Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fraggy0117 on February 03, 2022, 03:50:46 pm
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I've got a Knight 94SX700 amplifier that I'm planning on converting to guitar. I snagged the schematic and while looking through I noticed something interesting was going on with the heater circuit.
(https://i.imgur.com//uotQyq5)
To me, it looks like all tubes except v1 is connected conventionally to a 6.3v supply. However, v1 is connected to a second 10.6v secondary and it's connected in series (I think), but still AC.
(https://i.imgur.com//vXOI8Gw)
My question is; am I correct in my analysis above, and then why would they do that? Just to try and decrease any noise from v1?
Thank you!
(I attached the images incase the inline doesn't work)
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There is a long-time belief that running a preamp tube 10% or 20% below full rated voltage will lower the hiss. Maybe particularly 1/f random rumble. I'm not sure I know if this is 10-something or 5-something, the pin numbers blurr my eyes and SAMS is known for typos. But yeah, they probably ran V1 on the cool side.
The separate humdingers is interesting, and makes sense, though maybe less so with 12V-10V heat.
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look to this old Geloso Recorder schematic
(http://www.assabbi.com/instrumentos/gelosog257/images/GelosoG257Schem.jpg)
as you can see there is a 4R resistor on the Heater of input tube, reduced voltage = reduced humm
read Sluckey posts here
https://el34world.com/Forum/index.php?topic=17100.msg170616#msg170616
Franco
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The Knight 94SX700 schematic seems to show a 5V heater winding connected across pins 4&5 of V1!
Whereas its parts list notes the extra winding as being 10V, which makes a lot more sense.
I think that series connected, AC powered heaters may not be good for an input stage?
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The Knight 94SX700 .... ..... series connected, AC powered heaters may not be good for an input stage?
Maybe, but DC heaters sure had no business in a mid-price 1957 amplifier. The only DC heat from those years I recall in home hi-fi was working the power valves past 135mA (so more than two 6V6) to heat the preamp. This includes nearly all professional microphone preamps. Oddly compressors often ran DC heat on the gain tube.