Fisher tube chassis balance controls

Some Pictures of the X-101C Working

Dave;

Thanks for those suggestions on equalizing the drive through the output tubes. The adjustments are probably not far off now. I will pull out my 100W load resistors and signal generator plus the meter and make the adjustments you described. It is nice to know the best way to make this adjustment for minimum distortion.

I do have a suitable generator that can produce the 1kHz signal (and any other from below 10Hz to 1mHz). I just completed a final check by injecting a 1kHz signal into one channel while looking for any crosstalk into the other channel. This also served to check the full operation of the mode switch and all worked flawlessly.

Here are some pictures of the amp working.

Thanks again to you and to all the great people here on this forum! I could not have accomplished the fine results without all the help.

Joe
 

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Some Final Issues

I still need to do a few things:

Get a complete set of 4 correct tube shields for the low level input circuits.

Get one more new 12AX7 for one low level preamp tube.

Some additional paint touch-up of the inner front panel behind the door.

Get a perforated metal panel to use in a cabinet.

Clean the chassis top and remove all old lettering and then replace with custom made decals that I can do here at home.

Get a wooden cabinet to display the unit in.

Joe
 
Dave;

I do plan to send him some final photos of the working unit. He has seen some preliminary pictures of the progress I was making. He will be pleased as he has appreciation for good equipment and hates to see anything go to waste and ruin.

Joe
 
Dave;

What maximum power should I expect with the tubes running about 32mA?

Should I be feeding a signal to both channels at the same time or one channel at a time?

Use a sine wave and look for flat topping and back off?

Or, should I set the current in the tubes at 34mA and do the AC balancing adjustments and expect results close to what you achieved with Don's unit and then back off to 32mA for the long haul?

Joe
 
Those notes are packed away for the moment, so I would take the information you need from the thread. In short however, I recall the unit having no problems making 35 watts easy when driven individually, and maybe even that much with both driven. I think the quiescent current was set for 33 or 34 ma -- if it's not in the thread, I'll go fetch it (some work on the downstairs has some files packed away for the moment). I just remember that the power increase -- among other things -- was rather dramatic, as that thread also documented the original stock performance as well.

I typically measure power (which is always measured with a sine wave) at the onset of clipping -- that is, just at the point where further increase would cause visual deformity. Start at 1 kHz, check at 20 kHz, and at 30 Hz.

The results you achieve should be very close to what I achieved with Don's unit -- with one exception however: His unit had very good NOS tubes installed as I recall. The EH tubes are good -- but not quite as strong as the original American pieces.

Let us know!

Dave
 
Review Improving the Fisher X-101C

Dave;

Thanks for your response. I understand about packing things away. I will review the original thread and look at the statements about power levels.

The power measured just short of clipping is pretty much what I thought. One thing I can do is to drive one channel at a time (so I won't have to keep swapping meter leads from one pair of tubes to the other. I can start at a lower level and gradually walk up on the maximum power level, equalizing the cathode voltages for as close to null between them as I keep increasing the power level. That should be a fairly safe way of reaching the maximum power and balancing the tube currents without damaging the tubes in the process.

Once I find the maximum power level just short of clipping I can measure the voltage at the load resistor and determine the power level at 1kHz then do a test at 30Hz and 20kHz as you mentioned.

I looked at my load resistors yesterday after locating them (they had been packed away). They are Dale wirewound 8 ohm 1% tolerance load resistors rated at 250 Watts, not 100 Watts as I had thought. They are finned and designed to be mounted on a larger heat sink if needed. They can take anything the amp can deliver and far more. A friend who used to work for Marantz gave me these years ago. Now they will come in handy.

Joe
 
Joe -- I always verify on 8 ohms, but do my hard core testing/design from 16 ohms. This was standard practice back in the day, as using the 16 ohm winding means that the entire winding is used so that the complete characteristics of the transformer are accounted for.

It is not uncommon for performance to be slightly different on the 8 ohm tap from that of the 16, as the 8 ohm tap is rarely an exact "8 ohm" tap but rather, the closest they could conveniently make it to that impedance within the wind of the transformer. Because of the way output transformers are wound, the taps can only be made at the connection between two interleaved winding sections -- which may not happen to be exactly be 8 ohms. A 4 ohm tap is usually easy to make, since it represents exactly 1/2 of the winding. However, using the 16 ohm tap makes full use of all windings within the transformer, with the load reflected back to the output tubes always being exactly the intended impedance value without question.

Dave
 
Testing the output

Dave;

Thanks for those added comments. They will help.

I can put the two large resistors in series for 16 ohms. I still have two 8 ohm 25 watt resistors that I can put in series on the channel not under test (as a safety measure).

The Electro-Harmonix tubes will not be quite as good as the US made original tubes, such as the Sylvanias, but should be decent enough. I will report back what power levels I see. After re-reading the original improvement thread, I see that some of the tubes were biased to draw 38mA instead of 32 or 34mA.

With the bias set for 32mA at present I can hold my finger on the top of the output tubes if the amp is only operating at a moderate volume level. The only time they get hotter is if they are driven to higher levels. The power supply in the X-101C is not current starved like some amps with tube type rectifiers under higher output conditions.

Joe
 
Report on AC Balance Tests

I took time to take the 25 watt 8 ohm resistors I had and mounted them on a large aluminum heat sink with heat sink compound for best heat dissipation. I used these for one channel and the other channel had the two large 250W 8 ohm resistors connected in series for 16 ohms.

I connected my Fluke 8600A cathode to cathode on each channel one at a time. While connected to either channel I could adjust the AC balance control with the channel just short of clipping as seen on the oscilloscope across the 16 ohm load. I could never see a definite null or minimum on the DC voltage difference between the two cathodes being monitored.

