Fisher 480a questions

[Thocom]

Is this the book?

7189 tubes are best suited for that amp-they're rated for higher voltages. GE-7189A tube is highly regarded, I have a few quads. Many other companies re-branded tubes manufactured by GE, but you can always tell a true GE tube because GE etched its logo and dots on the glass. Look at some pictures on ebay and you'll see what I mean.

For EL84/6BQ5 tubes (lower voltage rating than 7189 but will work)-Sylvania Black Plates seem to command a very high price on ebay. I have two quads that I haven't listened to yet. Before I try the Sylvania's in my SA100- I wanted to get familiar with my GE 7189A's (very even sounding, nothing seems to phase them) and then my Amperex 6BQ5's (very clear, brighter than my GE's and a really fun tube to listen to). I might try the Sylvania's later today before I sit down with a friend for a listening session (and scotch, of course).

In your amp's chassis the Amperex tubes are amazing! Blew my Aragon 2002 MKII out of the water. Much better than NOS RCA, Magnavox and Sovtek. My 12" 45RPM copy of Creedence's "Susie Q" put out by MOV sounds like Fogerty is in the room, no kidding, everybody says the same thing. I have three friends that want to buy that amp from me now (that's why I have 4 of them).

If you want to learn more about tube amps-pick up a copy of "Tube Amplifiers" by Morgan Jones. Really good, chock full of theory, easy to understand. He also put out a companion book "Building Tube Amplifiers" that has more to do with technique and layout and testing. Very good book-one of my favorites.

Is this the book you are referring to?
Paperback $62 bucks? Wow, I wanted to make sure before I shelled out 62 clams?
http://www.amazon.com/Valve-Amplifi...qid=1399810029&sr=1-2&keywords=tube+amplifier

 

[heyraz]

If you want to modify the amp to Stand Alone, the first thing you'll need to do is eliminate the need for the external speaker selector switch. This won't affect the operation of the amp while connected to the console, you'll only be bypassing the need for the speaker selector plug to be connected.

If you look at the left side of the photo-you'll see a Yellow and Orange wire coming from the tag board on the bottom leading to the speaker selector connector on the top. These are the signal wires. If you tie all of the wires on the top row of the plastic Molex Connector together (blue+white+orange) and then the bottom row together (yellow+brown+green) you have just eliminated the need for a speaker selector switch and all external speaker screws and plugs are active.

Initially I did this with home made top side jumpers. Later on I mounted two sets of 5 way binding posts over the speaker screws and moved the wires around. My apologies for not shooting an updated photo yet.

When I moved things around underneath, I left as much wire coming from the plastic molex connector as I could. The topside connector was useful as it provided me with a way to monitor some voltages underneath without having to turn the amp upside down. So don't snip the wires flush at the connector to tie them together, jumper them for now until I provide you with a photo and explanation of what I did.

Also-you'll need a 250 ohm 10 watt resistor to take the place of the three 12ax7 heaters (from the tuner/preamp) once the unit is converted to stand alone. The 250 ohm resistor drags the voltage down just as the three preamp tubes would have done when connected. You can see it in the photo. Don't order it yet, there's another resistor you'll need elsewhere to further drag the voltage down (I just don't remember the value off hand).

Rich

 

[gadget73]

You found NOS Mullards for 8 bucks a pop? Even new production Mullards at 8 per is cheap, but NOS is an absolute steal. Wish I'd thought to ask where they came from, I'd have cleaned the guy out at that price LOL.

 

[Thocom]

New Mullards

You found NOS Mullards for 8 bucks a pop? Even new production Mullards at 8 per is cheap, but NOS is an absolute steal. Wish I'd thought to ask where they came from, I'd have cleaned the guy out at that price LOL.

It was the last guy out there, I had to dig but found a box with 8 of them in it. I only bought 4, I didn't know I was getting that good of a deal or I would have bought all of them. I posted a pick of them.


Rich,

Thank you for taking the time out and writing this out in a very easy to understand way for a noob.. This is fantastic!

 

[larryderouin]

Rich; Sorry to pop your bubble, but the tubes you got are repro's. Note the "MADE IN RUSSIA" and the date on the tube. 12-07 is either December 2007 or July 2012. NOS Mullards are made in ENGLAND. These aren't bad tubes. Actually they are fairly good for new stuff. But they aren't NOS. I like the 6P14P-EV Russian tube for about 11 a tube. Equivalent to the 7189's which you can use in 6bq5/el84 amps. The Russian tubes are old stock, and they made a boatload (or three) of them, as the Russian military used tubes in almost everything to combat EMP from Nukes. Solid State takes extra hardening/shielding to guard against EMP, which adds weight, and takes up space for other items in the planes/tanks/etc. and costs more rubles, which the Russkies didn't have much of.

