• Re: switching between 2a3, 45 and 300B for Joplin - Kuei Yang Wang 06:31:34 02/20/03 (1)
  In Reply to: switching between 2a3, 45 and 300B for Joplin posted by Rasim on February 20, 2003 at 00:01:50:

  Hi,

  > I was wondering if you have completed your mod and
  > incorporated a switch to interchange from 2a3 to 45
  > to 300B?

  I did not find the time last WE. I have been discussing this with Brian, I actually suggested not to use a switch but instead to use internal hardwiring.

  > Can you pls tell me how you plan to put (locate) the switch

  Inbetween the two Hum Pot locations, there seems sufficient space.

  > and where to connect the pins?

  This is harder. I think you are better off simply to use internal hardwiring. All that changes for the different Valves are the (common) Cathode Resistor Value and the heater wiring.

  > You especially focused on 2a3 and 45. What is the configuration
  > for 300b?

  Same 2A3, but the two heater windings are connected in series, instead of parallel for 2A3's.

  > What is the final material list needed?

  Most should be included anyway, I'm working on a full list with Brian, please contact him. I'm not really in the Kit business and such, so you don't get detailed Bills of Materials from me, moreso as often perople have their own ideas. I suggest you get the circuits from Brian and try to understand how they work, this will then allow you to decide these things for yourself.

  > I would like to keep the hum pots on.

  In order to be able to use 300B's you MUST wire the Valve sockets in parallel. This means they are parallel allways, disregardless of the Valve used. This also means only one hum balance pot is needed/used.

  However, as I would recommend to also fit the "bias balance" pot to the top it effectively replaces the now redundant second hum pot.

  For 45's use only ONE of the two 2.5V Heater windings (otherwise the Heater Voltage is too high), for 2A3's parallel the two 2.5V Heater windings, for 300B's connect the two 2.5V Heater Windings in series.

  > A weird question, is it possible to switch the operation of the
  > given tubes in class AB1 and A1 modes by means of similar multi
  > point switch? Is it too complicated? Can it be possible on top
  > of your mod?

  Well, Class AB1 operation needs more Bias Voltage, more HT Voltage and lower current. I suspect you COULD make the circuit switchable between fixed bias for Class AB1 and self Bias for Class A1 or if Brian gets the mains transformer spec changed you could switch the HT in addirtion to other parameters, but the switching would get VERY complex.

  My current discussions with Brian are more towards including wiring instructions for the Joplin that would allow the Amplifier to set up (by changing cathode resistors and adjusting some wire bridges) to use the following valves and approximate operating points:

  Type Class Anode V/I Bias RK +B V/I Power

  45 A1 275V/36mA 56V 780 335V/ 72mA 4 Watt
  45 AB1 300V/32mA 65V 1K 370V/ 64mA 7 Watt

  2A3 A1 250V/60mA 45V 375 305V/120mA 7 Watt
  2A3 AB1 300V/40mA 62V 780 365V/ 80mA 10 Watt

  300B A1 250V/60mA 45V 375 305V/120mA 8 Watt
  300B AB1 300V/40mA 62V 780 365V/ 80mA 11 Watt
  300B A1 300V/50mA 60V RK 365V/120mA 14 Watt

  Please note that current Output Transformer will NOT support the full power even of the 2A3 Class A1 PP at 20Hz, I have suggested to Brian that maybe a little more Iron may be called for. So the extra power from going to Class AB2 or to higher operating point 300B's will not be available at low frequencies unless the Output transformer is changed from it's current core size.

  In addition for the above to work with the higher power A1 and AB1 operating points to work we need a +B selectable between a little over 300V (like now) around 360V (we need a new added section and tap for that or rewind the Bias Winding).

  I suspect we could get the count of possible operating modes up further for 2A3 and 45 if add four S&B Gridchokes (no space in chassis though) and include Class AB2 operation, but I must admit that even the Class AB1 operation strikes me making little sense, as the increases in power are modest.

  With fixed bias we can get a little more power in AB1 but we now need very low value grid leak resistors (50k max. for 2A3 and 300B according to datasheet) which will require large value coupling capacitors etc. I would personally really only recommend the various Class A1 operating modes....

  Anyway, I hope all the above clarifies a little what is needed and possible.

  Sayonara
  
  

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  • Re: switching between 2a3, 45 and 300B for Joplin - Rasim 07:09:44 02/20/03 (0)
    In Reply to: Re: switching between 2a3, 45 and 300B for Joplin posted by Kuei Yang Wang on February 20, 2003 at 06:31:34:

    Hi Brian,

    Are you planning to modify the OPT on Joplin?
    Are you going to include the wiring instructions for various operating points as mentioned in the package?

    Thanks,
    Rasim.
    
    

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