Bias Circuit Modification Version 1; Bias Circuit Modification Version 2 - Pride Amp Series Service Manual

Pride DX300 CHAPTER 7
7.5 Bias Circuit Modification Version1 (Nomad Radio Mod)
The Bias voltage is a Negative 82 Volt supply which is set between -65 and -68 volts to the tube.
The most common part failure is the bias pot (RV1) 500ohm 2W linear pot and an 82-Volt 5-Watt zener (D25).
One substitute for the stock (D25) setup is three 20 Volt 5 Watt zeners in series.
For people who want a bias control that will turn all the way down past cutoff, use a string of six for the
"-R" version of the tube, and five of them for the stock "-B" tube. It looks a little over-the-top, but never comes
back blown.
Figure 7-1 Zener Bias Mod
The space the zeners occupy here is normally where the two very-hot 10-Watt resistors R38 and R39 go. You
can locate a single 5K 50W resistor off the board to do that job. It runs cooler and leaves room for five or six
zeners in series on the low voltage power supply PCB.
With six zeners installed the pot has an end-to-end range of 40 Volts, more or less. For the '250R, this is from -
120 to -80. For the '250B, with just five zeners installed, is -100 to -60. This permits biasing the tube past cutoff,
and provides a "rollback" or carrier control for a radio that doesn't have one built in.
And it helps to reduce the heat load on the tube from the carrier power.

7.6 Bias Circuit Modification Version 2

Another way is to use a transistor and zener. The following uses two TIP50's (you can use one transistor as long
as the one you use can handle the current and voltage). Every shop has a pile TIP50's laying around and they
are cheap.
You can use the stock D25 for the zener or series as many lower voltage 1W zeners you need to get the voltage
range you like.
Figure 7-2 Transistor / Zener Bias Mod
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