Laney LC50 Mk1 and Mk2 - B+ dropping resistor

[ryanuk]

Hi
A recent problem with my LC50 Mk1 amp caused me to open up it up and examine the PCB. I discovered a concerning difference between the PCB and schematic - see attachments as follows;

- Extract of the schematic - see top left for a 10K 25W nodal resistor in the B+.

- A pic of a LC50 PCB with a 10K 25w wirewound.

- A pic of a LC50 PCB which looks like the one I have (note the 10K (3w?) carbon comp almost in the centre of the pic)

My concern is that my amp (like the one in the pic) only has a carbon comp lower power resistor in this position. Further this R has drifted out of tolerance to 12K.

Been pondering over the correct rating for this Res. It drops approx. 87V (from 397V to 310V in the next node of the B+ supply). So using ohms law, draws 8.7mA current (87/10,000) to power preamp and PI. So in terms of power, it dissipates 0.75W. Is my maths/thinking correct? If so, a 3 or 5 watt would be plenty. Seems overkill to apply a 25W wirewound for this job.

So my questions are;
1) Is this Resistor over spec'd to begin with?
2) Should I replace the component given the amp is over 20 years old and has drifted out of tolerance?

[phatt]

If the the unmentioned problem is not related to this part and there is no evidence of overheating and the amp has been running for years like that,, then it's fine.
3Watt is a common value at that point as it's only driving the preamps.
10k,,12k nah close enough for a guitar amp,,,leave it,, you will never hear the difference.
Phil.

[ppluis0]

Should I replace the component given the amp is over 20 years old and has drifted out of tolerance?

You can connect a 56K or 68K 1/2w in parallel with that drifted resistor to compensate this aging and keep playing

Cheers,
Jose

[ryanuk]

So was my maths and logic in my post correct?

If the the unmentioned problem is not related to this part and there is no evidence of overheating and the amp has been running for years like that,, then it's fine.
3Watt is a common value at that point as it's only driving the preamps.
10k,,12k nah close enough for a guitar amp,,,leave it,, you will never hear the difference.
Phil.

Phil. The problem was a noise in one of the inputs. Turned out to be environmental. But whilst I had the amp apart I began to think about the age and fragility of some of the components.

You can connect a 56K or 68K 1/2w in parallel with that drifted resistor to compensate this aging and keep playing

Cheers,
Jose

Jose. Many thanks for this. I hadn't considered that

[george giblet]

Drift isn't a good sign. I'd just replace it. All cheap parts.

There's some argument to use a 3W carbon comp as it will fail open if you get a PSU short.
A 5W is unlikely to fail.

[ryanuk]

Drift isn't a good sign. I'd just replace it. All cheap parts.

There's some argument to use a 3W carbon comp as it will fail open if you get a PSU short.
A 5W is unlikely to fail.

Thanks george - can you confirm the maths and application of ohms law was correct in my post?

[george giblet]

The calculations look OK. Since the ambient temperature in the amp is relatively high you would use a higher power resistor than calculated in order to get the same life as a resistor at room temperature. You already know a 3W resistor is starting to fail before the amp does.