|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
108.70.59.216
In Reply to: RE: and you're still wrong...and so is Doc Hoyer...:) posted by PakProtector on May 07, 2016 at 05:16:38
...to support the claim. See below for repost of it. Notice the gain of the output stage is shown as 1.7; entirely consistent with the theoretical gain of ~2.It seems the problem this time is a measurement set up not consistent with the definition of gain.
Edits: 05/07/16Follow Ups:
I didn't look at it that close. That was somebody else diagram which was based on an original design supposedly.Here ix the thing.If the measurements are lying to us,there has to be another issue at play going on inside that we don't see in relation to the circuit.Doc Hoyer is the only one I know of that winds the Mac transformers other than Mcintosh themselves.There has to be a reason why.I think there may be more involved than what we see with the naked eye.
I still want to know why would anyone bootstrap a driver,when the stepup can place in the output and save the added stress on the driver? There has to be a reason.
A friend you get for nothing,an enemy has to be bought
Edits: 05/07/16 05/07/16
The diagram you posted is essentially the Mac unity coupled output stage minus the CF driver and utilizing a Sowter OPT. It appears to be reasonably representative of the Mac circuit.
The measurements you posted neglected the contribution of the cathodes. As you know, the cathode and anode windings of the OPT are in a 1:1 ratio. Therefore, the cathode-cathode measurement will be equal in amplitude but opposite in phase to the anode-anode measurement. Without getting into the math involved, if you had measured the anode-anode AC voltage and added it to the cathode-cathode voltage you would arrive at the total voltage developed by the output stage. This value divided by the g1-g1 input voltage gives you the "gain" of the entire stage....which even by your numbers is > 1.
Bootstrapping was/is employed primarily to obtain the very high voltage swing nec to drive the output stage using a relatively limited B+. This would be necessary if the output stage has a gain of 2 OR 1. The drive voltage could also be obtained by using a B+ ~600-800VDC for this stage but isn't really practical. The nec drive swing can also be obtained with a driver transformer as was done with Mac's early commercial offerings such as the 50W2.
Now the way you explain is making much more sense..I did measure the impedance of the output trafo on the Mc275 and as I recall the cathode and plate windings were of identical impedance and the third winding was a but more or a but less like about 30%.
Let me ask you this..Even tho the total impedance is halved between the plate and cathode of the output tubes,it is still loading across the whole output tube on the transformer primaries so why shouldn't it at least give us a relative reading on measurement across both plates.
THe reason I didn't want to add what was at read at the cathodes because that was already present and is identical to the voltage on G1 and this is with or without the output tubes installed.
A friend you get for nothing,an enemy has to be bought
You must include both the cathode and anode in your gain determination because that's how a tube operates. If you examine the Dyna circuit carefully you'll find that the cathode is involved here too. The big difference is that in the Dyna output stage the cathodes of the finals are effectively at ground potential. Thus voltage measurements of g1 input and anode output referenced to ground are the same as if referenced to the cathode. You can't reference directly to ground in the Mac circuit because the cathodes of the finals are not at AC ground potential.
As Doug has mentioned a couple of times, the 12BH7 bootstrapped stage is a means to an end and is really pretty irrelevant to the discussion of the gain of the unity coupled output stage.
I'm going to pull the 12bh7 and then insert the signal at the grids only the output stage so nothing else is influencing it..
A friend you get for nothing,an enemy has to be bought
You can measure things separately too...:) set up the grid voltage input and measure cathode response. Then measure plate response. Add the cathode to the plate measurements and divide by the grid input. Really simple...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: