Reply to thread

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Message: <blockquote data-quote="vintageampfixer" data-source="post: 1129550" data-attributes="member: 128568">The lamp's used as a current limiter - so that when the current in the output stage passes a threshold, the lamp's filament starts to heat up and would present a dramatically higher resistance, limiting the output stage current.  IF you're lucky, you might see a glimmer from the lamp but in normal operation I would expect it to stay dark.  A simple resistor or a diode cannot be used to sub here because they would behave differently.  I agree these aren't readily available but at least here in the UK, Google finds a few suppliers although generally the lamps are in LES packages rather than wire-ended.  I'd try any bulb that's similar - a good auto store will have small lamps to backlight dashboards, etc.  If you can find something similar - 1w to 2w for the test would probably be fine - put the new part in circuit and see what happens.  For my money, you'll either see 'nothing much' and find the amp works ok-ish or you'll see the lamp illuminate strongly and possibly blow given the 24v rail.  If the lamp is bright, that would indicate a fault, probably in the output stage or that something's upsetting the output stage's bias.Beware of 'testing' with (say) a 10w lamp because that might be fine (for a while) if the amp does work - to JR's earlier comment, lamps do randomly fail - but it won't have the right shape knee in its resistance curve to give the protection the lamp is intended to bring to that circuit.</blockquote>

Verification