Weber copper cap rectifier - Blown?

[DEATHRIPPER]

Greetings all!

I'm new here but i'm hoping somebody could help me with some troubleshooting.
I recently took delivery of a Weber Hiwatt clone, upon build completion i fired the amp up (which sounded excellent, i may add) and played at a reasonable lounge-room volume.

I noticed that the kt-66's were producing quite a lot of heat and glow, i attempted to test the bias with my biaspro but didn't get a reading on my multimeter?

i set the bias pot below (or so i would have to assume was low by backing off both bias pots) until i noticed an odor being produced by the amp, then suddenly the 2 amp/250v mains fuse blew.

i let everything cool down, replaced the fuse and instantly blew the new fuse, i removed the amp from its cabinet and began so inspect solder joins, looking for scorched caps etc when i realised that the sticker logo on the weber copper cap was flaked and charred, is it possible this is burned out causing the amp to short upon start-up?

also, how is this possible when the bias was set as low as possible? (without being able to obtain a reading)


any information that can help me out is much appreciated

[Mick Bailey]

If the rectifier is shorted then the fuse will blow instantly. The best way to investigate your amp is to use a series lightbulb current limiter until you've got it fixed.

When you say the KT66 were producing a lot of heat and glow, were the actual plates glowing? Output tubes produce a lot of heat and they glow anyhow - do you know that it wasn't working correctly in the first place? You could have been turning the pots the wrong way and reducing the bias voltage (therefore increasing tube dissipation). Adjusting the bias without measurement is risky.

My advice would be;

1. Remove the KT66s for the time being. You may have a shorted tube.

2. Power the amp with the light bulb, both with and without the rectifier. If the bulb glows brightly without the rectifier then you may have a shorted PT. If it glows brightly only with the rectifier inserted, then it could either be a shorted rectifier, or shorted cap. You would need to disconnect the feed from the rectifier to the first filter cap to see if the bulb stays lit with the rectifier in place and goes out with it removed. If it stays dull when it's inserted with the cap feed removed then you've likely got a shorted cap.

3. Replace the shorted components and re-test without the tubes.

4. If all is well to this point, reinsert the tubes and re-test - again with the bulb limiter.

5. Assuming you get this far OK, establish the position of the bias pot to give minimum tube dissipation (do this with the lightbulb connected).

6. Power up the amp off the mains and set the bias correctly.

[Steve Conner]

If they're "glowing" like this...

that's bad!