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    KT88's in a MkII?

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    terry37932

    Posts : 22
    Join date : 2011-05-18
    Location : Knoxville

    KT88's in a MkII?

    Post by terry37932 on Wed May 18, 2011 9:38 pm

    I'm obviously the new guy here so here goes, I recently purchased a pair of rewired MkII's on ebay. The driver boards are stenciled Jkk Mk3/6, there are 2ea. Sovtek 6922 tubes on each board, the rectifier tubes are Mullard (ServiceMaster) 5U4GB's. The power tubes are Chinese KT88's, and there is a sticker on each amp that says: BIAS .42V, both amps sound fine to me through my JBL L-65's. I also got 4ea, 6550's with the amps, my question is which pair of tube should I be using?

    Any information would be greatly appreciated. Terry

    Bob Latino
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    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88's in a MkII?

    Post by Bob Latino on Thu May 19, 2011 7:11 am

    Hi Terry,

    You can probably use either KT88's or 6550 tubes in your Mark II's. AFAIK all Mark II's came with 5U4 rectifiers and EL34 output tubes which were biased pretty "high" at 65 milliamps per each EL34. Given the fact that the power transformer is only responsible for one channel, you shouldn't have any problems using either a KT88 or a 6550 in your amps.

    As for generalized sound differences between KT88 and 6550 tubes, my ears tell me that 6550 tubes are a little more "tubey" sounding than KT88's with more mid-bass content. Which one you prefer in your amps is probably more of a personal decision based on your listening tastes ...

    Bob

    terry37932

    Posts : 22
    Join date : 2011-05-18
    Location : Knoxville

    Re: KT88's in a MkII?

    Post by terry37932 on Thu May 19, 2011 8:04 am

    Bob,

    Thanks for the information, I've read that replacing the 50ohm 10watt resister with the C354 choke is also a smart move, I'm also considering replacing the 5U4's with the GZ34, your thoughts on this?

    Thanks,

    Terry

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: KT88's in a MkII?

    Post by Bob Latino on Thu May 19, 2011 11:32 am

    Hi Terry,

    If you replace the 5U4 with a GZ34 the B+ high voltage will rise somewhat but I don't think it will really cause a problem unless your quad cap or any other parts that deal with high voltage are marginal.

    Yes - You can replace the 50 ohm resistor on the quad cap on the Mark II with a C-354 choke from an ST-70 or a modern Triad C-24X choke. If you do, any residual hum or AC ripple should be completely removed from the amp.

    Bob

    terry37932

    Posts : 22
    Join date : 2011-05-18
    Location : Knoxville

    Re: KT88's in a MkII?

    Post by terry37932 on Wed May 25, 2011 7:14 pm

    Bob,

    Now I'm concerned....I removed the bottom covers from both mono blocks, it turns out instead of the standard 50ohm 10w resister there is a pair of 300ohm 10w resisters in parallel for a value of 150ohms. The quad cap was replaced by several smaller caps siliconed to the inside of the chassis. Both amps play fine as far as I can tell and hear, the local Radio Shack has the 50ohm 10w wire wound resister for $2.99 a pair. Should I replace or let it slide until I replace them with the C-534 chokes?

    I have pics of the underside of both amps if you wish to see.

    Thanks, Terry

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
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    Re: KT88's in a MkII?

    Post by Bob Latino on Thu May 26, 2011 7:01 am

    Terry,

    Whoever did that (replaced the 50 ohm resistor with two 300's in parallel for 150 ohms) probably did that to drop the B+ high voltage on the amp. The tubes will probably last longer like that BUT > overall power of the amp will be reduced a little and the tubes *may* now be out of their proper operating voltage range.

    My suggestion is as you mentioned ... Don't change anything until you replace those resistors with two chokes.

    Bob

    Greg_M

    Posts : 40
    Join date : 2010-09-18
    Location : Poulsbo, WA

    Re: KT88's in a MkII?

    Post by Greg_M on Sat May 28, 2011 10:46 am

    Bob Latino wrote:.....my ears tell me that 6550 tubes are a little more "tubey" sounding than KT88's with more mid-bass content.

    Just to clarify....

    does your statement mean that 6550s sound more tubey and have more mid-bass?
    or that the 6550s sound more tubey and the KT88s have more mid bass?

    Thanks

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88's in a MkII?

    Post by Bob Latino on Sat May 28, 2011 1:49 pm

    The 6550's have more mid-bass content ..

    Greg_M

    Posts : 40
    Join date : 2010-09-18
    Location : Poulsbo, WA

    Re: KT88's in a MkII?

    Post by Greg_M on Sat May 28, 2011 1:56 pm

    Thanks

    terry379
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    Re: KT88's in a MkII?

    Post by terry379 on Mon May 30, 2011 7:52 pm

    Bob,

    Can you tell me what diode is normally used to replace the 20ma selenium rectifier in the MkII?

    Thanks,

    Terry

    Bob Latino
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    Join date : 2008-11-26
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    Re: KT88's in a MkII?

    Post by Bob Latino on Mon May 30, 2011 7:57 pm


    Terry,

    The voltage that this diode will look at is so low (about 55 volts AC) that almost any diode will do. I use a 1N4007 diode which is 1000 volt rated. You can get them at any Radio Shack. Diodes are very inexpensive ..

    Bob

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