• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Fisher 500C distortion

Fitzgreen505

Fitzgreen505

Member
I just finished upgrading my Fisher 500C with metalbone's restoration kit, and I have to admit, the sound now coming from my speakers is pretty remarkable. I also replaced just one of the tall can capacitors (C98, 200uF, 250V), and I replaced the 7591s with some NOS RCAs that seem to be very well balanced. Ninety percent of the time, the sound is quite detailed with no problems. The tuner seems to be functioning well.
My problem is that when I play CDs through the Aux input, I hear distortion when the music is loud and in the upper registers. The loud bass never seems to distort. Just the high brass when played at loud levels. The sound seems to tear apart. Is this fixable? If so, how?
 
Register to hide this ad
Its not just a case of trying to play the amp louder than it can go, is it? Keep in mind you're dealing with about 30 watts of power output here. Playing it very loud on speakers that aren't real efficient is going to cause the amp to run out of power and clip on you.
 
Is this distortion just in AUX while playing a CD at "LOUD" volumes? In FM? Phono? Tape?
What speakers are you using?

Why only the one can cap. That is 1/2 of the Voltage Doubler circuit and should have both 1/2's replaced to ensure good voltage. take voltage readings of all the CAN CAP taps (C91 a-b-c-d, C92a-b-c-d, C97A/B) and post them. Poor or sagging voltages can cause distortions at high power levels. I would recommend finishing the power supply and getting the voltages in line with spec.

Could be also that even tho your RCA tubes are "well matched" they are tired or plain worn out causing distortion. Good way to check tube health is to determine the current draw thru a 10 ohm resistor on pin 5 of each output tube, the result being in mv. Determine bias voltage(Pin 6) and post here along with plate voltage pin 3 and screen voltage (Pin 4/8).

Let us also know if the bias circuit is (a.)factory stock,(b.) has a 10k pot installed, or (c.) IBAM or IBBA boards installed.
Post the voltages per pin for all tubes, by tube.
 
Last edited:
The last time that I had an issue like that, it turned out to be the capacitors in the speaker crossover. Have you tried other speakers?
 
The power supply is definitely one area that can run out of energy if the electrolytics are not up to snuff. Ditto about trying other speakers. Do the recommended current checks on the output tubes at idle. This will tell you how much each tube is conducting and helps point to the next step in finding/eliminating the cause. Using a scope and 16 ohm load resistors (with your speakers disconnected from the amp) and a sine wave generator will help you determine the clipping point of your amplifier channels. It is normal for one channel to be a little weaker than the other one due to tube variances. Sometimes you can help the situation by moving the output tubes to alternate channel positions, checking for output tube idle current as you do this. Advancing receiver volume control with the sine wave displayed on the scope screen will show you what the peak power output actually is with the tubes you have. Advance until you see clipped sine waves at the load resistors and then back off until the clipping just stops and measure the voltage across the 16 ohm load resistor(s) then calculate the actual power out.

If you do not have a scope, then the process may take more time and effort to diagnose and correct, but we are here to help you.

Joe
 
I had similar issues with a 400 a long time ago. For me it was because the AUX and Tape monitor jacks were electrically connected together in each channel via a resistor and capacitor. (I did have a tape deck connected to my 400 at the time.) I broke that connection and it cured the issue.
 
First, thanks to everyone for their help.
The distortion did occur when I was playing at loud volume through my Vandersteen 2ce speakers, which ask for a minimum of 100 W. Since my 40 watt McIntosh has no trouble driving these speakers, I thought I would test them out with the 500C. At lower volumes, there was no problem with these speakers, and when I backed off the volume, the problem never recurred. I haven't had the problem with smaller speakers. I was never able to get the radio to create the problem, just the aux input.
So, it's probably just my turning up the volume too high.
All the same, I did the measurements. All the pin 5s have a 10 ohm resistors attached, so I just measured from the non-tube side of the resistor, with the chassis as ground. Here's what I found:
Tube #1 Tube #2 Tube #3 Tube #4
Pin 5 0.4 mVrms 0.7 mVrms 0.7 mVrms 0.7mVrms
Pin 6 -17.9 VDC -17.9 VDC -17.9 VDC -17.9 VDC
Pin 3 439.2 VDC 440.0 VDC 439.0 VDC 440.0 VDC
Pin 4/8 ------------------398.5 VDC---------------------------------------

As you know, pins 4/8 are all connected to each other, so they each had the same reading.

The schematic says that pin 4 should have 430 V and I don't know if this is a problem. I think all the other readings are acceptable, except perhaps Tube #1's pin 5.

I have a scope, but no functioning sine wave generator, so I haven't followed audmod01's directions, as yet.
 
