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Fisher X 101 B help

dsndblm

dsndblm

Super Member
I'm hoping some other Fisher owners can ease my concern about the temperature that my X 101 B output transformers run at. I got the amp about a year ago and it was all there but in need of some help. The filter caps were both swollen so they were replaced with correct mf units from AES. Two of the 7591's tested with shorts so I bought a new quad of matched JJ 7591's. The JJ's are biased at 50.4 V instead of the 44 V that the Fisher service manual recommends. I read that this will make the 7591's run a bit cooler. I also replaced the .047 mf coupling caps with new. The amp sounds good but after about 45 minutes or so the output transformers are pretty hot. The power transformer is pretty toasty also. I borrowed a digital thermocoupled thermometer and the power transformer is running at 160 degrees. The output transformers are a cool 150 degrees. I have built two other amps from scratch. One is a SE 6B4 and the other is a p/p EL34 amp. Both of these can run all day and barely get warm. I have good basic understanding of tube theory and can follow a schematic but I am at a loss on what direction to go now with the Fisher. Sorry for the long post.
Thanks and regards
 
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50V is HOT... especially with JJs. Assuming the circuit hasn't been changed, anyway. Set it to 40V and see if it's a bit cooler.
 
Yeah something is wrong there. I have an indoor outdoor thermometer with a sensor on a long wire. I have had it stuck on my 800c power transformer to see how it runs. With the ambient temp at 65° the tranny temp is only 87°. In the summer it was closer to 100°, but 160° tells me there is something amiss. Don't have experience with that fisher, but I would double check what the bias setting should be.
 
Welcome to AK.

Those temps are right about where my Scott LK-72A used to run.

After I started using a fan to cool the unit, those temps have fallen way off to under 100°F.

However, I noticed you mentioned replacing the original vintage tubes with JJ 7591S's. This is a tube that definitely would not be one of my choices for a replacement. I had many issues with those tubes. . .mostly in the quality control area, and lack of being a very robust tube.

Scott and Fisher amps were notorious for overspecing their grid leak resistors (connected to pin 6 on the 7591) which may be ok for vintage 7591's, but certainly not for the new production type. They generally standardized those resistor values at 330K, which is higher than the maximum value.

Since you're using new production 7591's, you need to consider dropping those resistors from 330K to around 200K (180K to 220K), and be sure they're matched. Then to compensate for that alteration, change out the four .047µF coupling caps with .1µF caps.

If not, the new production power tubes will generally start going into a runaway condition. This happens when the tube continually heats up and forces the bias voltage to go more positive, which makes the plate heat up more, which forces the bias voltage to go more positive, which makes the plate heat up more. . . .

. . Falcon
 
Your output iron is most likely picking up radiated heat from the 7591's. My sansui 1000A does the same thing within an hour. Do your usual tune-up and voltage checks.
 
I made the changes listed above by FalconEddy. 200k ohm resistors and .1uf coupling caps. I myself used the EH7591 tubes and they run great. I also use a small fan to keep the air moving, but I think you need to check those resistors and caps ad replace if necessary. John
 
cademan said:
Your output iron is most likely picking up radiated heat from the 7591's. My sansui 1000A does the same thing within an hour.
Click to expand...

Which is also exactly what my LK-72A is doing. Plus, my GZ34 is also sitting right next to the right OPT. They create a lot of heat in close proximity, but moving air across the area REALLY helps keep the temps down.

sloober said:
I made the changes listed above by FalconEddy. 200 ohm resistors and .1uf coupling caps. I myself used the EH7591 tubes and they run great. I also use a small fan to keep the air moving, but I think you need to check those resistors and caps ad replace if necessary. John
Click to expand...

That should read 200K ohm resistors. :)

. . Falcon
 
Thanks for the welcome and the information guys. I checked the amp out again and I want to make sure I'm understanding the bias function correctly. I measured 35 VDC on the 7591's grids to ground. Changing the bias setting on the amp to say 45VDC leaves the grid -10 VDC with respect to the cathodes, right? I also noticed that the cathode current in the X 101 B powers the filaments of four of the six 12AX7's. Am I understanding this correctly?

I also tested the JJ 7591's and two of the four are toast already. These tubes have around 20 hours on them. This is the second quad that I have had for this amp. Two of the first quad I got had shorts right out of the box.

