4cx300 amp build update.

[Naysayer]

My 10-80 grid-driven 4cx300a x2 amp is proceeding. I had to order parts for input. Last I reported, I had almost 100w AM from an old CB using W8Ji’s Resistive Input circuit that splits the 50 ohm swamping resistance. It was first application of drive so I used old tubes I got for free so I was happy for the moment. The G3SEK tetrode boards work properly except I cannot adjust the ALC Out. I’ll deal with that after I settle on an Input circuit.

For me, the appeal of Tetrode amp was low drive for grid-driven. That’s why I’m trying to improve upon the input circuit because it presumes 100w drive. The input impedance formula to re-determine swamping and drive values is ‘over my head’. Imaginary values are hard for me to understand and math is my weak point. I can copy OK. Pride’s parallel input is too large and the parts hard to replicate.

So, I found a grid driven 813 10-80 amp diagram that placed a T-50 red core (11T 20ga) across 2 Variable caps preceding a swamped input so I’m trying to copy that. I settled on a 110 ohm swamping value from a 2 x 4x250b diagram from another 10-80 amp from 1960's ARRL Handbook. The article stated that the 110 ohm value was calculated for 30w max drive. Major presumption here being that the 4x250b is same as 4cx250b/4cx300a. Guess we’ll see but it should be Ballpark and I can always go back to 50 ohm. I may need to rewind the T-50 red core for 1:2 given the 110 ohm resistance.

After looking at many Swamped input diagrams, i saw that some circuits rely on full DC grounds for the resistors while others only use RF ground via capacitors.
Is there any advantage to using one or the other grounding methods?

I’m almost ready to apply RF thru new input circuit. I need to reset the Screen current trip level on the G3SEK boards. It’s a day long pia but I had to relocate the 50w adjustable screen current resistor to make room for tuned input. I also re-arranged the swamp resistor section (in airbox) I can change resistors after I get all my voltages in place.

Neil
n2eye, NYC

 

[Naysayer]

Yesterday I made the 1:2.5 unun (center screen picture below)

and swr analyzer said it's OK for matching the 110 ohm swamping scheme for 30w max drive. I used another T-68-2 red core and 20Ga magnet wire. That 110 ohm grounding scheme came from a 1958 QST: A Compact 650-Watt Amplifier.

Unun fed by coax from T-R relay. Unun attached to the small T-match attached to outside of blower-box. A pass-thru drives grids inside box. I placed swamping resistor (N.I. 110R 50w) after the Blocking cap.

The placement of swamping R keeps nagging me: Before vs After Blocking cap? Diagrams I found seem to use different locations as variously as DC vs RF grounding is used. Anyway my swamping R is RF grounded only so I made it robust with 4 parallel caps, 2 large ceramics and 2 small polyesters.

Before I can apply RF again, I need a day to reset the shunt regulator circuits controlled by Tetrode Boards. Next amp will certainly be a Triode.

neil, NYC

 

[Naysayer]

I apparently installed the wrong bias Zener (56v) for the 300a tubes so I'm waiting on replacement (51v). I have 2 sets of new & used 4cx300a & 300y and placed a small toggle to switch Zener voltages (-55 & -70). Looking back it was silly idea. How often would I change tubes, right? This project a mess but I learned a lot.

While re-setting the standing-current & trip levels for Screen & Grid I found that my Screen voltage is low for the 300Y tubes. I'm a 32v shy of 400v on datasheet. Maximum Screen voltage I have at tube pins, regulated is 363v. It's 13v more than the 300a (reachable via Tetrode Board adjust range) but too low for the Y tubes. Remains to be seen if any advantage from the 300Y over the 300a.

Added some LEDS to my Dentron ATU meter (which I just repaired & calibrated). Pictured glowing away below. Makes evening use easier on the eyes.

Hope everyone here has a Merry Christmas and far better if the 25th is just another day for you.
Neil
South Bronx, NYC

 

[nomadradio]

Found these at alldatasheet.com:

Turns out they're pretty different. The "Y" is rated for more power, higher G2 voltage. Reminds me of the 4CX250B compared to the 4CX250R.

73

 

[Naysayer]

Had to revise grid-driven input by removing UNUN. SWR analyzer helps to set input SWR with different grid resistance layouts as I have what amounts to an L network preceding the swamping resistance to help compensate for the mismatch. My goal is to lower the drive requirements.

My Screen voltage is bit low from datasheets but Screen current shows good and is adjustable but it's a tedious process. This is going to take some time.
neil

 

[Naysayer]

10-80 Tetrode amp project:
Scrapped the T-match and made it an L-Network. Improved input SWR's went down 10-80. Reduced turns on torroid from 11 to 10. Other side of pass-through is 1:6 unun w/ 300ohm swamping resistor.

Waiting for a low mA meter to set standing current. Have all voltages needed for the 4cx300a tubes (2k plate. 350 Screen, -55 Control Grid).
The 300y tubes require 400 on the Screen but present Transformer can only make 365. Have remaining volts for 300Y (plate, -68 control grid).

I had to use diodes in reverse parallel to reduce filament voltage. Waiting on MUR860 (8A TO-220 style so I can heatsink) meanwhile, I'm using 6A 5400 series diodes and they get Hot.

neil, nyc