Low pf shielded cable

[Structo]

Do you guys have a favorite source for low pf shielded cable that you can buy, by the foot?
I'm talking about the type you would use for input runs in a amp.
Thanks.

[mlp-mx6]

www.apexjr.com

[stelligan]

topic covered here:
https://tubeamparchive.com/viewtopic.php?t=3461

[Funkalicousgroove]

I have 2 types available, one is 11pf/ft with a foil shield, one is teflon coax that is 15pf/ft.

[Bob-I]

I like the apexjr cable myself, but it's fairly high capatiance. My cheepy meter measured 27pF/foot. Even giving a 10% tolerence for the meter that's a tad high.

In general if you use it in the typical locations, it doesn't degrade the highs. In fact my amp was quite a bit brighter than Dogear's amp the last time we got together and it's loaded with apex cable.

[Normster]

+1 for apexjr. I still haven't tried Brandon's ultra-low pF cable yet, but I've never had a problem using Steve's teflon shielded. I've also used RG174/u but I think it's kinda flimsy.

[Structo]

Say a guy used what came in the D'Lite kit ( I don't know the specs on it) and then added a couple runs of different cable (different pf/ ft), would this cause any problems?

[Funkalicousgroove]

Shouldn't cause "problems" may make for Brighter or darker.

[martin manning]

Exhuming an old thread here...

A quick analysis (-3 dB corner points) of the six runs typically seen in an ODS-type amp shows that the one most susceptible to signal loss in the AF range is the volume wiper to V1b, where the signal path sees the top of the pot to the wiper as a series resistance followed by the resistance from the wiper to the bottom of the pot paralleled with the cable capacitance as a shunt to ground. I believe the corner point for this low-pass network is (R1+R2)/(2*pi*R1*R2*C).

Given that, the lowest corner frequency is found when the pot is at mid-resistance, where R1 = R2 = 500K. Then, assuming 9" of garden-variety RG174 at say 30pf/ft, or 22.5pf for C, -3dB occurs at 28.3 kHz.

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

[llemtt]

+1 for martin

your analysis is correct!

cheers
Teo

[heisthl]

That's the problem with math - it doesn't account for tone - you can definitely hear the difference between say RG58 and cheap shielded cable, I like the sound of the cheap stuff!

[ayan]

Exhuming an old thread here...

A quick analysis (-3 dB corner points) of the six runs typically seen in an ODS-type amp shows that the one most susceptible to signal loss in the AF range is the volume wiper to V1b, where the signal path sees the top of the pot to the wiper as a series resistance followed by the resistance from the wiper to the bottom of the pot paralleled with the cable capacitance as a shunt to ground. I believe the corner point for this low-pass network is (R1+R2)/(2*pi*R1*R2*C).

Given that, the lowest corner frequency is found when the pot is at mid-resistance, where R1 = R2 = 500K. Then, assuming 9" of garden-variety RG174 at say 30pf/ft, or 22.5pf for C, -3dB occurs at 28.3 kHz.

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

[BobW]

I appreciate the effort to workout the math, but I agree with Gil, if the math does not account for issues such as miller effect and thevenin equivalents of the circuit, it does nothing for predicting tone. Sometimes it's just easier and quicker to try different components, then understand why the math does not correlate.

Case in point:
My SANO / Dumble build initially used RG-174, and suffered from harsh highs and not enough lows, but after switching to a lower grade RG-59 (all that was readily available at the time) the amp came alive with what Gary Glasman calls "The Sparkle". The RG-59 cable capacitance was higher, but the wire gauge was increased from 24 stranded AWG to 20 solid AWG. The tone bandwidth opened up with this only change, and believe the tonal change was mostly due to a lower impedance. The higher pf / foot was not a factor.

The following was pulled from the web, and defines pf / foot usually at the 100kHz range. This is why you can't, as a rule of thumb, determine good/bad coax for use in a guitar amp on just pf / foot alone.

"The frequency where the impedance matches to the characteristic impedance varies somwehat between different cables, but this generally happens at a frequency range of around 100 kHz (can vary)."

IMHO

[rmb550]

I have access to a reel of shielded cable at work.....it's so old the label is unreadable but there must be 1000' there. I tried to measure the capacitance of it with a Fluke meter but it was too low to measure.
At any rate, can someone tell me what the capacitance of "typical" lead cords are? Surely this run has way more impact than the short pieces inside the amp?

[butwhatif]

Mogami 2964