Thanks again for all the help.
 
Once you have addressed any problems with coupling capacitors, incorrect bias and shorted transformers, things get better. You seem to have already done that. One thing that is a problem with using our equipment today is the line voltage. It is often around 123 VAC. These amplifiers were designed to be run on 117 VAC. Doesn't sound like much but it does make a difference. While I don't have a bucking transformer connected to my Scott LK-72, I do use a fan and have no cabinet on the amp. It runs much below 100 degrees F also.

Hope that helps...
 
dsndblm said:
I also tested the JJ 7591's and two of the four are toast already. These tubes have around 20 hours on them. This is the second quad that I have had for this amp. Two of the first quad I got had shorts right out of the box.
Click to expand...

Hence my comments earlier about why I won't purchase them ever again. The amount of money spent on a couple sets of these things would have nearly been enough to buy a quad of used vintage Westinghouse in very good condition.

Without those changes I mentioned earlier, many Fisher and Scott owners will see those new production 7591's go belly up pretty quickly. I, too, had at least two JJ's dead right out of the box and another that had a heater pin connection/wire loose inside the tube.

. . Falcon
 
I would check the resistors and caps. R85-88 should be changed from 330k to 200k. C32-35 should be .1uF. If these were not updated when you went to the new tubes, it is likely the problem. Going through new tubes like you are is very odd. 20 hours? I'd recommend sending a note to jim mcshane and ask his advice. He set me up with the caps, resistors and matched EH tubes, and will be able to help you get it working correctly. But if those values were not changed, I would start there. John
 
Thanks again to all. You guys are great. I will take your advice and do the cap and resistor changes.

If I was to get a quad of NOS 7591's in the future, what will the cap and resistor mod
do to them??

The JJ's have no more than 20 hours on them. Pretty sad..... The 6B4 G's in my SE amp that I built 1 1/2 years ago have been used every day. They were used when put into service and still test as they did when I bought them.
 
I am pretty sure that you can still use NOS tubes after the modification. But without doing the mod, you shouldn't use new production tubes. I bet this is why you have had poor life with those JJ tubes. When I did my Fisher I bought the EH tubes, resistors and Russian k40y caps from McShane. I would send him a note, he will be glad to help.
http://pages.prodigy.net/jimmcshane/
 
Sloober
Thanks for the link. Is there anything else I should do to the X-101-B while I'm in to this?

Is there any way to "fuse" the output tubes in a fixed bias amp like mine? I fused my EL 34's on my p/p amp that I built last summer with a 1/4 watt 10 ohm resistor . Its a great way to protect the tubes and easy to set bias. It would be nice to have the X-101 set up to bias each 7591.
Thanks again
David
 
Yes. Add a 10 ohm to each 7591 pin 5 to ground. There should be a wire jumper there now, just replace with a 10 ohm. Make sure you don't grab 10k by mistake as I did once. Darned if that didn't cut down the level! On my 800c I have added a 5k pot so I can adjust the bias voltage. Trying to do it with individual fixed resistors is a big pain. You could take that one step further and do individual pots for each 7591, but that seems like a lot of work.
 
Sloober
I don't think I can add the 10 ohm resistor on this amp. Pin 5 of all tubes are connected on a common rail and the current runs through a 5K pot. The "fixed" bias then feeds the filaments of four 12AX7's. I read that Fisher did this to supply the filaments with clean DC. I posted the schematic for this amp on the yahoo fisher tube audio group last summer if you would like to take a look.
 
Yeah I just looked. Never seen that before. Might want to ask McShane as he has much more experience, although I don't know if he knows this model. John
 
You CAN add 10 ohm resistors, but they won't be to ground - they'll be from the cathode pin to the common bus. You COULD put in four bias pots, but they will interact a LOT through the cathode resistance (preamp heaters). Turn one up, the others all go down...
 
Would adding the 10 ohm still act as a fuse to protect the output tubes? If not could you simply add fuses? John
 
dsndblm said:
If I was to get a quad of NOS 7591's in the future, what will the cap and resistor mod do to them??
Click to expand...

You could check with Jim McShane on that, but I believe it will simply allow you to use either vintage OR new production tube sets without issue. (Not mixed sets)

. . Falcon
 
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