giorgiosecco wrote: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
Depends what you mean really. Bnwitt is right that higher voltage generally mean more headroom, and therefore a cleaner amp. On the other hand, a lower screen voltage pushes the output stage more towards triode-like, class A operation, which might be described as "cleaner" in a different way, like smoother, more hifi.
Chokes are traditional; they were readily avaiable, kept the screen voltage high so you could maximise power (those old amps weren't supposed to be overdriven remember) and big caps were expensive. Nowadays resistors and big caps are cheap, chokes aren't. Personally I don't find any useful difference between the two. I understand Ken was a bit of a subjectivist, so maybe he did think that 25W resistor had some choke-like properties? Or maybe he just bought a big box of 'em once and then got sick of them taking up space on his bench...
Last edited by Merlinb on Mon Dec 01, 2008 6:18 pm, edited 2 times in total.
Maybe you could do some research. Use regular 2W or 5W, (whatever), resistor to begin with, and to get you amp working. Then later, swap in the 25W and tell us if it makes any difference!
stink wrote: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.
I have measured the drop across that 1k resistor when hitting some loud power chords (idle doesn't really matter), and found a 40 volt drop very regularly. That equals 40ma which has the resistor dissipating 1.6w
A 10 watt rating is plenty IMO. Screens would be melted FAR before the 25w gets close to failing.
I have found on my original TW (and two clones I've built for myself) that the 25w power resistor actually gets quite warm. Especially when playing sustained chords and solos. Hot enough to almost burn the finger if touched, and hot enough that on my two clones I made sure that the PVC wire running under the resistor didn't actually touch the resistor.
giorgiosecco wrote: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: https://tubeamparchive.com/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...)
This has been discussed quite a bit; do search on "25W" and you will get two pages of hits.
One thing to consider is that the voltages listed are DC; AC is still going on.
Know of two reputable builders that tried using a -10- Watt resistor; both had difficulties. For one it ran -it ain't gonna last long- hot and the other it actually blistered the Power Supply board.
I use a 15w resistor with no problems but it's heatsinked and pop riveted to the chassis so I guess it's dissapating heat a lot better than your average 25w resistor.
