Using wirewound resistor on a high power (very hot) circuit.

[bsas]

Hi all,

I was doing some mods on my Fender Hot Rod Deluxe and I decided to change three resistors on the circuit that are infamous to heat up and cook the board (sometimes even messing up with the soldering around it).

In case anyone is curious, here is the schematic:
https://support.fender.com/attachments/token/Mhsz8yRhIdx5KMe1y4Nh61ST2/?name=Hot+Rod+Deluxe+III+Schematic+and+Layout.pdf

The resistors are R78 and R79 and R97. In the original specs R78 and R79 are 470R 5W and R97 is a 1.2K 2W.

So, after some research (and some thinking after and old thread I did (https://www.eevblog.com/forum/projects/modifying-a-fender-hot-rod-deluxe-amp-wirewound-vs-ceramic-resistor/msg1590874/#msg1590874) I decided to just put more powerful and more precise resistors on those spots, and a big "spacer" between those and the board so the heat cannot cook the board (in theory). So, I bought 3x Vishay Wirewound Resistors (2x "470R 7W - https://www.mouser.com/ProductDetail/72-RWM06224700JA15E1" and 1x "1.2K 3W - https://www.mouser.com/ProductDetail/72-RWM410-1K2-5") and looks like everything is going great.

I am just curious about some stuff and I would love some engineering inside to understand those phenomenons better

1) When I use a infrared thermometer to check those guys temps, looks like the wire itself (that is inside the spacers) are quite hot (like 100-140C hot) but the casing of the resistor is not. The thermometer shows something around 50-60C for the casing (aka, the resistor itself but measured from the outside case). Is that normal or is that something in the casing material that makes the infrared thermometer not being able to measure it accurately? It is very hot to the touch by the way.

2) If those get constantly 100-150C, is that OK? I am just worried about cooking those resistor really bad, even if I am far bellow the power spec of the circuit, which I measured and it is a voltage drop from 50V to 17V on the 470R, so, around 2-3W max.

3) Are those resistors good ones for that application, or should I go for the cement ones? I love those resistors, they are neat, very accurate, perfect size for the board, but I am not sure if I am on a "fire hazard" situation

Thanks!

[janoc]

From what I can see, R78/R79 are just part of the RC filter in the power supply to suppress mains hum and limit the current through those Zeners D13 and D14. No wonder that that bit gets smoking hot - it is dropping whopping 32V!

Replacing those components with a small buck converter + a linear post regulator to eliminate noise would likely completely solve the problem. Given that that supply only drives 3 opamps, a very small easily shielded converter would be more than enough.

R97 detto - it is getting so hot, because of the two 10V zeners there dropping 38V or so, only to bias two opamps inputs (i.e. totally negligible loads, completely swamped by the zener currents). If you add the buck regulators needed for the +16/-16V rails, you can add two +10V/-10V linear regulators for this powered from the bucks and get rid of the resistor and the circuits around the zeners D28 and D26.

Problem solved, no more fire hazards.

[MagicSmoker]

Wirewound resistors are designed to run hot, frequently spec'ed for a surface temperature of anywhere from 150C to 250C at rated power (so the wire temperature will be much hotter). Consequently, it is fairly normal to ridiculously derate wirewound resistors just to protect nearby components from excessive heat.

The Sfernice resistors you selected are excellent and will work fine for R78 and R79 - these resistors are just dropping the supply voltage for the op-amps, after all. While I couldn't find R97 on the schematic, I doubt there will be a problem with your substitution there, either.

[bsas]

From what I can see, R78/R79 are just part of the RC filter in the power supply to suppress mains hum and limit the current through those Zeners D13 and D14. No wonder that that bit gets smoking hot - it is dropping whopping 32V!

Replacing those components with a small buck converter + a linear post regulator to eliminate noise would likely completely solve the problem. Given that that supply only drives 3 opamps, a very small easily shielded converter would be more than enough.

R97 detto - it is getting so hot, because of the two 10V zeners there dropping 38V or so, only to bias two opamps inputs (i.e. totally negligible loads, completely swamped by the zener currents). If you add the buck regulators needed for the +16/-16V rails, you can add two +10V/-10V linear regulators for this powered from the bucks and get rid of the resistor and the circuits around the zeners D28 and D26.

Problem solved, no more fire hazards.

I though about that a lot janoc, but I decided to not change the original circuit. But thanks for your input

[bsas]

Wirewound resistors are designed to run hot, frequently spec'ed for a surface temperature of anywhere from 150C to 250C at rated power (so the wire temperature will be much hotter). Consequently, it is fairly normal to ridiculously derate wirewound resistors just to protect nearby components from excessive heat.

The Sfernice resistors you selected are excellent and will work fine for R78 and R79 - these resistors are just dropping the supply voltage for the op-amps, after all. While I couldn't find R97 on the schematic, I doubt there will be a problem with your substitution there, either.

Thanks MagicSmoker (best name ever for this forum ), but why the temperature on the leads are so much higher then on the cases? Is that the case is working nice on dissipating it or just my infrared thermometer cannot measure it for some reason?

[MagicSmoker]

Thanks MagicSmoker (best name ever for this forum ), but why the temperature on the leads are so much higher then on the cases? Is that the case is working nice on dissipating it or just my infrared thermometer cannot measure it for some reason?

Aye, much magic smoke have I set free in my time, hence the sobriquet.

The material making up the body of the resistor - whether it be silicone, vitreous enamel, cement, etc. - has a relatively poor thermal conductivity, so there will be a huge temperature drop from the resistive element to the surface. In contrast, the lead wires will have a relatively good thermal conductivity so they will reach a similar temperature as the resistive element (and won't cool off much from radiation/convection due to the minuscule surface area).

[janoc]

I though about that a lot janoc, but I decided to not change the original circuit. But thanks for your input

Well up to you. I just found it rather shocking that someone considers this an acceptable design practice. Granted, it is a tube amp, so some energy efficiency is likely the least of anyone's concern, but resistors scorching boards should be.

All that to save one winding on the (already) custom transformer ... Well, expensive audio gear, why am I not surprised ...

[TimFox]

A construction hint from the past:  Heathkit used to specify cutting a small piece from the cardboard box (4 to 6 mm thick) and use it as a temporary spacer between a wire wound resistor and the circuit board, removing it after soldering.
In vacuum tube circuits, condemned to low efficiency due to cathode heating power, the heat inside the chassis, with proper thermal design (being careful with sensitive passive components such as polystyrene capacitors and with envelope temperature on the tubes) can be a good thing, keeping the circuits dry in humid environments.