4-Tube Elkin Repairs

 This is a 4-Tube Elkin amp a friend picked up at a hamfest. It was in need of a bit of repairs and some updates. The Elkin amps are grid driven, link coupled, push-pull Class Ab2 designs. Very clean spectrally, well made, and were regionally popular on CB in the 60's through the 80's. It was named for the town in was built in, Elkin NC. 

Some of the later mobile models were band switched for 40 through 10m, and the tank in input circuits of the fixed station amps were tapped. I ran a 12-tube version on 40m for many years, with 0 problems. 

Here's a batch of pics, some vids, and descriptions:

A few pics above of the finished amp...

An initial look under the chassis shows a bit of a mess...

The negative voltage pass transistor is literally bridged B to C... smh...

An attempt at screen regulation was also all fubar...

Another transistor was bolted in, collector grounded, and nothing else.

Incorrect tubes installed... One had literally melted the glass and started to implode.

Correct tubes installed, note size difference with the small tubes that it came in with...

Bleeder resistors incorrectly wired, and some were bad.

Filter caps were shot...

Keying circuit for contact closure output was incorrectly wired and kinda hanging in the wind...

Delay cap blown open...

2 Plate caps were broken free of the aluminum tank coil. That stuff has to be soldered with a combination aluminum rod. 

New filter caps in...

New bias regulators installed and wired correctly...

Replacing bias caps...

Plate choke replaced...

Split Plate tune cap needs padding...

Spot drilled and smoothed for variable 150v screen supply...

Dual Filament switch installed to enable use of 6v or 12v tubes...

Variable screen supply installed...

Split input coil adjust and sealed...

Split plate tune cap padded properly...

A number of new parts installed :-)

Swan 500C Restoration

 This is an old Swan from a local SK's estate. It had been out of service since it died in 1998 (according to notes inside the cabinet). It was pretty grungy. I gave it a good cleaning, serviced all of the controls and switches, recapped it, changed a handful of out of spec resistors, replaced the RF choke in the PA, and replaced one of the shorted tubes in the PA. The cases got a new coat of paint, too. She's back up and running, and looking good! Here's a few pics...

829 Amp Build

 This is a small RF amp for use with QRP, small SDR etc rigs to bring the low 2-5 watt output up to the 100w+ range. By swamping the input with a 100 ohm resistor, installing a tapped tank, tuned inputs are avoided and easy multiband operation is achieved. 

I designed this one as a monoband amp, and set it up on 29.000 mhz on the bench. I added an RCA port on the rear to key down stream QRO amps for full legal limit operation with a QRP rig. Here's a few vids and pics...

Finished pics...

It requires very little ventilation, so I fabricated a perforated rear panel.

A look inside frome the rear...

The project begins...

I love this wrinklefinish...

Spray it on wet, and wait...

Testing the Filament current...

Opening the neon lamps and installing incandescent bulbs...

Perforated base and tube sockets installed...

Transformer mockup...

Wiring begins...

RF sensing circuit built...

3-wire safety cord installed...

Chassis clearance for switches...

RCA port to key QRO amp...

Setting idle current...

Sweeping the input and outputs circuits...

Nearing completion...

Choosing from misc coil stock :-)

Cutting rear ventilation panel from stainless steel...

Packing for transport...

Tuned monoband input to allow for very low drive levels...