Low pf shielded cable

[Structo]

Mogami 2964

Is that what you use?
2964
[img:300:56]http://www.eventhorizon-srv.com/assets/images/w2964.jpg[/img]
Specs:
Mogami 2964
75Ω coax
Dual Served
24AWG Center Conductor
.189" dia.
27Ω/1000ft
65pf/ft

I was at my local surplus house today and he as a huge reel of RG-59 but it's the small stuff about the diameter of George L cable.
Is that stuff any good for these amps?


Right now in my amp I used some teflon coax that Moss sent me.
It is small diameter white with a brown stripe.

I'm thinking it is low capacitance cable and might be a source of some harshness in my tone.
Although I wouldn't think that as short as the runs are that the capacitance would be that big an issue, right? wrong?

[lbradshaw]

Some of the older amps -- the holy grail ones to some -- with the Radio Shack shielded wire probably have in excess of 100pF per foot... add it all up, and it begins to make an audible difference.

I am using bunch of shielded wire removed from a set of old organs. I just tested it. Comes in at 90+ pf per foot. Kind of high. I guess I don't need to add pf caps to ground in my built-in D'lator.

[martin manning]

...Even given the vagaries of this simple analysis, I don't think there is anything to worry about using 30pf/ft cable, is there?

MPM

Hard to tell because of the preceding tone stack, I guess your analysis assumes the output impedance of the TS is 0, which is not actually the case. Also, you analysis ignores the contribution of the Miller effect, which will tend to make effect of the capacitance greater by a factor equal to the gain of the following tube stage. In any case, striving for the lowest cpacitance shielded cable, in my most humble opinion, is not the answer. I know some of you have measured the capacitance of the cables used by Dumble, and the numbers shouldn't be low -- they are not as per my findings. Shoot for something super low and you will end up with a harsh sounding amp, which you will then need to tame hanging caps to ground or using larger output resistors, etc. Some of the older amps -- the holy grail ones to some -- with the Radio Shack shielded wire probably have in excess of 100pF per foot... add it all up, and it begins to make an audible difference. Easy enough to do, start hanging caps to ground and listen to determine when it starts to make a difference to you.

Cheers,

Gil

I was looking for a back-of-an-envelope estimate to see whether it was worth chasing down low-capacitance cable. The answer is no, I guess. Sticking with the simple approach, 100pf/ft cable would put the corner point at about 8.5kHz...

Re the circuit analysis (and I know just enough to be dangerous), this "volume" cable run feeds a parallel R-C (220K||500pf), and then the V1b grid. Re the Miller effect, I thought it was only the inter-electrode capacitance that is multiplied by the gain. Am I wrong about that?

All that aside, I know what is important in the end, and that it is not always possible to reduce it to numbers. The numbers can help steer you in the right direction, though.

Tom, if I had an existing amp that seemed too bright or harsh, I wouldn't go tearing out cable just yet, but do as Gil suggests and tack some small caps across the ones you have in your amp and see what that does.

"You got to remember the name of the game is 'what does it sound like?'"

-Ray Charles speaking about studio recording technology in the film "Tom Dowd and the Language of Music."

MPM

[llemtt]

...Re the Miller effect, I thought it was only the inter-electrode capacitance that is multiplied by the gain. Am I wrong about that?...

The Miller effect here has to be referred only to the capacitance existing between the anode and the grid:
given that the signal at the anode is opposite (180°) phase and "gain" times the amplitude of the grid signal, that capacitance can be modeled as a capacitance between grid and (AC) ground having capacitance equal to grid-anode capacitance multiplied by "gain".

Grid-anode capacitance is about 2pf for a 12AX7 tube and gain stage is around 50 so resulting capacitance is something like 100pf. Although the circuit is much more complicated due to the NFBL and the cap across the 220k grid resistor, the approximate reasoning is good enough.

BTW math of these circuits is very complicated but luckily spice analog simulators are near perfect for such "simple" circuits...

cheers
Teo

[BobW]

Remember guys since coax cables are generally used for RF applications they measure their capacitance (reactive) typically at 100kHz. You can't assume their (reactive) capacitance quoted in the data sheets is linear from DC to blue light.

I used Belden 8241

[Structo]

Remember guys since coax cables are generally used for RF applications they measure the capacitance typically at 100kHz. You can't assume the capacitance quoted in the data sheets is linear from DC to blue light.

I used Belden 8241

So the Belden 8241 is a RG-59 right?

I sure like using the Teflon shielded cable because of the heat resistance of the center conductor insulation.
But I'm not sure if it is available in the higher pf rating.

I have had problems before with melting the center insulation on PVC coax.
Maybe I need to turn my iron down a bit.....

[BobW]

So the Belden 8241 is a RG-59 right?

Correct, I really like the teflon too, but this all I had in my garage at the time.

[ApexJr.]

Saw this post and should let you guys know I do have in stock
RG 174/U in stock and not listed on my website...
at .15ft

Also my 22 Awg PTFE can't use the word *eflon in stock
too..1 conductor shielded also at .20ft

[rmb550]

Remember guys since coax cables are generally used for RF applications they measure the capacitance typically at 100kHz. You can't assume the capacitance quoted in the data sheets is linear from DC to blue light.

I used Belden 8241

OK now explain to me how the capacitance varies with frequency.

[butwhatif]

Also, true coax is slightly magnetic---it has an impedance, most audio cables don't

[martin manning]

All coaxial cables, including those used for audio, will have a characteristic impedance (i.e. the 50-Ohm or 75-Ohm value you see listed). For cables that are to be used at audio frequencies, though, it is of no consequence and so it is not specified.

Magnetic? My snake oil sense is tingling...

The dielectric constant of some insulation materials (PVC and Rubber) can vary somewhat with frequency, causing a decrease in capacitance per unit length. PE, PP, and PTFE do not vary significantly, and those are the materials commonly used in the cable we're talking about. I don't know, but I'd guess that this is only important for frequencies well above the audio range anyway.

MPM

[butwhatif]

all of the coax i have, can be attracted by a magnet, rg59, 162, and i can't say that about any of the audio cables i have. I can't offer an explanation why, but i have had issues with use of it in amps.

[martin manning]

all of the coax i have, can be attracted by a magnet, rg59, 162, and i can't say that about any of the audio cables i have. I can't offer an explanation why, but i have had issues with use of it in amps.

The explanation is simply that there is some ferrous material in it. That is definitely not the case with all RF coax cable. I'm pretty sure most audio cable will have (non-magnetic) copper conductors.

MPM

[Structo]

Saw this post and should let you guys know I do have in stock
RG 174/U in stock and not listed on my website...
at .15ft

Also my 22 Awg PTFE can't use the word *eflon in stock
too..1 conductor shielded also at .20ft

Would you happen to know the specs on the PTFE cable?

I'm pretty sure I have it in my amp since I got it from Brown Note and Moss said he bought it from you.

But I'll ask again.
Since the shielded runs in my D'Lite are so short with the longest being less than a foot long, does the capacitance or impedance even enter into the equation as far as tone shaping?

[ApexJr.]

I have sold the shielded to Moss at Brown Note along with quite
a few others too....
Do not know the impedance, however I also stock some of the
PTFE RG type coax too, ie. RG 178, RG 187, and RG 196 and
those are military numbers and can be looked up.

Steve @ Apex Jr.