Fisher 800c line stage distorted phono stage good why?

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Wilryn

New Member
I have a Fisher 800c which has some distortion on the line stage but not the phono stage.
Is it possible that modern CD players or tape players output at a higher level than the devices that would have been connected to the line stage in 1962 which could cause overloading. Since the phono stage also goes through the line stage on the way to the output tubes I can’t think of another reason for this. The CD player is a JVC XL-V311 from 1989 and it has been confirmed working by comparing it to my other CD player a Tascam CDRW-901 and it’s not distorted when played on my other stereo. The output and 12AX7 line stage tubes have been tested and the unit has its bias set to within 10 percent of the recommended 300 mA. The grid coupling capacitors have also been replaced to the output tubes.
 
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Wilryn said:
I have a Fisher 800c which has some distortion on the line stage but not the phono stage.
Is it possible that modern CD players or tape players output at a higher level than the devices that would have been connected to the line stage in 1962 which could cause overloading. Since the phono stage also goes through the line stage on the way to the output tubes I can’t think of another reason for this. The CD player is a JVC XL-V311 from 1989 and it has been confirmed working by comparing it to my other CD player a Tascam CDRW-901 and it’s not distorted when played on my other stereo. The output and 12AX7 line stage tubes have been tested and the unit has its bias set to within 10 percent of the recommended 300 mA. The grid coupling capacitors have also been replaced to the output tubes.I"
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Welcome to Audio Karma.....I'd be surprised if the new cd's have higher output that can bring about any distortion. Your question is very thought out and explained your hooking up the CD to another amp to find out if there is some distortion that great expiation I'm not expert on tube amps other than what sounds good compared to sounds like crap..have you used any other input straight from the cd ? or maybe into tape player inputs, low volume at first, just to kind of nail down which input has the distortion !! tuff without a schematic to view signal path. So it could be a cap, resistor or any wire near the input that is touching the - and + or s solder point., do you have simple test equipment try a multi meter looking at capacitance so you can hear the beep...good luck..
 
I am making progress. The Nakamichi CR2A cassette deck has a variable output control which had been set to maximum. I moved the Nakamichi to the Aux input and the JVC CD player to the tape monitor input. I then Put in a tape and listened, it sounded distorted then I reduced the output level to 75 percent and the distortion disappeared. I then tried the JVC CD player on the tape monitor input and that is also free of distortion! Vinyl continues to sound good on the phono input.
 
The output of the phono stage is probably 300mV to the line stage at best, not sure what the official specs are for the line stage but it doesn't surprise me that the full output of a hot modern source overwhelms it. Many old sources were in the 0.25 to 0.5 volt range. Thats why the volume control tends to be barely cracked when running any source that isn't the phono.
 
Wilryn said:
The Aux for the CD player, Tape monitor for my Nakamichi CR2A cassette deck and Phono Low for the turntable. Record out was connected to a subwoofer but it was turned off.
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This is likely to be all or part of the problem. RCRDR OUT is driven through a .02uF capacitor from the anode of a 12AX7. Even if the subwoofer amplifier is specified as having a high impedance input, powering it off changes that. It might appear electrically as a diode across the signal path in that condition. or some combination of diode junction, resistance and capacitance. It's not clear to me why a sub would be connected that way (it won't track with the volume control), but did you ever try this with it disconnected?

Jack
 
It makes no difference whether subwoofer was on or off or disconnected. This particular subwoofer actually turns on when it detects a signal even when switch is off. A line attenuator may help but everything is working good now without distortion with the JVC CD player on the tape input and the Nakamichi with its variable output on the aux input.
I am listening to a CD now as I type this and am impressed with the sound quality of that 37 year old CD player on a Fisher 800c with the tubes warmed up. There is no hint of distortion as currently connected.
 
Wilryn said:
It makes no difference whether subwoofer was on or off or disconnected. This particular subwoofer actually turns on when it detects a signal even when switch is off. A line attenuator may help but everything is working good now without distortion with the JVC CD player on the tape input and the Nakamichi with its variable output on the aux input.
I am listening to a CD now as I type this and am impressed with the sound quality of that 37 year old CD player on a Fisher 800c with the tubes warmed up. There is no hint of distortion as currently connected.
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Actually the volume does change slightly if subwoofer is truly off or disconnected but no difference in distortion.
 
If it were mine, I would look at modifying the high-level inputs for a more standard sensitivity. That would eliminate the possibility of any future problems with sources that might not be adjustable. Anyway, glad you got it sorted!

Jack
 
Wilryn said:
Actually the volume does change slightly if subwoofer is truly off or disconnected but no difference in distortion.
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Do you mean no difference that you can hear? There may well be a difference in distortion characteristics that would show up in a bench test. If so, that would mean you're introducing some degree of unwanted solid state characteristics into the signal path. Ugh.

Jack
 
TriodeLuvr said:
Do you mean no difference that you can hear? There may well be a difference in distortion characteristics that would show up in a bench test. If so, that would mean you're introducing some degree of unwanted solid state characteristics into the signal path. Ugh.

Jack
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Would you recommend disconnecting the subwoofer or leave it connected and on?
The speakers are KLH five (1969 version not modern)
I would assume it is best to disconnect the subwoofer for purest sound quality but that would be at the expense of reduced bass response unless you feel 1960’s KLH speakers provide adequate bass response.
Those speakers have been recapped about 5 years ago and do sound good.
 
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TriodeLuvr said:
This is likely to be all or part of the problem. RCRDR OUT is driven through a .02uF capacitor from the anode of a 12AX7. Even if the subwoofer amplifier is specified as having a high impedance input, powering it off changes that. It might appear electrically as a diode across the signal path in that condition. or some combination of diode junction, resistance and capacitance. It's not clear to me why a sub would be connected that way (it won't track with the volume control), but did you ever try this with it disconnected?

Jack
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I just disconnected the subwoofer, you are right about loading down the signal path since volume changes. This receiver does have center channel speaker terminals. I am not sure if that can be safely used or how to connect the subwoofer. The KLH speakers have a fairly large woofer so I will just use that for bass.
 
the center channel outputs are wired off the output transformers so its low impedance. So long as there is no connection between the sub and chassis on the Fisher it should behave.

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It's not clear to me that the center channel connections produce a summed output that would be appropriate for the sub. I think this might need a resistive adapter that can be attached to both 16Ω outputs. I seem to remember that some subs have a switch and can be connected that way.

Jack
 
It will give the stuff thats common to both channels, not precisely summed in the usual way. Should be acceptable for a sub but its easy enough to try out. It would only work on a sub with a single high level input though, if it had the usual L and R input pair I don't think it would function correcly.

On that note, if a resistor divider setup were to be used, if the two negatives tie together, both - will need to connect to the 4 ohm taps, and the + to the 16, otherwise it will short things out.

or just use speakers that go deep enough and don't bother with the sub.
 
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