[Drake] T-4XC Plate current not rising bad finals?

[Garey Barrell]

Frank -

Possibly finals, but if they test good on a reliable tube tester they
should set to 0.7A bias at -55 to -60 VDC on the grids.

Check the Final cathode resistors, (15 ohm, 1W across each final tube
socket,) and the meter shunt resistor R59, (3.3 ohm, 0.5W).  Also check
the Final Screen resistors, (68 ohm, 0.5W).

With 650 VDC (700 VDC unloaded) on the Plates, 250 VDC on the Screens,
-60 VDC on the Grids, and < 1 VDC on the Cathodes, good tubes WILL draw
0.70 A of cathode current regardless of the rest of the transmitter's
condition.

If you have a DMM, you can check the voltage across each of the 15 Ohm
resistors, use Ohm's Opinion with the  _measured_  resistance of each
one to see the "real" current.  IF they're close to 15 ohms, you should
read about 0.5 VDC across each one.  Measure across each resistor, not
from the Cathode to chassis for this check.  Absolute value of the two
resistors not real critical, but they should be within a few percent of
each other for best Final balance.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>



Frank Castelluccio wrote:
> Hello,
>  
> I recently aquired a T-4XC and the AC-4 supply. I
> rebuilt the ac4 with the heathkit shop upgrade kit and
> all viltages are good.
>  
> I am having an issue with the T-4XC though, upon
> tuneup I cannot get a good indication of plate current
> (doesnt even get to half scale with the gain all the
> way up) and no visible dip.
>  
> When I try to set the bias I can get it to spec
> however the pot is all the all the way up and the bias
> voltage is about -42 volts. 
>  
> I swapped in a new driver tube and no difference. I
> verified HV on the final plates.
>  
> I did test the 6JB6's on a tube tester and the test
> good, perhaps this is not a good way to tell?
>  
> Can anyone offer any suggestions as to how I may
> troubleshoot this and determine if the finals are
> indeed shot?
>  
> These tubes are expensive and I dont want to buy a set
> and ruin if the rig is faulty.
>  
> Thanks in advance,
>  
> Frank
>
>