Wiring a tube preamp

[Jeffrey Landgraf]

I've completed the circuit board and stuffing of a vacuum tube preamp for my
guitar rack and it's ready to wire up the control potentiometers. What kind
of wire is recommended for the pots? I've seen similar designs that call
out small gauge coax (ex. RG-174/U) and twin-ax cables but I'm having
trouble finding the small gauge twin-ax cable. The lengths will be no more
than 8". Is there a benefit to using coax cable over a good shielded cable
such as the Mogami Quad? Or is shielded even necessary at these lengths?


Thanks
Jeff

[Ed]

Never ever use rg coax. Under certain conditions, rg coax will go
microphonic on you. It's nasty with audio.

Look inside of a Fender or Marshall. 20-something(maybe 22) gauge
stranded PVC insulated wire. Teflon insulated will give a nicer finish
but is more expensive. If you feel the need to run shielded wire to
volume/preamp stage controls, small gauge shielded audio cable is
fine(Belden, Alpha, or equiv).

Check with Mouser or Newark.


On Sun, 16 Nov 2003 20:29:28 GMT, "Jeffrey Landgraf"
> wrote:

>I've completed the circuit board and stuffing of a vacuum tube preamp for my
>guitar rack and it's ready to wire up the control potentiometers. What kind
>of wire is recommended for the pots? I've seen similar designs that call
>out small gauge coax (ex. RG-174/U) and twin-ax cables but I'm having
>trouble finding the small gauge twin-ax cable. The lengths will be no more
>than 8". Is there a benefit to using coax cable over a good shielded cable
>such as the Mogami Quad? Or is shielded even necessary at these lengths?
>
>
>Thanks
>Jeff
>

[Mike Rivers]

In article > writes:

> I've completed the circuit board and stuffing of a vacuum tube preamp for my
> guitar rack and it's ready to wire up the control potentiometers. What kind
> of wire is recommended for the pots? I've seen similar designs that call
> out small gauge coax (ex. RG-174/U) and twin-ax cables but I'm having
> trouble finding the small gauge twin-ax cable. The lengths will be no more
> than 8". Is there a benefit to using coax cable over a good shielded cable
> such as the Mogami Quad? Or is shielded even necessary at these lengths?

Whether shielding is necessary or not depends on the circuit and how
well shielded the whole box is. Sometimes two pieces of shielded wire
(coax such as RG-174 falls under the category of "shielded wire" here)
are used, one to the top and one to the wiper, with the bottom contact
of the pot almost always going to ground, and therefore connected to
the shield of the cable.

But you can just use unshielded wire for starters if your box is well
shielded and you don't have a power transformer nearby.


--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

[Scott Dorsey]

In article <znr1069021241k@trad>, Mike Rivers > wrote:
>In article > writes:
>
>> I've completed the circuit board and stuffing of a vacuum tube preamp for my
>> guitar rack and it's ready to wire up the control potentiometers. What kind
>> of wire is recommended for the pots? I've seen similar designs that call
>> out small gauge coax (ex. RG-174/U) and twin-ax cables but I'm having
>> trouble finding the small gauge twin-ax cable. The lengths will be no more
>> than 8". Is there a benefit to using coax cable over a good shielded cable
>> such as the Mogami Quad? Or is shielded even necessary at these lengths?

How can you use Mogami Quad on such a short run? It's MONSTROUSLY large
compared with the little coax. Your fingers must be much more nimble
than mine.

You probably need a shielded line. Use small shielded line, or coax (don't
use RG-174, which has a steelclad core), or take a wire and wrap a coil of
bare wire around it for a shield (like the Citation III kits had you do).

>But you can just use unshielded wire for starters if your box is well
>shielded and you don't have a power transformer nearby.

Yup, but you can have trash floating around in the chassis, say from AC
filament wires or power transformers. Best to use shielded stuff on a
long run. It's not like it's hard to get or costs a lot.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

Ed > wrote:
>Never ever use rg coax. Under certain conditions, rg coax will go
>microphonic on you. It's nasty with audio.

RG coax?

>Look inside of a Fender or Marshall. 20-something(maybe 22) gauge
>stranded PVC insulated wire. Teflon insulated will give a nicer finish
>but is more expensive. If you feel the need to run shielded wire to
>volume/preamp stage controls, small gauge shielded audio cable is
>fine(Belden, Alpha, or equiv).

All that stuff is in the Radio Guide also. There are something like
a thousand different kinds of cable with RG designations in the book.
Most shielded configurations are in there.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Jeffrey Landgraf]

Thanks to all for the advice! The Mogami wire I have is short pieces of
quad cable left over from a patch bay wiring job. It's actually not much
that much bigger than the coax cable and not difficult to work. The
transformer and power supply are both close enough that I think I go ahead
and try the shielded Mogami wire and see how it works.

Jeff

[Ed]

On 16 Nov 2003 21:38:18 -0500, (Scott Dorsey) wrote:

>Ed > wrote:
>>Never ever use rg coax. Under certain conditions, rg coax will go
>>microphonic on you. It's nasty with audio.
>
>RG coax?

I was refering to the 50 ohm stuff designed for RF use. I used it back
in the day when I was modding guitar amps and ended up having to
replace it with standard plenum audio cable due to various noise from
it. Oh well.

Ed


>
>>Look inside of a Fender or Marshall. 20-something(maybe 22) gauge
>>stranded PVC insulated wire. Teflon insulated will give a nicer finish
>>but is more expensive. If you feel the need to run shielded wire to
>>volume/preamp stage controls, small gauge shielded audio cable is
>>fine(Belden, Alpha, or equiv).
>
>All that stuff is in the Radio Guide also. There are something like
>a thousand different kinds of cable with RG designations in the book.
>Most shielded configurations are in there.
>--scott

[Scott Dorsey]

Ed > wrote:
>On 16 Nov 2003 21:38:18 -0500, (Scott Dorsey) wrote:
>
>>Ed > wrote:
>>>Never ever use rg coax. Under certain conditions, rg coax will go
>>>microphonic on you. It's nasty with audio.
>>
>>RG coax?
>
>I was refering to the 50 ohm stuff designed for RF use. I used it back
>in the day when I was modding guitar amps and ended up having to
>replace it with standard plenum audio cable due to various noise from
>it. Oh well.

If it's RG-58C, it will go microphonic bigtime because the sleeve can shift
around too easily around the dielectric, and because the dielectric itself
is triboelectric. Not a problem at all at RF, but a big problem for audio.

I think there is an RG-58 variant (Is it A/U?) that doesn't have as much of
this problem, but with very high-Z lines there are special coax cables that
are intended for the job with graphite around the shield and special
dielectric.

Most of the plenum cables are teflon-insulated, which is wonderful stuff for
audio. No triboelectric problems, great to solder, wonderful stuff. I think
we use RG-416A around here for hookup work inside chassis; it's teflon and it
is very tiny.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Frank Vuotto]

On 16 Nov 2003 21:16:20 -0500, (Scott Dorsey) wrote:


>You probably need a shielded line. Use small shielded line, or coax (don't
>use RG-174, which has a steelclad core), or take a wire and wrap a coil of
>bare wire around it for a shield (like the Citation III kits had you do).
>

... and be sure not to make the mistake of using the shield as a signal
path.


Frank /~ http://newmex.com/f10
@/