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I got my Fisher KM-60 back from Mike Zucarro yesterday and this tuner sounds wonderful. Thank You Mike!
This tuner has some scary depth to it when the material has it. Nice wide sound stage. The sound is really nice from top to bottom and this is stock. Even the compressed stations sound lot better but the uncompressed station are wonderful sounding.
While I was listening yesterday one channel started to die out and I fixed it with a new 12AX7. After the swap all was good and the output on both channels were balanced and way more quiet than before (back to this a little later).
I was very surprised at how well it picked up distant stations like that KJAZ 88.1 in Long Beach and was very quiet.
Now, back to the dying tube part. I was reading on another forum about the Fisher KM-60 that the output on this tuner or most other tube tuners like to see 100k inputs on the gear they are connected to and really don't like the impedence SS gear have at their inputs and this could cause issues. I'm about half ways done with my Transcendent Sound Masterpiece preamp which should be fine but for now I'm using my SS preamp. Will this hurt the Fisher tuner and could this have caused the original output tube to go bad?
I have also read that increasing the output capacitor values would help out in this regard to low impedence inputs. Any truth to the impedance SS issues and possible fix?
Follow Ups:
Yup the KM60 is awesome, I had two for years. It's one of the simplest too.
Edits: 12/29/18
After so many decades in this hobby, the only tuners I now have left are one perfect KM-60 I've had forever and a rough spare for donor parts. That is how bad OTA FM has become. The KM-60 is far more tuner than anyone presently needs to get it all off the air today.
You have no worries as to any output impedance mismatch causing any damage whatsoever, as the KM-60s output stage is capacitively coupled. YMMV, but the best output tube I've heard in that tuner has been a Tele smooth plate X7, as originally equipped, although a 5751 will work very nicely in it with more sensitive preamps.
The most important tube in that tuner, as in any tuner, is the first one. For the KM-60, that is a 6DJ8. If you want to splurge on tubes, do it there first. Although again, the quality of OTA may not warrant dropping in a prized NOS Amperex or Mullard anymore.
Mike still does good work and is one of the few reliable tube alignment techs left. He recently helped me out on a piece, as I have also gotten rid of most of my bench gear in recent years and needed a scope trace. Just tell him not to write all over the chassis when he works ;)
would be the first choice tube replace. Hope I won't need to do a total realignment again.
Thank You for the info!
You can roll tubes to your heart's content in the front end, and it will not affect the alignment in the least. You can also roll away on the MPX subchassis all you like. V102 is the stereo audio amp stage and is very responsive to rolling 12AT7s. Of all I've rotated through that particular tuner, an early 60s Siemens sung best, and offered nice tonal balance with the Tele line out amp, but your mileage may vary.
Just an aside, but substituting AT7s for the X7s used on the later subchassis version more often used in the receivers can also help firm up a flabby stereo beacon lock when so equipped.
Just don't mess with the IF tubes or mix their sequence up, as that could affect the alignment.
Hopefully, you sent it to San Diego with fresh IF tubes installed, or Mike did you a favor and dropped in fresh ones for you.
Aligning around old IF tubes whose best days may be behind them is not the best value. Always align on new IF tubes if you can. Even RCA Command IFs are cheap. Alignments plus shipping costs both ways is not.
I say aligning rather than realigning with your KM-60, as it was a kit tuner, and its first true alignment was likely the one Zuccaro just did on it. That is why the KM-60 was a notorious 'sleeper' tuner for many years. A good tuner when meter-aligned. But once expertly bench aligned, it was transformed into a very competitive giant killer on a good signal. Again, far more tuner than you will ever need in 2018.
in the audio output today, nice tube with nice depth to it. So far it has 5 hour on it but it's getting better and better. This Fisher KM-60 is still in stock form too!
here is the manual with schematic
http://www.fisherconsoles.com/non%20console%20manuals/fisher%20km60%20km61%20sm.pdf
The outputs on the KM60 use both sections of the 12AX7, one for L, other for R. It is not a cathode follower, but typical inverting gain stage, but with feedback to lower gain and output impedance. The output coupling caps are C19 and C23, 0.1uF, with 85V on one side.
If your preamp has an input impedance of 100K, you will have in the neighborhood of ~16 Hz for a low end -3dB point. This uses 1/2*pi*R*C and ignores the 12AX7 output impedance, so its "ballpark".
