Fisher Console Amp Mod?

[cademan]

Crswanson, if you want to get your amplifier to power up, the blue & white wires coming off of the power transformer, are the 120volt taps.

Go from there! :thmbsp:

 

[analog addict]

That was it!!!! All is working as it should and I'm loving it. I ended up connecting qty 2 100ohm and 1 50 ohm in series. Project 481A = done

Do you have a slightly better picture of where you have the resistors connected?

Thanks....:thmbsp:

 

[auxtech66]

Do you have a slightly better picture of where you have the resistors connected?

Thanks....:thmbsp:

I can post a pict later today. There is a yellow wire which goes from the multi pin tuner connector and daisy chains to each of the 6bq5's. Remove the wire from the connector and attach one end of the resistor to the wire. The other end to ground.

 

[seamas]

So i am assuming there are no schematics for the 460-a and 460-t around?

 

[vanmeterannie]

I've been posting about my newly acquired 690A amp in another thread here at AK but am wondering about getting the amp up and running alone and holding off on the tuner/preamp I have since it needs far more money and work. Is the 250 ohm resistor the same on a 690A? So all I'd need to do to use it as stand alone if so is to bridge the on/off wires from the phono plug together (actually, I want to remove the plug and bridge them underneath) and add this resistor in place of the the yellow wire on the tuner harness shown here, attached to where the yellow wire connects and then to ground on the other end?

 

[Tom Bavis]

This is a hybrid amp - I REALLY doubt it's the same...

 

[larryderouin]

But the tuner / pre is all tube. His pin 11 & 12 on the amp, feed v-8-9-10 heaters on the tuner/pre, which are all 12ax7's. I'm thinking if they are doing that on the 460a's to counteract a lack of heater resistance from the missing 3 12ax7s, and his setup is basically the same, maybe it's a possiblity. His is a 690T(63Ambassador) which is similar to my 49T(Custom Electra VIII). The 690A amp is basically a 59A(65Futura) which the ambassador and futura also used in 64. then in 65 Fisher changed the Model #'s and both used the same amp and tuner (59A/59T).

 

[analog addict]

Sounds about right....

Some research I did online on the 460A (of which I have 2) suggests using a 300-330 ohm/10 resistor to replace the tubes on the preamp that would otherwise be driven by the amp's PS.

However, I never did dig far enough into it to determine exactly where the extra load went.....:stupid:

I spent a lot of time looking for the 460A/T schematic set up. I don't recall if I found it, but if I did, and I come across it, I'll post it up here.....:yes:

 

[WarrGo]

Sorry to drag up an older post such as this.

I am getting ready to power up a 440a like what is pictured here. I didn't find a local 250 ohm resistor. Is 225 close enough?

I have read that incorrect bias can lead to red plating and hum. I am having both problems. Just cleaned the bias pot so maybe that will help.

Will puting the 10 ohm resistors on the cathode as discribed earlier on page one make it not run right? Seems to me since the yellow wires go in series from pin 3 of one el84 to the nest adding the resistor would cause problems.

Lastly can I short across the RCA inputs to isolate noise? Or will this damage something.

 

[WarrGo]

Forgot a question. To use thi amp as a stand alone there is a yellow an a black wire coming from the power transformer that is used to power a couple light bulbs. Can I just tape these off or do I need to put a load here

 

[cademan]

Those wires are the pilot lamp taps. No load needed. You can use a lamp to tell if its on, or tape 'em off. Your choice!

 

[larryderouin]

Series up 2 100 oHm and a 50oHm resistor to get the 250.

Larry

 

[crswanson]

Hey guys. Sorry to dig this back up but I still could use some help. How do I make my amp stand alone? Pictures of my amp are on post #8 of this thread.

 

[larryderouin]

Put a switch between the BLUE and WHITE leads coming out of the transformer (see pic #3, post 8) per cademan(post 21). Hook up your preamp/source to the RCA inputs and the speakers to the outputs FIRST(you have to have a load on the output trannies) before powering it up.

Larry

 

[seamas]

This should help.e-46-x

 

[oldtexasdog]

I’ll take the chance of creating more confusion by sharing some pictures of my modified 440a amplifier and an untouched 481a amplifier. The amps look very similar except the 440 uses 7247 preamp tubes and the 481 uses 12AX7.

The first picture shows my 440 amp. The yellow wire coming from the tuner \ preamp plug is wired with the EL84 cathode pin 3. You can see in the picture the white 250 ohm 10 watt resistor that I added. At the very bottom of the same connector you might be able to see where I jumped two black wires. This bypasses the preamp \ tuner switch and the amp now powers up when plugged in. I added the 250 ohm resistor based on advice in a different thread that I can't find now. Eventually I’ll add a power switch.

The second picture is the unmodified 481 amp. The amps look very similar but the connectors are a little different. When I get around to it, I plan on adding the same 250 ohm resistor to this amp. I think it should fit nicely on the terminal strip lug where the yellow wire goes now. I would just ground the resistor to the closest power supply cap.
Good luck. The 440 amp sounds nice.

I also have a 660a amplifier that uses 7591 power tubes. I'm using this amp right now and also added the same 250 ohm resistor. In addition to the resistor I added 10 ohm 1/4 watt resistors in series with supply going to the cathode. This allows me to measure voltage across the resistor and adjust the bias appropriately.

For the 440 amp I'll add these resistors by moving the yellow wires going to pin 3 of the EL84 to the unused pin 1. I'll then add the 10 ohm resistor between pin 1 and pin 3. I've read that the resistors also add some protection to the output transformers. They can also be used to match tubes. Below is a link that describes the tube matching process.

http://enginova.com/Tube_matching.htm

So you not only add the 250-300 ohm resistor but also put in the 1/4w 10 ohm resistors on each EL84? I tried the link for tube matching to see how this works but it is broken.

 

[heyraz]

Might not be a good idea using socket pin 1 as a tie point for the 10ohm resistor to socket pin 3. Tube pin 1 on el84, 6bq5 and 7189 is normally not used, however on the 7189A tube pin 1 is internally connected to pin 2. I found this out the hard way with a quad of vintage GE produced 6bq5's that were internally connected as 7189A's. A little research revealed that GE apparently labeled some of their 7189A's as 6bq5's. BTW-GE produced tubes for RCA, so the same warning applies.
Better to use a terminal strip for the 10ohm resistor rather than risk a short between tube pins 1 and 2.