I went back and tried to duplicate what I had already done measuring the AC voltage from the bias control to either grid on each pair of tubes. I found I could not duplicate the measurements I had seen previously.

Finally I connected the two channels of the scope across each grid of a given channel and moved the traces on the screen of the scope until they overlapped. Then I adjusted the AC balance until I got the best match in level comparing one to the other. This seemed to work best.

One problem I have is that I live in a high RF location. I am only 4.5 miles from a 10kW FM station at 100.7mHz and it couples into any wires I connect. The condition varies some depending on weather conditions. I suspect that is part of the problem. The scope probes help to eliminate some of the stray RF pickup I have to deal with since their ground clips and probe tips have very short lengths.

My best estimate on power out is about 32W on one channel and 30 on the other with just one channel being driven. I think about 28W and 26W with both channels driven. That is based on the observed P-P voltage measured on the load resistors after peak voltage is found without flat topping.

Joe
 

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Joe -- That sounds about right, as extensive testing with my power output tester has shown that the EH tubes -- while good performers -- typically output about 15% less power than the classic 7591A American tubes could. Of course, line voltage levels figure into the maximum power that can be produced as well -- itself typically having seasonal values to it.

RFI is insidious stuff. I too used to live near a radio tower, and getting rid of it was worse than getting rid of gum stuck on the bottom of your shoe. Once you've dealt with it, you never forget it. When I moved a few years ago, proximity to transmitting towers was a definite consideration.......

Dave
 
Operating and Enjoying It!

Dave;

Thanks once again for all your help along the way.

Yes when I first moved here I did not realize that the close FM station would be such a problem. I have to use my Kenwood KT-7500 tuners in narrow IF width to minimize the encroachment of that station on WRR FM which is 101.1mHz while the relay FM station is on 100.7mHz. There are few FM tuners that can deal with the situation and that is with an outside FM only 9-element antenna with its back to the offending station. WRR's signal level is about -59dBm while the other one is at -9dBm. I often have to add grid stopper resistors in amplifiers I use here because of the RF interference.

I added some photos to the post showing it back in its OEM metal cabinet. It is not the most beautiful thing, but does protect the amp from some damage. I plan to order a perforated metal piece to replace the missing one from this cabinet.

Joe
 
A Safety Precaution!

One thing I thought it best to pass on to others is the tight space around the terminal strip where the AC wires go to/from the front of the unit. This is the area right under the power transformer at the rear of the chassis. This same area is where the added LV supply and EFB feedback circuitry plus the fixed bias is located. This tight space could lead to potential AC wiring shorts to the chassis when feet are fastened to the bottom of the chassis or a cabinet.

When I began to install the chassis back in its cabinet I noticed the wires being quite close to where the screws that held the feet on the bottom of the cabinet.

I did two things:
1) I shortened the screws I was using so that with feet added, the top of the screw would not protrude above the captive nuts of the inside bottom chassis panel.
2) I carefully redressed the wires and re-formed the grounding lug of the terminal strip holding the AC wires. The result was all those wires moved somewhat closer to the vertical left side wall of the chassis.
Both measures help assure that installing feet on the bottom of either the chassis alone or on a cabinet would not short to the AC wires.

Be observant of your particular installation of the modifications to assure that there will be no shock hazard regarding the AC wiring.

Joe
 
One other possible change

I noticed when I was biasing the output tubes that there was a consistent difference between the current through one pair of the tubes and the other pair. This was true even if I swapped them to the opposite channel positions. A possible solution for this would be to have separate controls for the fixed bias for each channel. A concentric type bias control and a change in the value of the resistor to ground from the two controls would do the job nicely.

Although the tubes I bought were supposed to be a quad matched set, the matching really depends on what kind of equipment is being used to match the tubes. The chances are that most companies supplying tubes just use some sort of tube tester, which is not applying the same conditions as the amplifier in which they will be used.

Joe
 
Static Problem after restoration and modification:

I have been having intermittent problems with my X-101-C after finishing the restoration and modifications. There was intermittent static in the right channel output stages. It finally reached the stage where almost anything around the reverb jumper jacks or jumper cables (the original links were missing when I got the unit) would cause static. I also discovered the two phase inverter/driver tubes could be wiggled in their sockets and cause static. I cleaned the sockets with Deoxit and plugged and unplugged both tubes at least 10 or more times. That seemed to eliminate the static from the tubes being wiggled.

I then started probing with a plastic rod in the general area and was rewarded with a major loud pop and saw a huge spark jump twice near one of the phase adjust controls. I turned the unit off and began to check for a solder bridge or loose wire. I discovered that after I had installed the added LV power transformer for the EFB & Bias mods that the reinstallation of the phase adjust controls did not go quite as expected. There was a solder drip from the middle terminal of the right channel phase balance adjust control that was almost touching chassis ground. I used my soldering gun and applied heat with the chassis standing on one end until the solder blob moved onto the solder tip and removed it from the amp. I plan to continue listening to the amp the rest of the day. If no more static presents itself, I will consider the problem cured.

Any time a variable control with B+ on it is worked on or replaced it is a good idea to check each terminal very closely with good lighting and even a magnifying glass to make sure there are no potential shorts to ground that might develop. This also brings up the issue of voltage ratings of any replacement parts. Make sure the internal resistance element and terminals have good high arc-over resistance before selecting and using one. The ones in this unit were not original Fisher parts. I do not know what company made them, but they are definitely after-market replacements.

Joe
 
Another 6 hours of listening this morning showed the X-101-C to be working correctly. I never heard any static or pops from cold start until I turned it off after lunch. This is a great sounding amplifier! Its refined sound is not at all tiring to listen to.

Now if I can just find a set of Fisher XP-3 speakers - .

Joe
 
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