The 1K ohm 'choke' resistor in the Express

[rooster]

Hey, just wanted to put this out there. The 1K value is pretty large relative to a lot of other amps, and most of us here would agree that it helps create the compression that happens in the Express. Well, I just ran into a '60s Magnatone 260A amp, called a 'Twilighter', and I found an original wire wound resistor in it that also performed as a choke - who's value was 4K ohms. The amp runs with about 460 VDC on the plates (6L6s) using today's AC voltage, BTW, but 425 in the day. And yes, this amp has that very smooth compressed feel to it.

But my point here is that this is a 10 watt wire wound resistor and its been in service for more than 40 years without a failure, and its still going strong. Which is to say that if I ever had a doubt about the 25 watt 1K ohm resistor in the Express, I will stop worrying right now.

[passfan]

I have done the math on some of the output bias resistors used and have come no where near 25 watts either , what gives ?

[gearhead]

This has been a popular topic over the years.

Good rundown: https://tubeamparchive.com/viewtopic.php?t=2379

[Cliff Schecht]

As Ken himself has stated, everything part used in the Express is overrated to reduce stress on the entire system. Since no part of the amp is ever struggling for current, the amp is able to have a very quick dynamic response. One of the first things that stands out with the Express is how quick it reacts to changing playing dynamics.

[PCollen]

As Ken himself has stated, everything part used in the Express is overrated to reduce stress on the entire system. Since no part of the amp is ever struggling for current, the amp is able to have a very quick dynamic response. One of the first things that stands out with the Express is how quick it reacts to changing playing dynamics.

Like using a gallon bucket to transport a pint of water. One thing for sure: that 25 W'er will never burn up.

[gearhead]

Like using a gallon bucket to transport a pint of water. One thing for sure: that 25 W'er will never burn up.

But it'll definitely get pretty warm. A 10 watter's been known to blister the board on some builds.

[JamesHealey]

I use a 15w without any issues, Mine is on it's own tag strip elevated into the air though so it's got nothing to burn if it got hot.

[Zippy]

As Ken himself has stated, everything part used in the Express is overrated to reduce stress on the entire system. Since no part of the amp is ever struggling for current, the amp is able to have a very quick dynamic response. One of the first things that stands out with the Express is how quick it reacts to changing playing dynamics.

Resistance is resistance - having greater heat dissipation does not lower the resistance. There is no enhanced current flow through a resistor just because it dissipates heat better - no effect on dynamics. That's "just" a characteristic of the design.

[PCollen]

Like using a gallon bucket to transport a pint of water. One thing for sure: that 25 W'er will never burn up.

But it'll definitely get pretty warm. A 10 watter's been known to blister the board on some builds.

That resistor could be mounted either on the other side of the board, or off-board on a terminal strip and the chassis itself used as a heat sink. But it wasn't, and so it isn't.

[Cliff Schecht]

As Ken himself has stated, everything part used in the Express is overrated to reduce stress on the entire system. Since no part of the amp is ever struggling for current, the amp is able to have a very quick dynamic response. One of the first things that stands out with the Express is how quick it reacts to changing playing dynamics.

Resistance is resistance - having greater heat dissipation does not lower the resistance. There is no enhanced current flow through a resistor just because it dissipates heat better - no effect on dynamics. That's "just" a characteristic of the design.

I didn't state that the 1k resistor has magical properties that change how the amp sounds, it's just a resistor in a voltage dropping string. My point may have been misleading in that post.. I think it was chosen as a convenient low-range value (i.e. easy to find for service reasons). The choice of a 25W part is obvious as well, those ceramic wirewounds will COOK if you start pushing them near their power rating. To prevent his amps from failing constantly, he put in a bigger part. Otherwise, as someone else stated, the boards would warp and eventually fail.

[gearhead]

Like using a gallon bucket to transport a pint of water. One thing for sure: that 25 W'er will never burn up.

But it'll definitely get pretty warm. A 10 watter's been known to blister the board on some builds.

That resistor could be mounted either on the other side of the board, or off-board on a terminal strip and the chassis itself used as a heat sink. But it wasn't, and so it isn't.

Was just trying to point out that the 25W rating is closer to the edge than a gallon-to-pint comparison. It's more like using a gallon bucket to transport 1.5 to 2 quarts of water. It's still more than enough, absolutely.

[alanp]

Resistance is resistance - having greater heat dissipation does not lower the resistance. There is no enhanced current flow through a resistor just because it dissipates heat better - no effect on dynamics. That's "just" a characteristic of the design.

Resistors do act a bit strange when they heat up, and their characteristics can change. I don't know the figures for them, but perhaps a 10W heats up more, to the point where it changes from 1K to some other value to an unacceptable degree.

[Cliff Schecht]

Ceramic wirewound resistors have a low TCR, usually less than 50PPM/degC. That I don't think is the issue. It most likely has to do with the fact that without some sort of heatsinking, the wattage rating of ceramic resistors should be derated. This is especially in a no airflow situation like inside an amplifier.

[billyz]

I like this one.

[M Fowler]

Yeah but isn't the idea of using that type of resistor is to mount it to the chassis surface helping dissipate heat.