<AGC in the Drake amp is developed from the transmitters ¡°input¡± to the tube(s)
<When you key the transmitter it doesn¡¯t see the AGC signal until it goes through the comparison circuit then develops the required negative DC voltage to pull back the RF coming from the transmitter.
<So there is a reoccurring front end RF spike that is pulled back by the AGC circuit each time you key the transmitter as the AGC voltage disappears during receive.
<AGC control process is ¡°Sample, compare, voltage generation, then transmitter output reduction¡± each time you key the transmitter.
<If you set the AGC voltage to cut the transmitters output to 60watts (In the example of my L7) I get 1000watts carrier out.
<Looking at the output on a scope the signal is nice and clean with a two tone audio for a test signal and transmitting into a dummy load.
IOW,? you are closing the barndoor......after the horse has bolted, and every time you trip the VOX on SSB.? ? And the ALC time constant in the amplifier has to match the ALC time constant in the xcvr / tx.? Drake had a tech note out about matching the time constants if using a T4XC with a L7....or using a? TR7? with an L4B.? It was? a cap / resistor change.? ?The mic gain in the T4XC is not a PO control....when using SSB.??
The advantage of using the external, adjustable DCV method is..... the ALC voltage has already been developed, and is sitting there all the time, at a constant level.? ?The drawback is, if whispering into the mic, you won't get desired PO out of the T4XC / XCVR. If an external processor is used, or external audio compressor / gating system used, or a boom headset used,? it's a non issue.?
Where you would really get into trbl, is when using a TR7 / T4XC on SSB mode, and try and drive an amp that requires very little drive, like most of the SS amps.? You are putting out 130 watts pep initially from the T4XC, until the derived ALC voltage developed in the SS amplifier, is sent back to the TX / XCVR.? On the Merc-3 SS amp, it has a built in event counter, and will know asap, if you ever applied > 60 watts pep into the amp.....warranty denied.
If that 5K @ 7 watt resistor below the chassis in the L4PS / L7PS opens up, you will end up with full B+ on the ALC adjustment pot in the L4B / L7.?
In my case, the 7K pot, along with the pair of 50K bleeders was tossed, to reduce heat in the L4PS.? Since the yaesu xcvr already has adjustable PO, it's a non issue.? Still, the external DCV can still be used, if the SS xcvr has the dreaded overshoot / initial spike issue...typ of some of the Icom xcvrs, like local buddy's? ICOM? 775DSP, which puts out a 200 watt spike, regardless of? where the PO is set at....which wreaks havoc when driving his Ameritron AL-80B, single 3-500Z amp.? The ICOM 706MK2 series also put out a 140 watt spike, again regardless of where the PO is set.? ?A few other SS xcvr's have a similar issue, to varying degrees.?
On a? sorta related sidenote,? I just ordered a? 40 kv? HV probe.? ?It uses a long, large value resistor, in series with a low value resistor.? Cold end of low value resistor is clipped to the B- of the HV supply...via it's 3' test lead + clip.? ? The V drop across the lower valued resistor is fed to my fluke 87 DVM...via a 2 x conductor cable.....with dual plug assy.? ? ?40 kv dc? will be? 40.0 vdc on the? DVM.? (1000:1 ratio).? 2650 vdc? will be 2.65 vdc on the fluke 87.?
IF that return test lead ( that goes to the B- of the supply)? ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM.? This freaks me out.?