According to my calc's The tubes are running thusly; I did not compensate for the little the screen draws but in any case you need to verify these numbers. They are way too HOT.
Tube #1 439V x .04ma = 17.56W
Tube #2 440V x .07ma = 30.8W
Tube #3 439V x .07ma = 30.73W
Tube #4 440V x .07ma = 30.8w
Something is obviously wrong as your #2,#3, & #4 tubes would be 3 melted blobs normally. . Recheck the Pin 5 to ground with the RED PROBE on pin 5 and the black on a convenient chassis ground with the resistor in between the probes. The way you described it " the non tube side of the resistor to ground has both probes at chassis ground. You're picking up stray dc on the chassis and not the voltage from the tube. Just move the Red probe from tube to tube.
.
The increased plate and screen voltages are indicative of your higher than normal for 1963 wall voltage. Both are well within 10% of nominal. (430V & 375V) It's NORMAL. FISHER allowed for a 10% variance in voltages from the schematic nominal due to manufacturing tolerances and variances in individual components. Take 10 fresh off the factory floor 500C's and plug them in and check votages from a stable 117V supply. No two will be identical. Check your filament voltages on the small signal tubes (12ax7's) for no more than 6.4v or so. Ideally they should be between 5.8 and 6.2 as FISHER ran them a bit cold. If high you can put a R/C dropping network in the filament supply to drop the voltage down to 5.8 or so.

A buck Transformer (6v drop) will drop the wall voltage down to a more comfortable level for the unit.

Please re-check the output tubes pin #5 again and post.
 
I re-measured for VDC across the 10 ohm resistor (red on pin 5, black on ground with the resistor in between). Results (in VDC):
Tube #1: 401 mV
Tube #2: 505 mV
Tube #3: 403 mV
Tube #4: 417 mv

So, the current and wattage across the resistor should be:
Tube #1: 401 mV/10 ohm= 40.1 mA = 16 mW
Tube #2: 505 mv/10 = 50.5 mA = 25.5 mW
Tube #3: 403 mV/10 = 40.3 mA = 16.2 mW
Tube #4: 417 mv/10 = 41.7 mA = 25.5 mW

I double-checked them with two different voltmeters. I hope this makes more sense.
 
That's better but tubes #2 and #4 should be paired and #1 & #3 paired. Rotate them until they match as close as you can in pairs. Tube 2 is still way above maximum dissipation of the 19.5W spec'ed for the 7591. Tube #4 is actually running @ 18.23W or about 95% of max diss. Tubes 1 & 3 are running about 82-83% of max. It's recommended that they run between 70% to 80% of max dissipation.

Your best bet to do that is to build an Individual BIAS/BALANCE Adjustment board(hereafter IBBA). This board will balance each pair for current draw (0% difference between them) and then bias them to approx 75% or 14.6watts per tube. It's better when biasing tubes to bias them for current draw than to rely on a set voltage as the current draw can be all over the place as your tubes are now. With the current draws you have now installing a 10K variable resistor inline with the bias supply won't drop them equally. You'll still have 1 pair that is running higher (hotter) than the other.

With the IBBA You 1st balance the current draw between the pair of tubes (1&2 or 3&4) nulling out the difference. Then bias them to a set current draw. With the tubes balanced and biased this way, sound quality is enhanced as both tubes are working equally, and putting out the same wattage. Plus with them running @ 75% of maximum dissipation, they run cooler, and last much longer (5+ years is not unusual). With the way they are now, I would expect the hotter tubes to die shortly (<1year) with the cooler tubes lasting somewaht longer. With the cost of a quad of 7591's approaching 1/2 of a mortgage payment (old stock), it's a no brainer.

Move the decimal to the LEFT 1 space when converting mv to ma using a 10ohm resistor. Then multiply the result by the plate voltage. Result is wattage (rough estimate as it does not account for screen draw). I used 440V in your example as it's easier to type in.

TUBE #4 .417mv / 10ohm=.0417a .0417 x 440v=18.348W

The imbalance between the tubes can account for an overdriven condition when played loud. Especially when the imbalance has one tube over max dissipation at idle. Build the IBBA board and balance / bias the tube to 75% and you'll notice a lot smoother sound quality, less distortion, longer lasting tubes, and a cooler running receiver.
 
I think you'll have a hard time balancing tube no 2 with any of the other ones, at least that's been my experience. Personally, I'd go with the IBAM.
 
They both get the same job done at the end of the day, but personally I find bias/balance adjustments to be easier to deal with than individual bias adjustments. One feature with the IBBA setup is that it is fail-safe, meaning that if one of the adjustment pots fails, the tube gets full bias and doesn't melt down.
 
What gadget says is right, however, I've found in more than a few situations where one pair will not balance with the IBBA if their cathode current varies a great deal. I don't know if 9 mA is one of those problem situations or not. It's not that hard to bias individually with an IBAM. You do lose the fail safe feature but after 6 or 7 years of doing IBAMs I haven't seen one fail yet, plus the 10 ohm cathode fusistor might save the day. I like the balance/bias scheme and use it for matched or simi-matched pairs. Sansui 1000A did a factory IBAM in their later models. US mfgs, e.g. Fisher, Scott, Sherwood used the IBBA a lot.
 
Fair point. The non-matched tubes on my Sherwood 5500 require some trickery to get it to behave with the balance controls. I had to fiddle the bias supply voltages in such a way that one pair ends up close to the top of the adjustment range, one near the bottom and I think its the left channel that has the balance pot about 3/4 of the way to one side. The amps I own with matched tubes don't do this though, both channels end up with the balance pot set very close to center and the bias level controls are fairly close to one another as well.
 
You must log in or register to reply here.
Back
Top Bottom