 305VDC which in more in line with a 1482 circuit. The Hammond chassis looks mighty wide, but it may not be? Possible problem if you was wanting to mount in a head cab or combo type cab or if you just left in the form of a chassis, the width might be a problem sitting on a speaker cab as being wider than the speaker cab. Just things to consider. Platefire

    BTW--Here is a 1472 Schematic
     

    Attached Files:

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  14. Nickfl

    Nickfl Friend of Leo's

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    That mic input is pretty redundant. Personally I would change it to a grid leak biased input stage by removing the cathode resistor and adding a grid stopper and coupling cap at the input and 10M resistor to ground at the grid. That would give you a distinctly different sound on that channel while you would still have the stock sound on the other inputs. I've put grid leak biased inputs on a couple of builds recently and like them a lot. They are high gain and don't play well with pedals, but you have the other channel for that so its a nice way to have a different flavor on that second input channel.
     
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  15. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Good point regarding the width of the Hammond Chassis. It's 22" wide.
    I'm either going to trim it to about 14" wide, or else build a tall combo with the amp mounted vertically - as in the original, but larger.
    My plan is to make this a super 1482, better than the original, using a solid pine cab that's deeper and larger than stock, a birch ply baffle, and the Hammond transformers.
    Unfortunately, this is no longer a Webcor project. I took a look at the old Webcor cabinet (cheap thin plywood) and I just can't justify the hours to build something that will likely turn out mediocre.
    Probably should start a new thread.
     
  16. ArcticWhite

    ArcticWhite Tele-Afflicted

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    I like this idea a lot.

    Questions:
    A. Looking at the schematic for the two amps, the circuits are almost identical. One difference is the 1472 has 330k grid stoppers on the power tubes. That's a high number according to Aiken amps page I just read:
    In general, the grid resistor at the grid of the power tubes can be as high as 56K to 100K before any noticeable loss of high frequencies occurs. Higher values can help in reducing "blocking" distortion as noted above, and can also take some of the "edge" off of an overly brittle sounding output stage. If the resistor value is made too low, it may not be enough to prevent parasitic oscillations, and the amplifier may exhibit instability in the higher frequency range.​

    The 1472 is a dark sounding amp. Perhaps this is why? The 1482 doesn't have any grid stoppers on the power tubes. Should I add some?


    B. I also notice that the caps in the power supply are very low in value; 20/10/5mfd. Is this due to the use of the 6X4 rectifier? Or a cost saving decision for a cheap amplifier? Should I raise these values? Or should I use a 5U4, and a more typical 50/40/40 cap scenario?

    C. The coupling caps are .01mfd. I read somewhere that .1 might be a better number. Thoughts?
     
  17. Platefire

    Platefire Friend of Leo's

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    The higher to value of the coupling caps the more bass is passes is my understanding. I personally
    think a 1482 is dark because of the tone stacks are simple and do not boost treble but passes
    more bass to ground as you turn the treble up. That's why I recommend a 5E3 tone stack on one of the channels. That actually gives you a bright channel.

    But I would say try the stuff you want to try. If it's not pleasing to your ears, you can tweak it a little more or even tare it out and rebuild it. I'm not all that technical in knowing all the ins and outs of how tube amps work---I mostly follow proven well known circuits that I know will work.

    Here is the schematic of all the mods I did to my 1482 just as a reference. With the exception of
    the 5E3 tone stack on the mic channel, all the mods are switchable that when turned off return
    the circuit to 1482 original. Platefire
    Silvertone 1482 #2 Oct 2015.jpg
     
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  18. ArcticWhite

    ArcticWhite Tele-Afflicted

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  19. ArcticWhite

    ArcticWhite Tele-Afflicted

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    That's a great idea. I've taken a crack at drawing what you described below.
    Is my schematic correct?
    If so, what values should I use for the grid stopper and coupling cap?


    20180812_201406.jpg
     
  20. Nickfl

    Nickfl Friend of Leo's

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    Close, but the grid stopper and coupling cap should be in series, not parallel. The purpose of the coupling cap on the grid is to retain a bias voltage on the grid that is created by the very large 10M grid leak resistor. So, from the input jack it will be: resistor, cap and then the grid leak resistor at the grid. Check out the top input in the schematic on by recent build here. In that build I used a 33K grid stopper and 0.01 coupling cap, but those are just standard values and can be tweaked to fit your design.
     
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