I have a Tentec 425 as well (Golden Series) and I love this amplifier.
I have had this plate current problem twice in the last two years and
both times it was a bad Chinese Matchlett tube.
I hope it turns out to be a bias problem as K2XX has described. I tried
the tube swap first as it was easier and I had some spare Eimac tubes on
hand.
When using this amp watch the grid current at all times. I like to keep
mine at 30mA. If you are showing too much grid current when tuned then I
increase the Tune setting by maybe 0.5 and then reload for max RF. This
gives less amp gain and you will see less grid current. I keep a table
handy with all the settings for the different bands and spot
frequencies My amp is a later model and has grid current protection.
I don't think some of the earlier models had this protection. It is a
quite simple circuit where the current monitoring voltage is applied to
the base of a transistor through a resistor and once the current drive
voltage gets too high it turns off the bias to the tubes.
73 Mark N1UK
On 04-Feb-17 11:03 AM, Joe via Amps wrote:
While I sold my two 425s some time ago, I believe that this is a
common problem when the amp is over-driven. R17 on the bias board
opens up. It is a 4.7 ohm, 5% resistor and is accessible by removing
the small metal plate from under the RF compartment. Then disconnect
four Molex connectors and three(?) mounting screws to remove the board.
THERE'S A 50:50 chance that this is your problem.
I agree with Jim that these are excellent amps. They're easy to work
on because each lead is numbered and keyed to the schematic. Jim's
complete one page schematic is also a real boon.
73, Joe
K2XX
P.S. If it does turn out that you need new 3CX800s, I have two unused
for sale. They were checked out in my 425s before I sold them.
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