The AM Forum

In Spain I did build a linear amplifier with two toshiba equivalent tubes of the 4-400 in grounded cathode. Anode voltage 4 kV. The input uses a broadband network with two twisted line transformers to go up to 800 Ohms, resistive loaded. The amp need 40 watts drive to produce 1 kW.. Vacuum cap in the anode tuning
I used it for approx 4 years and did build an other one with 8 BLF177 FETs in two modified EB104 boards from Communication Concepts. (nice stuff) Also 1 kW. I was a little worried about reliability, but I can transmit full power into a short, into an open antenna, into the wrong lowpass fiter, sparking and arcing antenna feedthrough etc. and it stays alive. So I started to use the FET amp

Lately I got the tube amp out of storage because my friend TI7XP, Michael needed an amp. So I tested the amp after 3 years no use, transport in the container, moist en more abuse. There was an open resistor in the HV measuring circuit. Replaced the resistor and fired it up at half voltage to see how the electrolitic caps behaved. After half an hour, no leakage, so to full voltage A little higher mains than in Spain. No load 4300V, loaded 4000 V. The 5 kV doorknob DC anode blocking cap started to spark a little, so replaced for a 7 kV, 8 kVA disk to avoid nasty surprises. And the amp still worked very fine. 1 kW at 40 Watts drive at 20 meters.
It made Michael happy. I hope we hear him soon. Unfortunately, he doesn't work AM I convinced him once an we made a contact from my side with a WS19 and he with his 7300 I am happy that the amp has a new owner and will be used again. Better to give it away that let it rot away

It's nice that you passed it along for someone else to use.

I built an amp using three 3-500's and I wish I still had it. I tore it apart like I do most things I build.

Jon

I got  lot of times the most fun out of designing and building, so I understand.... But this one was build quite rugged to be used for years. Pity to part out.