Silvertone 1485/Twin Twelve

[jaredcohen]

User trad3mark posted a schematic he called "Eighty Seven Percent - De Stijl" on FSB.  He said it's basically a FET conversion of one of the channels in the Silvertone 1485 Twin Twelve.  His schematic is below.  The red rectangle is a resistor.



I created a vero layout and figured I'd post it here.  It sounds pretty rad.  I know a guy with a Silvertone 1472 and this pedal sounds an awful lot like his amp.  I built-in a 9->18v converter.  It's just |v|ark's standard 9-18 board built in.  Sounds better to me at 18v.  I think most of these old circuit emulations do.  I assume this should sound a lot like JHS's new hyped "Twin Twelve."  

SEE UPDATED LAYOUT BELOW

The only issues I've had are:

1. I was getting a weird, fizzy sound underneath my guitar audio.  My voltage out of the 7660S was 17.8v and I originally biased the J201's to half that - 8.9v).  I re-biased all transistors to about 10-10.5v and that seems to have gone away. I have no idea where it was coming from or if there's something wrong with the schematic or with my layout.  Tried with a bunch of J201's.

2. A couple of the caps are only 2 rows across.  Sorry, I know that can be tight.  But they're low values.  I used the green mylar caps for those spots and they fit fine.

3. Ran the original values through Duncan's TSC and I used a 390p in place of the 680p in the top right.  I also swapped the 1M Treb and Bass pots for 500k.

If you build it, enjoy.  If anyone has any ideas on the fizzy J201's, any advice is appreciated.  Thanks!

[jaredcohen]

By the way, the trimmers that fit are this type:

[Frank_NH]

Nice one Jared!

I'll have to build this once I get more 22nF caps...

I've been thinking that I'd like to get into doing more 18V builds, as some of these amp sims tend to sound a bit flabby at the low end.  Maybe the headroom offered by an 18V supply voltage would help cure that.

(Which reminds me that I also need to source some more charge pump ICs...)

[Neil mcNasty]

Cool!
This one I have to build first thing in the morning!

I have 4 Silvertone amps so far:
1448 (Amp in Case)
1481 (4 watts of blues/rock dirt)
1483 (exactly the same as 1484/1485 but without the Tremolo & Reverb, with a 15" Cab. Buy this one, if you're getting yourself a Silvertone Amp! Way cheaper and more compact size!)
1484 Twin Twelve

They all sound amazing and I'm really curious if this circuit sounds similar and has the same magic compression/drive as the amps (might be difficult)
The JHS Superbolt (or should we say ROG Supraux Deux?) actually sounds quite similar to a Silvertone Amp.
I built that one and use it every time when Fender style amps is my only option. Hopefully this one is more suitable and can replace it in these cases!

Regarding fizzy J201's:
I've gotten some low quality J201's lately that has been a bit difficult to bias, and has had a bit more fizz than normal when mis-biased, but my general understanding is that it is simply a mis-bias thing, nothing more, and that JFet's are all over the chart in specs and will not always bias at half voltage, that's just a general starting point to work from.
The sweetspot is almost never at 1/2 voltage even if that's the theory. Go by ears more than numbers!

Thanks for sharing!

[jaredcohen]

I should also say that I haven't boxed this.  My unboxed build is very noisy.  I chalk it up to having four J201's with nothing to block RF interference or anything else (and I live in the city - there are a ton of waves in the air).  If you have the same issues after building, let me know, and I'll try to look for a cause.

[jaredcohen]

Yikes.  Played twice and the 100uf cap from pin 2 of the IC is bulging.  I'll replace it, and based on my experience with electro's, I'll assume it was just a bad cap.  That seems to happen sometimes - but I wanted to check.  I just took the 9->18v from this site so I assume that part of the circuit is all good. But if that electro cap is doing something it shouldn't be doing, let me know.

I used a 35v.  I figured that would be enough on an 18v circuit - but maybe I'll use a 50v next time just to be safe.

[Ciaran Haslett]

Hi Jared

That 100uf cap is shown the wrong way round on your layout.  It should be cathode to pin 2.  That could be your problem.

