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In Reply to: RE: Fisher 800-C questions (newbie) posted by dadbar on June 30, 2008 at 20:18:48
Thanks to everyone for all your helpful advice and suggestions these past few days. I've got a lot to think about! I've also got some penny-pinching to do, since it's looking like a complete overhaul is probably the way to go.
I listened to my system again last night, and I must say I really enjoy the combination of the Fisher and the Klipsch even in the current state of things, though admittedly I don't have a lot of other hi-hi experience with which to compare it. (At least one person responded to my threads saying they did not like the sound of the Fisher, and even recommended some solid-state units instead!) I realize I may have given an impression that the system is unlistenable, which would be far from the truth. I put on the LP of Andreas Vollenweider's "Caverna Magica" at low volume last night and got chills...it really felt like I was in a cavern with musical instruments moving all around me! I never "got" this sort of music before owning this system, but all that sense of space and "live" instruments is just amazing. All except for that muddy, boomy bass that is...
Hate to sound like a crazy guy here, but now that I'm considering a complete overhaul I'm starting to worry that I may regret the work once it's been done. I've had guitar amps which were overhauled similarly and though the were cleaner-sounding, etc. afterwards, they never had the same "magic". Would that there were a way of hearing the difference before I decided to do the work!
Anyway, thanks again for all your help and advice. I'll be sure to post an update once any work has been done but with finances being what they are that may be a while...
A guide I use is if the sound is very pleasing and one is impressed everytime the gear is used, it must be good.
If you must keep the 800C and I can understand that, it must be upgraded with new parts for reliability. The new parts will have a heavy hand in the end result. I like Russian K40Y-9 for coupling caps & the Fisher will have plenty of coupling caps including ther tuner section. The PS is also a must for cap replacement.
Thanks as always for the thoughts, Neff. I'm definitely not impressed every time I play my system, so it looks like the time for doing something about it is undeniably here. I've got a lot of good suggestions to work with, and will definitely give the Russian K40Y-9s some thought. For the record, what do you mean exactly by the "PS" cap?
PS is power supply. I must say I am more of a quality sound vs vintage collector person. I seem to have this issue with being sensitive to harmonic distortion. Out of our of us brothers & sisters ages of 55-64, only two of us seem to have these ears that are not depreciating much with age including my 62 year old sister. I still hear to 16kHz and my ears are quite sensitive.
As some others have suggested, invest in a matched quad of EH 7591's for ~$80. It won't hurt to put them in briefly and see if the sound quality improves. If there is no change you can easily put things back and go from there. The matched quad would be easy to sell on Ebay if you don't think you need it. If the change makes you happy then it will point you in a different direction...toward modifying the grid leak resistors & coupling caps to reliably handle these tubes (a very cheap mod).
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...
"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.
If the bias on the tubes isn't correct it will smoke $XX dollars worth of tubes! The modification of the grid resistors and caps is NOT something that should be considered optional. You may get some EHs to tolerate it, but you won't know until you have one red plate on you that the ones you have didn't tolerate it! Then you're out some money.
Wow, there's some strong dissenting option to the idea of testing out the Electro-Harmonix tubes (BRIEFLY) in order to decide whether to go through with the rest of the modifications.
I've also just found this about the current Electro-Harmonix substitutes on thetubestore.com: "Electro-Harmonix 7591A: This is a new tube that replaces 7591 types without the resistor mod that is required with the 7591XYZ. Their height can still be an issue, though, as they measure 3 3/8" (about 9 cm) from the bottom of the base to the top of the glass."
Does this mean anything for our current conversation? Also, is the height of these going to be an issue with my wooden cabinet? Will it pose more of a fire hazard, etc.?
Thanks as always for your help!
Look at the pic I linked to. It's no problem at all using EH tubes in an 800C.
"Wow, there's some strong dissenting option to the idea of testing out the Electro-Harmonix tubes (BRIEFLY) in order to decide whether to go through with the rest of the modifications."
How are you going to tell what the amp sounds like unless you let it run long enough to be fully warmed? If you do that, there is a chance that a tube or tubes could act up.
There's no one who has posted that what I described can't occur, they likely just didn't think to mention it. I did think of it because I have to be sure to caution anyone who buys 7591A EHs from me that it can occur, so it comes to mind automatically so to speak...
It's up to you if you want to take a chance. But since you'll need to make the change no matter what, why don't you make the changes before the tube swap? The parts you need are under $10.00 (at least from me), and it's not a huge task.
Thanks, Jim. I didn't realize before that you are the guy everyone else keeps recommending!
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."
That sounds like a pretty big hassle to me, as a novice. However, I am interested if as you say it's not a big deal to make the necessary changes myself (using your kit/tubes) in order to use the new output tubes myself. But the only experience I have is basically using a soldering iron to replace guitar pickups, so it would have to be a pretty basic project indeed!
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.
Thanks again for your help!
"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!
Yeah, what he said.... Seriously, you will certainly change the tone of your Fisher when you FIX it. There are some tubes and parts in there that are running on about 5 out of 8 cylinders. They are way past their designed service life. You are not listening to the amp circuit at its full potential, like when it was new. It is crippled. Sure, it sounds pretty good. Welcome to the world of tube audio! BUT, please consider that it will sound better when all the parts in there are operating properly. Change the resistors, the caps and the output tubes. Test the small signal tubes and simply change the ones that aren't testing strong. A new production 12ax7 that is very good is only about $15. Really, they are very good. The tungsol reissue or the new mullard reissue 12ax7 are nice tubes. But you may not even need any...just the output tubes. If you re-cap the power supply, the coupling caps and fix the grid resistor problem and put in new output tubes as Jim suggests, you will be amazed at how much better that amp will sound. Also, remember that all the new parts take 30 or 40 hours to burn in at a minimum so don't judge it right away after you do it.
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