2W carbon composit ressy

[richwalters]

Supplies are getting very thin with scattered odd values....anyone know of a non spiral equivalent ?
This ought to be in parts_components section but since this for a tube amp, perhaps other tube_ampers would know.

richj

[Colt45]

Why would you want that drifty junk?

[Conrad Hoffman]

Probably for the same reason people like Black Beauty capacitors? Anyway, I thought carbon comps were easily available again, but a search didn't turn up any 2W parts other than NOS. IMHO, one shouldn't be too concerned about the spiral pattern. It really has no effect at audio frequencies. If you're convinced otherwise, I can think of one other thing. There are thick film power resistors on ceramic substrates with no patterning, just a single cut to trim the value. The ones I've seen are 5W, but they're not too big. Flat and compact, but do *not* bend them back and forth much, as they'll break off the board! edit- do a search for MPC52103J to get an idea of what these are.

[richwalters]

Originally posted by Colt45
Why would you want that drifty junk?

Drifting isn't an issue. Impulse resistance to the spike caused by primary output transformer leakage inductance, yes. This voltage spike can be quite large.
High power p-p UL amps with a response up to 40Khz are prone to parasitic ringing in the screen grid-anode, especially when driving difficult loads. This is a difficult design area and I find carbon comps do reduce square wave ripple. I will get a pic or two to show how this is so.

richj

[richwalters]

Anyone tackling the worst case p-p o/p stage waveform as shown in the upper waveform needs experience and stacks of patience. Often considerable "luck" is required, but it may seem hard to convince that with many p-p circuits the type of resistors play a big part. The blame isn't always the output transformer but repairing a friends amp it was due to sloppy layout and wrong resistor types.

This is dangerous terroritory. Be very aware what one is doing when tampering with the p-p primary side components; a wrong fiddle or misplaced probe can result in severe burns and shock.

richj

[Conrad Hoffman]

That makes sense. Can you parallel or series some lower wattage carbon comps that are more easily available? If the stray capacitance of the resistor is an issue, series combinations are better. If not, parallel gives a smaller package size. BTW, what values would be most useful to you?

[316a]

Originally posted by richwalters
Supplies are getting very thin with scattered odd values....anyone know of a non spiral equivalent ?
This ought to be in parts_components section but since this for a tube amp, perhaps other tube_ampers would know.

richj

Painton resistors sound like what you need . These are enamel dipped carbon comps with non-magnetic end caps . What values do you require ?

cheers

316A

[richwalters]

Hi 316a; values 390R,470R.

Perhaps someone in the FE or elsewhere will re-judge the market and perhaps restart carbon composition stuff again.
The problem with the amp design my friend built, that standard m/o resistors rated at 350V were used with a circuit running at 500V. Many constructors often make this mistake without realising it. This over-voltage stress combined with high frequency created varactor effect which was upsetting circuit parasitics and ringing. The variation of resistance with voltage effects carbon types more but other parameters i.e impulse behaviour with frequency is far more stable.
This explains why the replacement carbons were a radical improvement. Observing with a 1 KHz squarewave is a decoy and doesn't show the problem whereas at 10KHz upwards the harmonics make underrated components lossy. The cue is to choose correctly rated components !
Not every design will highlight problems. However worth mentioning that this particular design used a top flight o/p tranny allowing higher frequency operation. When compared with a standard or vintage o/p tranny the loop reponse has to be reduced for stability and this avoids other problems..

So ended the fault finding session.

richj

[Magura]

How many do you need?

Magura

[Poindexter]

You sure it's 'Painton', my friend? I just Googled around and came up empty.

Aloha,

Poinz