[Hallicrafters] Replacement for a PL-172 in HT-33A?

[Roger (K8RI)]

Paul Kraemer wrote:
> Roger
> Can you elaborate a little on "tune it like a tetrode"?
I hope I phrase this correctly:

Normally we tune Triodes for maximum output while keeping the grid 
current at the desired levels.
Normally while going through the tunup procedure the Tetrode and Pentode 
is tuned for  maximum output  with the plate tune and maximum screen 
current with the load control while staying within the limits of the 
screen current. Each commercial amp may tune a bit differently depending 
on the tube and how much power it is running and what it has on it for 
voltages. Some Tetrode and Pentode amps will not draw excessive screen 
current as long as the drive/grid  is kept to the proper levels and they 
can be tuned for maximum output.

The 33B is a bit different as the amp has an adjustable bias control so 
the bias is changed with the mode of operation. Also they measure 
cathode current so screen and grid currents must be subtracted from the 
reading to get plate current.  Filament warm up is 3 minutes before 
applying plate voltage.

As it's only an excerpt, it should be permissible to post the tuning 
procedure:
For CW:
1. Bias Adjust to max CC position
2. Plate voltage on after 3 minutes of filament warm up.
3. Idle current for CW operation may be set from minimum to 200 MA.
4. Apply enough drive to increase cathode current to ~250MA
5. Set meter switch to RF Output (RFO) and adjust tune and load for 
maximum output.
6. Increase drive to obtain a cathode current of ~300MA
7. Repeat step 5
8. Increase drive and adjust tuning and loading for a *Plate Current* of 
~ 370 MA is reached as maximum output.  (Remember Plate current is 
cathode current minus screen and grid currents)

NOTE these are for 1KW DC input (2700 V * 370 MA = 999 watts)  They 
placed a lot of faith in those meters.
At these settings the tube is just loafing along at roughly 50% of its 
capabilities depending on the bias setting which is probably how they 
got away using a regular fan under the tube for cooling.

For SSB:
1. Bias for 200 MA cathode idle current (IE no drive applied)
2. set mode to CW and increase drive until ~ 250 MA of cathode current.
3. Meter switch to RFO & adj tune and load for max output.
4. increase drive for 300 MA cathode current
5. Repeat step 3
6. Increase drive and adjust tune and load controls until  Plate current 
reads 550 MA at max RF output.
    (The manual is not clear when they say the meter reads plate current 
as it actually reads cathode current)
7. Switch exciter to SSB. The drive should be set to just below the 
point where grid current appears on the loudest voice peaks. This should 
result in cathode current peaks of 250 to 390 MA

They add the note this is for the rough tuneup and an oscilloscope 
should be used for the final tune.




> Thanks
> Paul K0UYA
>