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Improving the Fisher SA-100 with EFB II

I noted a couple of differences in the pics and the schematic, what is the proper circuit for this? In the pic with the diode and cap, the cap value is 50 uf @ 100 v, the schematic says 50 uf @ 150 v, which is the correct value? Also, I see 2 10 K resistors in the same pic, one across the bias supply cap, which matches the schematic, and a 10 K in series with the diode, on the schematic it shows 1 K. What are the proper values for this? Thanks.
 
dcgillespie said:
If you can point me to the thread number for the pics you're referring to, I'll try to help clear things up.

Dave
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Thanks Dave! For R56, the schematic says 1000 ohm, but the pic in section 30, the last pic shows a pair of 10 K resistors. It may be that the orange band is really red but showing orange in the pic. In the same pic, you bias cap is marked 50 uf @ 100 volts, whereas in the schematic it shows 50 uf @ 150 volts, can I use a 100 volt cap? Finally, the FQP9N50C was not available, so I got a FQP9N90C, is that an acceptable replacement? Thanks!
 
In the pic, R56 is in fact a 1KΩ resistor. The color bands show up fine for that value on this end, but different displays can always affect colors just enough to raise questions. Glad you asked for confirmation to prevent any hiccups in your project!

A 100 volt bias cap is plenty sufficient -- I think the discrepancy is due to the fact that the schematic I drew identifies that cap as C2 on the Sams schematic, so I just brought over the specifications for the original C2 rather than what I actually used.

Your suggested replacement for the original MOSFET I used is fine, with an even higher voltage rating than the one I used.

I hope this helps!

Dave
 
dcgillespie said:
In the pic, R56 is in fact a 1KΩ resistor. The color bands show up fine for that value on this end, but different displays can always affect colors just enough to raise questions. Glad you asked for confirmation to prevent any hiccups in your project!

A 100 volt bias cap is plenty sufficient -- I think the discrepancy is due to the fact that the schematic I drew identifies that cap as C2 on the Sams schematic, so I just brought over the specifications for the original C2 rather than what I actually used.

Your suggested replacement for the original MOSFET I used is fine, with an even higher voltage rating than the one I used.

I hope this helps!

Dave
Click to expand...
Excellent Dave, thanks so much!
 
I am looking at building this clone but have a couple of questions as a hobbyist.

Do you need any unobtainium output transformers ? I have several old 8k transformers in stock.

Could you use the Russian 6p14p EL84 tubes? I believe these tubes have a higher plate voltage of 400v but my Russian is not great:) perhaps the plate and cathodes voltages can be lowered a little ?

Regards

Richard
 
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You certainly could use the Russian 6p14p-ev tubes. The lesser 6p14p tubes might work, but to my knowledge had not been proven to work in this clone with its relatively high plate voltage.
 
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spantou01 said:
Welcome Richard,

The clone build is described here:
[https://audiokarma.org/forums/index.php?threads/teaser-pics-fisher-sa-100-clone-in-progress.466229/

The clone, as designed, needs the output transformers of a Heathkit AA-100. The power transformer of this amp has also been used since it has a bias supply coil, unlike the Fisher X-101ST power transformer Dave used in his initial build.

Good luck and keep us informed of your progress.
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Thanks for the reply

If I have to hunt for old Heathkit transformers all the way from downunder then I fear the option of building this clone is dead in the water
 
You can try other output transformers but will have to tune the feedback specifically for the OPT you choose. What transformer choices do you have?

There is a pair of the Heathkit OPT's up for auction right now but are listed as Stancor outputs for 7591 tubes. Not sure if seller will ship to Aus or how much that might cost.
John
 
+ 1 What @nerdorama said. You could build your own “clone of the clone” using the best 25 watt 8k primary transformers you can find downunder and afford. Tweaking the feedback components might be challenging but doable if you have access to an oscilloscope.

P.S. Your profile does not show your location.
 
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spantou01 said:
+ 1 What @nerdorama said. You could build your own “clone of the clone” using the best 25 watt 8k primary transformers you can find downunder and afford. Tweaking the feedback components might be challenging but doable if you have access to an oscilloscope.

P.S. Your profile does not show your location.
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Thanks for the reply. Hopefully my location is visible now.

My motivation is to try and use what I have scavenged but equally can afford to buy new if essential. I have built several solid state amps and have plenty of test gear and 2 scopes and all the safety gear etc.

A have several pairs old output transformers. Two were taken from an old Hammond H100 organ (6bq5 pp) and are push pull with 8k impedance into 8 ohms and probably 15-20W. They look really solid. Also, 2 from an old projector built of 6bq5 pp also. Projector was a rare Bell and Howell (filmosound) model 8399. These are bit smaller and are 10k into 8 ohms.

Richard
 
Coitboy2000 said:
Thanks for the reply. Hopefully my location is visible now.

My motivation is to try and use what I have scavenged but equally can afford to buy new if essential. I have built several solid state amps and have plenty of test gear and 2 scopes and all the safety gear etc.

A have several pairs old output transformers. Two were taken from an old Hammond H100 organ (6bq5 pp) and are push pull with 8k impedance into 8 ohms and probably 15-20W. They look really solid. Also, 2 from an old projector built of 6bq5 pp also. Projector was a rare Bell and Howell (filmosound) model 8399. These are bit smaller and are 10k into 8 ohms.

Richard
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Williamson works well with moderate gain 6L6/5881/807 tube family, but demands good OPTs with low distortion for stability and optimal bandwidth performance. A pair of decent transformers from organ amps using 6L6 may work with those tubes with the right stabilization tweaks re feedback and bandwidth limiting.
EL84/6BQ5 tubes are high gain sensitivity tubes that prefer a simpler drive topology for fine results.
 
Pio1980 said:
Williamson works well with moderate gain 6L6/5881/807 tube family, but demands good OPTs with low distortion for stability and optimal bandwidth performance. A pair of decent transformers from organ amps using 6L6 may work with those tubes with the right stabilization tweaks re feedback and bandwidth limiting.
EL84/6BQ5 tubes are high gain sensitivity tubes that prefer a simpler drive topology for fine results.
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Thanks

The two sets of output transformers I have come from EL84 PP circuits and as I have quite a few EL84 tubes that is why I gravitated to this circuit thread. I have no 6L6 tubes
 
Coitboy2000 said:
Thanks

The two sets of output transformers I have come from EL84 PP circuits and as I have quite a few EL84 tubes that is why I gravitated to this circuit thread. I have no 6L6 tubes
Click to expand...
The common input voltage amp into phase splitter dual triode driver circuit is the ideal recommended topology for these tubes, it is a sonic revelation when working correctly.
 
Organ amp transformers often have limited hi frequency capability. Probably good bass. The feedback tuning might be a little tricky to get a clean hi end without too much early rolloff.
 
nerdorama said:
Organ amp transformers often have limited hi frequency capability. Probably good bass. The feedback tuning might be a little tricky to get a clean hi end without too much early rolloff.
Click to expand...
Williamson can be tricky to stabilize due to the number of phase shifting elements, organ amps typically use a much simpler driver topology that can tolerate the addition of a less than perfect OPT.
 
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