Fisher 500c burned resistors

[audiodon]

I agree with NOSValves that EHs are hard to get to bias close and not to drift, even with 200k or 220k grid resistors.

I'd give JJs that are known good a try but I haven't. Since the internet rules, whether right or wrong, JJs in units for sale drop their value.

But my solution has been to snap up vintage 7591s when possible or buy lots of EHs until I find good matched pairs at 15 minutes, 24 hours, and 96 hours.

 

[drewbolce]

This is the main reason I quit using EH years ago the bias drift was tremendous and very inconsistent tube to tube. It's possible they have remedied the problem but I doubt it.

Craig

The problem you refer to is for tubes dated before 2000, so you quit using them many, many years ago. Tubes dated later were burned in, remedying the problem. The only bias drift (other than normal gradual aging) I see now is due to circuit issues which over stress the tubes, such as red plating, letting the bias go to zero volts, bad coupling caps, etc..

P.S. I will admit I haven't seen any JJ's for awhile now, since I quit using them about 3 years ago. At that time I couldn't jump fast enough to stop "bias runaway" which occurred pretty consistently within the first week of use. It meant that you would have to throw away at least one tube in a quad and the remainder would be badly mismatched.

 

[NOSValves]

umm nope I quit using the EH when the JJ came out which if memory serve me right was less then 5 years ago. I have always bought JJ from the same source and have used a boat load of them. This seller contacted me to try out some of the samples he was sent when they first came out so I know all to well about the initial problems. I think I have had to have about 10 replaced prematurely and almost of those were from the very first few batches. But my tube dealer burns in and tests the dog out of all the power tubes he sells.

If EH has fixed the problem then why is everyone told to change the grid resistor value by such a huge amount below the spec of a real 7591?

If the tube was a real 7591 no changes would be required at all.

 

[NOSValves]

Funny I just pulled a bag of old EH's I have here that prematurely failed... they are dated 2- 02, 3- 03 and 1- 04 I believe the first number is year correct?

 

[drewbolce]

I agree with NOSValves that EHs are hard to get to bias close and not to drift, even with 200k or 220k grid resistors.

I'd give JJs that are known good a try but I haven't. Since the internet rules, whether right or wrong, JJs in units for sale drop their value.

But my solution has been to snap up vintage 7591s when possible or buy lots of EHs until I find good matched pairs at 15 minutes, 24 hours, and 96 hours.

I should be fair, when I talk about the bias values I am using my bias circuit which allows me to set the bias individually for each tube. If you are using a circuit like the stock fisher or a modded one with one pot or even a pot on each pair you cannot track the drift accurately even though you can measure it across each cathode resistor. The reason is quite simple, the bias circuit is interactive, and each tube will change somewhat differently and therefore change the reading on the others.

Another issue is what is acceptable drift. With a good voltmeter giving reading of millivolts across a 10 ohm resistor we resolve 0.1ma. If I set my tubes at 33ma then I accept 30 to 35ma as good. If you are calling 1ma too much drift then you are too critical. Also remember you can only measure the tubes while idling, no music and you must have a speaker load.

Why are so called NOS tubes so stable? Because they are really OS. New Old Stock tubes are extremely rare in power tubes. Why? Because they lasted only a year or so, particularly as hard as Fisher and some of the others ran them. So a tech's tubebox may have had many true NOS 12ax7's, etc., however, most of the power tubes would be pulls at the end of their useful life or close. By the way, I found some failure data on tubes and calculated that the average 12ax7 in a Fisher would last 80 years, I feel sorry for those who felt the need to get rid of those old Tele's and put in some "new" tubes.

How do I know this? I have watched Terry put many of them to full power with his load box and a known good amp. Many cannot make full power (standard 3% distortion reading) or even close, the real sign of a worn out power tube. All of these tubes would test great on almost any tube tester.

Does anyone have a list of Vendor's who burn-in and match JJ's, I would be interested.

 

[drewbolce]

If EH has fixed the problem then why is everyone told to change the grid resistor value by such a huge amount below the spec of a real 7591?

If the tube was a real 7591 no changes would be required at all.

I'll let Jim chime in here, but if I recall his suggestion applies to all 7591's and is based on RCA tube data.

Sorry, I just don't share your experience with the EH's.

 

[NOSValves]

The tube data is 300K for the 7591.... 330k is just over spec but folks are being advise to lower the value to 180K to 220K.... something fishy about that.

The reason it need to be lowered IMHO is because its a close cousin to a 6L6...which has a grid resistor value rating of 100K...

I think you need to quit watching videos about EH and taking them as gods word.. Do you believe the BS about the reissue tungsol, mullards and Genelax also? Those tubes are not even remotely like the originals...Also NOS Output tubes lastes more like decades... Many of the 50 year old pieces of gear I rebuild still have original good working output tubes in them.

That burn in rack shown in that video is a joke for a major tube manufacturer they produce thousands upon thousands of tubes it would take way to many man hours with that small setup to truly burn in the entire production line of a large factory. Also that is not the factory that is New Sencor in New York the factory is in Russia and who knows what goes on over there.

I have to head out the door or I would of comments more.