The Mcintosh MC-30 and the Fisher 80-Z units look in their schematics eerily similar. Does the 80-Z have the same or similar type of output transformer to the McIntosh?
My guess would be no because Frank's patents may not have expired by the type the 80-Z was designed.
My guess would be no because Frank's patents may not have expired by the type the 80-Z was designed.
I once owned both MC30 and Fisher 80AZ.
Thru my experience, Mc30 easily blown that fish out off the water!
This Fisher is to me a PITA, instability problem. One moment,it's fine and then it oscilates. Cathode current goes up and down un-accountable !? 😕
It still prevails and wish it drops dead, then dump it, and moves on with a new page.
MC-30 is a sure win decision!
Thru my experience, Mc30 easily blown that fish out off the water!
This Fisher is to me a PITA, instability problem. One moment,it's fine and then it oscilates. Cathode current goes up and down un-accountable !? 😕
It still prevails and wish it drops dead, then dump it, and moves on with a new page.
MC-30 is a sure win decision!
With some work I may be able to correct the whistle issues. But you did not answer my question. The unit belongs to a friend, not mine.
I do not own either one, so please take the following as speculation: I think that the OPTs on the two amps are different, since the Mac's were famous for its output transformers, without them, the amps would not work nearly as well as they did.
Sorry! I mis-interpreted your question!
MC30 OPT looks and weight so much more than Fisher's!
I think that Fisher is your typical 1950's cheeseball. The circuit looks unstable so I am not at all surprised it sporadically oscillates. They added that pot and switch gobbledegook to switch resistors in and out of the feedback loop to give user control over the dampening factor. That probably did not help matters at all. What average Joe user would know and care about dampening factor? Just connect a speaker and the music plays?
Why do you think Fisher added all that stuff to give the user fancy control over negative feedback? Was it some kind of sales gimmick?
Why do you think Fisher added all that stuff to give the user fancy control over negative feedback? Was it some kind of sales gimmick?
It was no sales gimmick but a cover-up of it's prevailing instability.
With LF oscilation, you can see the woofer pumping but, with HF oscilation, it's almost un-audible. Worst still, it heat-up the speaker's magnet.
If amps are allowed to oscillate supersonically, wouldn't it have the propensity to radiate garbage that would cause it to fail statures such as FCC section 15, yet alone ruin tweeters?
Is the Fisher either partially or fully cathode coupled?
The McIntosh patent is about "unity coupled" output stages, both halves of the output tubes (plate and cathode) have equal windings on the output transformer's primary.
In the case of the McIntosh, there is no gain in the output stage, so all the voltage gain has to happen before the output stage. This means that the driver has to be able to swing the full swing of the output tubes, so if that is 500vdc, then the drivers have to swing that. With a standard output stage, the drivers do not need to swing more than the grids need to drive the output tubes (with their gain) fully.
The McIntosh patent is about "unity coupled" output stages, both halves of the output tubes (plate and cathode) have equal windings on the output transformer's primary.
In the case of the McIntosh, there is no gain in the output stage, so all the voltage gain has to happen before the output stage. This means that the driver has to be able to swing the full swing of the output tubes, so if that is 500vdc, then the drivers have to swing that. With a standard output stage, the drivers do not need to swing more than the grids need to drive the output tubes (with their gain) fully.
The unity coupled output stage has a gain of ~2 so the driver needs to swing about half the B+.
It's an apples oranges , the Fisher is a conventional output circuit that has a cathode feedback winding an early form of ultralinear . The other is a unity gain cathode circuit .
With minor modifications the Fisher can be a fine amp , the PPP 6V6GT version of that circuit was a very stable and worked well that was used in there hi end consoles .The out put transformer in the Fisher was made by Todd and several very fine amps where made with Todd output transformer .
http://audiophool.com/Schem_A/Fisher_80AZ_spec.pdf
With minor modifications the Fisher can be a fine amp , the PPP 6V6GT version of that circuit was a very stable and worked well that was used in there hi end consoles .The out put transformer in the Fisher was made by Todd and several very fine amps where made with Todd output transformer .
http://audiophool.com/Schem_A/Fisher_80AZ_spec.pdf
Can you kindly elaborate on that minor modification needed to stabilize this Fisher.... thankss.
Can message me at : zkwan@singnet.com.sg
Here is the thing..Both amp need updates and a 35uf polypro filter cap is paramount off the rectifier tube.The Mc30 only has one filter can and it isn't nearly enough to properly supply energy to the circuit when you have 4 tubes driving a pair of 6L6gcs.The phase inverter should be changed to a 6cg7 in the mc30 as well as the AZ80.Here is how I did this pair of mc30s and I put new chassis on each of them as well as an all film cap power supply.
I also did everything without damaging or cutting the chassis's so they can be reversed if you ever want to put it back to original..Anyway,you pretty much have to recap AZ80s with the poly and a pair of dual 100uf FT or JJ caps.Just keep that 35uf poly for the charging cap in each case.
I also did everything without damaging or cutting the chassis's so they can be reversed if you ever want to put it back to original..Anyway,you pretty much have to recap AZ80s with the poly and a pair of dual 100uf FT or JJ caps.Just keep that 35uf poly for the charging cap in each case.
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I have two of fisher 80AZ mono amp, which I replaced input cap.22 , 2 bypass caps .022uf on 12AT7, and 2-bypass caps 0.1uf on 12AU7 (all Mcap supreme silver and gold). I adjusted the cathode bias resistance on V1 12AT7 according to the fisher schematic, which I set both amps to read 100K ohms from pin8 to ground. The amps has the same output tubes are EL37 and Telefunken 12AT7&12AU7. It has made much better sound stage and I love it.