Elkin 8-Tube Repairs...

 To preface this, this project spans a bit of time, so the videos might seem a little disconnected...

This is an Elkin 8-Tube mono-band amp, originally built in Elkin NC in the late 60's. It uses a link coupled input and output, is grid driven, and is biased in AB to AB2 territory.  The tube complement is 8ea 1625 JAN surplus tubes from the late 1940's, and a pair of 0A2 cold-cathode, gas-discharge tubes for screen regulation. Bias regulation is via a series stack of zener diodes. Elkin amplifiers are extremely spectrally clean when properly set up. I think the cabinets were made by Hammond, but I'm not 100% sure on that. I was fortunate to have met and known Ray (the designer / builder), beginning  sometime around 1974 and a few times over the following years. 

Being grid driven, these require very low drive. I set this unit up on 29.000 mhz, though I've put these on 40 and 20 meters in previous years with a simple addition of inductance to the ends of the link coupling hi-z windings. The smaller, mobile versions (same design) had switchable tanks for 10, 15, and 20m. 

A friend picked this up in a batch of gear in a purchase, and it needed some attention. Another friend recapped the amp and got it running for him, and I took the challenge of bringing it to peak power. Part way through the project, the owner did some major cleaning and cosmetic work, and I then resumed the repairs and updates. 

The Elkin design morphed and improved over the years, partially due to design updates, and partially due to various parts availability to the designer/builder. This particular early version had lower plate transformer HV (around 700vac), though the screen in all of this ilk was regulated at 300vdc and the HV derived bias was around -70vdc fixed. Later models had an adjustable bias supply, HV transformer output of over 850vac, an adjustable solid state screen voltage regulator, etc. As a result, these early models typically topped out around 400 watts output when at saturation. They were like the Alpha Amp adds back in the past...they could take a brick on the key all day, effortlessly. Let's get started on this 55+ year old girl...

A little inspection found the values of the screen limiting resistors to be all over the map.

The power cord needed replacement due to some cuts or possibly doggie chew damage.

I subbed in some bias and screen voltages to test the amp at the correct voltages...

This 47 ohm resistor in the bias distribution had shot up to several hundred ohms and needed replacement.

A handful of resistors were far out of tolerance and needed replacement.

New power cord!

New screen dropping resistors installed...

A new zener stack was installed to bring the bias to the correct level...

I tapped and installed a new setscrew into the main split-stator tuning knob...

An additional low voltage filter cap was added to the 12vdc control circuit and secured with a little hot melt...

Modern electrolytics had replaced the bulky old caps, and increased capacitance, to boot!

Next, a boost transformer was added to raise the High Voltage ac from 700 to about 850vac. This pushed the unloaded DC plate voltage upwards of 1100vdc. 

The 0A2 screen regulators are working beautifully!

A trimmer (Arco 303) was installed across the output coupling coil to cancel some reactance and bring the output up.