Tetrode Board amp build, last step.

[Naysayer]

I've got about 2 years into this project and reached the end of the Instructions.

This past week was focused on the ALC circuit which is the last step before applying RF. The setup requires 40 minutes and I've lost count how many times I finished only to realize I must do it again. The Warm-up timer imposes a 3-minute wait each On-Off cycle so that accounts for a lot of the time.

The problem was no ALC voltage. I followed the printed suggestion of increasing R122 to 1K with no affect. After several failed attempts I proceeded to the next Step which required disconnecting the inter-board Trip-In and Trip-Out wire whereupon the negative ALC voltage appeared. Somehow, the ALC remained after reconnection of the Trip-wire. §9 presumes more familiarity with the circuit than the earlier sections and that increase in complexity requires a more careful reading of the Instructions.

Builder needs to measure .5mA and single digit mA's several times so an appropriate meter is a must. I find it remarkable that G3SEK published 30+ pages of general instructions that apply to many Tetrode configurations yet, they all produce the same current in §9.5.

For anyone building the Tetrode Boards --I recognize I'm likely one of the last being over a decade late to the Tetrode Party-- I suggest decreasing the brightness of the Alarm LED before you start the Power On process so it's easier to note Bright vs Dim. I used the HV Interrupt circuit to illuminate a red lamp when the HV relay closes. The relays are quiet & fast. Once I could distinguish between the Trip & Reset states, it got easier.

I would not make another Tetrode amp. The increased work for the extra Grid is enormous and it did not play-out the way I anticipated. My original intent was a low drive HF amp I could power with a QRP rig but that's not what happened. it is, however, a story for another year.

neil, nyc
n2eye

 

[nomadradio]

I would not make another Tetrode amp.

The allure for a couple of decades was that the cost of tetrodes per Watt got stupidly cheap after the soviet breakup. Now that Vlad the Inhaler has shaken things up, cheap russky tetrodes have or will soon become a lot more expensive. Enough to make a chinese radial-beam triode look more attractive.

The power gain is almost as good. The era of the cheap RF tetrode is drawing to a close.

73

 

[Naysayer]

It appears my thoughts of being near the end were premature. An on-board issue requires trouble-shooting beyond what I've done to date. I've been fortunate but I now need to take several steps back to address a rapidly chattering K-1 relay on the G-2 board, which precludes plate function. Fastest relay chattering I've ever heard, it's like a sewing machine.

There will be more but not for awhile unless I get lucky by replacing parts.

 

[nomadradio]

I don't have a diagrams handy for the board in question, but this sure makes it sound as if the tube's grid bias voltage isn't reaching the socket's grid post. If that relay trips from excessive anode current, this is where I would look first, for an open grid choke or some other open circuit leading to the control grid.

I remember downloading diagrams for some "tetrode" boards decades ago, but I can't even remember the callsign of the ham who sold them. Time flies when you get old.

73