JCM200 DSL401 BIAS Qestion

Discussion in 'The Workbench' started by drstrange, Aug 5, 2009.

  1. drstrange

    drstrange New Member

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    Hi from Germany

    After repairing the DSL401 I will now setup the Amp.
    That means I will configure the BIAS.
    It seems to me, that this Amp has a combined BIAS.
    Once it has Cathode-Resistor and Cathode-Cap in the Powerstage,
    on the other Hand it seems to me that this Amp has an additional
    negative fixed BIAS Circuitry and an pot to configure this external
    negative BIAS Voltage on the grids of the power tubes.
    Ok, I saw something like that in old Silverface Fender Amps too.
    Normally in a Class A cathode BIAS Amp with EL84 the BIAS is configured
    automatically around 50 mA (AC30, Ruby Riot Framus etc.)
    Does anyone know the range of the BIAS setting and the related Sound?
    On the Schematic there is written that there exist an extra document
    which describes the range of the BIAS an how to BIAS this Amp.
    Is there someone out there, who can help me with that paper or give me
    any hint to BIAS this Amp properly? Every Hint will be appreciated.

    Best Regards
    drstrange
     
  2. American Viking

    American Viking Well-Known Member

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    There is a video on this page with the DSL and how to bias any amp from the DSL/TSL series. Watch the video and read the information located at this link. Good luck.

    euro-q
     
  3. MartyStrat54

    MartyStrat54 Well-Known Member

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  4. drstrange

    drstrange New Member

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    Hi Viking Hi Marty

    Thanx to you for your replies. The Amp is from 1998.

    I found the paper with the BIAS Infos and also now I am able to
    connect my BIAS Probe to the Powertubes. I made an adapter because the
    tubesockets are embedded in the Chassis and it was not possible to
    connect the probe. Now it is. I saw without changing something, that the
    idle current of the Tubes was 36mA, 34mA, 28mA and 19mA. So I suggest
    that either the tubes are not matched or the two last mentionned tubes are
    weak. In the afternoon I will measure the Voltage on the Pins and configure it
    to roundabout 1000 mV. After this I will oncemore read the idle current onn
    my BIAS-Master probe.

    May be I have to buy a quartett of matched powertubes.

    (BTW: The bridgerectifier which was hot I replaced with another 25Amp type
    and mouted it on the Chassis with heat sink paste. Now its becoming
    handwarm only. After the bridge I mountet 30000 µF (yes 30MILLI-farad)
    and now no hum is there anymore.

    Regards
    drstrange
     
  5. MartyStrat54

    MartyStrat54 Well-Known Member

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    Swap your power tubes around and see if the low readings move with the tubes. Sometimes a low reading like that can mean the output tranny is getting weak. If the low reading moves with the tube, you'll need a new set of matched tubes. If you can ever post a clear picture of the cap mod and the different BR, I'd like to see it.
     
  6. JohnH

    JohnH Well-Known Member

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    The original spec as mesured at the bias test point was 1.375V, across a 10Ohm resistor which equates to 34mA per tube - rather hot I think. Post august 2003 they changed a couple of resistors and factory set the bias to 0.55V - very cold! I think the best sound is around 0.8V. Theres a tendency for the overdrive channel to be louder at higher bias, and very slightly fuller in tone but theres not much in it.

    John
     
  7. drstrange

    drstrange New Member

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    Hi Marty, hi all

    I swapped the tubes around and the low values swapped also. (Phuu).
    So the outputtransformer seems ok. I ordered a quartett of SOVTEKs for
    50 € and I will put the BIAS to 800 mV, which means here roundabout 30 mA
    per tube. The other value (0,525V) I am not able to configure. I am really
    shure, that this value is valid for the DSL201 only. Lateron I will explain why.
    The value which was configured in the Amp was 1000 mV.
    Due to my adapters, I can see the voltage on the BIAS set point and at the
    same time with my bias master, I can see the idle cathode current from every
    tube. The Marshall BIAS set point measures nothing more than the voltage
    drop about the cathode resistor, that means 1000 mV for DSL401 4xEL84 and
    roundabout 500mV for the DSL201 2x EL84. Thats why I am shure, that this
    ominous value of 0,525V must be related to the DSL201.

    But I am not shure anymore, that this amp is Class A. If I see the idle current
    configured with fixed BIAS Pot is ca 30 mA instead of 50mA like the AC30 so I
    belive now this Amp is Class AB even if it has the Cathode resistor in the
    powerstage.

    The minimal value here is 720mV (the Max Value is 1,45V) 720mV is here ca.
    25mA idle current per tube. 1450mV is here ca. 50mA idle current per tube.
    This is the range I can configure without changing the BIAS-cirquitry. And I
    do not want to change the BIAS cirquitry.

    Two Pots are damaged and I have to replace them
    1.) The Gain_Clean VR1 A1M
    2.) The Gain_Overdrive VR5 A1M

    Does anyone know the right industrial Name of this PCB Pots and may be
    where I can buy them?

    Next question: The phaseinverter is a 12AX7 (ECC83)
    Did someone ever tried a 12AT7 (ECC81) here?
    Makes it sense? Is there a remarcable benefit in Sound?

    I would like to give you some Pics but where and how? In this forum?
    Or send by mail?

