The Fisher 400 Restore.

Well I said I would and here it is. Pictures of what to replace in the audio chain of your Fisher 400. And also how to rebuild the bias properly.:thmbsp:

It's simple to see what needs doing and it wasn't even that hard to do. :yes:

IMPORTANT!!!! Tube amps can KILL YOU. Always discharge all capacitors before working inside of your amp! If you don't know how to properly do that don't bother doing any of this!!:nono:

Alright first up is picture #1:
Circled in green are 10 ohm 1/4watt flameproof cathode resistors installed for safety.(They replace nothing, and are installed to protect important/expensive parts by acting as fuses.)
Circled in yellow are the grid resistors. Apparentley for original fisher tubes the original 330k resistors will do just fine. But if you plan on installing new tubes this step is a must!! It will stop your amp from oscillating and/or being unstable. Replace with 220k resistors.
In blue are the coupling capacitors. They are originally .047, but if you switch the grid resistor value to 220k it is necessary to replace with .1 uf to keep/boost low frequency response.
In red are caps that I did not replace. But Maybe should be replaced......you can figure those values out for youreself.

Step 1 parts needed:
4x10ohm 1/4watt flameproof resistors
4x220k 1/4watt resistor
4x.1 uf 630volt Good quality coupling capacitor.


Step 2/ picture 2:

This is the phono/tapehead pre amp section. Located at bottom right corner when looking from the front of the amplifier, already opened up and flipped over.

Here you see 4 coupling caps and alot of resistors. The resistors should be changed if the highest quality sound is desired.(i'll probably do it some time)

In green are .022 250v
In the blue are .01 ??v

So you need:
2x.022
2x.01 (You should just make all capacitors 630volt ones, unless cost is a huuge issue)


Step 3/Picture 3

Here you see 5 and a half coupling capacitors. And one other ceramic cap circled in red
In yellow .047.
In green .02
The red ones I did not replace, but the ones to the far right definatley have sound pass through them.
I !think! they are .001.

So you need:
2x.047
2x.022
2x.001????

Step 4/Picture 4

This is just a better picture of the half capacitor from picture 3.

I've also cicrcled another capacitor that other people apparently have replaced. I'm not sure of it's value and I did not replace it.

Once you've done this your amp should sound very nice. With slightly deeper bass. The biggest sound difference comes from replacing the ceramic caps. But I enjoyed changing all of them and listning to the difference along the way.


Now the last picture is the bias supply. Many people have wrote how to change the bias from -17 to -20 and said that rebuilding the bias causes a 27 volt to be fed to 12ax7 heaters. Well its all true and all the voltages affect eachother.

So listen close here cause this is the real deal. On how to get everything into spec.

Firstly, replace bridge rectifier circled in yellow.(this is a must!! if you have the selenium rectifier) Mine was a newer Silicone rectifier already so I did not change it.
Second, in green are 1000uf 35 volt capacitors. The ones shown are the new ones I installed.(originally its a very large yellow multicap)
Third, in pink/purple is a 100 uf 25v cap....replace it.

Fourth, This step is actually two steps that must both be done before repowering..... In red is a 15 ohm 5 watt resistor. This resitor will change the 12ax7 heater voltage. I've read that it should be changed to 20 ohm. I did this and it was not enough. In reality you need 25 ohms to bring it down to 24 volts. And if you change this resistor you will affect the bias supply! It would probably drop to -15 volts! So circled in blue is the resistor that controls the bias voltage.(It's hidden behind the wire) You wanna change this to 3.3-3.7k This will get you to about -18.5-20.5 volts.

Now my line voltage is 120 vac. So 20 ohms may work for some people! Maybe!.... But if your heater voltage is 27 after steps 1-3 then I don't see why things would be any different.:yes:

Update: If you have a variac and one of the hgiher model # fisher 400s(it may apply to all of them I'm not sure) You want to set the variac to 115.4 vac wild the amp is powered on. This will put the output tube heater voltages at exactly 6.3 volts, giving longer tube life and more listening hours

ALSO: Using a 22 ohm resistor for adjusting the 12ax7 heaters is just about perfect. After adjusting the wall voltage to 115.4 you have 23.75 volts. very close to the 24 it is supposed to be.

More pictures coming!!

Those pictures show you the new parts installed and also labels where each section/step is.

