Fisher X-100-2 question

[Patrice B]

Hi,

I'm in the process of rebuilding my Fisher X-100-2 for the second time (replacing the radial leads electrolytics with cans and also the mistracking volume pot).

Looking carefully at the schematic (page 8 on the linked pdf), I've noticed that the red-yellow and red wires coming from the power transformer are have their position reversed.

The R/Y is supposed to go at the junction of C4, SD1 and SD2 but is tied instead at the junction of C1 and C2A. The red wire is supposed to go there but it goes instead at the junction of C4, SD1 and SD2...

It looks it was this way when I acquired the amp as the R and R/Y wires are cut to the appropriate length to reach their actual positions.

R/Y reads 4 VAC at junction of C1 and C2A. Red reads 160 VAC right after SD2.

I also have the needed 390 VDC after SD2 BTW.

Are the locations of those leads that important or it doesn't matter?

Thanks in advance,

Patrice

Schematic: http://fisherx100a.files.wordpress.com/2008/11/fisherx100ax1003.pdf

 

[Patrice B]

Oh, BTW...

It was a good idea to replace the cheap electrolytics I've put there last year as a 220 uf@100v was already bulged. Maybe it suffered some heat from the power tranny...

 

[audiodon]

gugusse,
That's a good idea. If you are suggesting that the electrical field might be reversed, then why would you change it back now that it's broken in this way?

 

[Patrice B]

gugusse,
That's a good idea. If you are suggesting that the electrical field might be reversed, then why would you change it back now that it's broken in this way?

Huh!

Not sure I follow you...

You mean that the fact that it ran reversed for several years (I guess so, however, there's a repair sticker apposed to the PT so maybe it was replaced in the past and connected reversed...) has changed the direction of the electrical field inside the power transformer? Is it what you meant?

Thanks.

Patrice

 

[Patrice B]

BTW, the new (NOS) Centralab power switch/pot I bought is worse tracking than the original. Meaning that there's a difference of around 3-4 dB on low level (2-3).

It was something like 2 dB with the original one.

Anyway, I listen mainly around 4-5 level, where this pot is equally tracking. The old one was off by 1 dB around 4-5. Kind of a compromise!

 

[audiodon]

I looked in my X-100-3 last night and yes, your wires appear reversed. Switch them if you want. I'm not sure you'll gain anything though.
Extend them as required with comparable stranded wire.

 

[Dadbar]

Oh, BTW...

It was a good idea to replace the cheap electrolytics I've put there last year as a 220 uf@100v was already bulged. Maybe it suffered some heat from the power tranny...

Oops....is that 220uf being used for the voltage doubler or for cathode bypass duty? If it's being used for the doubler you might want to look at some of those axial 220uf@300V caps for a better margin of safety. You might be exceeding the 100+V rating of those....

 

[Patrice B]

I looked in my X-100-3 last night and yes, your wires appear reversed. Switch them if you want. I'm not sure you'll gain anything though.
Extend them as required with comparable stranded wire.

Is there any advantage to do so? It looks to be working fine as is. I'll think about it...

Oops....is that 220uf being used for the voltage doubler or for cathode bypass duty? If it's being used for the doubler you might want to look at some of those axial 220uf@300V caps for a better margin of safety. You might be exceeding the 100+V rating of those....

Already replaced with a JJ 100/100 in parallel (500v, should be enough). The original (in the old can) was 100uf@70v. Strange that a 100v one didn't survived.

Thanks to you two.

Patrice

 

[audiodon]

I don't see an advantage actually. One can argue opposing direction of current flow in a multitap transformer may be detrimental, cause saturation, hysterisis or eddy current problems and all that, but I don't know much about it. It has worked so far.

 

[Patrice B]

Well, wiring was done accurately against schematic today (reversed the r and yr).

No changes obviously apart the fact that I feared myself :

There was a cold solder joint on my new 20/20 can and thus, no voltage going to the phase splitters=no sound!

The damn thing is now playing finely after a couple of hours debugging, reflowing solder joints, etc...

Patrice

**The new volume pot needs to be higher to achieve similar settings than with the old one but it's fine like that as the tracking is more even around 4-5 (half point). The old was CTS and this one, Centralab.

 

[audiodon]

Using 5751s in the preamp and/or tone control section also helps with fussy volume controls because they have a .7 (mu) gain factor compared to the 1.0 gain factor of a 12AX7. With tubes with a lower gain factor, you have to turn the volume higher for equivalent volume.

 

[Patrice B]

Using 5751s in the preamp and/or tone control section also helps with fussy volume controls because they have a .7 (mu) gain factor compared to the 1.0 gain factor of a 12AX7. With tubes with a lower gain factor, you have to turn the volume higher for equivalent volume.

Good idea, I'll just have to buy some now...

Anyway, I mainly listen at 4-5, where this pot seems to track equally on both sides.

Thanks.

Patrice

 

[audiodon]

I like RCA triple mica black plates in V1, V2 and V3 in this amp.
Remember a few things:
1. Absolute phase is reversed. Black is + on the speaker terminals. Since there's only one tone control amp tube, there's no second tube to invert the phase again.
2. Even one 5751 on V3, which covers the whole tone control section in this, will produce an audible improvement.
3. Make sure your filament voltages to V1, V2 and V3 are high enough to run the tubes properly.

After this thread came up, I pulled mine out and upped the filament voltages to V1-V3 by removing the voltage dropping resistors from the filament supply.
If you look at the schematic, there are two, though I don't recall the number offhand.
Pulling them upped the supply voltage to about 33 from about 29. But, don't ever pull a tube while the amp is running if you try this.
That really lit this thing up. It's sailing along at about 10.8Vdc on the filaments now and has lots more life and muscle.

 

[Patrice B]

I like RCA triple mica black plates in V1, V2 and V3 in this amp.
Remember a few things:
1. Absolute phase is reversed. Black is + on the speaker terminals. Since there's only one tone control amp tube, there's no second tube to invert the phase again.

Yes, I think I've read here and it was you maybe in another thread. My speakers leads are reversed (+ to - and - to +) to overcome this "bug".

2. Even one 5751 on V3, which covers the whole tone control section in this, will produce an audible improvement.

There's a strong Sylvania here but I'll contact my favorite tube seller and ask for some 5751's...

3. Make sure your filament voltages to V1, V2 and V3 are high enough to run the tubes properly.

After this thread came up, I pulled mine out and upped the filament voltages to V1-V3 by removing the voltage dropping resistors from the filament supply.
If you look at the schematic, there are two, though I don't recall the number offhand.
Pulling them upped the supply voltage to about 33 from about 29. But, don't ever pull a tube while the amp is running if you try this.
That really lit this thing up. It's sailing along at about 10.8Vdc on the filaments now and has lots more life and muscle.

I've paralleled another 22ohms with the previous one last year (thus around 11-12ohms) to up the voltages on the 12AX7's a bit (I have now around 10.5 volts I think). This is R65 on the schematic. I heard on the Fisher group that the 2.2k (R66) is here to protect the tubes in case one of the 12AX7's is removed.

 

[Patrice B]

For the record, the exact post on the Yahoo Fishergroup (about R65 and 66):

http://launch.groups.yahoo.com/group/fishergroup/message/1955