Flex 6300 RF-Gain Slider

Steve N4LQ

Steve N4LQ

On the 6300 it appears to me that the RF-GAIN slider (under the ANT button) is really an attenuation control. When it is move to the right (20) it seems to insert 20db of attenuation. I have had some owners here on the forum insist this is a pre-amp however I've also been told that it has no pre-amp. The manual isn't any help. So what's the facts?

Ken - NM9P

I don't know if there is much difference on the 6300, but on the 6500 and 6700 the control has 4 positions, from left to right..

-10 dB (attenuator)
0 db (flat)
+10 dB preamp
+20 dB preamp

You will probably never need to run it beyond 0 dB unless you are on 10 Meters (when you might need +10 dB), or 6 Meters (When you will need all +20 dB).  
The noise figure on these rigs is fantastic.  If you use the higher gain settings when they are not needed, all you will do is add noise.  NOTE..If you add gain, your S-meter reading will NOT change, as it does with other rigs.  This keeps your signal reports constant, regardless of Preamp/attenuation setting.  This may add some confusion as you experiment.  The rule of thumb I use is.... if you see an increase in the noise baseline on the panadapter when you connect your antenna, then you don't need more gain in the front end. 

Mike

Steve: the 6300 has 2 preamp gains. None and 20 dB. Its probably working correctly. Tune in a beacon or cw signal on a low noise band, like 6 meters. Look at the signal level. It might be -100 dBm. This is the power at the antenna terminals. Now, look at where the noise is. Might be -105 dBm. Now turn on the preamp. Signal will still stay at -100 dBm, but the noise will drop, maybe 20 dB, but probably a bit less due to atmospheric noise. What you see is an improvement in signal to noise ratio. You don't see the preamp gain, beacuse the scale is the signal level at the antenna terminals, and the preamp is after the antenna terminals. --Mike, WV2ZOW

Steve N4LQ

This can't be right. Try tuning in on the AM broadcast band. Now flip the slider over on 20. Most of the signals disappear. How is this a pre-amp? Steve N4LQ

W9OY

The pre amp in the 6300 is a 20 db preamp that works full tilt at the higher freqs and decreases in gain as freq decreases until there is no gain.  The slider is not a slider it is a switch that turns the preamp on and off

73  W9OY

Steve N4LQ

Ok...Whats it doing on the AM BC band? It pretty much kills all but local signals. Is that how it's supposed to work there? N4LQ

W9OY

I'm just telling you what Gerald told me.  It's likely giving negative gain on BCB but you're not expected to use the pre-amp below about 10mhz.  

My guess is the gain is controlled by a transfer function and not a fixed integer and it may go negative at some point.  The idea is for most people the strongest signals their radio will see and therefore the greatest chance of overload is from broadcast band signals so removing strong signals from the mix increases the effective dynamic range.  BCB signals in my experience are not the strongest.  Even though I have 50kw 10 miles away and a 10kw 5 miles away my strongest station in the HF spectrum is radio Havana on 6 mhz.  Even that station is more than 40dB below where my front end crunches 

73  W9OY

Steve-N5AC

With most radios, you can only listen to one frequency at a time.  When you turn on the preamp, it sticks some amount, 20dB or whatever, in line.  The radio doesn't care where you are in the spectrum or what you are doing or whether gain is appropriate.  Many hams will punch the preamp button when they are having trouble hearing a signal on 40m, but the preamp will almost never help at 40m because of the atmospheric noise.

On the FLEX-6300, you can listen to two places in the spectrum at the same time.  Let's say that you are listening to the broadcast band at the same time you are waiting for a 6m opening.  If you turn on the preamp, you may overload the radio with the broadcast signal -- plus you don't need any gain in the broadcast band.  In fact, below 20m there are very few situations where you would ever need gain.  Knowing that you are likely to do this with the FLEX-6300 (listen to two bands at once), the preamp puts about 20dB of gain on 6m and it slowly rolls off as you approach 20m.  There is actually some attenuation in the 40/80/broadcast bands because the attenuation will not hurt you: the receiver noise figure is around 15dB better than typical band noise is in these bands.  So the objective here was to allow operation on multiple bands at the same time and for the radio to be "smart" about how the gain is applied.