I hope this helps some one save some time and lets them buy the right parts the first time

Now I'm off to enjoy my hard work and this awsome 90's rock weekend my local radio station is having. (power 97! Listen live online...It's good stuff!!)

Cheers,
Rudy

Nice work, & that's pretty cool of you to simplify things for other people. :thmbsp: I have a 600 & a 800C, but no 400 yet. I understand there were a couple-few different versions, but I have no idea which were the better ones yet. :scratch2:

Ooooooo shoulda thrown that in there . My model is 3000+. So it's one of the newer ones.

And I'd have to say the newer models were better. Only because they included a silicone rect instead of selenium. But Also they changed something with the tuner also. I think it involved one more tube and it made things simpler and better.
My 2 cents
Cheers,
Rudy

Good stuff. I have parts, now I have clear instructions. Now, how to discharge the caps.........................................

I've never seen those ceramic caps replaced before. Did you do a before and after? I was under the impression that they weren't in the signal chain.

Nice work and great write up! :yes: :thmbsp:

Scott

Very nice work. And nice pics, too!

Re: ceramic caps. These are almost never changed out, because they tend to keep their values with minimal change over time.

I assume the serial number is a typo? Instead of 3,000 it should be 30,000. Yes?

Well the ceramic caps that I replaced ARE in the signal chain. I am sure of it. I verified from the schematic and also by useing my own judgement. (they either go to a grid or plate...)


And discharging the capacitors is a simple process.....but be very steady and careful while you do it!

You need some sort of probe/clip that is persice and insulated so there is no chance of you touching metal while you use it.
Then you connect a resistor to this, something lower ohm 15-100. (I used the 15 ohm 5 watt resistor from the bias).

You ground one end of the resistor(attach to metal frame of amp) then connect the wire coming off your probe/clip to that resistor. Then you take your probe/clip and attach it to the circled red areas for 4-10 seconds, depending on what size resistor you used. Then it should be safe :yes:


I also have a question for everyone..... I received my 5 new 12ax7lps tubes a couple days ago. They sound really good, but when I put them into my tester they wont even go above 20%. Which is very disapointing after spending 80$ to get them here. Do the tubes need some time to rejuevinate or are these tubes all duds.......??

Edit: Yes the serial number would be 30000+, mine actually has a 50000#. And as for the cermaic caps, your right. All of them tested in just fine on a capacitance meter. But in a quest for the best sound. I replaced them

Nice job!

are the newstube Russian? I had one like that, tested weak on a simply emisson tester.

Oh and I should mention this I guess. What do I think Of the Change?

When I first bought this amp I thought it sounded good. But after some critical listening I was leaning towards reselling it. It didn't have the strong deeeeep bass I like and sounded distorted. (I was very close to adjusting the IM distortion pots)

After the new parts and new tubes. It sounds just like I thought it should. If not better. And now this amp will not be leaving my home any time soon:nono: And I may even buy new output tubes for it And of coarse, it's much more satisfying listning to something after you've put some time into it :thmbsp:

Cheers,
Rudy

Yes the new tubes are sovtek russian tubes.

And I do use a simple emmision tester........Are they still good?????????
Rudy

For the mostpart, emissions testers are fine for most jobs. I wouldn't use them if I were selling tubes, and every tube gets the 'in-circuit' test, but for years that was all I had.

I've got a EICO 625 and not one of the 2dozen + 12AX7's, 12AU7's, & 12AT7's I have go above 62% on the meter. Put them on a Hickok and they all show above 80%. SO I use 60% as a baseline for 12A*7's.

Larry

I think if the tubes sound fine, then that is the answer. I know what you mean about expecting the higher emissions, but at least you know other folks got the same results (brand new tube with low emissions).

Another fact about about the fisher 400. This may not apply for the earlier fisher models but it does for mine and just might be the same for the earlier models.

To get the correct heater voltage and increase the life of the precious valuable output tubes you'll need a variac set to 115.4vac wild the amp is running.

The output at my socket wild the amp was not turned on was 116.7. After turning it on the voltage drops to 115.4vac thats why it must be done turned on.... And that is the sweet spot that gets your heater to exactly 6.3v

Enjoy :thmbsp:

Cheers,
Rudy

Edit: If you have changed the 15 ohm 5 watt resistor that controls the 12ax7 heater voltage to 22 ohms as suggested. You will have 23.75 volts after adjusting the wall voltage to 115.4. That is pretty darn close to 24.