Larry

 

[Thocom]

It's cool

Rich; Sorry to pop your bubble, but the tubes you got are repro's. Note the "MADE IN RUSSIA" and the date on the tube. 12-07 is either December 2007 or July 2012. NOS Mullards are made in ENGLAND. These aren't bad tubes. Actually they are fairly good for new stuff. But they aren't NOS. I like the 6P14P-EV Russian tube for about 11 a tube. Equivalent to the 7189's which you can use in 6bq5/el84 amps. The Russian tubes are old stock, and they made a boatload (or three) of them, as the Russian military used tubes in almost everything to combat EMP from Nukes. Solid State takes extra hardening/shielding to guard against EMP, which adds weight, and takes up space for other items in the planes/tanks/etc. and costs more rubles, which the Russkies didn't have much of.

Larry

Yes, I kinda figured they were repros. That's ok for me now, I have two amps that take them and needed some regardless of quality. Once I get this Fisher 480 operational again, I'll worry about finding a good quality match for it. The next show is in Sept, so maybe I'll get my act together by then .

-Tom

 

[heyraz]

Sovtek puts out an EL84-M tube, which is rated heavier duty, more like the 7189. I think that's the safest tube to use on these amps.

 

[heyraz]

Here's some photos of one of my 481A chassis modified to stand alone. I changed a couple of coupling caps, added 2 resistors, a power switch and speaker jacks. I also changed the output resistors to KOA Spear carbon films- really nice sounding resistor on this amp.

I also utilized the speaker connector and the tuner connector to monitor the voltages when setting the bias. Look at the under chassis pictures, they say it all.

Hope this helps. Rich

 

[Thocom]

Thanks Rich!

Here's some photos of one of my 481A chassis modified to stand alone. I changed a couple of coupling caps, added 2 resistors, a power switch and speaker jacks. I also changed the output resistors to KOA Spear carbon films- really nice sounding resistor on this amp.

I also utilized the speaker connector and the tuner connector to monitor the voltages when setting the bias. Look at the under chassis pictures, they say it all.

Hope this helps. Rich

This helps ALOT. Thank you so much Rich! I owe you one..

/Tom

 

[heyraz]

Let me know how your voltages read when you're done. I found a huge discrepancy between two of my amps, one of which had an electrolytic change. I'm beginning to think the old electrolytic had a higher resistance and was dropping more voltage than the new cap. Really has me boggled.
Any questions-let me know.
Rich

 

[Thocom]

Will do

Let me know how your voltages read when you're done. I found a huge discrepancy between two of my amps, one of which had an electrolytic change. I'm beginning to think the old electrolytic had a higher resistance and was dropping more voltage than the new cap. Really has me boggled.
Any questions-let me know.
Rich

I really like what you did with the speaker selector molex for a test point, I would have never thought of that.. Let's face it, I wouldn't have thought of everything you did
I'm going to copy everything from your design, with the exception of the the power switch. I happen to have a neat little panel rocker switch that fits perfectly into one of the plug (power) receptacle switch rectangular punch outs.
Other than maybe a mini phono preamp, I can't think of anything I would actually plug into these so loosing one shouldn't be to big of a loss.

 

[larryderouin]

This should be chocked full of dumb questions so please bear with me

-Tom[/QUOTE]

In answer to the above.....My wife gave me a plaque years ago with the following on it.

1.) Answers................................$1.00
2.) Answers which require thought..$2.00
3.) Correct answers.....................$4.00
4.) Dumb Looks are STILL FREE

Still waiting on a question from her that will make me a couple of Bucks.

Larry

 

[Thocom]

Can I start a tab?

This should be chocked full of dumb questions so please bear with me

-Tom[/QUOTE]

In answer to the above.....My wife gave me a plaque years ago with the following on it.

1.) Answers................................$1.00
2.) Answers which require thought..$2.00
3.) Correct answers.....................$4.00
4.) Dumb Looks are STILL FREE

Still waiting on a question from her that will make me a couple of Bucks.

Larry

I'm going to owe about $1000 bucks by the end of this project!

-Tom

 

[Thocom]

Progress..

Rich,

I got the test points done tonight, see pic. I should be able to finish the rest tomorrow. Where did you pick up the resistors you were referring to? I have some ceramics that are equivalent..

