Fender Tweed: Faux CT

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UnlikeKurt
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Fender Tweed: Faux CT

Post by UnlikeKurt » Tue Sep 21, 2010 4:23 pm

Im building a Fender 5F2A and the 6.3V winding on my PT has no CT. I'm going to use 2 100Rs to the 6V6 cathode. Is it ill advised / is there any downfall to tacking the resistors directly on the tube socket (ie: 100R between pin 2 and 8 and 100R between pin 7 and 8)?

Thanks

James

The Scum
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Post by The Scum » Tue Sep 21, 2010 4:35 pm

Um, the schematic I found doesn't show a center tap on the 6.3V.

http://www.freeinfosociety.com/electron ... php?id=802

Are you working from something else?

UnlikeKurt
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Post by UnlikeKurt » Wed Sep 22, 2010 7:37 am

Same schematic, except the heaters. Heater secondary winding is 6.3V across two 3.15 wires. Twisted pair, one side to 6V6 pin 2 other to pin 7 then off to the 12AX7 pins 4/5 and 9 respectively. Since I have no center tap on my heater winding I am going to go the 2 100R resistor route and instead of referencing it to chassis ground, elevate it to a dc reference (6v6 cathode - pin 8). My question is to whether or not there is any potential problem with doing this directly on the 6V6 socket by soldering one 100R between pins 2/8 and one between pins 7/8? In the past, when utilizing the 2 100R faux ct method, I have used a terminal strip and connected the heater windings to the strip and then soldered the resistors to the strip and then twisted to ground and ran my heater wires from the strip to the tubes.

themagicmanmdt
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Post by themagicmanmdt » Mon Sep 27, 2010 9:01 pm

the only thing to think about is current draw through your ground path.

after the first filter cap's current draw, the heaters are the second most through their ground reference, then the rest of the tubes and such from the B+, from power tubes downward to preamp.

this is the theory of a 'star ground' and a 'buss ground' - you hook your different stages up seperately and independently to a common ground location (star), or in order from least current (preamp) to most (power tubes then heater balance then first filter stage) to ground.

why? otherwise, if you connect a high current ground location (let's say first filter cap negative) between that and any other stage, you'll induce hum due to the high 'flow' through the sensitive preamp stage caused by the first psu filter cap.

in short -

personally, I consider the heater ground reference to be grounded either 1) far away from most other things - ie most fender amp designs grounding them at the pilot or 2) at the second/third filter cap ground in the filter section. usually, if you think in terms of the 'stream', it goes like this:

master ground --- first filter cap after rectifier ---- second filter cap ---- heater reference (can be tied with second filter) ----- power tubes ---- phase inverter ---- other preamp tubes in order

you might have success with grounding to cathode....or you just might end up inducing hum in one output tube...!

what happened to the 6.3v pilot light?

marshall
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