Anybody notice how temperature sensitive Piher resistor are.

[Charlie Wilson]

Hello,
I have been purchasing and replacing the Xicon carbon film resistors with Pihers in my ODS. Last night it was pretty cool in my room and I started measuring the Pihers and was surprised to find they all measured higher than when I first measured them. I then put some heat on them and found the resistance drops pretty quickly. I know 100w ODS amps get pretty warm. I hope my amp doesn't drift on the bright side after an hour or two of being powered up.
CW

[tele_player]

I'm surprised, I'd expect them to show a positive thermal coefficient, resistance rising with temperature.

Robert

[Synchu]

I have a C-tone BM HRM purchased before I started the building obsession. It looses brightness and gain after few hours. I suspect rather a cold solder joint, than resistors being hot, but it's anyway pending "surgery"
Good point though, I'll measure the resistors while on the bench.
Niki

[Structo]

I'm surprised, I'd expect them to show a positive thermal coefficient, resistance rising with temperature.

Robert

That is how I remember it.

[Charlie Wilson]

The 1W ones I have definitely drop in value when heated by a couple K. These resistors are going to be my overdrive controls series resistors and overdrive preamp tube grid resistors so I expect the overdrive to brighten up as the amp heats up.
CW

[ic-racer]

What was wrong with the Xicons?

[JazzGuitarGimp]

The datasheet for the pihers you're using should define the limits of the temperature drift over heat. I would start there, and make sure the drift you're measuring is within these limits.

By "putting some heat on them", what exactly do you mean? For example, are you doing this close enough to your DMM that you are actually raing its temperature? If so, it could be your meter that is changing as it warms up. I suppose it could even be an effect of heating your test probes connected to the resistor under test.

[ToneMerc]

FWIW, the ones seen in Dumbles were the 60's and early 70's original manufacturer resistors and specifically labeled "hi-stability".

TM

[Charlie Wilson]

ic-racer, actually I think the Xicons sound good. I just wanted to try NOS resistors. Putting the Pihers in the phase invertor definitely smoothed the high end and took the harshness out of the OD. Jazzguitargimp, yeah I was careful not to heat the meter or leads. I used a heat gun at a distance and heated just the resistor and then I measured it. I'm not sure where I would get a spec sheet for vintage Pihers but my method is not really very scientific anyway. I know Dumble used Pihers so it is not a deal breaker. I guess what I should do is simply leave the amp on for a couple hours and just measure them. On the subject of thermal drift, I took out my 1 ohm bias resistors because they were drifting down in value. I also found that the chassis itself changes resistance as it heats up. So putting a single ground test point and four cathode test point spread out across the chassis did not seem like a great idea to me.
CW

[Structo]

Some people think that the drift of values is actually a good thing.

As suggested by vintage Fender amp owners.

Those old carbon comp resistors drifted big time so it may be prudent to measure those old resistors to see what the "m0jo" is.

[Charlie Wilson]

As long as they mojo in the right direction.
CW