Tube DIY Asylum

Both channels have a problem.

75.168.108.86

Ralph

In Reply to: RE: Scott 299B balance problem posted by Steve Franklin on January 20, 2025 at 06:48:04

The 'tube side' Voltage should be 0V and not something just north of it.

Do the Voltages vary with the tone control settings?

I'm suspicious C24 and C55 0.1 caps are defective. I've been seeing a fair number of coupling caps failing in equipment this old.

If I were you I'd go through the amps and check the Voltages on the output of every coupling cap in it.

Follow Ups:

RE: Both channels have a problem. - Steve Franklin 10:44:31 01/21/25 (3)
Those caps had been replaced by the previous owner. Those and the other caps I checked have no DC voltages. Tone controls dont change anything. Today I disassembled the volume control, cleaned and lubed it. There is no wear I can see on the tracks. It operates smoothly, and is within about 5% channel to channel.
sf
- RE: Both channels have a problem. - Ralph 11:21:49 01/21/25 (2)
  In both channels the Voltage closer to the 12AX7 section after the tone stack is lower. That means the source of the Voltage is on the other side of the tone stack.
  What sort of caps were installed by the prior owner? If oil filled, that might be the problem. Try disconnecting one end of one of those coupling caps and see if the mysterious Voltages goes away or not.
  
  Edits: 01/21/25
  - RE: Both channels have a problem. - Steve Franklin 14:26:41 01/21/25 (1)
    Film type caps except for those going to the output tubes. They are k42y. I put them in.
    Once the volume control is back in I can follow your advice.
    Thanks
    sf
    - The K42Y is an oil filled part - Ralph 09:18:19 01/22/25 (0)
      I've encountered many oil filled capacitors that exhibited electrical leakage, which is to say they didn't block 100% of the DC Voltage they are supposed to.
      
      That is why I avoid oil filled parts (except for the V-Cap ODAM part), because I like things to work without weird problems after I've done the refurbishment.
      
      Right now I'm very suspicious of the caps that are installed! Imagine what can happen to an output section if all the oil filled caps were messing with the bias point. You could damage power tubes and even the power transformer.

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