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In Reply to: RE: Rebuild of MC30s... posted by Steve O on April 30, 2016 at 14:10:03
This is a simplified version of the original mac but notice how the 12bh7 already puts signal on the plate of the output tubes as well as g1 and thru the 10uf on the cathode and this is why I feel it's just swinging those existing voltages and working as a follower.
Steve
' I know we had that discussion and I still don't buy the 2 to 1 gain because of the existing voltage on the screens,G1,and the cathodes before we even insert the 6L6 tubes..I will measure again but I measure G1 ref to gnd and the plate ref to gnd and it was a drop..Measuring from the cathode it appears to have a gain but if that were the case, there would be no need to boot strap the driver because you would already gsin in the output stage..I will disable the cathode signals on the output tubes so we can leave that part out of the equation to make it more conventional..I can safely reference the dc component of the cathode just to see if it moves the voltage higher at the plate..It's just that this circuit is so difference from a conventional driver and I don't know Doc Hoyer would plainly tell me that it amplifies current and not voltage. You should give him a call.I talk to him at least once a month. I sent him some old triad chokes I had he wanted to play with.
A friend you get for nothing,an enemy has to be bought
Edits: 04/30/16Follow Ups:
Are you aware that your attached diagram fully supports the gain ~ <2 position? Notice at the bottom of the diagram just below the EL86 the notation "A=1.7/4.5dB". Clearly, 4.5dB is "gain" and is greater than 0dB. Keep in mind you must measure from a-k and g1-k for your voltages because that's consistent with the definition of gain.
Technically, the 12BH7 does not "put" signal on the plate of the output tube. The 12BH7 plate supply voltage is derived from the plate of the opposite phase output tube. This is a form of positive feedback and increases stage gain and increases effective plate supply voltage to ~> 2X DC value thus greatly increasing AC voltage swing capabilities. This is what "bootstrapping" is all about
Regarding Doc Hoyer; I've spoken with him in the past (pair of custom transformer winds) and he is surely knowledgable so maybe there's a misunderstanding somewhere in your discussions with him. It may also be possible that he misunderstands the unity coupled output stage although that seems unlikely.
Steve
There is no misunderstanding I wouldn't think because we discussed this very subject on several occasions..Give him a call and talk to him and ask him. I will try the method of measuring that I outlined in a previous post. There is signal coming off the 12Bh7 because this is why you can remove the output tubes and the amp still plays but at reduced power. I realize the signal comes in at G1 but there is signal on the plate of the 6L6 when measured with no 6l6l installed. I guess this is why there is so much controversy among so many on the circuit's operation.
A friend you get for nothing,an enemy has to be bought
What's your take on those gain numbers on the diagram you posted?
Also, if you remove the finals you've significantly altered the circuit operation to the point where the function of the 12BH7 is completely changed. In effect you're backfeeding a signal from the 12BH7 into the OPT. This won't occur to any significant extent if the finals are in place. Instead of removing the finals to determine the "contribution" of the 12BH7, remove the signal on g1 of them instead. I haven't done this myself but I suspect any signal you get is the result of leakage thru stray capacitance.
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: 05/01/16
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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