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Re: [Amps] Hercules II With SD 1405 Bias Current Setting

Subject: Re: [Amps] Hercules II With SD 1405 Bias Current Setting
From: Steve Thompson <>
Date: Mon, 24 Aug 2020 22:36:58 +0100
List-post: <>
Can I suggest you check some other voltages? Looking at the schematic, with the bias pot at minimum the base voltage on the SD1405 should be equal to the voltage across the bias reference diode. The reference diode is 1N4002 with 0.5mA going through it, I'm struggling to imagine that would be enough voltage to bias the SD1405 into conduction.

I've used pretty much the same bias circuit in commercial amps in the past except with a smaller reference diode running at higher current (so probably higher reference voltage) and there was zero current in the rf devices at minimum bias setting.

What are the diode and base voltages?

Steve G8GSQ

I have my Hercules II amp on the bench for a repair and I decided to check the 
power supply voltages and the PA transistor bias settings.

The power supplies with the undriven amp keyed was 14.03V,  13.98V, 14.05V and 

I then lifted the 4 red Collector jumpers from each pair of PA transistors.  I 
put a DC ammeter across one collector jumper and measured the collector current 
on that pair of transistors with the amp keyed and a dummy load connected to 
the input of the amplifier and a second dummy load connected to the output of 
the amplifier. I measured the collector current on each of the four PA 
transistor pairs.

I was expecting to see a bias current of 250 or maybe 400 mA but I measured

Top Left       1.795A
Top Right     1.760A
Bottom Left     1.460A
Bottom Right   1.486A

My amplifier doesn't have the MRF 458 transistors but is fitted with  SD 1405 

I tried turning down the Bias adjustment pot on the 81405 Bias Board but the 
lowest I could see was around 750mA with the pot set at minimum

I checked D2,  the 1N751A  5.1V reference zener on the bias board and it was 
reading 5.05V.  The DC supply to the bias board is 12.7V

What bias current should the SD 1405 transistor pair be set to?  Is it more 
than the bias setting for the original MRF458 transistor pairs ?

If I want to get the bias voltage down more then I probably need to lower the 
reference zener voltage.

73 Mark N1UK

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