Fisher 70A design vs Fisher 30A design

[AnyDayNow]

Sam Cogley: Yes, I plan to review your series of posts on the 70A amplifiers before I tear into them. Right now I’m just trying to understand the circuits before I do anything. I didn’t realize that available myself of some kits that include circuit boards for replacing the power capacitors, as well as coupling caps and a thermistor to get me started.

My amps did not include the Z-Magic control itself. From the reviews I’ve read, those that have tried them day the amp sound better without them. So I plan on following your suggestion.

ZacktheDog: I checked the schematics on the 80AZ, and they are similar but different, especially at the output section. The 80AZ is rated at 30 Watts; the 70AZ is rated for 25 Watts. The 80AZ uses EL37 tubes at the output section while the 70AZ uses 5881 (or 6L6). The 80AZ includes a 4 Ohm speaker tap, which the 70AZ lacks.
Finally, the output transformer is very different, it lacks the ultralinear taps noted by DirtyErnie; it has the cathode winding like the 70AZ, but the upper half of the primary side is a simple center-tapped portion like the Fisher 30A I’m getting my “Elmer-ducation” from. The 80AZ also has a higher component count, and not all of it has to do with the ‘peak power’ indicator. Although I notice that it includes a fuse, which I may add to my 70AZ. So, although they may look similar, there is good reason to say they may sound different. I guess I’m trusting in the Fisher brand to deliver the sound my Fisher 80T tuner did back when I was in high school.

Thanks again to everyone who has responded to this thread. I’ll keep you posted on developments, especially if I have anything to add to Sam Cogley’s journal.

 

[gadget73]

The 80AZ output stage is a pentode design with cathode feedback. Screens are fed DC from the power supply rather than from a tap on the transformer.

Driver circuitry is pretty much the same, though I didn't compare all the values. Same topology at least.

 

[GordonW]

yeah UL and cathode feedback are an unusual combo for sure. Honestly don't see a ton of cathode feedback amps but I don't think I'm aware of any others that do both.

currently listening to a pentode amp with cathode feedback. Completely different front end arrangement though.

Yes, and it makes sense that it needs the additional gain of 12AT7s (vs. the usual 12AU7/6SN7 type tubes used in most WIlliamson designs) in the Fisher 70 amps, for enough open-loop gain, having both UL and cathode feedback. That takes a substantial drive voltage and gain in the driver stage, to drive output tubes with that little gain once UL and cathode feedback are both applied!

Regards,
Gordon.

 

[Sam Cogley]

The 70A-2 and 70AZ are the "boxcar" form factor?

Yep. The 70A was the only one made on the more-square aluminum chassis with the tubes in a line in front of the transformers. The 70-A-2, 70-AZ, 80AZ, and 100 all used the same plated steel “boxcar” chassis. The 70-AZ had the Z-Matic control on a plug-in remote cable, the 80AZ and 100 had a faceplate bolted on with the control knob and a cage wrapped over at least part of it. The apparent intent was for the 70AZ (or a 70A/70-A-2 with the retrofit kit) to be installed inside of a cabinet, the Z-Matic control had a little brass faceplate that could be mounted next to the tuner / preamp. I guess Fisher realized that some people just wanted to put their amp on a shelf, or didn’t need to tinker with the variable damping, and put the faceplate on later amps so the control was integrated. The only post 70-AZ Fisher amps to retain such an industrial look were the ones designed for their integrated console systems.

 

[gadget73]

For a long time hifi gear lived in a cabinet, largely out of sight so I guess that isn't too surprising. I guess they got the message when that started to change and made the later amps a little nicer looking. That is one down side of many console pulls, they just aren't very attractive in stock form.

 

[zackthedog]

Sam Cogley: Yes, I plan to review your series of posts on the 70A amplifiers before I tear into them. Right now I’m just trying to understand the circuits before I do anything. I didn’t realize that available myself of some kits that include circuit boards for replacing the power capacitors, as well as coupling caps and a thermistor to get me started.

My amps did not include the Z-Magic control itself. From the reviews I’ve read, those that have tried them day the amp sound better without them. So I plan on following your suggestion.

ZacktheDog: I checked the schematics on the 80AZ, and they are similar but different, especially at the output section. The 80AZ is rated at 30 Watts; the 70AZ is rated for 25 Watts. The 80AZ uses EL37 tubes at the output section while the 70AZ uses 5881 (or 6L6). The 80AZ includes a 4 Ohm speaker tap, which the 70AZ lacks.
Finally, the output transformer is very different, it lacks the ultralinear taps noted by DirtyErnie; it has the cathode winding like the 70AZ, but the upper half of the primary side is a simple center-tapped portion like the Fisher 30A I’m getting my “Elmer-ducation” from. The 80AZ also has a higher component count, and not all of it has to do with the ‘peak power’ indicator. Although I notice that it includes a fuse, which I may add to my 70AZ. So, although they may look similar, there is good reason to say they may sound different. I guess I’m trusting in the Fisher brand to deliver the sound my Fisher 80T tuner did back when I was in high school.

Thanks again to everyone who has responded to this thread. I’ll keep you posted on developments, especially if I have anything to add to Sam Cogley’s journal.

You're right, the 80AZ had pentode outputs, I forgot about that.

 

[Sam Cogley]

Looking forward to seeing your restoration progress! It’s been a long time since I worked on those, but I’ll be happy to help if I can.