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About6 months ago, I refurbished my Fisher 500B. I replaced coupling caps, added 10 ohm cathode resistors fuses, replaced the selenium rectifier, bias filter caps and diodes and EH 7591A tubes. It's been in use for at least 5 hours a day for 3 months. It worked fine. The power trannie ran cooler and so did the OPTs. I had some hum but noticeable only at high levels.
Yesterday I decided to reconnect my yamaha turntable while the amp was on. When I tried to plug in the AC plug into the aux power outlet in the back, it wouldn't go in easily so I nugged it a little and thus jarring the whole unit. When I did that, the amp started to make an oscilating mechanical sound and I could see some flashing from the output tubes. Then smoke started to rise out of the ventilation holes near the OP tubes, I pulled the plug.
The inner two tubes tested very low 20 and 30 or so on my TV-7 the outer two tested 85 and 60. When I flipped the amp over I can see that the cathode resistor for the tube that tested 20 was blackened and it measured 70 ohms. The resistor on the other inside tube tested 36 megs and the outer two resistors tested 10 ohms as they should. I checked the output tranformers (blue wires) they measured 230 ohms each.
Now, where do I begin to get her back to health? Any help would be appreciated in helping me fix this unit.
Follow Ups:
So far the EH7868's have performed well in my restored Fisher 400 and JJ7591's in my Scott 340B though I am now considering the grid resistor change the next time I open them up. I did modify the neg. bias voltage on my 400 to about -22v and it runs much cooler.
I'm sorry to hear that your B got nicked. But it does appear that the cathode fuses did their job, albeit somewhat sluggishly. I would replace all four resistors at this point. Use metal films, as they'll open the fastest, usually instantly with a loud and reassuring pop.If you already had some hum building, you might want to check the main filter caps to make sure they survived the stress. At 45+ years, all bets are now off with them.
While I've been down this road before with Jim over the Fisher grid resistor values as they relate to new construction 7591s, you can certainly bump the values down if you must. There is still an army of old Fishers chugging along just fine on 45 year old original outputs, so I don't consider Fisher's specs too aggressive for the tubes of the era.
But I would exercise some restraint in bumping the main coupling values as high as .1 uf, regardless of the outputs used.
But the reason your 7591s shorted was because of a hard jostle when running. That's a no-no with the newer production outputs, which I find less mechanically hardy than the older stuff. A 7591 has never been an easy tube to make, and I would have lingering concerns with the survivors, regardless of the tester results. For that reason, you might want to start fresh with a new quad. Outputs are still cheaper than OPTs. That, or find a set of used/test strong Westinghouses and rest easy.
Good luck!
Even with the new JJ and EH tubes and the McShane mods, expect some "incidents."In the past 2 years I've purchased 3 matched quads of JJ 7591's for different amps and 2 matched quads of EH 7868's...all from different reputable suppliers which are all AA sponsors. In all of my amps I reduced the grid leak resistors to 160k. With two of the matched JJ sets, one tube in each set went "south"....one after 9 months, the other after a month of use...the third (newest) set is fine for now after 3 months. The EH 7868's have faired better for me in my statistically insignificant sampling....all of the tubes have been fine (although one tube started out noisy and settled down in use).
In the future I may buy a matched sextet to have a spare or two...
The new 7591 won't tolerate the high value grid leak resistors fisher used. They must be made lower in value. Likely you will want to bump up the coupling cap size one notch to maintain frequency response. The limit is 0.3 megs and NOS will handle that but the new ones won't. I'd say no more than 220K. Jim McShane has discussed this before so search the archives. It is real (I promise) and don't discount it.
Russ is correct. Check the link below for a look at the resistors in question. They were 330K from the factory, and many have drifted up to 360K or more over the many years of use.There have been some cases where the new production 7591A tubes (EH and JJ) have had trouble in Fisher circuits using a very high value grid resistor. The best solution is to change all the grid resistors to 150K to 220K as Russ mentioned (from the old value over 300K - they even exceed the old RCA tube manual recommendation!), and change the coupling caps to .1 uf.
Your amp might have been right on the ragged edge and just the small disturbance kicked it into failure.
The diagnostic advice from Mike Samra is very good too, don't overlook it!
Also, if the main B+ filter caps are tired it is time for those to go too. Reduced capacitance from age/heat/etc. can reduce the amp's stability as well due to poorer decoupling.
In my Fisher 680-A "console" amp (from Futura console) I have put in 4 JJ 7591S.
Should these have grid resistors changed also?
Yes, you need to make the changes. JJ 7591s are just as "touchy" about grid leak resistors as EH 7591s are.Fisher used the "same old" circuit again and again. A 12AX7 section is VERY marginal in the "concertina" phase splitter role, when driving PP 7591s. EL84s - qualified yes, 7591s - resounding no. The drive VOLTAGE requirements of the 2 types are the same, but ... Replacing the 'X7 with a 12DW7 permanently resolves all the electrical issues. Send Jim an EMail and he'll explain, in detail, the whys, wheres, and hows.
Eli D.
Sidewinder,You will be ok and its sounds as tho you did the troubleshooting yourself..Good Job!! Like corerosin said the resistors for the cathode opened as they were suppose to.
Now you probably want to know what caused this..I cant believe the tubes got that weak from the quick short that opened the cathode resistors by having the tubes over conduct.Put the new resistors in but take some readings at pin 6 of each of the 7591s and dont touch the bias until you take the readings..If you see that the two tubes that have weakened have a lower negative voltage reading at pin 6 than the two good tubes,you need to find out why and correct it.
Your 10 ohm cathode resistors did their job. They now need to be replaced for the one channel. You measured 230 ohms for the whole of either output transformer primary. Ensure that each half of the primary, the red center tap to one blue lead, then the red center tap to the other blue (blue/white) lead are comparable between output transformers. If they are, then replace the two weak output tubes or spring for a whole quartet and you should be back in business.Also, remember that newer 120 VAC plugs are polarised, but older sockets aren't. The one pole is substantially wider than the other and is wider than the socket you tried to utilise. You might check to see if your utility 120VAC socket is cracked or just floating on the inside of the chassis. Also, check the suicide cap connected to the 120VAC line to chassis. Just replace it. Ask Ron Oehlert what an appropriate replacement might be. His is the unintended expert opinion on such matters. I might guess about a correct value for that cap, but if Ron tells you the right value, I'll bet he'll be correct.
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