Identifiying The Tubes In My Fisher 800B

Hi -

About a month ago I posted a thread asking for suggestions about what might be done to get the most out of my new Fisher 800B. AK member Sony6060 asked me to identify which tubes were in which positions, and I wanted to do that now. For while I'm getting a relatively good sound out of the 800B, I am certainly not getting the "holographic" effect that I've heard many tell about. To restate, my tech did say that the tubes were overall running at about 70% capacity, and that the 7591's would need to be replaced in a few years. He also did a lot of work to put the receiver in very good running condition, however was also up front about there perhaps being a few more things that could be done under the chassis that might improve its sound.

I guess what I am thinking here, is that before trying to do any further refurb on the receiver, that after identifying the tubes that are identifiable here and having described their condition, that depending on the feedback I receive from those of you who are more knowledgeable about this, perhaps I should try some new tubes first. I also noticed that one of the 7591s in the output section seems to be missing a guiding pin. I would imagine that that might effect the sound somewhat.

Anyway, here it goes. I was only able to identify some of the tubes, in that some of them didn't have anything written on them. The tech said that he thought many of them might be RCAs though. Here also is a quick pic...




v1 - ?
v2 - ?
v3 - ?
v4 - ?
v5 - ?
v6 - Bogen
v7 - ?
v8 - ?
v9 - ?
v10 - ?
v11 - Fisher
v12 - ?
v13 - ?
v14 - Fisher
v15 - Fisher
v16 - Fisher
v17 - Fisher
v18 - Fisher
v19 - Fisher
v100 - ?
v101 - ?
v102 - ?

Well, there are a lot of question marks there. But my basic questions are:

1) if the tubes are at 70% capacity, should I change them all out for NOS or brand new ones that are at full capacity?

2) Should I perhaps change the 7591s first, in that one is missing a guiding pin, and also because the tech told me that they would have to be replaced within a few years of normal use, anyway? (BTW, are 7591s the same as 7591a's? Noob question right there.)

3) If I were to change the tubes, would I be changing the sonic signature of the Fisher in anyway dramatic way?

So, thanks in advance for any advice anyone can give me.

Best -

Gannon

1.) Not necessarily. Unless you're one of those purist's that are anal enough that all the tubes have to be perfect in everyway. I'd kill for a set of tubes that ALL rated 70%or above.

2.) You can get a pin locating adapter that slides over the pins (it has a new center pin) from AES, and other tube stores. 7591a's are a newer subset of the 7591's. No discernable difference for the average user. Run them till they die!!!

3.) Not that I've heard. More like subtle tuning of what you have.

Until you have the unit completely gone thru and replace all the worn out caps, and out of spec resistors, and get the unit back to spec's voltage wise, doing anything with the tubes is a futile exercise. Caps and resistors will give you the most improvement. The last thing to change is tubes. If you change them.

It's like sending a 57 Chevy in for a tune up. ( 2bbl 283cu in. V-8., FACTORY STOCK IGNITION SYSTEM). spark plugs are analogous to tubes here.

The only thing that ends up changed is the points and condenser (a capacitor) inside the distributor cap. Nothing else. The engine will run, but unless the cap, rotor, and wires are changed out it won't run as well. The Plugs look clean and slightly brown but the center electrode is about 1/4 worn ( good for our purposes here, clean and regap). If the plugs were changed but not the cap, rotor and wires, the engine still runs but again not as well as it should becuse they aren't being supplied full designed voltage.

So the right way to do it is change the Points, condenser, cap,rotor, wires, plugs, reset the timing, and reset the idle mixture and idle speed on the carburator.

NOW the engine will run as efficiently and powerful as was designed at the Factory.

Same thing for the 800-B. Unless the various systems (circuits) underneath are ALL brought up to factory spec (or as close to it as you can make it) it won't work right. And unless you can afford to send it to a shop that does this for vintage gear, the only other way is to do it yourself. There are as many ways to do this as there are guys doing them. And there are more parts out there that are more alike than the marketing dept's of the companies would have you believe.

Best way is to set it up with basic caps and resistors to get a baseline sound and response. Orange drops and Nichicon UPW series(general lytics) and UKL (signal paths) are good for this. Then if you want to experiment, others will chime in with other types. But get a good baseline sound 1st. And that means replacing all the caps (except the ceramics as they are a special type and don't usually go bad.) and out of spec resistors. The can caps will also require replacement, either due to electrical leakage (you can't see this except on a scope)as they dry out or physical leaking/bulging. Figure about $200.00 for parts including can caps. (this doesn't include any tubes).


TUBE Circuit breakdown with tube numbers.

V1 thru 7 inclusive. TUNER TUBES LEAVE THEM ALONE unless one dies, then change it. Indiscriminate changing CAN/WILL affect alignment and tuning. A re-alignment is almost always required. This is something you CANNOT DO! The reason's are long drawn out, and involve Radio frequency theory, and expensive equipment that you'd probably use once.

V8-V9. Tuner indication tubes NO NEED TO CHANGE unless bad.

V100-101-102 MULTIPLEXER Tubes. DO NOT CHANGE OUT (SEE TUNER TUBES).

The following are AUDIO SIGNAL TUBES. These can be changed or moved around on the chassis to sockets that have the same tube.

V10-V11 Phono Preamp.

V12-13 Tone Control Amps

V14-15 Voltage amps/phase inverter

V16-17-18-19 POWER AMPLIFIER If the 7591's are running now, leave them. Improvements in parts underneath and circuit improvements can back the power output off a bit and extend the life of the tubes. FISHER ran these at the "REDLINE". Now aday's we tend to run them idling @ between 75%-80% of REDLINE and they sound fine. Dave Gillespie is working on a bias circuit that eventually will fit in all 7591 based units and have them idling @ about 50% and giving full power and sound.

Larry -

THANK YOU for your detailed response. I now know what my priorities are. I didn't realize that 70% was good for a tube. I obviously have a lot more to learn about them and electronics in general. Pretty much everything.

I contacted Mike Urban up in Connecticut a couple of weeks ago and had a very nice conversation with him. Again, some work was already done on the Fisher, but more can be done. I will try to get up there in the next month and have him work on it.

Thanks also for clearing up the 7591 issue. I will write down some of the recommendations made on my earlier thread about certain caps that could be used, etc. and run them by Mike when I see him.

THANKS AGAIN.

Gannon

If it's not too much trouble, can you take a couple of pictures of the underside? Give us an idea what was done, and can point out items that need to be addressed.

Larry

larryderouin said:

If it's not too much trouble, can you take a couple of pictures of the underside? Give us an idea what was done, and can point out items that need to be addressed.

Larry

Click to expand...

Here is my original thread on this Larry. There were a few suggestions made like installing Russian K40Y-9 capacitors and a few other things. I'm going to have go over the thread again before I go see Mike Urban. Of course, he'll probably be way ahead of me anyway. A lot of detailed pics of the underside of the chassis:


http://www.audiokarma.org/forums/showthread.php?t=533813