dropper resistors on the heater, lamp circuit? (in the Ruby)

[dms]

Hi - I am finally getting around to my Two Rock kit... why is there a terminal strip with dropper resistors that seems to be sitting inbetween the heater wires and the 6.3 volt winding? looks almost like the pilot lamp is getting the direct feed from the tranny, and then the heaters go via a couple of resistors. The pilot lamp is incandescent, and seems to be hooked up first. Marshalls ave lamps and heaters that tap dierctly in to the 6.3 winding... why would anyone have resistors here... any clues? If you look at the files section, Two Rock Ruby - Inside 1 photo, you can see this along the lwft edge of the photo.

Thanks!

[loverocker]

A band-aid fix for a slightly out-of-spec PT? Can't see the harm in it - just a tad more heat in the chassis. I've got a PT that gives about 6.9V when used in low power amps.

[dms]

I wouldn't think the heaters would mind a slight overvoltage - the odd thing is, is that I've seen it on some of their other models too, which makes it look a bit more "intentional." I have a shot of a Two Rock Custom, that clearly shows the tranny wires going directly to the pilot lamp, and then two 2watt resistors coming off off each and seeming to converge together - unfortunately, you can't really see the heater wires back to the tubes . Hmm... will try giving them a shout for an explanation.

thanks, dms

[tubedogsmith]

Lots of different builder have used two 100 ohm resistors to ground the filament circuit. My guess is that's what these are. Your amp most likely doesn't have a seperate ground wire for the filaments.

[Bluebell]

Is there a center tap on the heater supply? If not, it could be an artificial center tap.

[loverocker]

OIC. Yup, if they're joined together, and not in-line with the heater supply, it's a "sort-of centre tap". Might be connected directly to ground, or elevated above ground (e.g. to power valves' cathode on cathode-bias amps, or some B+ voltage divider on other amps).

[dms]

Thanks for all the replies! K&M responded, that the purpose was indeed to balance the heater windings, which is not necessary on later transformers as included in the "kit."

Hat's off to all of you who had already guessed this from my description!

thanks!

[Funkalicousgroove]

If those are not there and there is no center tap the Winding will HUMMMMMMMMM. You can actually "Float your heater voltage on your power tube cathodes, or even the B+ if it's not too high. That's a real good trick to reduce the Hum in single ended circuts.

[loverocker]

If those are not there and there is no center tap the Winding won't make any voltage. EMF (Voltage) is always a potential,

Now I'm the first to admit that I'm 'theory-lite' but that's wrong - surely? The potential difference is developed between the two ends of the winding, and the valves' heater elements make the circuit, so current flows.

The reference point is just a nicety for quieter operation.

Please correct me and fill in my blanks if I'm wrong here.

[Funkalicousgroove]

You are correct, I was thinking rectified DC voltage ala B+, which requires a reference to the chassis to make voltage. I don't know where my head goes sometimes. with AC the CT or Resistors are for Hum. I don't know what I was smoking, but I'd like some more please !