[AMPS] DJ5RE 2m Amplifier Tuning - Longish

[Ian White, G3SEK]

Steve Thompson wrote:
>
>
>-----Original Message-----
>From: Phil <PhilHSS@Breathemail.net>
>To: Amps <amps@contesting.com>
>Date: 18 May 2001 14:13
>Subject: [AMPS] DJ5RE 2m Amplifier Tuning - Longish
>
>>The initial DC conditions are as follows:
>>
>>2.6Kv Anode Voltage (on load) and 2800V off load
>>400mA Anode Current
>>300mA Grid Current
>>80w input RF
>>300w output RF
>>150mA Anode Standing Current (giving 80mA of Grid Current with no drive)
>I can't claim any experience with GS35, but 80mA grid current at idle seems
>very high. The data suggests 30ish. I know grid current isn't set in stone,
>but nearly 3x different strikes me as wrong. Low gain and 300mA grid current
>for 400mA anode current also seems far too high, suggesting too low an anode
>resistance, although that wouldn't affect the dc conditions. I'd be inclined
>to check the grid meter and try and find another valve to try.
>

Sorry I wasn't available to come back to Phil yesterday.

Significant grid current in a GS35b at idle current seems very unusual.
Assuming there is no deliberate RF drive and no oscillation at any
frequency, grid current should read zero in a good tube.

My best guess is that there is a low-resistance path in the power supply
between chassis and B-minus. Many power supplies contain a safety
resistor to prevent the B-minus rail from floating away from ground, in
case the supply is ever totally disconnected from its load. The Triode
Board manual suggests 1k for this resistor, because too low a value will
affect both the grid and anode current readings.

Another source of strange meter readings and efficiency values might be
that a trimpot for meter calibration was accidentally altered, sometime
later in the setup procedure (been there, done that, and others have
too).

The easy way to check this is to remove all power from the amp, and then
connect an external low-voltage DC supply with positive to the cathode
and negative to B-minus point in the HV supply. Don't assume anything
about the existing wiring - connect directly to the place where the
negative poles of the rectifier and capacitor join together. When the
low-voltage supply is adjusted to force a typical idling current of
150mA around this circuit, as indicated by a reliable external meter,
the anode meter should read 150mA and the grid meter should read
precisely zero.
 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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