[Amps] TenTec 425 Plate Current at Standby

[Mark Robinson]

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.
>
>