ZL1RS - Amplifiers

A Compact 2m 8877 Amplifier


A "carry-on" sized 2M 8877 800W Amplifier.

All aluminium construction except the anode capacitors and clamp. Good efficiency and 800W output on CW. Unfortunately the demands of JT65b digital operation were too much for the small HT transformer ... and the smoke came out! The 8877 has obviously been removed.

You can see the similarities to the W6PO design that I took the ideas from. I removed all unneccessary height by cutting the anode lines off at 6" (being the effectively used length) and fastened them directly to the chassis base. The anode tuning and loading are done with much simpler flapper capacitors driven by vernier drive mechanism's on the front panal (after K1AGB design).

The old voltage doubler power supply rectifier and capacitor bank. The C-core HT transformer will be replaced by a toroid and so I will probably have to re-organise the rest of the HT supply too.

Here is the input circuit. Both 2M 8877 amplifiers that I have made have required a change in the cathode input matching circuit to get low input SWR. In both cases one turn had to be removed from the cathode side inductor to achieve this and get "full power" output.

All of the common cathode contacts on the socket are shorted with a strip of finger stock which happend to be handy.

You can just make out the home made grid ring. Another length of finger stock soldered to a piece of PCB. The socket standoffs and grid earths are BNC connector clamping nuts.

UPDATE Dec 2008 : HT Power Supply Repairs!

An AnTek toroidal transformer of 800+800V @ 800VA was a good fit into the existing box. A new bridge rectifier stack and new bank of HT filter capacitors was fitted giving better HT regluation than the old voltage doubler HT power supply.

Removing the HT filter capacitors from below allowed better airflow to cool the toroid and tube.

The 8877 tube has been re-fitted and the repaired amplifier is ready for testing.

The rebuilt amp provides a comfortable 800W output in SSB and of continuous carrier in JT65b ... without any sign of excessive heat!! The toroid is as cool as a cucumber after 5 minutes of continuous carrier at 800W and over 1 hour while testing on FSK441 Meteor Scatter :-)

It has had "a good thrashing" from A35RS, E51EME and 3D2RS as well as from the home QTH on Meteor Scatter and it has proven to be very reliable.

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