Author Topic: JHS Twin Twelve Clone debug  (Read 216 times)

Dclubb83

JHS Twin Twelve Clone debug
« on: September 13, 2019, 04:47:56 PM »
Alright.. So I'm working on the JHS Twin Twelve Clone from tagboardeffects

Page link: https://tagboardeffects.blogspot.com/2017/10/jhs-twin-twelve-v1.html

Layout Image: https://2.bp.blogspot.com/-YZX8s-Qokas/WgT2dXQSu9I/AAAAAAAAB4w/gJzpArKlTLEyKsfRbWByexI9meYk7TX8wCLcBGAs/s1600/JHS%2BTwin%2BTwelve.png

Schematic: http://guitar-fx-layouts.42897.x6.nabble.com/file/n18170/destijl%2520revision%25202.jpg

From what I can tell.. this is NOT actually the JHS Twin Twelve, but just another version of a Silvertone 1484 clone.

I used 2N5485's in place of J201s

So I AM getting sound out of it but there are a couple of problems I'm having. Problem 1 is the "Bass" control is definitely not working at all. I'm still pretty bad at knowing where to stick an audio probe to find problems, especially with tone controls and such so I don't know exactly how to debug this. I have a feeling this is the source of another problem which is..

This thing sounds BAD. Like really bad. I can barely get any usable sound out of it at all as it is right now.

Some other info: The schematic says to bias the transistors to 50% or 9v. When I did this I got no sound at all, but when I went in and tweaked the trim pots with audio coming through I got sound out of it, albeit bad sound. Also it is pretty quiet. Gain and volume both have to be pretty close to maxed out to get unity level out of it.

Thanks for any help guys!!

PRR

Re: JHS Twin Twelve Clone debug
« Reply #1 on: September 13, 2019, 09:54:18 PM »
1) Post DC voltages at all JFET pins and battery voltage.

2) Get the right JFETs. 2N5485 is very different from J201. (This would explain very low gain.)

KarenColumbo

Re: JHS Twin Twelve Clone debug
« Reply #2 on: September 15, 2019, 03:59:05 AM »
FWIW: When J201s and 2N5457s are too heavy I found that MPF4393s are the ticket, if you can get them. Pretty "elastic", biasing-wise
"In the beginning there was nothing - which exploded" - Terry Pratchett

Dclubb83

Re: JHS Twin Twelve Clone debug
« Reply #3 on: September 21, 2019, 04:01:44 PM »
These are the voltages as of now.

Q1 - D: 5.87  S: 1.35  G: 0
Q2 - D: 14.3  S: 1.9   G: 0
Q3 - D: 3.2    S: 1.5   G: 0
Q4 - D: 15.8  S: 0.3   G: 0

Also, any idea why the bass control on this wouldn't be working at all?

Dclubb83

Re: JHS Twin Twelve Clone debug
« Reply #4 on: September 21, 2019, 04:16:19 PM »
Supply Voltage is 9.3v and the voltage coming out of D2 is 17.8