    The bridge rectifier is mounted inside of the Frontpanelchassis and screwed
    with one screw through the "M" of the "JCM" label. It is only on PCB soldered
    but on the BR directly it is connected via cable shoe, that you can put out
    the PCB without soldering anything. The 30000µF is 3 x 10000µF. The first both
    10000µF are in the default place and they are going to ground on the
    centertapping. The third 10000 µF is glued upside down on the socket of V1
    and connected directly between + and - (6,3V) of the heater filament.
    Even if in case of switch-on, a heavy current is flowing because either of the
    cold heaters in the three preamp tubes and also because the Elkos will be
    loaded, the rectifier I took, (25A heavy duty) is only laughing about.
    (KBPC2502 200V max on 25 Ampere

    [www].reichelt.de 1,15 € very cheap.)

    But also I saw, that the rectified filamet voltage is always ca. 0,4V less than
    the unrectified due to the silicone bridge rctifier behavior.

    [www].datasheetcatalog.net/de/datasheets_pdf/K/B/P/C/KBPC2502.shtml

    BR

    drstrange

    PS: Under tonehunter.de you will find a Amp Mod section and there you can
    find lots of sound examples and also the Sound of the DSL401 Do you know
    which mods they made? :)

    [www].tonehunter.de/custommade/ampmod.php
     
    Last edited: Aug 7, 2009
  8. drstrange

    drstrange New Member

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    Hi

    Pictures first part


    1.) additional preamp heaterfilament cap
    2.) additional shield_plate
    3.) Bridge rectifier new
    4.) Bridge rectifier_connected
    5.) Fan with shock absorber
     

    Attached Files:

  9. drstrange

    drstrange New Member

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    Pics part 2.

    1.) BIAS with idle current direct on the tube and voltage on the BIAS_set point
    2.) hole to be drilled bigger because the Sovtek where to thick to fit the hole
    3.) Output tubes on the four BIAS adapters
    4.) Frontpanelscrew from the Bridge rectifier
    5.) reverb in the middle of the bottom of the cabinet

    Regards from Germany
    drstrange
     

    Attached Files:

  10. drstrange

    drstrange New Member

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    Hi

    So today I got the Amp ready and now it sounds good to me.

    When the Amp's Input was not connected to anything there was no hum at
    all. During Test with the Humbucker_Iris and a Kinman PU CBS Strat I saw,
    that there was still an anoying hum small in the clean channel, middle in Ch2
    and loud in Ch3. But the Amp without guitar did not hum. The Guitars on
    other amps do not hum either. Both together -> hum. I walked with my
    guitar around the Amp and I recognized, that the hum becames stronger near
    by the transformators (mains and output). I know that in this Amp the "Drake"
    Trafos have no shielding covers and so, if you draw near than 1 Meter with a
    approved noiseless guitar, it was humming. I already mentionnen the shield
    plate on the top of the Amp. Now I equipped on both trafos a shielding cover
    on the top of the trafos but not inside the chassis. Inside the Chassis you
    have the cables of the trafos and furthermore inside the Chassis it is already
    shielded. To mount the shielding covers you have to remove the housing of
    bobbin coil former as it is shown in the pictures. I used a metal shears for this
    but be carefull not cutting a winding.

    After Tests now, the hum is nearly thrown away because of the shield plate
    on top of the Amp and the both shielding covers belonging to the both trafos.
    You have to draw near than 10 cm to the transformers to hear now again remarcable hum.

    Conclusion after 4 Weeks pain an calamity
    If you want to have a good DSL401 (build before 2003) you have to do this:

    1.)Check (and change) the tubes because the Amp is biased hot and was always overheated.
    2.) Put a 7025 in the first Preamp stage.
    3.) Change the filament Bridgerectifier to a 25 Ampere type and mount it on the Chassis
    4.) Change the both 10000 µF filament Caps after the BR and take 105° Celsius typs
    5.) Add an additional 10000 µF filament Cap near the socket of V1.
    6.) Change the B+ Filter Cap to 150µF 450V 105° Celsius.
    7.) Mount a 80mmx80mm Fan (230V) and use it with 120V
    8.) Mount the Fan on rubber Shock absorbers near by the outputtubes on the chassis.
    9.) Resolder the whole PCB carefully because there are lots of cold/bad solder pins during the "hot" years of the Amp.
    10.) Enlarge the holes for the outputtubes in the Chassis, that it becomes possible to choise between EI's and other EL84 Typs.
    11.) Mount a shieldplate inside the whole top of the cabinet of the Amp to get rid of trafo strew hum.
    12.) Be carefull screwing all screws hard because all screws here where lax. Either Chassis either Cabinett either Loudspeaker either reverb pan.
    13.) Mount on every Trafo a shield cover on the upper side which is not inside the chassis, to get rid of the trafo strew hum.
    14.) BIAS the Amp roundabout 1150 mV on the BIAS Set Point in this case you have roundabout 31 mA on every outputtube.
    15.) Do not try a 12AT7 (ECC81) in the Phaseinverter it will not sound good. Remain a good 12AX7 (ECC83)

    16.) Plug in your Guitar and now you can enjoy a good Marshall.

    Regards
    Drstrange
     

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    Last edited: Aug 22, 2009

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