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In Reply to: RE: Fisher 800-C questions (newbie) posted by dadbar on July 01, 2008 at 08:09:50
Thanks, this is exactly the sort of advice I've been looking for! It seems pretty common-sense, but I wasn't sure whether putting the tubes in for only a brief period as you mentioned would hurt the unit or not.
How long do you think it is safe to leave Electro-Harmonix 7591A tubes in with no changes?
Also, is there a way to mod the receiver to handle these tubes without doing any conversion to the 12AX7 tube types, etc.? Someone previously mentioned that "The comprehensive solution to the problem is reduced grid leak resistor value and reworking the driver/splitter from a 12AX7 to a 12DW7." Sounds like a pretty big hassle...
Follow Ups:
"reworking the driver/splitter from a 12AX7 to a 12DW7." Sounds like a pretty big hassle... "
Yes, it is, and it is foolish for anyone to suggest it. Come on people, help this guy instead of giving contradictory opinions. I knew almost nothing when I started my 800c (purchased unworking), but it turned pretty out well.
"Yes, it is, and it is foolish for anyone to suggest it."
No it isn't foolish. Not at all. In fact, you can COPY the circuit used in another Fisher model THAT USES THE 12DW7 without much trouble at all. And the gent who suggested it would be happy to help him, I know that for a fact.
"Come on people, help this guy instead of giving contradictory opinions."
You expect everyone here to have the exact same approach to the problem?? Come on, get serious. There are a MILLION different things you can/can't do, the budget for him may be different, the skill level may be different, he may indeed opt to use a $500.00 matched quad of NOS 7591A and then the resistors and caps don't need to be changed until the next time he needs tubes, he may choose to preserve it in totally stock form. It's up to him.
Just because an operation doesn't fall within your skill set doesn't mean it isn't worthwhile. And the guy you said made the "foolish" suggestion has helped as many people on this and other forums as anyone I know of.
I'm sorry for the rant, but unless you personally intend to communicate with the owner exclusively and intend to help him out at every single step, then you are going to have to consider the opinions of others. And Eli Duttman's opinion carries a lot of weight with me - and MANY others here. He's a good soul who wants to help, and a very smart man.
I am not as experienced as you Jim S. However, how about the next step up from a 12DW7 to a Holland E80CC or 6085 PQ and replacing that 12AX7 side of the 12DW7 tube plate & cathode resistors to appropiate values. I have taken Eico and a few other amps from so-so to very good sounding.
If it must be a 12DW7/7247, the RCA long plate is best per my experience. An RCA short plate was also manufactured of late, but more rare & less desireable.
BTW- I wish others could hear the Eico 2510 I upgraded. It is insane sounding for a receiver. Yes, E80CCs & other 'tricks'. I am waiting for new Transcendar SE audio transformers to finish it.
> > In fact, you can COPY the circuit used in another Fisher model THAT USES THE 12DW7 without much trouble at all. < <
Attached! :> )) The 7247 = 12DW7. Keep the fact that circuitry that drives "12" W. tubes also drives 7591s in mind. Both Fisher and Scott took full advantage of the opportunity and used the same driver circuitry in their 7591 products as they used in their EL84 products.
Eli D.
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