Grid-driven Tetrode Input Circuits

[Naysayer]

Been winding Baluns, tidying messy wires & tweaking this & that. Replaced the small core with 1-1/8" core (still mix 43). I'm focused on the RF Input section now. Changes take time to try and what works on a DL is not always good on an antenna.

The attached "now" pic file is a diagram of the Input that is working best so far. Max drive is mid-20w's pep on 40. <12w pep on 10M. I'm not content with only 1 balanced output feeding the tubes. Feel like I'm wasting half my drive. I'm looking for some suggestions/criticism. Unfortunately, my test equipment is limited to an swr analyzer but I've come a long way. Input SWR is nearly perfect from 10-80 and the tank pi shows optimal settings in the mid-range of each component so I can fine tune. (Mr. Hacksaw removed some air cap plates) 40M PO is impressive, >kw pep easy with <30w drive.

To swap tubes I placed a toggle to switch Grid Bias and the multi-tap Screen Transformer has bullet connectors so I can swap secondary terminals from 350v or 400v. It's far from fast but do-able in about an hour. 10M performs the worst. If I short the tank coil & switch for just 10M, the PO goes up a lot, so it comes with multiband territory, I guess. These tubes are tougher than I expected. I made a few mistakes that did not yet destroy the tubes (leaving blower Off, etc). I need an Idiot Light for the Blower so that means...... IDK. I re-adjusted Buck-Boost for lower plate voltage & got a bit more usable plate current so progress continues just slow.

Back to RF Input, I'm trying to decide what circuit to try next. I think there's more efficiency to be had extract from the balun/swamped resistance arrangement. With one balanced lead going to RF ground I'm losing almost half the drive. the plan is to try connecting the other output ...... somehow. The other diagram is my next attempt as of now.
neil, NYC

 

[nomadradio]

Thinking back to the Pride DX300 input circuit. The control-grid's capacitance prevents the 10-meter input from being resistive, even with swamping resistors as shown. The Pride deals with this by putting a toroid inductor across the grid. The inductance is chosen to 'cancel' the grid's capacitance and become broadly resonant across the 10-meter band. Lower bands require an added capacitor to resonate those bands' grid coils. So long as you have a broadly-resonant parallel tuned circuit across the grid-swamping resistors, the input impedance remains largely resistive.

That amplifier uses an untuned 3-to-1 voltage step-up transformer wound trifilar on a toroid core. Never have discovered the ferrite type they used, just bought them from RF parts as needed over the years.

Takes very close to 1 uH to get the input SWR down on 10 meters with a 250B tube. I'm feeling too lazy to look up the 300's grid capacitance just now, but you get the idea.

I know we tried using a transmission-line style input transformer on a 250B tube decades ago. What I remember was that it required a trimmer cap from the its output to ground. It was sufficiently long ago there are probably no bench notes about the setup.

I'll admit to being a bit puzzled over the diagrams. Only one end each of the red, blue and yellow wire is show. I would ground the 'start' end of the blue, connect 'finish' end of the blue to the start end of the red wire. This will be the 50-ohm input. The finish end of the red goes to the start end of the yellow. The finish end of the yellow will be your 450-ohm output to the tube grid and swamping resistor. This is how the trifilar step-up toroid in the DX300 is built.

73

 

[Naysayer]

Thanks for your time to comment. I need to re-read your first para a few more times to fully understand it but I will do so. The very idea of a transformer for RF is fascinating.

I looked at the Pride input on paper but could not decipher the diagram. Think I need to see the part, visual search online I guess.
Adding Capacitance, sounds like a good idea. Wish I thought of that before I closed up. I'll investigate that next. Amp is buttoned-up now so next time I open it up I'll have another go at it.

The drawings were meant only to show the different connections on the balanced outputs. It's an ordinary 50:300R balun with a Star and Dot to show polarity. The attached pic is a close-up of a 'practice balun' just like it except I used different wire. The 2 middle wires (twisted pairs) are 50R input & the 2 outside wires are the outputs. Works good 20-80 but my 10 problems could be most anything,

Edit
I get you now: "10M on these tubes is notorious for Input Capacitance issues".
That provides direction. Thanks Nomad.
Later,
Neil, NYC