Japanese tube amp - wirewound resistors?

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I just opened up an 7189a based Japanese tube amp (a Tiesco). Unless
I'm really mistaken, every single resistor in the thing is wirewound.
They all look like air core coils dipped in some kind of bluish
plasticy coating. My guess is that this would be extremely costly and
difficult to design around due to the inductance of the all wire
windings. Is that inductance too small to matter? Is there any other
kind of resistor that looks like a hollow tube? Or is this really as
wierd as it seems to me?

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apa wrote:
> I just opened up an 7189a based Japanese tube amp (a Tiesco). Unless
> I'm really mistaken, every single resistor in the thing is wirewound.
> They all look like air core coils dipped in some kind of bluish
> plasticy coating. My guess is that this would be extremely costly and
> difficult to design around due to the inductance of the all wire
> windings. Is that inductance too small to matter? Is there any other
> kind of resistor that looks like a hollow tube? Or is this really as
> wierd as it seems to me?
>
Could be wirewound. Those resistors are expensive but not out of reach.
They can also be machine wound. One of the key advantages of wirewound
resistors is precise ohm values with little variance. In the hands of a
good designer it can lead to a less noise and better performance in a
number of areas.

--

J

www.urbanvoyeur.com

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That's gotta be it. A hollow cylindrical metal film makes much more
sense - didn't know they existed. I thought I saw evidence of windings,
but it was probably just wrinkles in the coating.


Andy Eng wrote:
> Interesting... Could you actually see the wire shapes under the
> coating? I do recall metal film resistors where the resistive coating
> was deposited on a cylindrical shape.
>
> An X-ray or cutting one apart is one way to find out... :-)

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apa <tacoma57@hotmail.com> wrote:

> That's gotta be it. A hollow cylindrical metal film makes much more
> sense - didn't know they existed. I thought I saw evidence of windings,
> but it was probably just wrinkles in the coating.

It might be a spiral cut applied to the coating to increase the overall
resistance and/or trim it. The extra inductance was considered
negligible at audio frequencies.

This was done with an abrasive wheel and it usually cut a shallow groove
into the surface of ceramic tube as well.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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They seem pretty solid. There's a thicker ring at each end and I assume
there some extra material under there to make the lead connection
tougher.

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> Thinking back, I seem to recall that our service people
> reported near-zero failure rates for the *odd* resistors in
> this pacific rim product.

....thinking about how resistors fail...

Resistor failure is almost always secondary to another component failure, or
a bad design, right? Just wondering if these odd resistors were more
tolerant of excess power or something.

-John O

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John O <johno@!noSPAM!heathkit.com> wrote:
>> Thinking back, I seem to recall that our service people
>> reported near-zero failure rates for the *odd* resistors in
>> this pacific rim product.
>
>...thinking about how resistors fail...
>
>Resistor failure is almost always secondary to another component failure, or
>a bad design, right? Just wondering if these odd resistors were more
>tolerant of excess power or something.

Today resistor failure is almost always secondary. But back in the seventies,
carbon comp resistors failed all the time, often from thermal damage. They
would develop tiny cracks from heating and cooling, and suddenly the value
would start to rise.

Film resistors don't do this quite so easily. Which may be the reason that
the Japanese went to those goofy film resistors while folks in the west were
still using carbon comps.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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> Today resistor failure is almost always secondary. But back in the
seventies,
> carbon comp resistors failed all the time, often from thermal damage.
They
> would develop tiny cracks from heating and cooling, and suddenly the value
> would start to rise.

Ya know what, you reflashed a memory. I used to work on old Heathkit ham
gear, transceivers like SB-101, linears, and such. Those CC resistors *did*
fail, especially in the power supplies and near the finals. You could see
the cracks, and bust them apart while poking around on an initial
inspection.

I had completely forgotten about that.

-John O