Thanks,
Tom

 

[heyraz]

Mouser,
The KOA Spear resistors I'm talking about are to the output tubes (look at my photo, they are smaller and different looking than the Fisher resistors). You don't need to change them to make the amp work, I happened upon them because one of the chassis I got had been altered and I thought they sounded better than stock. Really sweet and clear. I can't remember if this is one of the values, but it's the right series. You can get 100 of them for $5.00.
http://www.mouser.com/ProductDetail...=sGAEpiMZZMsPqMdJzcrNwgK4/J7COtxbcexiPY6/vv4=

You will need two resistors to convert the chassis to Stand Alone. Both Wirewound, the values are visible in the photo. Most people mount the big one under the FP caps (from the yellow wire to ground), but I moved it under vent holes because it gets so hot. Heat is the enemy of capacitors, so I tried to keep the hot resistor from under them.

Eventually you'll want to change the coupling caps (0.047uf)...that can come later as you get familiar with the amp. I tried a few different brands, amazing how they can affect the sound so much.

Enjoy it! I got my first one for $125 and it is the best $125 amp I have ever heard, and I've been listening to amps since the 70's. That amp is el84 based, The Beatles recorded using el84 amps, so they sound great on this one. This amp is a great starter project-always brings a smile. I have much better amps now, but I can still enjoy the 481 as much as ever. It's a great backup.

 

[heyraz]

I'm going to copy everything from your design, with the exception of the the power switch. I happen to have a neat little panel rocker switch that fits perfectly into one of the plug (power) receptacle switch rectangular punch outs.

I couldn't find an exact fit rocker switch to replace the AC receptacle, at least not without a little trimming. And then I couldn't find one that was rated high enough.
What was an exact fit (and I use now) is a DPDT slide switch. Completely lines up screw to hole. It's all I use now.

 

[dcgillespie]

Hey -- If I may, you might want to consider reconfiguring your speaker connector to monitor cathode current from each output tube, rather than the Grid No.1 voltage as currently configured. Since there is only one bias control on these units, the (positive) voltage on these grids will necessarily be the same on all four tubes, and it doesn't tell you anything of the relative matching of the quiescent current between each channel's output tube pair.

Along with the usual 10 ohm cathode resistors and TP connections, it will take a 5th connection to the cathode buss line, since that would be the connection point for the negative lead of your meter with such a configuration.

Just a point to consider. Those are great little Fisher amps!

Dave

 

[heyraz]

Dave
Great suggestion to monitor the cathode currents, but I think it's dangerous to re-configure any speaker connector for that purpose. I think whatever connector is used must be idiot proofed for the future.
I considered ways to put meaningful test points on that amp, and really the only safe way was to run wires up to those molex connectors on the top. Not impossible, the problem was finding tie points for the 10 ohm resistors. Couldn't use the adjacent unused tube socket pin, because some of the 7189A tubes are internally connected. The big reason I nixed the idea myself was because to put it bluntly...it's a console pull. It would have been great if I could have found the matching plugs for those topside connectors, but there were none to be had. I couldn't even find a modern molex to fit in either hole.
On one of the amps I mounted solder tags with resistors that were wired to measure each tube and found that because of differences in the output transformers, one channel always pulled more current than the other.
I was going to split the bias circuit to left and right channels, but lost interest when I came across the SA100.
In the end I decided to keep it simple as designed by Fisher and measure voltages. Cheaper to buy a matched quad too. The most I would do is split the bias supply. I've given this amp a lot of thought.
Rich

 

[Thocom]

so far...

"Idiot Proof". Yup, thats what I'm all about.

Glad im not the only one who could not find that molex connector. i just wanted the pins and had trouble.
I got the speaker connections done this afternoon. made up a cool little panel on the milling machine at work (actually a buddy made it for me) I even personalized it with my last name.
I added the rocker switch, and i you were right Rich, i ended up nipping a little bit off the plug panel. Also added a blue LED, well, because blue LEDs are just cool.
Still need to wire them in, and add a resistor to the LED once i figure out where I'm going to pick up the power.
If you like the speaker panel I made Rich, let me know.. i can make more now that i have the drawing done (of course Id have to change my last name on it. lol)

-Tom

 

[Thocom]

stalled

So I think i have finished everything I need to do until my resistors arrive, 10W 250 ohm, and a 22K 10w Wirewound.

I haven't asked any stupid questions today but the day isn't through..

two things:

1. there was a couple of wires coming off this chassis to a tweeter and midrange that were by themselves above a larger driver, there isn't any crossover on this amp? I removed those wires, I assume I should not worry about that?

2. whats the best way to tighten up a tube socket? Dental pick and patience? I did order those tube tensioner springs that Dave pointed out for $1.75 each.

thanks,
Tom