Three possibilities come to mind.
- Defective breaker. Possible but quite rare.
- Leaky filter caps. A very good possibility if they are original. Leave the
amp on for a half hour or so and then shut off. Make sure the HV is down to
zero and pull the plug. Feel the caps. If they are more than just barely
warm they should be replaced. The plate meter will not show the capacitor or
bleeder current. Get 220 MF 450V Cornell Dublier 381LX series caps from
Mouser. They are snap locks but all can be oriented to fit existing holes.
Replace the 8 bleeder resistors with 75K 3W MOX, also from Mouser.
- Bad tube or tubes. Actually not bad but not compatible with the SB-200
design. Some Chinese and especially Russian tubes have different
interelectrode capacitances and lower mu than the original Cetrons and
become unstable. Unless you want to install a neutralization circuit and
possibly increase the operating bias the best step is to replace the tubes.
There are still many good and even NOS Cetrons around. BTW, what brand are
they?
Carl
KM1H
----- Original Message -----
From: "Tom Osborne" <w7why@verizon.net>
To: <Amps@contesting.com>
Sent: Sunday, August 26, 2007 6:16 PM
Subject: [Amps] SB-200 circuit breakers
> Hi All
>
> On my SB-200, the front circuit breaker trips after half hour or so of
> operation on RTTY. If running RTTY, I usually keep it down to 200-250
> watts. It is the same breaker, the front one, that always trips. After
> it
> cools down a bit, it will come back on. The plate current is only running
> about 300 ma or so.
>
> The tubes are fairly new (4 months) and don't run red when operating RTTY
> at
> 200-250 watts. I also have some muffin fans on top with the lid open to
> take out some of the heat.
>
> In looking at the schematic, the breakers are in the primary of the power
> transformer. What causes them to trip, heat? There doesn't seem to be
> that
> much with the extra fans on top. 73 and thanks
> Tom W7WHY
>
>
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>
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