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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Rebuild of MC30s... posted by Michael Samra on May 1, 2016 at 17:31:09:
The output stage in a Mac has gain of ~2. It is functioning just like a split load inverter. Grid to cathode voltage change is about half the anode-cathode change. If you're measuring gain any other way, it is wrong.
On the Mac, aside from PS improvements Mike has developed, the circuit needs some attention.
The PI should have a better tube than the 12AU7. It will also work better with a CCS in the tail, with its plate loads equalized.
The 12BH7 positive FB can be removed. Install CCS-es instead of the 12k plate loads. The loop of global NFB can then be removed.
Install adjustable bias onto the cathode follower grids. Fixed, fixed bias is just silly( I mean idiotic, ridiculous, stupid...foolish ). Measure idle current through the DCR of the cathode winding. While you're in there, use 12AT7 for the CF...much better than the 12AX7.
Finally, the 12AX7 can be swapped for a 12AY7. These are fantastic tubes, and the 12AX7 gain is not needed. Slight bump to the cathode resistor puts the operating point where it needs to be.
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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Follow Ups
- RE: Rebuild of MC30s... - PakProtector 03:50:29 05/02/16 (2)
- RE: Rebuild of MC30s... - Michael Samra 13:01:42 05/02/16 (1)
- RE: Rebuild of MC30s... - PakProtector 03:05:31 05/03/16 (0)
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