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fisher 500B help

Fisher & NOS tubes

I have copied this from an earlier AA post:

"For what its worth, I have had horrible quality control problems with the EH 7591As - all "matched" quads - in my Fisher KX-200. Tubes going critical without any rational pattern: different tubes, different sockets, different times. Never could bias or balance them properly. My tech replaced every resistor, and other appropriate components, suspect or not, and still no help. One "going critical" EH 7591A took out one of my output transformers.

NOS, US-made 7591As bias and balance correctly every time, and keep their settings often after a thousand-hours of playing. Even used ones adjust well. And the sound - sorry, no comparison. The midrange is far more liquid and smooth, with clearer highs and more distinct bass."

My sunk cost in the KX-200, starting with about $350 for the original purchase and shipping, iand then adding six Telefunken 12AX7 smooth plates, two pair of matched NOS 7591As (and several spare NOS Quads), all new resistors, socket tightening and cleaning, testing, etc. can best be imagined.

But it sounds great!

Here's a photo:
 

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I have restored/upgraded a Fisher 400 and a Scott 340B, actually both worked well stock. The Fisher is more solidly built (heavier steel chassi) and I suspect some of the parts are better. The Fisher is also easier to work on but I have to say that after similar work performed on both units my Scott sounds better than my Fisher 400, though it is a different type of sound. My Scott has better 3d imaging. My Fisher is still one of my favorite vintage pieces and other than the imaging is at least equal to the Scott in sound quality.
 
Fisher & NOS tubes

I neglected to add in, as a sunk cost, the price and shipping for my KX-200 parts unit (which turned out to work just fine) of about $200 in my operational KX-200. Well, as a result, I now have a spare power transformer and one more OPT, in addition to several Tele 12AX7s, knobs, switches, sockets, etc.
 
That reminds me. There is a little 2-7pf ceramic capacitor across pins 2 and 7 of the 12ax7 phase splitters that must be there or the the output tubes will go critical. The NOS are more forgiving if this is missing. This is not shown on many Fisher schematics. Do not make the mistake of removing them.

If you have a link to the cathode resistor discusion please post it. I cannot seem to find it.

By the way I have had the opportunity of fairly comparing the GOS (known good old stock), EH and JJ tubes, both by ear and measured distortion and power. The EH come out a little ahead, believe it or not. The JJ's and GOS are about equal, with the power of the JJ's being a little less. But this was done in a fully restored circuit with an individual tube bias mod.
 
drewbolce said:
That reminds me. There is a little 2-7pf ceramic capacitor across pins 2 and 7 of the 12ax7 phase splitters that must be there or the the output tubes will go critical. The NOS are more forgiving if this is missing. This is not shown on many Fisher schematics. Do not make the mistake of removing them.
Click to expand...

Are you sure about this? Pins 2 and 7 are the two grids of the 12AX7, and 2-7pF seems an awful small value to have any effect in the audio range. As a matter of fact a small value cap like that would barely pass some RF (Radio Frequency).

I know we are talking about a 500B, but since it is very similar to a X202B, just checked under the hood of mine and there is no sign of these caps, nor on the Fisher service manual or the Sams Photofact.

Maybe is only used in the 500/800 amps to filter out some RF, or to prevent RF oscillations?

-Alex
 
Absolutly positive. It goes between the grids. It has nothing to do with anything in the audio range. I believe it has to do with the miller capacitance of the tubes. It is there to prevent a runaway oscillation. I have only seen one 400 schematic that shows it. I have seen it in most Fisher units which use the 12ax7 phase splitter.
 
I find it rather silly that a tech would not want to install cathode resistors in your 500-B "because the factory didn't do it that way".

Did he also want to install a NOS selenium rectifier??? Afterall the factory did it that way.

Fisher did not install cathode resistors into the majority of there units for a reason...COST. Four resistors times 200,000 = lots of pennies saved. Bean counting existed then just as much as it does now. Lose a fraction of a cent here and there...and the accountants are happy.

Plus it was a LOT easier in 1962 to get a replacement output transformer...and since the warranty period for a Fisher back then was 90 days...most likely the additional parts sold to a repair shop = more revenue.

Also...a 10 ohm resistor is such an incredibly low value...that it is almost not there.

At least your KX-200 has them!!!
 
EH 7591As

smollett5 said:
I have copied this from an earlier AA post:

"For what its worth, I have had horrible quality control problems with the EH 7591As - all "matched" quads - in my Fisher KX-200. Tubes going critical without any rational pattern: different tubes, different sockets, different times. Never could bias or balance them properly. My tech replaced every resistor, and other appropriate components, suspect or not, and still no help. One "going critical" EH 7591A took out one of my output transformers.
Click to expand...

uh oh! Now I'm getting cold feet about using a newly purchased set of EH 7591As matched quads in my Fisher 500C.
 
uh oh! Now I'm getting cold feet about using a newly purchased set of EH 7591As matched quads in my Fisher 500C.
Click to expand...

Install 10-ohm cathode resistors and there's nothing to fear ;-)

Just out of curiosity, why'd you buy the EH tubes and not the JJ's?
 
mhardy6647 said:
Install 10-ohm cathode resistors and there's nothing to fear ;-)
Just out of curiosity, why'd you buy the EH tubes and not the JJ's?
Click to expand...

I was ordering some items from Parts Express a month or 2 ago & noticed a "sale" on the EH tubes in their flyer. So I thought, "why not try them." Just convenient. I hadn't read or heard anything bad about the EH tubes up until I read this thread. Still using old stock 7591s as of now.
 
I would not call the EH 7591 bad by any means...it is just large....and does not fit in some applications (sherwoods or ampegs anyone???). It is a fine sounding tube (not as good as the originals..but what is???) and will fit in every Fisher application...even though sometimes it takes a little inginuity to install them when running a wood case on some models.

As for quality control problems...all the current companies have them...anyone remember when JJ sent out that entire run of 5AR4's that were pinned wrong???

Tubes are subjective just like anything else in audio...for everyone who likes the JJ there is someone who prefers the EH.

I just think it is nice that we have two brands to choose from!!!
 
The EH's have not had any major problems since about 2001 or 2. I think it may have been a bad run. Don't be afraid of them.

I have had the opportunity to test EH's, JJ's and good old stock in a known good 500c. The EH's produced the most power with the lowest distortion and sounded best to our ears. The JJ's and old stock were about equal, being only slightly lower in power and distortion than the EH's. I haven't seen any real NOS in so long I doubt if I will ever get to run a test on them.
 
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