fisher 800c clipping at ultra low volume

[slhoward]

I have all new output tubes but have been replacing caps in the powersupply. Could this be a powersupply issue where the output tubes are not getting the required power. Any places I should look

 

[Tom Bavis]

Clipping? Or crossover distortion? Both channels? Check the plate, screen, bias supply voltages. The output stage shouldn't clip until 30 watts. If one channel, check for a shorted OPT winding.

 

[slhoward]

fisher 800c clipping

Both channels clip at low bass levels. Would you be able to give me the right voltages to check.

 

[Tom Bavis]

Check the schematic on my web page: http://www.audiophool.cjb.net/Audio.html

(under the picture of the 800C)

I will put up a better scan ONE of these days...

 

[Dave Cigna]

I can't be sure that I understand you exactly, but if low levels of bass are causing distortion, then one guess is that your output tubes are drawing grossly different idle currents. This is creating a large DC imbalance in the core of the transformer causing it to saturate at low signal levels.

Why would it happen in both channels? Coincidence maybe? You said they are new tubes. Could be that you got a couple of bad ones and put one into each amp.

-- Dave

 

[Miles Prower]

There's another possibility: an RF oscillation. That could also cause premature clipping.

 

[slhoward]

fisher 800c clipping problem

i opened the darn thing up and tested the cathod voltages. All tubes read roughly 200mv except one which reads a little over 8 volts. Does this mean that there is a short in one of the tubes. Also it looks like the clipping is just one channel.

Steve.

 

[Dave Cigna]

Re: fisher 800c clipping problem

All tubes read roughly 200mv except one which reads a little over 8 volts.

40X is a BIG difference! I suppose it could be a bad tube, or a bad coupling cap, or a problem in the bias circuit (check the voltages at the grids.)

I'm not familiar with the 800C, so I don't know exactly how that 200mV is generated. If it's across a 5 ohm sense resistor, then 8V would imply that one tube is drawing 1.6A (1600mA) while the others are drawing a more expected 40mA. I would be surprised if a bad tube (or a good tube with a bad coupling cap or bias circuit) would draw 1.6A. I doubt the power supply could provide that much current even if the fuse didn't blow. And then the resistor would probably be a 1/2 watt piece; 1.6A would cause it to blow...

So... I'm thinking you might want to look for something awry in the cathode circuit of the misbehaving tube. If there is a small resistor there (like 5 ohms or something) then it might have gone bad. Might be as simple as a bad solder connection....

Check the grid voltages first. If there's a problem there that caused a cathode sense resistor to open up you want to fix that problem before anything else.

-- Dave

 

[Eli Duttman]

I'm not familiar with the 800C, so I don't know exactly how that 200mV is generated.

Dave,

The only difference between an 800C and a 500C is the AM tuner section. AAMOF, when it comes to Fisher power setups, see 1 and you see all. Fisher used a 12AX7 section "driver" into a 12AX7 section cathodyne splitter over and over.

Fisher 500C schematic here.

 

[Dave Cigna]

hmmm..... in that schematic the cathodes are grounded. Where is the 200mV of the presumably normally operating tubes coming from?

-- Dave

 

[Jeb-D.]

Check biasing- as mentioned earlier. If you can't hear any hum, more than likely it's biasing or faulty tube.

Check resivor Caps- With a signal applied, check the supply rails for any AC would be an indication of the supply caps. Either low frequency audio or 60-120Hz hum.

Check cathode bypass caps- With a signal applied, stick a scope on cathode resistors and see if any with a bypass cap show AC. If the cathode bypass caps are starting to loose capacitance it would cause low frequency roll-off. It would change the gain at low frequencies but wouldn't dramatically effect the clip point.


Output transformer- Excessive DC idle current in the output tubes will impair the low frequency of the transformer. Also, I'm not familiar with the specific amp in question, but some vintage amps simply weren't designed to go as low in frequency as modern amps.

 

[slhoward]

CATHODE VOLTAGE IS ODD ON ONE TUBE

mY 800C IS ONE THAT HAS BEEN REDONE BY THE FISHER DOC, THEREFORE IT HAS 10 OHM CATHODE RESISTORS TO GROUND. ALL OUTPUT 7591S CHECK AT ABOUT 250MV AT THE CATHOD PIN EXCEPT ONE WHICH IS ABOUT 8 VOLTS. WHAT WOULD THIS MEAN????

 

[slhoward]

800c clipping

My fisher was purchased from the fisher doctor and he put 10 ohm cathod resistors on each cathode. One end connected to the cathod of each output 7591, the other to ground. This safeguards the ouput transformers from a bad tube shorting out and blowing them up. Instead the resistor blows up. I'll take the resistor over a output transformer.


I believe I found the problem though. One of my cathod resistors reads 295k rather than 10 ohms. I do recall a tube going bad in that spot, so the resistor did its job. If I replace the resistor the new tube should operate normally and the clipping will go away.

However what would having a bad or open resistor at the cathod do to the tube, and why are the cathods grounded anyway. If possible please explain this. No offence.