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Thread: 6J1 Pre-amp modifications (hum removal)

  1. #1
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,432
    I'm Alan.

    Default 6J1 Pre-amp modifications (hum removal)

    As bought from here:

    http://www.ebay.co.uk/itm/2917862715...%3AMEBIDX%3AIT




    As standard there is a low level hum present, this is cured by fitting an extra capacitor. You need a 470uF 35V electrolytic, the NEGATIVE lead is connected to the junction of two 47 ohm resistors on the top of the board as shown below:




    The POSITIVE lead is wrapped around the side of the board and connected to the 0V or 'earth' connection of the input sockets:




    Modification to covert to a buffer stage.

    The circuit of the board is this:


    ...although the 100K output resistors are actually connected to 0V not -28V as shown.

    To alter the function from a gain pre-amp to a zero gain buffer stage the circuit is altered so:




    Three new components need to be added to give the correct valve biasing. 2x 150K resistors and a 1uF decoupling capacitor.

    The two 4.7K anode load resistors are removed from the centre of the board and replaced with wire links:




    Replace the two 220 ohm cathode resistors with the two 4.7K resistors just removed.

    The two 1uF output capacitors have the front leads lifted from the board (closest to the volume control), then extended and connected to pin 2 of each valve, shown below:




    The extra biasing components are fitted to the front of the board once the two 470K grid resistors also have their front leads lifted from the pcb.
    The connecting points can be seen in this photo:



    Job done
    Last edited by Firebottle; 21-02-2017 at 12:16.
    Firebottle - for the no bottleneck approach.

    I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan

  2. #2
    Join Date: May 2016

    Location: Much Wenlock

    Posts: 283
    I'm Gary.

    Default

    Cool, waaay over my head

    I appreciate you guys that do this "electronic stuff" , you may have well written this in Egyptian glyphs.

    See you Friday...lol

    Gary

  3. #3
    Join Date: Jan 2017

    Location: Kansas City, Missouri

    Posts: 590
    I'm Nathan.

    Default

    Mine are somewhere in Hong Kong right now. Thanks for the guide!

    Sent from my Z981 using Tapatalk

  4. #4
    Join Date: Nov 2015

    Location: Wolverhampton

    Posts: 2,362
    I'm Oliver.

    Default

    Lmao, I have tried a couple of times to follow that but I still don't know what's going on haha.

    I shall endeavour to understand.

    Looking forward to hearing it !!!

    Sent from my EVA-L09 using Tapatalk
    Mission 774 OG Tonearm > Toshiba SR 370 Turntable > ZYX R50 Bloom cartridge > Firebottle OTP Phono Stage > Firebottle Monoblocks > Creek OBH-11SE Headphone Amp > IXOS Speaker Cables > Pioneer CS-77A Speakers

  5. #5
    Join Date: Mar 2008

    Location: Dunfermline, Scotland, UK

    Posts: 12,141
    I'm inthescottishmafia.

    Default

    How does it sound Alan?
    There are times when you canít do the sensible thing, when you canít act like a responsible adult at all; you just have to do whatever insane thing comes into your head. When bad people do it they end up murderers, when good people do it they end up heroes, and when the rest of us do it we end up looking like total idiots. But whenís that ever stopped us?

  6. #6
    Join Date: Jan 2017

    Location: Kansas City, Missouri

    Posts: 590
    I'm Nathan.

    Default

    He did a much better job of explaining it than I could. My diagrams look like a foreign language to everyone but me, and I don't use the right language when I'm explaining it. I learned how to do most of it by trial and error before I could even understand the math behind it.

    Sent from my Z981 using Tapatalk

  7. #7
    Join Date: Nov 2015

    Location: Wolverhampton

    Posts: 2,362
    I'm Oliver.

    Default

    Quote Originally Posted by nthall View Post
    He did a much better job of explaining it than I could. My diagrams look like a foreign language to everyone but me, and I don't use the right language when I'm explaining it. I learned how to do most of it by trial and error before I could even understand the math behind it.

    Sent from my Z981 using Tapatalk
    Lol, I'm starting to understand it a bit more now.

    What's that silver block next to the volume control ?

    Sent from my EVA-L09 using Tapatalk
    Mission 774 OG Tonearm > Toshiba SR 370 Turntable > ZYX R50 Bloom cartridge > Firebottle OTP Phono Stage > Firebottle Monoblocks > Creek OBH-11SE Headphone Amp > IXOS Speaker Cables > Pioneer CS-77A Speakers

  8. #8
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 33,039
    I'm Grant.

    Default

    Looks like a ceramic capacitor
    Regards,
    Grant ....
    Sometimes incompetence is useful. It helps you keep an open mind.
    .... ..... ...... ...... ................... ..... ..... ..... ..... .....
    IMUTO TAURUS X4 BATTERY SUPPLY. - RPI2B & AUDIOPHONICS DAC running MOODE 3.6- TONBRUX BATTERY SUPPLY & CHROMECAST2 AUDIO DONGLE no1 - JBE SERIES 3 SLATEDECK & EMOTIVA XPS-1 PHONO - DENON DV2900 & HEAVILY MODDED MF X10v3. BERESFORD SWITCHBOX. - TWIN PRO MONOBLOCK AMPLIFIERS. - XIANG SHENG DAC\PRE\HEADPHONE AMP. -AUDIO TECHNICA ATH-MSR7 HEADPHONES - WIN10 JRIVER22, SPOTIFY PREMIUM- OPPO PM-3 PLANAR HEADPHONES, KINPS CABLES, SMSL M6 MINIDAC, CHROMECAST2 AUDIO DONGLE no2 POWERED BY ANKER BATTERY PSU - FULL RANGE TWIN & SINGLE TELEFUNKEN's - TANDBERG TCD310 - CARTS.. AT95 Shibata, SHURE M55E original, JVC MD1029. - RCA CABLES BY BRIAN SPKR CABLES BY MOI HAIR BY MARCO FEET BY ELECTRICBEACH - MAINS REGENERATED BY POWERINSPIRED AND FILTERED BY BELKIN PUREAV.

  9. #9
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,432
    I'm Alan.

    Default

    It's a polycarbonate cap, it's the 1uF decoupling capacitor for the 2x 150K bias voltage divider. The lower lead is connected to the nearest 0V ground point.

    Ali it sounds very neutral, I shall be trying it on the end of a lesser quality CDP later to see if it takes any of the digital glare away.

  10. #10
    Join Date: Jan 2009

    Location: Essex

    Posts: 15,283
    I'm openingabottleofwine.

    Default

    Is there any advantage in removing the 'blingy' blue LEDs?
    Have you listened to this month's choice in the Album Club?

    Barry

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