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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: Fisher 800-C questions (newbie) posted by Larry1304 on July 1, 2008 at 10:14:18:
Hi Larry,
"I'm wondering what your thoughts are on this earlier response:
'Fisher "cheated" in the value of the 7591 grid leak resistors used. Current production 7591s tend to misbehave in the out of spec. regime. The comprehensive solution to the problem is reduced grid leak resistor value and reworking the driver/splitter from a 12AX7 to a 12DW7.'"
I totally agree. The RCA manual says that for fixed bias applications (that's what you have) 300K Ohms is the limit. Fisher often used 330K Ohms! And being carbon composition resistors, over time they tend to drift up, up to the 360K or even a bit higher range. Excessive grid resistance can cause the tube to run away, and while the old tubes were tolerant of high grid resistance the new ones aren't.
Now if you change the caps and resistors as I mentioned you can then use both old stock and EH (or JJ if for some reason you want to use them...). The result is very satisfactory. It DOES make the job of the 12AX7 more difficult though, because (long explanation omitted) it requires a bit more drive power from the 12AX7. You can use a 12DW7 (which is 1/2 of a 12AX7 and 1/2 of a 12AU7 in the same bottle) instead of the 12AX7 and the "12AU7" section of the new tube has much better drive capability.
My suggestion at this point is to 1) change the caps and resistors, 2) make sure you have a healthy 12AX7 in that driver position, and 3) use the new EH tubes. Then, after your skills (or budget, in case you want to have it done by a pro) increase then go back and rework that section. If you intend to do it yourself that's the easiest path to take.
"One more question: after making the changes to use the Electro-Harmonix tubes, would it then be possible to go back and use the original or NOS tubes if I desired? As I mentioned before I don't want to sound like a crazy guy around here, but I'm worried that the receiver may lose some of its "magic" with new tubes, since I've had that experience with guitar amplifiers in the past."
As I said, you can use ANY 7591A in the output after the changes. Will it change the tone some? Sure, anything you do will change the tone some. But if you choose to do nothing you won't be able to use the 800C at all in a very short time. By carefully selecting the parts you use you can stay quite close to the "vintage" tone, but some change is inevitable.
As you'll discover over time, all things audio related are compromises. You are looking at a couple compromises right now!
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Follow Ups
- RE: Fisher 800-C questions (newbie) - Jim McShane 11:55:45 07/01/08 (1)
- RE: Fisher 800-C questions (newbie) - dls123 08:28:20 07/02/08 (0)
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