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    mark 3 questions

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    natoe

    Posts : 34
    Join date : 2010-03-06
    Location : Pittsburgh, PA

    mark 3 questions

    Post by natoe on Sat May 01, 2010 6:56 pm

    I had asked sal to post some of the pics that i sent to him all help would be great with what someone did with these..

    thanks pete

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: mark 3 questions

    Post by Sal on Sat May 01, 2010 8:05 pm

    Here you go Pete, the 2 photos of your Mark III's

    What is the problem with this amp?

    Regards,
    Sal





    Last edited by Sal on Sat May 01, 2010 8:13 pm; edited 1 time in total

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: mark 3 questions

    Post by Sal on Sat May 01, 2010 8:11 pm

    Pete,
    1st off I would replace the selenium rectifier with a 1N4004 diode or greater. Also it looks like you are paralleling additional capacitors to the quad cap, you should electrically disconnect the quad cap if it is bad.. Actually if this amp will be a daily driver, buy a new quad cap from Kevin at Dynakitparts.com and get rid of the old can all together before you kill the 5AR4 tube or power transformer.

    What are those 2 green 10v 1000uf capacitors doing there?

    Sal

    natoe

    Posts : 34
    Join date : 2010-03-06
    Location : Pittsburgh, PA

    Re: mark 3 questions

    Post by natoe on Sat May 01, 2010 11:58 pm

    That is whay I asked for the pics to be posted sal I scratched my head about it too.I spoke to kevin the other day and put a big order in and now these fall in my lap but what can I say. Kevin is the man for the parts.

    The 2 green caps are comming off of pin 8 and 2 other resistors? got me, thought someone on here might be able to shed a little light..

    thanks again for your time sal.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    MK3 bias

    Post by tubes4hifi on Wed May 19, 2010 11:32 am

    looks like someone tried to convert this MK3 from a fixed bias to a cathode bias configuration.
    Those 2.2K resistors are connected to ground with the 10uF caps across them.
    Take them out. Replace the selenium rectifier with a diode. Replace the 100uF caps in the bias circuit, and make sure the bias pot is working correctly, and/or replace it with a new one.
    Replace the quad cap. Then all should be well!!

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