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In Reply to: RE: I had a nice talk with Doc Hoyer. posted by PakProtector on February 25, 2015 at 15:24:53
The output has no voltage gain..We talked about that again as well..All the voltage gain is from the boot strapped driver and you have to measure in reference to ground being there is existing voltage on the cathodes already.
The circuit is 100% feedback in output stage so how much gain do you think you will get from that?..Its basically works more like an impedance follower. If it had a 2 to 1 voltage gain,why would you have to bootstrap the driver?
Scroll down to page 10 and look at the output trafo and explain how it can be a voltage amp.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/25/15Follow Ups:
go ahead and reference to ground...sum the voltage changes across the outputs...you will get either zero or ~2x the input voltage...and since zero is obviously wrong...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
If its a 2 to 1 amplification,why bootstrap the driver? I will do it again with the measurement but you need to talk to Doc because he is very clear about this.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
sounds good to me Mike, but go and see for yourself...:)
but try this thought experiment first:
apply a 1V signal to the grid of the finals( out of phase with each other of course)
measure this 1v to ground, and it will not matter if it is rms, or peak.
what do you expect the cathode response to be?
---I will expect to see ~.9v on the cathodes...this is approximately the cathode follower loss, right?
now, what do you see on the plate?
---I will tell you I expect to see the same, but opposite voltage on the plate as what I expect to see on the cathode.
as far as your 'why bootstrap' question goes, it is obviously needed since the output stage has a voltage gain less than 2.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Gotcha!
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Doc's favorite Peerless transformer is the one in W-6m Heath amps..
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
In the a-a load, it is arguably the best out there. Having two, I certainly parted with my Citation II in their favour. Those W6 are the only amps I have not running Class A.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The Peerless on the W6M is part number 16431.The 16431 was chosen by Heath because it had a 70 volt secondary tap and hence would have some applications in the pro sound sectors of the marketplace.
Here's some historical trivia. Peerless designed two other outputs for the W6m amplifier.
I actually prefer each of these two alternates over the 16431. Neither of these two had the 70 volt line tap on the secondary.
One of the two had a bifilar wound secondary (16, 8, 4, & 1 ohm) and the other had a really cool trifilar secondary (again, 16, 8, 4, and 1 ohm).
If you don't need the 70 volt line tap then either one of the alternates would be a better choice IMO.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Edits: 02/26/15 02/26/15 02/26/15
The output TX primary is working just like a split load phase inverter. which of course has a gain of ~2...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
There is no voltage gain when reference to ground,it is a swing from the cathodes.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
One way to look at the gain of the output stage is to reference everything to g1. The voltage gain at the cathode will be ~ +1 while the gain at the anode will be ~ -1. If meas betw A to K, gain will be approx +1-(-1)=+2.Bootstrapping has nothing directly to do with the unity coupled output stage. It's simply a means of generating a large AC voltage using a limited B+ voltage.
Edits: 02/25/15
Steve
It needed to be done tho because of the lack of voltage amplification in the output stage, or very little by comparison to a conventional output stage. One time I put in a pair of 6L6gcs in one of my Mc60z..It sounded very good but what happened was it sucked in the side of the glass and warped the inside of the tubes.
Now under most conditions a 6L6gc biases up like a 6550 as far as negative bias voltage and plate voltage goes..I had put 6L6gcs in the citation 2 and no such thing happened but I did reduce the current to about 50ma a tube.After seeing what I saw what the Mc60s did,I had no trouble believing doc when he said current amplifies thru the output stage of the unity coupled circuit..
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/25/15
...recall that the patent indicated use of an interstage transformer to drive the finals as did the production version 50W2. Bootstrapping was just a lower cost means of achieving the nec drive voltage compared to the high quality interstage transformer of the original concept.
Correct
The initial circuit used the innerstage as you said..Give doc a call when you get time..He loves to talk when he has time and he is winding things down so he has some time now.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
One of the things I will do to the next set of MC amps I get my hands on is eliminate that boot strapping. It is easily done with a CCS replacing the 12BH7's 12k plate loads. Then you still get the bootstrapping voltage headroom, but not the bootstrapping positive FB.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
...they're bootstraped too.
first things first...let's deal with the big fishes first...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Mike, do tell me what is happening at the plates please.
cheers,
Douglas
or, better yet, please look at why you think this stage is anything different than a split load phase inverter.
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I will check each G1 referenced to ground and then each plate referenced to ground.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
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