I'm building from Normsters bassman layout and I have the fender NSC-022921 reverb transformer in it. My question is that is it ok to the tranny to get B2+ (about 450V), or is it too much for it ?
Another question regarding the NFB. Should the 4ohm and 8ohm jacks share the same NFB line. Can I just put jumper from 4ohm+ to the 8ohm+ jack ? This might be stupid question but I'm going to ask it anyway.
Yes, the transformer goes to B+2. I know 450V seems high, but it works.
NFB comes from the 4 Ohm jack only. If I understand the question correctly, you cannot jumper the 4 and 8 Ohm jacks. Each comes from a different tap in the OPT.
Normster wrote:Yes, the transformer goes to B+2. I know 450V seems high, but it works.
NFB comes from the 4 Ohm jack only. If I understand the question correctly, you cannot jumper the 4 and 8 Ohm jacks. Each comes from a different tap in the OPT.
Ok, B+2 it will be
I know that 4 and 8 ohm comes from separate taps of the OT secondary but how should I connect the NFB to work both in 4 and 8ohm jacks ? It might be too obvious but I cannot figure it out
Ah, got it. The feedback circuit is designed to work from the 4 Ohm tap only, regardless of the speaker impedence. IOW, even if you're running an 8 Ohm cab, the NFB still comes from the 4 Ohm tap. The resistance values in the NFB circuit are calibrated to the tap, not the speaker load.
Normster wrote:Ah, got it. The feedback circuit is designed to work from the 4 Ohm tap only, regardless of the speaker impedence. IOW, even if you're running an 8 Ohm cab, the NFB still comes from the 4 Ohm tap. The resistance values in the NFB circuit are calibrated to the tap, not the speaker load.
BTW, nice looking build!
Thanks for the clearance but I'm afraid I'm still little unsure how to connect the 4 and 8ohm taps
I'm building from Your preamp and powerfilter layouts, so BIG thanks belongs to You
I guess the simplest way is to do what Dumble did, just use the 4 Ohm tap. (The Bassman only has a 4 Ohm tap and seems to work well with 4 or 8 Ohm cabs.)
Last, and least desirable, is to use non-shorting jacks with one lead going to each jack. The only problem with this method is that you have no protection if you forget to plug a speaker in.
I believe you can also wire it so that the primary connection is 8 Ohm and switches to 4 Ohm if you add an extension speaker. I'm sure there are other options as well, but that's all I've got off the top of my head. I guess nothing's as simple as it seems.
As far as the reverb tranny, the 450 is fine. Look at the Fender Super reverb and Twin schematics. The tranny was feed from the Screen grid. I would suggest adding a 27K to 47K resistor in series to the transformer to help isolate it a bit from the screen grids. You could also add a 10 to 20uf cap after the resistor for the final touch.
Gary
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
Normster wrote:I guess the simplest way is to do what Dumble did, just use the 4 Ohm tap. (The Bassman only has a 4 Ohm tap and seems to work well with 4 or 8 Ohm cabs.)
Last, and least desirable, is to use non-shorting jacks with one lead going to each jack. The only problem with this method is that you have no protection if you forget to plug a speaker in.
I believe you can also wire it so that the primary connection is 8 Ohm and switches to 4 Ohm if you add an extension speaker. I'm sure there are other options as well, but that's all I've got off the top of my head. I guess nothing's as simple as it seems.
Ok, I think I will be using the 4ohm jack for starters, thanks Normster !
glasman wrote:As far as the reverb tranny, the 450 is fine. Look at the Fender Super reverb and Twin schematics. The tranny was feed from the Screen grid. I would suggest adding a 27K to 47K resistor in series to the transformer to help isolate it a bit from the screen grids. You could also add a 10 to 20uf cap after the resistor for the final touch.
Gary
Gary, do You mean a 27K-47K from V4 plate2 (pin1) in series to 10-20u into the reverb transformer ?
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