![]() ![]() |
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
134.174.165.168
In Reply to: RE: Fisher 800-C questions (newbie) posted by Jim McShane on July 01, 2008 at 08:22:17
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!
Follow Ups:
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!
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!
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.
cheers,
Don
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: