Help!!25w 1 kohm

[giorgiosecco]

Hi everybody,need help with the big 25w 1 kohm resistor in the supply board.It seems really difficult to get one,I got one from mouser and here's the surprise:there was no image available and when I opened the package I found one reeeally small Caddock resistor,same size of a LM three pins regulator!!,which is anyway rated at 25watts 1 kohm.Where could I find this big size resistor?Any idea on substituting it with a choke or somethingelse?Sorry for my terrible english,thanx for your help.ciao

[fzfwyv]

Mouser Part number: 280-CR25-1.0K-RC

http://www.mouser.com/Search/ProductDet ... 25-1.0K-RC

If I had an extra I'd send it to ya, but I just used my last one in my new Liverpool build.

-Rich

[giorgiosecco]

thank you so much,I'm a i..ot!I wasn't able to find the right resistor on mouser,unless I used "cement"and "xicon"in the search field.Problem with your language.Thanx again

[joebob]

Fry's has them for $.99.

Dave

[Merlinb]

Out of interest, why is that resistor so big?? Seems to me a 5W resistor would be ample?

[doctord02]

Thats been often debated, but no consensus as to why Ken used such an oversized/rated resistor... But he was very consistent with doing so...

Oh, and welcome here Merlin. Your website and posts over at AX84 have always been very helpful.

[Structo]

Perhaps it is due to the inductive properties of the power rating?

Being wirewound it would have some inductance and maybe Ken thought it added some m0jo.

[jaysg]

Anyone ever come up with the TRWs? I have a couple in other values. The leads are heavier than the Xicons.

[selloutrr]

I have a few dozen of the NOS originals.

[giorgiosecco]

hi merlin,one question to you,or anyone so kind to answer:why do you think 5 watts its enough?In the meantime I found 15w 1kohm,who knows the proper calculation to find the right dissipation power for this resistor?
also,is 50 volts for the electrolitics in the bias section too low a voltage rating?

[selloutrr]

the bias does seem low 50 would tend to be cold and lifeless in my opinion. but it's depends on the tubes and voltage. if the math works it might be fine or Old tubes. KF was quoted in an interview saying TW's leave the shop set at ( transformer shunt ) at 30. alot of builders on here are changing the bias resistor and going for 42.

[stink]

giorgiosecco,
Try a choke in place of the cement resistor. I think you'll like the "feel" and the tighness the inductor will provide much more than the resistor.

http://www.mojomusicalsupply.com/item.a ... id=MOJO777

$16.95

My $0.02
________
Z9

[Merlinb]

hi merlin,one question to you,or anyone so kind to answer:why do you think 5 watts its enough?

Because this is a standard PSU topology. That resistor only feeds the screens and the preamp, which is, at most, going to suck about 30mA.
In fact, the schem: http://ampgarage.com/forum/files/twreck ... ic_421.pdf
shows the voltage on either side of that resistor as 395V and 380V, so the voltage across it is only 15V. The power dissipated is therefore 15^2 / 1k = 0.225W. Even a 2W resistor would handle that!
And you could still use a WW resistor, if there is any mojo it it. (I'm not convinced...)

[giorgiosecco]

ok merlin,thanx a lot,this is very clear.I'm trying to put some ideas together,about this "Resistor vs.choke"thing.I found an interesting forum page about this :http://music-electronics-forum.com/show ... php?t=9402
it seems amp builders use resistor because is cheaper than choke(cause choke turn out to be better,under many points of view),but,reading the calculations on this forum,I'm starting thinkin' ken Fisher used the 1kohm resistor on purpose,to lower voltage down a bit.Is the effect of this voltage drop in the express getting it cleaner?am I right?(sorry..english)for cleaner,I'm thinking clean headroom

[bnwitt]

Is the effect of this voltage drop in the express getting it cleaner?am I right?(sorry..english)for cleaner,I'm thinking clean headroom

Typically higher B+ voltages on a tube stage will deliver more clean headroom not lower voltages.