If you replace the output caps with 0.47uF values, you get ~3 Hz, 1uF gets 1.5 Hz.
BTW, I would also check voltages against that schematic to make sure all is OK on that stage. They are never match exactly, but you are looking for gross problems.
You won't have a problem driving a 100K impedance on the pre-amp, but you may get more deep bass if you replace the output caps with larger values.
changing out the output caps. I have some 1uf Audyn Copper foil caps along with some 1uf Jensen Copper Foil PIO caps. Both are and may be too big to fit in the bottom of the Fisher KM-60. I do have some .33uf Audyn Copper foil caps that will fit. I just remembered I do have some Jupiter .47uf square caps that sound really good too, they may fit in there. so I do have some good choices for caps.
I may also change out the resistors that feed that 12AX7 tube since I have 100K Takman resistors.
The problem with large coupling caps is overload recovery. And values like 1uF are completely unusable given the limited current capability of a 12AX7. My suggestion is to use higher quality parts, but don't try to out-engineer Fisher. Very few audiophiles are up to that task.
--------------------------
Buy Chinese. Bury freedom.
All good points, but I wonder how much of a problem it would be, in the case of a KM60? Have you actually tried it?
I have a KM60 here, and like it's sound too. I actually measured the response in my lab, and posted it back in 2004. It is down a few dB at 20 Hz.
You can always do a trial by soldering an additional cap across the existing 0.1uF cap and listening. If you do only one channel and listen in mono, you can compare stock to modded on L versus R.
here in my stash box to see how that type of cap would sound before I ordered the correct value or parts. I totally hear what you're saying always try to use the same value parts.
Added the sound stage is getting wider and deeper. I'm hearing more things than with the Dynaco FM 3. But in all fairness, the Dynaco needs an alignment. This is one remarkable sounding tuner in stock mode. I am going to change out the output caps and maybe some resistors. I will buy the power supply cap for this unit just in case in the future.
I still need to fix the cabinet that got damaged during shipping and I will clean up the face plate while I'm at it.
I can just imagine how this tuner will sound in my ALL tube system once the Transcendent Sound Masterpiece preamp is done.
Did you do the alignment procedure described in the manual? Maybe not as precise as an instrument alignment but better than none at all.
"I play Kind of Blue every day-it's my orange juice." Quincy Jones
I had Mike do it. I would like to try it next time.
is in cathode follower mode?
If it is, ask him if you could swap in a 12AU7 / if NOT, how much for him to do it. IE If there's enough heater/filament current reserve.
Then you wouldn't have a problem, into any likely load.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
No. Unless you are driving a 100m interconnect, there is absolutely no need whatsoever to modify this output stage for an AU7. There is no cathode follower employed here, and there is no need for one. All an AU7 is going to do is sag the supply into the MPX stage a little more, and you may have to recalibrate all the coupling values for the outputs.
Stick with an X7, or perhaps a 5751 if your preamp can tolerate the reduced gain. There is nothing wrong with this output stage as designed.
It's working fine, now.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
I did some research after I had posted this and found out that I'm good with the preamp I'm using and it has a 100K input. But it was good to see that post to alert me about tube outputs being used with solid state inputs that there could be issues.
My current pre-amp USED to be an Allen Wright Electronic FVP aka 4 valve pre!It used a 12AX7 in CF for its output after the VC. Even the line in tube was!!!!!
???? yep.
It doesn't anymore, it's a 12AU7 in CF mode which are blameless and the line tube is a 12AT7.
It has a BIG, remote, regulated PSU, and it doesn't hum anymore!
Kvyet it is, too!
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 11/12/18
I always wanted to build the Allen Wright FVP5 preamp with the super regulators. I hear that was one very nice sounding preamp along with its phono section!
Mine is better, the remote PSU being the main factor. The DC HT is regulated and the DC heaters.
The RIAA stage embodies all of AWE's ideas. A mixture of film caps were used.
We didn't go to Vishay laser trimmed Rs, but did use some Holcos for RIAA, IIRC the rest are 1% selected MFs with twice the wattage of the originals. Same wit the RIAA stage.
This helps with noise, I'm told.
It's in a QUAD 22 box, so no room for a PSU anyway!
The VC is an ALPs
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Switching absolute polarity from the listening seat, is far better than walking over to the pre-amp and back to yr seat.
So, I do it at spkr/power level.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
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