Ciaran

[jaredcohen]

Damnit.  What a dumb mistake.  Thanks Ciaran.  Here's an update for anyone who wants to build:

[giudis]

Hi JaredCohen,

Have you solved your issues with the Ciaran's indication?

[jaredcohen]

Giudis,

Yup the image above has the capacitor in the proper position. Should sound awesome now. Life got busy and I haven't played any pedals for a few months, but last time I had this one fired up it sounded great.

[Neil mcNasty]

Built this a while ago, but could not get it to work properly. Now I know why!
I was pretty stoked about this layout, since I own 4 Silvertone Amps. They all sounds awesome!

Been thinking about trying to build a version of the 1483 as well.
It breaks up alot more than the 1484/1485 and absolutely loves to be pushed hard with boosters.
It actually can reach fuzz territory when boosted hard...
It's a monster that makes all your overdrive pedals obsolete.

Thanks for solving the problem!
Now I can fix my board and enjoy this sweet sound when I'm away from my amps...

[notnews]

Hey Neil

Thanks for your reply in the DBA RM thread, much appreciated.

I also own a 1483 and 1484. There is something special about these amps for sure. I'd love to hear from someone who builds this circuit regarding how well it emulates that sound.

[tonymccallie]

I'm seriously excited to give this a try, but I don't know anything about biasing transistors to 50%

Does anyone have a good resource on this? I've been googling and having a hard time finding something simple enough for me to learn from. I've only built 3 pedals.

Thanks for any help and for your patience.

[Neil mcNasty]

Biasing to 50% is simply put, just using the trimmers to get half the voltage at the Drain leg of the JFet transistors, compared to the input voltage.
Meaning that if your main voltage is +9v, then the trimmer is adjusted so that you get +4,5v going to the transistor. If the main voltage is +18v (like in this case) then you set the trimmer to +9v
In order to measure this, you use your multimeter, and measure between ground and the Drain of the JFet transistor (Drain is the top leg of the transistors in this case, except for Q2 that is mounted uside down/fliped 180 degrees)

JFet transistors often vary alot in quality/specs (a bit like Germanium transistors), so half voltage/50% is not allways the perfect value/setting, but a good place to start. From there on I allways adjust by ear bringing the voltage up and down around this startingpoint to find the sweetspot, something that can take a bit of time if there ar many transistors to bias/adjust, as you'll need to go a bit back and forth before you get them all as you want them.

Remember to use sockets for these transistors, as you might stumble upon some that are so off spec that you never get them to bias properly. Specially with the J201's which are starting to become difficult to find with good specs.
Just like with Germanium transistors, you allways buy a bunch of them.

If you want to learn the basic technicalitys of the Fet transistors, then this is a quite nice tutorial: http://www.electronics-tutorials.ws/transistor/tran_5.html

[Beaker]

Slightly off topic, but I've just finished building another Runoffgroove Ginger pedal, and this time I used SMD 5457 JFETs. On a whim I threw in a voltage doubler to run at 18V as well as 9V. It really makes a difference, and in my opinion is well worth the effort.

I biased the JFETs by ear, and got it sounding just right, then measured the voltage. Q1 measured 4.52V and Q2 4.49V.

I know that is only a tiny sample, but to me that just confirms what we have been told about SMD JFETs being much more accurate and much more consistent.

It's SMD all the way for me now!

I would reccomend any beginner to get into using SMD JFETs as soon as possible. I eliminates the dissapointment of buying out-of-spec or fake  parts, and they are much cheaper.

[tonymccallie]

Neil,

Thank you so much for the thorough reply! That's fantastically helpful.

[j-pee]

"I would reccomend any beginner to get into using SMD JFETs as soon as possible. I eliminates the dissapointment of buying out-of-spec or fake  parts, and they are much cheaper. "

I would never have thought of it...
thank you for the advice!

[Jaysharpwilliams]

I don't see the connection to positive side of 47u next the the 5817.

[jaredcohen]

the 47uf cap just straddles those 2 rails. the + side goes in the row directly above the - side. right next to the cut.

[Jaysharpwilliams]

I dont get it? What does that do?