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What are the odds?

Thocom

Thocom

AK Subscriber
Subscriber
I found a 1963 Fisher Console (Electra) with a 480A amp in very good condition a few weeks ago in Salvation Army store.

Yesterday at a yard sale,I found one with the same amp but in very very poor condition. I still bought it for 5 bucks. It's a different model console, but same amp is used. I'm going to use some of the parts on the good one, but I think I may want to make a stand alone amp out of the water damaged unit. I brought up on the variac and behold, it powered up.

Now, to do this, I'm sure I'll have to add few resistors and a switch, etc.. I have the schematic and can probably figure it out.. Has anyone done this here (yes, I'm being lazy and asking, I did search a little bit, but didn't see any threads on it)

Thanks!
 

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You will need to research your project goal carefully, as the power amp and preamp chassis must remain connected together for the power amplifier to work properly. I'm not just talking about jumping out the power switch terminals in the preamp section to apply power to the power amp section. Fisher used the heaters of the 12AX7s on the preamp chassis to (in part) bias the output tubes on the power amp chassis. Therefore, the units must remain connected together, or appropriate modifications must be made to the power amp chassis to allow it to operate in stand alone fashion without the preamp connected. This topic has been discussed in a number of threads on the Fisher Forum.

Good luck with your project!

Dave
 
Yes

dcgillespie said:
You will need to research your project goal carefully, as the power amp and preamp chassis must remain connected together for the power amplifier to work properly. I'm not just talking about jumping out the power switch terminals in the preamp section to apply power to the power amp section. Fisher used the heaters of the 12AX7s on the preamp chassis to (in part) bias the output tubes on the power amp chassis. Therefore, the units must remain connected together, or appropriate modifications must be made to the power amp chassis to allow it to operate in stand alone fashion without the preamp connected. This topic has been discussed in a number of threads on the Fisher Forum.

Good luck with your project!

Dave
Click to expand...

Thanks.
Understood. I've read a little bit on this, and others have said exactly what you have.
In looking at schematic, I believe it will not take too much work or materials to separate them. This analog stuff is new to me, (I'm a tube noob) I've worked with digital and SMT technology for the last 25 years. Point to point shouldn't be too different then PCBs.
 
transistors glowing

I was told once by a young'un that my transistors are glowing and I should shut it off.

Paul
 
Glowing transistors

pdm4606 said:
I was told once by a young'un that my transistors are glowing and I should shut it off.

Paul
Click to expand...

:)


Well... This played very well for about 20 minutes with a CD player running into the aux input..I did bring it up with a variac. Then one of the EL84 "transistors" REALLY glowed!! It actually looked very cool, like a mini Jacobs ladder. Purple arcs running all trough the tube! It would be cool if that's what they are designed to do. I guess it was one of two things.. I didn't let the deOxit dry enough, or the tube just failed.
I have a couple other El84 tubes laying around, how important is it that they are matched? (Told you I'm a noob).

-Tom
 
And one hell of a vacuum pump! (I once knew this dusky young thing in bangkok who could pull a soccer ball thru a ..........well you get the idea).

SERIOUSLY. Don't power that one up until you've replaced the coupling caps for the output's. #1 or #2 reason that tube took off like a roman candle.
 
Thanks.

larryderouin said:
And one hell of a vacuum pump! (I once knew this dusky young thing in bangkok who could pull a soccer ball thru a ..........well you get the idea).

SERIOUSLY. Don't power that one up until you've replaced the coupling caps for the output's. #1 or #2 reason that tube took off like a roman candle.
Click to expand...

Gotcha. Thanks, do you think it may have damaged a thing else? Transformer? Other tubes, etc..

When it happened I had gently put my finger up against it, almost no pressure at all. I was just seeing how hot it was getting because it seemed much brighter than the other 3, that's when it looked like the Emperors electrical attack from Return of The Jedi!
 
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Ahhhhhh! The full story. Sounds like when you touched it one of the pins lost contact, and it went Chernobyl on you. Tighten up the pins on the socket, (very carefully) to ensure they are tight. put another tube in it. Don't touch them. If you feel one is hotter than the others, best to check it's voltages underneath.


AES has tube retainers that will keep the EL84 types tight and upright to their sockets. See this thread......http://www.audiokarma.org/forums/showthread.php?t=573073
Probably no other damage if you shut it off right then. But good idea to replace the coupling caps, and the power supply filter caps (if they haven't been done already).

Larry
 
Uh oh

larryderouin said:
Ahhhhhh! The full story. Sounds like when you touched it one of the pins lost contact, and it went Chernobyl on you. Tighten up the pins on the socket, (very carefully) to ensure they are tight. put another tube in it. Don't touch them. If you feel one is hotter than the others, best to check it's voltages underneath.


AES has tube retainers that will keep the EL84 types tight and upright to their sockets. See this thread......http://www.audiokarma.org/forums/showthread.php?t=573073
Probably no other damage if you shut it off right then. But good idea to replace the coupling caps, and the power supply filter caps (if they haven't been done already).

Larry
Click to expand...

So I tightened up the pins in the socket, and put in another tube (one from a magnavox, but an el84) and no sound. Everything lights up, just no sound or hum or anything from any input.. Time to pull it and check it out. Hopefully I didn't fry the trans...
 
I've modded four of the 480-A/481-A chassis to work as Stand Alone amps.

I prefer a power switch on my amps, so I widened the hole where the phono/turntable cables came out and put a toggle switch in there. On another unit I removed one of the AC convenience outlets and put a DPDT slide switch in its place to control the power.

On all of my units I put 5 way binding posts in where the speaker screw terminals were and bypassed the speaker selector switch.

I haven't been using those amps since I upgraded to a SA100, but the 480/481 is amazing for it's size. I'll have to dig one out and shoot you a picture of the underside-it will make the conversion easier for you. I can also show you where to put the resistors to make up for the missing tuner/preamp.

On that amp-matched tubes are essential for optimal operation. Even then, one channel may bias higher than the other. I was going to split the bias control into individual channels but lost interest once I found a SA100 for sale. Now I'm working on a SA300B, but when I'm done, I'll finish up the 480/481's and either sell them or gift them to family members.
My favorite output tubes on that amp are by Amperex-the ones with the "D" getter. Tele's, Amperex or Mullards make fine input tubes.
You should think about replacing the coupling caps. The large electrolytics are a little harder to find in the right values. The amps I got were in excellent condition, I brought them up to full voltage very slowly on a variac, but I know replacing them is in the future.

Wonderful amp-it's the one that got me into EL84 amps. Completely "wowed" all of my friends that heard it. Get the schematic and look underneath...it will start to make sense to you.

Rich
 
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