Amplifiers..

[VK3ZL]

Hi all, I havn't been posting a lot in the forums a lot these past few months but I generally have a browse every day so I am keeping up with all the shenanigans you all get up to....That aside I have been very busy these past six weeks building some amplifiers for friends and also preparing for the South East Radio Group convention...This is held in Mt Gambier VK5 each June..I like to take over some of my projects each year and enter them in the Homebrew competition...I have won first prize for the past 10 years or so...That aside I like to take homebrew ham equipment to inspire others to try it out...The SERG club has a big following and each year there is a stiff competition in the Foxhunting which is held over two days...

Like my presence on this Ham Island, I have many friends to meet up with at the convention and I always come home happy to have caught up again...As I am getting pretty old, I guess time will eventually run out for me, so I appreciate the friendship both here and with the radio group.

The following pics are my latest amplifier creations..I started these six weeks ago and have just completed them all ready for the competition.
On this page is a 1kw 2 meter air cooled amplifier designed specifically for EME work..The tube is a rugged GS-31B Russian triode, 1kw anode dissipation....I have used these tubes in quite a few amplifier applications and find them rugged and pretty much bomb proof...I have solved the cooling problem that many experience with tube amplifiers using "Brute Force" ...I use a 12-28 volt Papst fan that is rated 250cfm...This unit is 5.5 inches diameter and 2 inches deep...I seal off and pressurise the RF deck..The air evacuates through the tube cooler and Teflon chimney attached to the top cover....With approximately 50% loss under pressure I still get well over 100cfm through the tube anode and even at 800 watts WSJT minute long bursts the tube stays cool....The Cathode compartment is sealed and uses a 4 inch 240 volt computer type fan to keep the Cathode and filament area cool....

I used the big DC Papst fan rather than the 240 volt types as there is no 50 cycle hum which can be a noisy annoyance...All you can hear with the 28 volt Papst fan is air, heaps of air....During receive I have a relay in the PTT line which cuts the fan voltage down to 12 volts eliminating most of the air noise ...

The 2 meter amplifier is simple and straight forward..It has step start HT, tunable T input and a half wavelength output line...The HT power supply uses a robust custom built Toroidal transformer and it produces 3200vdc @ 800ma...I have these transformers built in China...Quality is excellent..As per all my amplifiers I have developed my own style of construction and all amplifiers are built using the same method..I machine many of my own parts and build the cases by hand...Homebrew is fun..

Pics are the 2 meter amplifier..Enjoy.

Bob..VK3ZL..

[VK3ZL]

Ok to carry on, here are a couple of pics of a water cooled GS-31B 20/15/10 meter amplifier I have built for a friend....This unit uses a heavier custom built transformer compared to the 2 meter unit and easily delivers 1500 watts output...Once more the construction methods are the same as the 2 meter amplifier but I build a copper water manifold in place of air cooling...This is very quiet and efficient....I use a 15 liter water container and an ordinary 12 volt Pond Pump 1000 liters per hour.This actually runs 4 liters of water through the manifold per minute....The cathode compartment is cooled with a 240 volt computer fan...

The rest of the amplifier is straight forward and standard design...Pi input networks and Pi output....This unit runs on 4kv, great combination with water however it is quite safe....We allow 1 meter of water hose per 1kv and the reservoir is grounded through a 1ma meter to monitor water contamination...With fresh distilled water normal leakage current is around 90ua and rises to 250ua at 40C...

Enjoy the pics.

Bob..VK3ZL..

[VK3ZL]

This is the third of the latest amplifiers...It is strictly experimental and uses a water cooled GI-7BT triode....I looked at some circuits on the internet but there isn't much real information available for this tube...However I found some drawings by RA3YO and decided to chance it and built up a complete amplifier adding my own techniques for tuning and loading the output....I built a special water manifold for this amplifier....Air cooling at 70cm and 23cm can quickly run a project into failure due to insufficient cooling of the tube and tuned lines...Currently this amplifier is running 270 watts output with no sign of instability or frequency drift...I have about 40% efficiency but I hope to improve this with a few mods...I am confident I will be able to get a stable 400 watts output eventually.

Enjoy...

Bob..VK3ZL..

[PA5COR]

Beautifull as always Bob, i admire your work building amps from the start, well thought out, very nice design, and you put a lot of effort in details as well in H.F. thinking as well layout design, no wonder i won't be surprised see you come back with another few 1st prices...

Thanks for the inspirational pictures of your work, nice tough to see you use watercooling ;)
Just picked up at the last ham fair a few 12 cm Papst fans 230 volt types for a nagging project that has me thinking for some time now...

Good luck there getting the VK hams all excited about homebrew projects, you certainly hav your followers here ;)

73,

Cor

[rot]

Beautifull as always Bob, i admire your work building amps from the start...
.....
Good luck there getting the VK hams all excited about homebrew projects, you certainly hav your followers here ;)

73,

Cor

What he said.
Cool stuff Bob and Thanks for pics!
rot

[NQ6U]

Beautiful work, Bob—as usual.

Just out of curiosity, how do you spec out those transformers? I've been thinking about building a plate-modulated AM rig for the local 80m net. I wouldn't need a lot of power—50 to 100 watts would be enough—do you think that Chinese manufacturer could wind a modulation transformer?

[W3WN]

Damn, Bob, you do nicer work that some of the commercial amps I've seen. (Ham amps, not those silly things the 11 meter crowds use)

Sure wish you lived nearby... I could use a nice amp... * sigh *

[KG4CGC]

It's more of an art piece really. Fully functional of course.

[KB3LAZ]

As always, I am at a loss for words when I see your work. Outstanding!

[N8YX]

Very nice work. I showed this thread to a friend over dinner; he remarked about the attention to detail...and he knows little about electronics himself.