Tubes Asylum

RE: McIntosh MC60 Bias Measurement

112.118.74.235

rupertbear03@yahoo.com.hk

In Reply to: RE: McIntosh MC60 Bias Measurement posted by Michael Samra on February 21, 2016 at 20:29:14

Thank you Samra,

I change the resistor from 1.8k to 8.2k the bias drop to below -10v(pin 5) and HV to about 250v( pin 4)

my other mc60s have the same circuit (1.8k and new silicon rectifier 1n4007) and same set of tubes performs normal (bias -46v and HV 430v)

I'm quite sure the parts and wiring are OK. Just don't know why, any ideas?

Joe

Follow Ups:

My mistake. - Michael Samra 13:12:46 02/22/16 (4)
I was thinking of the Mc30 where we use the 8.2k..In the Mc60s I use 4.7k.
What B+ do you get at pin 3 and pin 4 of the 6550s when you use a 1.8k on the amp you are having trouble with?

A friend you get for nothing,an enemy has to be bought
- RE: My mistake. - rupertbear03@yahoo.com.hk 19:31:45 02/22/16 (3)
  With KT88 of AC line 110V, bias reading pin 5 -16V, pin 3 and pin 4 (both tubes) below 300V.
  
  Thanks
  Joe
  - RE: My mistake. - Michael Samra 11:39:58 02/23/16 (2)
    You have a power supply issue.What voltage so you get coming off the 5u4 rectifiers going into the choke? Also voltage do you get coming out of the choke? Can you post of a picture of the under side of the chassis?
    
    A friend you get for nothing,an enemy has to be bought
    
    Edits: 02/23/16
    - RE: My mistake. - rupertbear03@yahoo.com.hk 01:32:15 02/29/16 (0)
      all voltage seems normal when no power tubes inserted.
      
      Thanks,
      joe
    - RE: My mistake. - PakProtector 04:40:13 02/26/16 (0)
      Something is going on under there that is *NOT* proper.
      cheers,
      Douglas
      
      Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

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