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Can an SWR analyzer be used for Impedance tuning a SS amp Input-Output network?

Naysayer

Member
Mar 6, 2020
27
20
13
New York
I'm ready to install four MRF422's in an amp project I'm working on.
The bias voltage is set and switched by the T-R relay (manual no COR).
Installing the transistors is a point of no return and this is my first time doing this.
As for Transformer capacitors I'm guessing based on diagrams I've found online.

Can I further tune the Input/Output T's without a VNA before I install transistors?
I do have an MFJ antenna analyzer (old model) and I've seen vids using resistors to tune Inp/Outp networks but they used VNA's.

As is now, I put 390pF on the output side of each Input Transformer and 1000pF on input side of Output Transformers. Have ceramic trimmers on Output Trans and plenty of fixed Micas to add or subtract as needed.

If I buy any more test equipment, my wife is gonna kill me.
neil
 

nomadradio

Analog Retentive
Apr 3, 2005
5,442
7,915
573
Louisville, KY
www.nomadradio.com
The analyzer's drive level is a good bit lower than your radio's drive level. So long as you have set the zero-signal bias current, the analyzer's tiny signal should 'see' the same impedance as the radio's transmitter will be driving into.

The amplifier's input impedance will also be affected by the drive-power level. Biggest mismatch will occur when the drive level is at or over the 'clean' limit where distortion begins to kick in.

Just beware that an unstable amplifier can backfeed RF wattage into an analyzer and blow it out.

Been there, done that. Was a bad day.

73
 

Naysayer

Member
Mar 6, 2020
27
20
13
New York
The analyzer's drive level is a good bit lower than your radio's drive level. So long as you have set the zero-signal bias current, the analyzer's tiny signal should 'see' the same impedance as the radio's transmitter will be driving into.

The amplifier's input impedance will also be affected by the drive-power level. Biggest mismatch will occur when the drive level is at or over the 'clean' limit where distortion begins to kick in.

Just beware that an unstable amplifier can backfeed RF wattage into an analyzer and blow it out.

Been there, done that. Was a bad day.

73
Thank you a good bit to consider.
 

Naysayer

Member
Mar 6, 2020
27
20
13
New York
So better to use an old radio with separate SWR bridge. More appropriate power level with less risk of damage to equip. Thank you it took awhile to sink in
 

Naysayer

Member
Mar 6, 2020
27
20
13
New York
I deleted a post with a pic for re-write.
MRF422 project is proceeding.
My Bias Meter is behaving itself a little better after I placed a multi-hole barrel choke near the meter itself. I was surprised as I already had a ceramic across meter terminals. I'm seeing about a 200w AM carrier into DL and whatever on SSB. I have not yet tried to adjust the mica dipped caps inside the Input-Output Transformers. Presently, I have 390pF on Base side and 1120pF Collector side of each pair.

I think I'm seeing less current draw from my PS than 4 2879's, uncertain of that may be my own wishful thinking as it was a reason that I moved to 28v.
neil
 

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