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In Reply to: RE: Rebuild of MC30s... posted by Steve O on May 01, 2016 at 13:26:54
Steve
You are right.I will remove the G1 signal but that diagram is different than the later macs because of 12ax7 CF driver but at any rate it's confusing because I also did another experiment one time.
Normally the operating voltages of a 6L6gc are close to that of a 6550 in circuit in that with the Mc60 you have a plate voltage of 435 and a G1 of minus 45 to 47 volts..When I put a 6L6gc in a 6550 amp with those voltages,the 6L6gc operates fine but produces less power. When I put it in the mc60s the tube actually melted the glass on the side once I turned up the volume.Now I realize the Rp in a 6550 is lower than a 6L6gc but it tells me that the output tube is serving a different function in this case being a current amp or pulling current thru the circuit from the Dc supply ? I have never had a pair of 6L6gcs melt the glass for that short time they were in the circuit playing in other amps such as the citation 2.In that amp I did have to rebias a tiny bit.
A friend you get for nothing,an enemy has to be bought
Edits: 05/01/16Follow Ups:
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.
Doug
I'm going to try the CCS in the PI being there is no balance adj.
A friend you get for nothing,an enemy has to be bought
Drop me a note Mike, I have some boards( about 1.5 x 2") for battery biased, depletion-mode MOSFET's. They can be done smaller if you are willing to forego the battery bias...though that makes setting current with 1% setting resistors easier.
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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