Hi all,
I have acquired an 18w clone built locally (for an insanely low price) from the family of the builder (trained as an electrical engineer). It sounds fantastic and is a really great build from what I can tell. I want to rehouse the chassis into a new cab. I'm trying to ascertain from the schematic supplied with the build whether I have a bleed resistor on the filter caps? I think I may need to create a discharge tool before I undertake this endeavour. I'd also like to add a standby (I don't think it's strictly necessary but the original has one).
It came supplied with two schematics (Gabi Bucataru and pedalmonkey confusingly) - so extra eyes on this would be greatly appreciated so I don't zap myself ...
Thanks!
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Do I have a bleed resistor?
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wookster76
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Do I have a bleed resistor?
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Bieworm
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Re: Do I have a bleed resistor?
It’s hard to tell from the pictures. Measure the 1st filter cap + to ground. What value do you read?
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Crabman
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Re: Do I have a bleed resistor?
As Bieworm has indicated, you can measure the voltage across the reservoir cap to see where things stand. However, before you decide to poke around inside the amp, how comfortable are you with measuring voltages in a tube amp?
As far as a bleed resistor goes …
Based on what I can see from the images, you most likely have a 560K(?) bleed resistor (green-blue-yellow-gold?) in parallel with the second 47uF filter cap. The quality of the soldering is a bit of a mess so it masks following the ground connection between those 47uF filter caps. Anyway, if this is the case, you will have a fairly slow discharge time with a 560K resistor, 47uF cap and 350V on the reservoir cap (another assumption here). It would take at least a couple of minutes to get the voltages down to a safe level after turning it off. But before jumping to conclusions, you need to take some measurements (safely).
Regarding the standby switch …. leave it the way it is. The Valve Wizard has covered this ad nauseam in his books and his web blog. You are better off not having a standby switch when using a valve rectifier for reasons you can read about. If you have a need to keep your heaters warm during gigging sets, then leave the power on and turn the volume to zero ... you will be surprised at how well that can work!
Also, the choice of 47uF filter caps is not what we normally see in an 18 watt using a tube rectifier ... but that’s a different discussion.
As far as a bleed resistor goes …
Based on what I can see from the images, you most likely have a 560K(?) bleed resistor (green-blue-yellow-gold?) in parallel with the second 47uF filter cap. The quality of the soldering is a bit of a mess so it masks following the ground connection between those 47uF filter caps. Anyway, if this is the case, you will have a fairly slow discharge time with a 560K resistor, 47uF cap and 350V on the reservoir cap (another assumption here). It would take at least a couple of minutes to get the voltages down to a safe level after turning it off. But before jumping to conclusions, you need to take some measurements (safely).
Regarding the standby switch …. leave it the way it is. The Valve Wizard has covered this ad nauseam in his books and his web blog. You are better off not having a standby switch when using a valve rectifier for reasons you can read about. If you have a need to keep your heaters warm during gigging sets, then leave the power on and turn the volume to zero ... you will be surprised at how well that can work!
Also, the choice of 47uF filter caps is not what we normally see in an 18 watt using a tube rectifier ... but that’s a different discussion.
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