KG4KGW's 813 Amplifier Project began in 2007

Kilowatt Amplifiers are common place in today's world of Hi-Tech gadgets and gizmos. In the days of old it took a skilled hand to make one of these beasts. This is my record of attempt to repair and restore a dual 813 tube type amplifier of unknown origin dated back to sometime just post WWII.

This is the front panel of our test subject. As you can see it has been in disrepair for some time now. It has a weight that is ungodly due to the large power transformers required to power this unit. My hope is to not only learn enough about its operations to repair it but also to be able to install some modern features.

 

Possible upgrades include:

1.     Safety interlocks for top panel.

2.     Inrush protection to extend the life of the pair of 813 tubes in the final RF output stages.

3.     Step start circuits

4.     Replace the grid block keying circuit with a low voltage modern circuit that will keep that high voltage away from me and my equipment.

 

The current status of the equipment is non-functional. At one time there was 204 volts on the Push to talk cable. This is very unsafe. I had taken the unit to a local repair shop to have a modern keying circuit installed, but the owner of the shop died before the work was completed. This is happening all too often in Amateur Radio circles. These guys are getting older and if the knowledge they posses doesn't get passed on to the younger generation it will die with them. The age of experimentation seems to be coming to an end very rapidly. Finding hams on the air in my age group is fairly rare even with the lowered testing requirements. I was licensed in 2000 and quickly upgraded to General class a month later. I waited far too long to upgrade to the top license, so it wasn't easy starting over again. I got my Extra Class Amateur License in February 2007. Now to share the knowledge and experiences as it happens.

The above picture is of a roller inductor located inside the amplifier. These are very rare now  days and require care and maintenance to keep them in top shape. This one is still in working order, but could use some much needed cleaning and lubrication. I'll have to investigate what type of cleaner and lubricant will be needed so as to not remove the silver plating or cause a short in the inductor.

This picture shows the band switch that is used to select what portion of the band to load the amplifier on. Bottom right you can see the air capacitor that is also used for fine tuning the amplifier.

In this picture you can clearly see the pair of 813's that is the powerhouse of this units RF amplification. Between them and slightly offset there is a large RF plate choke. This keeps the RF from entering the plate side of the amplifier and creating parasitic oscillations in the tubes and final output of the gear. The transformer next to the tubes on the bottom right is the power supply for the tube heaters that keep these big tubes glowing. Just above that is the back of the dual purpose meter that is used to measure the plate and grid DC amps.

This one tells a story about this amplifiers age. As you can see in the top left hand corner of the picture there is a huge transformer that provides the power to the amplifiers main input power. Center top is a Swinging Inductor transformer. The plate that is mounted above that is home to a relay that is used to key the unit. On the top right hand corner you have two very large oil filled capacitors that help to filter the power from the mains to the High Voltage board (I could be wrong on this, it could filter the power from the HV boards to the amp) I'm not sure what the power is on the board in the middle of the picture but to my understanding it is capable of 3KV into the amplifier. Notice the large door knob capacitor at the end of the roller inductor? The chassis is a negative ground and more than likely will have approximately 200-300 volts of negative power located on it. I'm not sure what the choke on the bottom left does, but I think its part of the tank circuit tuning. Slightly above that is a variable capacitor that is much larger under the RF deck for biasing the tubes.

This one is a little blurry because of the range I had to fit the camera in below the RF deck. I haven't had time to label the wires on the RF deck in order to remove them so the deck can be removed from the unit. As soon as I can I'll get up some better pictures of this part of the amplifier.

Here you can see some more of the bottom of the RF deck. I'm not sure again on what the transformer here does, but I'll be sure and let you know as soon as I find out. You can clearly see the bottom of that capacitor I spoke of in the earlier picture that looks like a volume knob from the top without the knob on it. The large capacitor on the right seems to be a replacement after the original date of construction.

This is the back of the front panel below the RF deck. On the left is one of the two lights you can see on the front panel and on the right is one of the two power switches. You can also see the base of the tube sockets here on the middle right hand side.

This is another picture of the bottom front below the RF deck. You can see one of the tube sockets on the left, another light in the middle and the secondary power switch on the right. Notice the rust on the bottom of the amplifiers case? 

Phase I

Disassemble and Identify Problem Areas

 As can be easily seen in our subject there is much work to be done. First off we have no known schematic to go from and thus must try to see what the original engineer had in mind when he built this amplifier. Second we must remove hazardous parts and obsolete design elements and give us a good base from which to start. After carefully numbering the wires present on the power block I've removed the RF deck and took a few more shots for us to ponder over.

This shows up our RF deck removed from the unit. As you can see time has left some nice dirt for us to clean up. The transformer on the left has a sticker that is peeling off. The sticker reads:

P-6461

Filament Transformer

Primary 50/60 cycles

117 volts''black

Secondary 10v@10 A'green

CT'..green-yellow

Insulation Test'..2000 volts

Chicago Standard Transformer Corp.

