bassman power transformer

[davebolden44]

hello again. I have another question regarding my 5f6a bassman build. I keep plugging away slowly and find myself constantly confused. I put the power transformer in the chassis today and I noticed a wire that isn't shown on the diagram or schematic or at least not that I can see. it is green with a yellow stripe. my first assumption is that it is a centertap for the filament winding. I checked the impedance with my radio shack meter between the filament windings to see if I could get a half measurement but it reads so low that it basically tells me nothing. thought maybe someone here would just know this answer as it is probably common knowledge to some. also if it is indeed centertapped, would I forego the two 100 ohm resistors coming of the jewel lamp to ground as I believe that is an artificial centertap. am I close or should I just give this up and take up drinking? thanks in advance

[Leo_Gnardo]

my first assumption is that it is a centertap for the filament winding.

Kee-rect! You would need a milliohm-meter for an accurate read but don't worry, your guess is correct. It's up to you whether to use the CT to ground, or apply balancing resistors (anywhere from 47 to 220 ohms is good, most use 100), or a "hum-dinger" pot. The hum-dinger lets you carefully dial in minimum hum from the filaments. Best to get a wirewound pot from 100 to 500 ohms for that.

On with the work. I'll have a drink on your behalf and you may too. Not advised to get tipsy and work though, you really need your concentration. Just a little mood enhancer.

[lord preset]

It's almost certainly going to be the filament center tap. But what kit and layout are you using? The original Fender schematic shows a CT on the filament winding. BTW you are right that the 100 ohm resistors imply an artificial center tap. Maybe the kit supplier uses different PTs from time to time. You can use either the real or artificial center tap, just not both.

[davebolden44]

Excellent, thank you for the replies. I guess some of this is sinking in. As far as the layout I'm sure it differs from the original fender. It's a mojo kit and i am just trying to follow the diagram to the best of my ability. As far as grounding goes they emphasize it a lot in the literature but give no examples on how to do it so i am going to attempt a star grounding scheme. My biggest concern is grounding the filter caps, i was going to run two wires directly to the star ground lug i will install near the power transformer but the wires will pass along some of the preamp which feels wrong to me. I have read bad things about oscillations because of things like this but i dont know the best way to do this as all i have done is reading. Could i ground them right on the bus bar that i will install by the pots or just go strait to the grounding lug?

[Reeltarded]

Everything from the PI forward (toward input) gnd at input (very close) everything else gnd near the transformer. That works good.

[davebolden44]

Right, the power filters rest physically behind the pi on the input side under a cap pan. Would you suggest soldering them directly to the bus bar where they come through the chassis?

[davebolden44]

I have also thought of grounding them under the cap pan but i still dont know if this is acceptable

[Leo_Gnardo]

Right, the power filters rest physically behind the pi on the input side under a cap pan. Would you suggest soldering them directly to the bus bar where they come through the chassis?

Best to have your filter cap negative ends hit the chassis at the same spot your hi voltage center tap does. Whether from above or below chassis. As Miles suggests, near the PT.

You may be able to get a little better power supply hum rejection by having the last filter cap in the string, the one that serves the input tube, land its neg lead nearer the input jacks. Same with the input tubes' cathode resistor/caps.

[pdf64]

See http://www.valvewizard.co.uk/Grounding.html

[davebolden44]

thanks for the replies. makes more sense to me now. takin me some time to retain all this so all of your help is appreciated.