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Hi all I've been asked to repair a Roost 50w guitar amp. Its a fairly typical kinda design with 2 x el34's. The bit I'm having difficulty understanding is that it uses a full wave rectifier (not a problem) but the negative side of the rectifier is connected to ground via a resistor then a fuse. I see the point of the fuse but why the resistor? The problem is that something is causing the resistor to fry (and therefore no HT)which I 've yet to find. I'm hoping its a chafed wire grounding out. The valves test ok and so far not found a short in the smoothing caps.
Any one got any idea why a resistor would be put in the rectifier? The only reason I can think of is to cause some kinda sag on the HT.
Cheers Just

Just out of curiosity, what value is the resistor, and how much power can it dissipate?

Kyle

antiquekid3 wrote:Just out of curiosity, what value is the resistor, and how much power can it dissipate?

Kyle

Its 1k although the schematic I've found says 1R0. Schematic doesent specify power but the one fitted was about 3-5watt. I had replaced this one some time ago due to the old one showing signs of ageing. When I replaced it this time it started to get very hot and another part of the amp started smoking. Guess I'm going to have to disconect stuff and work through it bit by bit until I find the culprit. Just currious as to the purpose of the resistor.
Cheers Just

The 1R0 (1 ohm) resistor is probably there to measure the idle current of the power tubes. You do not want to put a 1k in there. A 1/2 watt should be plenty high enough for a pair of EL34s.

Newbie --

With a value of 1 ohm, any surge protection it provides will be minimal. However, since the output stage pulls the vast majority of all the current drawn by the amplifier, the resistor could also provide a means by that manufacturer for checking the current draw the output stage (setting the bias), and provide a fuse for the entire PS at the same time.

If the existing resistor is a value of 1K, it surely would smoke it as the power would then be dropped across it, rather than the amplifier.

Dave

I've talked about this a few times elsewhere, but... When the diodes switch into forward conduction the capacitors look like a dead short to ground as the transformer dumps current into them to replenish the current that was drawn while the diodes were off. This makes huge spikes in the ground plane as the current returns to the transformer. It also contributes to heating of the transformer as the current spikes dissipate power in the resistance of the windings. The easiest way to address this problem is to put a small value resistor between the diodes and the capacitors. I use 3 ohm 10 watt myself. As a side benefit you can monitor total curent draw with this resistor as well. If you scope this resistor you can clearly see the spikes.

Don