Chicago, IL.    USA

I'd say that this is a sure fire way to tell its our tube transformer! I'm aware that the 813's need 10v@ 10 amps for the filament. So now we're making some progress.

 

Above we can see our Filament transformer wires coming through the bottom of the RF deck at center left. We know from the sticker that the green wire is our secondary with our required 10v@10amps. What do you think the block that its wired to does? I don't know just yet but its some sort of transformer as our filament connections are coupled to it and our transformer leads. The black wire we know to be 117v is going to our primary power switch and also wired with our secondary red front panel light and our secondary switch. Our green and yellow wire is going back through the RF deck to the meter located on the front of our amplifier. The rod like piece going across that block from our tube is connected to our RF input. The other phone jack next to it is our PTT line that is missing our voltage! 'Gears are turning here' That little orange and white wire running through the RF deck on the bottom left is coming from our PTT connector center and the outside of our RCA connector is chassis mounted. The other end of that wire goes to terminal 7 on our power block and is wired to our PTT relay as well as the number 3 tap on our main power transformer. (Let's take a minute to pray our main power transformer is alright)

With the RF deck removed we can see our High voltage power supply board. It almost looks like a kit of sorts and might be a unit from another piece of equipment from the amps construction era. We will be rebuilding this board soon replacing the 'top hat' style diodes with new 1N5408's. Behind this board to the right you can see our old oil filled capacitors. These guys will be saying goodbye since they contain an oil base with PCB's. PCB's were banned in the United States around 1977 and have been stopped from production in most of the world. I've read that the USSR was still using PCB's as late as 1990. The chemical is a Polychlorinated Biphenyl and was used as a insulator and fire retardant in many products during the 1950's.  This is some really nasty stuff. It has been known to cause cancer, reproductive damage, liver damage, and even nerve damage. It is closely compatible with most living organisms and is absorbed fairly easily by the body quickly reaching toxic levels. We will be using 40uF ' 60uF 450v snap in type capacitors in their place. This will take up less room because they are more compact. My understanding is that it will take about 12 of these caps to give us around 4-5 kV for our choke input power supply. Removing those items we will be eliminating that swing choke that is in the middle of the picture.

I just couldn't resist putting in another picture of our engineers fine handy work. This is a close up of the center of our HV power board.  This is going to be rebuilt using 1N5408 diodes.

The above picture shows our powerhouse transformer, swinging inductor, bleeder resistors, and oil filled capacitors. The only thing here we will be keeping is the main power transformer. Its rated at 1825v and should do well with our new capacitor bank. The swinging inductor isn't going to be needed as we are not running an AM station and for CW we can limit our drive power to about 50% PEP.

Swinging inductor removed from the chassis.

Main power transformer pulled. This is starting to get light now!

All is gone now leaving only the large resistors to pull and the coax connector. A lot of case clean up and we will be ready to sandblast and paint our cabinet. I have set the RF deck aside and began to order parts. We will be redesigning the RF deck in the next phase of our project. Meantime I am on the hunt for our step-start relays, HV board diodes and resistors, and our filter capacitors to complete the rebuild of our power supply. It pays not to buy the first thing that comes along. I've saved hundreds of dollars shopping around and have high hopes of getting a much needed air capacitor for our plate tuning circuit.

This is a work in progress and is a partial diagram schematic of our Power supply and rectifier board as it was originally installed in our amplifier. I will be completing these drawings in sections and hope to be done within a couple of weeks. Just like anything else I do work and have a family to take care of so be patient if the drawings are not up fast.

 

T1 pinout is

662-0279-00

Power Transformer

292-5648G1A

PRI 1-2-3-4

TERMS    VOLTS    AMPS AC     TV

 1,3 . 2,4      115                          1500

1 . 2 , 3-4     230                          1500

     5 . 6        460           0.54        1900

       7-8      1825           0.44        5500

MAX ALT 10000 FT

Raytheon, Waltham

T2 is our swinging inductor is

 668-0280-00

Inductor Swinging

292-5660G1

TERMS     HY   AMP DC    RES DC

        1-2          7.2      0.500      100??MAX   

30.0      0.025     

250V   60 CY  TV RMS  5500   

MAX ALT 10000 FT

Raytheon, Waltham

 

Yet another draft of our tube layout as seen from the bottom of the chassis.  Strange enough if you look at the keying circuit you will see that the filament shares the same connection with our keying system. The filament transformer is rated 10V@10A . When this rig was inline I metered 204V on the PTT line. It will be interesting to see where this voltage was coming into our keying circuit. I'd say it was a hot chassis voltage.

Contact:
Elevated Wire Radio Group
763 North 79th Avenue
Myrtle Grove, FL 32506