Late model 400 question

[RS Steve]

This 400 is a time capsule I restored a while back, only thing not installed was a bias adjustment. So I wanted to keep it simple with this unit and just install a trimmer that adjusts all output tubes at once. I have done this on other units like the 500-C and 800-C which was resistor R-132 with 10K trimmer, from what I see on the 400, it would be R-35 and a 2K trimmer needed. Looks to be a 1K resistor and located just to the right of the green can cap. Any help to confirm this would be much appreciated.

 

[thornev]

One trimmer that adjusts all tubes at the same time? Would you mind posting a schematic that would work on a 500-C (which has 4 power tubes)?

 

[larryderouin]

I don't recommend the single adjustment unless you have tightly MATCHED cathode draw on the tubes within 2ma maximum, and then as a temporary measure until you install an IBAM or IBBA board. On the 400 (late model serial 48001 and above) The resistor to replace is R35. Then Remove R34 and C42. These 3 are located onthe terminal strip next to the back corner UNDER THE TWIN LEAD 300 ohm leading to the front end of the tuner.

Thornev; on the 500c, The POT should be a 10K pot to replace R132(5.6K). Remove R131 and C86. These are located in the same place as on the Late 400.

 

[thornev]

Larry - Thanks. I'll stick with my IBAM. And yes, for each tube I used one 10K trimmer pot, each with a 12K 1/4 watt resistor, 33uF 50V e-cap and 200K 1/4 watt resistor, and R121-R124 in the chassis removed. Each 200K is connected to a pin 6 on the power tube, and also there are connections to the B+ power supply and to chassis ground. I think that covers it all, maybe not described exactly as installed. I considered Dave's IBBA design. I forget why I went with the IBAM.

 

[RS Steve]

I don't recommend the single adjustment unless you have tightly MATCHED cathode draw on the tubes within 2ma maximum, and then as a temporary measure until you install an IBAM or IBBA board. On the 400 (late model serial 48001 and above) The resistor to replace is R35. Then Remove R34 and C42. These 3 are located onthe terminal strip next to the back corner UNDER THE TWIN LEAD 300 ohm leading to the front end of the tuner.

Thornev; on the 500c, The POT should be a 10K pot to replace R132(5.6K). Remove R131 and C86. These are located in the same place as on the Late 400.

Thanks Larry, trying to refresh my memory. My mind has been on my boat restoration lately.

 

[RS Steve]

No room for the IBAM board I built for the rear external locations on the bigger models. I will have to create a smaller version to go underneath. It's been a while since I played a 400, a bit whimpy pushing AR 17's, but certainly sounding great.

 

[RS Steve]

Looks like my IBAM is just the right size to mount on the 400, this should help save the original output tubes that still test strong.

 

[RS Steve]

Hooked up my IBAM and all is good, (no sparks or smoke)

One thing I over looked on this unit was removing the noose, boy did that wake up the sound!
I hated to change much on this 400 much since it was a working original including tubes, but now it's better than new with no worries about old parts causing damage.

 

[AlTinkster92]

Nice Job Steve! Wondering if that noose removal is possible on my 800C?

 

[RS Steve]

Nice Job Steve! Wondering if that noose removal is possible on my 800C?

I believe it is, been away from electronics for a bit restoring my boat so I have to refresh my memory on this stuff. Removed the resistor from pin 3 of the phase inverter tube.

 

[Dave451]

Nice work, Steve. I did all Dave G's recommended mods for my 400 ("Improving the Fisher 400") including integrated IBBA/EFB, driver mods (including losing the noose), and the Miller effect cure and have been VERY satisfied with the results. The only thing I didn't do was re-purpose the "Space Expander" jacks as "amp in/pre-amp out" and I'll probably end up doing that too. Also aligned the tuner and MPX--great receiver!
Dave

 

[RS Steve]

Nice work, Steve. I did all Dave G's recommended mods for my 400 ("Improving the Fisher 400") including integrated IBBA/EFB, driver mods (including losing the noose), and the Miller effect cure and have been VERY satisfied with the results. The only thing I didn't do was re-purpose the "Space Expander" jacks as "amp in/pre-amp out" and I'll probably end up doing that too. Also aligned the tuner and MPX--great receiver!
Dave

I have much more studying to do on the EFB before I try that, but I've come a long way from where it all started.

 

[thornev]

Dave451 - What is "losing the noose"? I"ll search, can't be too hard to find with such a title, eh?

 

[RS Steve]

Dave451 - What is "losing the noose"? I"ll search, can't be too hard to find with such a title, eh?

Read this.

http://audiokarma.org/forums/index.php?threads/fisher-400-service-bulletin.671281/

 

[Dave451]

That's it!

 

[larryderouin]

It's not as noticable on the 500c/800c as FISHER didn't strangle them as much. With the 400, they strangled the shit out of it. But in any case, I'd use a domestic/EURO tube. NO RUSSKIE tube in the Phase Inverter.

 

[AlTinkster92]

Thanks Larry,