Computer Control Of The Last Dinosaur Amplifier


Introduction

 

Amplifier spare parts are a mixed blessing. After a while you may begin thinking that what could you do with all these components. One thing leads to another and then you begin building a computer controlled amplifier (Fig. 1). That is what happened to me. This article describes system level choices I made and some computer control related specifics of this amplifier. You propably don't want to replicate the design as it is, instead you might want to copy some ideas and improve your amplifier otherwise.

 

Figure 1: Front panel

The basic design is proven in many combats. The tube is Russian GU-84 with 2800 anode voltage, and the anode choke is split into two relay switched parts. You can find several schematics and construction instructions on the internet for this tube, so I won't go much into details. I am assuming that you do know fairly well how to build tube amplifiers. Grid and screen power supply is of GM3SEK design. It has the necessary protection circuits for various odd happenings.

Things don't always work out. The usual mistakes were made, like having a screw too close to the anode choke which caused some impressive arcing inside the amplifier. Or forgetting to short out the lower band taps of the tank coil when operating on a higher band. The natural consequence was some arcing around the 80 meter coil tap and, after making the wire clearances wider, over the 80 meter band relay when operating on 10. The solution is to switch to on position also the lower band relays. The biggest mistake, however, was trying to build the amplifier into a too small cabinet. Because of that I had to build the amplifier twice.

You may wonder why I call this one The Last Dinosaur Amplifier. Firstly, I have firmly decided that this is the last amplifier I build. This time it is true. Trust me, I know what I am doing. Secondly, I think that finally the time for solid state amplifiers has come. Tube amplifiers are slowly becoming history.


A Few Words About Component Selection

For the most, to build a computer controlled amplifier for remote use is not that different from building an ordinary manually tuned amplifier. What is basically needed is a way to switch the amplifier on and off, and to be able to tune the tank circuit in computer control. In addition one needs some remote metering. Not much, but some.

Let us begin with tank coil's band selection. If you already have a conventionally built manually tuned amplifier, you would use a reasonably strong stepper motor that would turn the knob for you remotely. When building the amplifier from scratch it gets easier, just replace the band switch with relays.To avoid arcing remember to connect the lower band relays when operating a higher band, but do keep open the relays of higher bands than the operational band. I used Russian V2V relays for the high bands and V1V relays for 160 and 80 meter bands. In hindsight it would have been better to use V2V relays for everything. They are faster to install and wire properly as the relay coil is at the other side of the chassis than the high voltage contact. To mount such a relay, just drill a 31 mm hole and a couple of smaller holes for the screws and you are almost ready.

 

Figure 2: Control software

I had earlier made an amplifier that switches banks of fixed capacitors for tune and load. This time I wanted to try motorized capacitors. There are ready-made vacuum capacitor assemblies that contain all necessary control components, a motor, a position indicator, and limit switches. The only problem with these is cost. I happened to have a vacuum capacitor that seemed like just the right one for tune capacitor use. I bought some reasonably priced DC motors from Surplus Sales of Nebraska. A position indicator is easy to build using an ordinary multi-turn potentiometer and some gears. The limit switches were a little different story. With the right gears that would have been doable, too. However, Pena OH2BMQ solved it by giving me a linear screw actuator that had been in a decomissioned spectrometer. The mechanical design is shown in the right side of the amplifier in Fig. 7. The black switches on top of the actuator are limit switches. For load capacitor I bought an ordinary air variable capacitor.


Control System

There are many ways to build computer control for an amplifier. Commercial manufacturers usually use microcontrollers for which they design special printed circuit boards of their own. That was clearly too complicated for me. An option would have been to use some of the widely available microcontroller cards, like Arduino. That would surely work. However, I chose an even easier path. I designed the amplifier based on the Velleman K8061 USB interface card. The reason was, that I was then able to do all programming and debugging in a Windows computer using Microsoft's easy-to-use tools, i.e. free version of Visual Basic. You see the result in Fig. 2. It is a completely normal and simple Visual Basic program, except that there is a continuously running timer loop in which the measurements are read from the Velleman K8061 card and shown in the screen.

 

Figure 3: Motor control principle

A companion product of the K8061 is the K6714 relay card. It has 16 relays that I use to control all the 27 volt relays of the amplifier. Note that every single relay absolutely must have a reverse diode to protect from spikes. Once I had forgotten to add a diode to a 27 volt relay. It did produce a spike that traversed all the way through the amplifier so that the USB card went nuts for a moment, i.e. required a boot. The relays of K6714 have the following functions:

K1 = control power on-off

K2 = high voltage on-off

K3 = trip reset

K4 = ptt operational

K5 = tune/load capacitor select

K6 = capacitor right

K7 = capacitor left

K8 = unused

K9 = unused

K10 = 1,8 MHz coil select

K11 = 3,5 MHz coil select

K12 = 7 MHz coil select

K13 = 10 MHz coil select

K14 = 14 MHz coil select

K15 = 21/18 MHz coil select

K16 = 28/24 MHz coil select

Probably it was not worth the trouble, but I have built the amplifier so that it can also be controlled manually. All of the relays of K6814 are also backed up with a manual switch. See Fig. 8. which shows that for example switching relay K1 at the Velleman relay card equals to pressing S2 which is located at the RF deck's front panel. Not shown is how band selection is done when in manual control. The idea there is to give to the relay card (K6714) similar signals from a rotary switch that would otherwise have come from the USB interface card.

 

Figure 4: Measurements


Metering

By far the easiest way to implement remote meters is just to build a manually tuned amplifier and install an IP camera in front of it. I wanted something more elaborate.

The very most essential meters to move to a computer are the position indicators of the tune and load capacitors. In addition, it is also a good idea to be able to see anode current and the fault trip indicator of the protection circuit (GM3SEK board) so that one knows if the amplifier is ok or not. That is pretty much it. I decided not to implement screen and grid metering remotely. When the amplifier is tuned to preset capacitor positions these meters do not really provide much value, as the protection circuit takes care of any abnormal situations. Note that I do have a separate remote power meter which is not part of the amplifier, so I see remotely if the amplifier is producing power as it is supposed to do.

I used the following analog inputs of the Velleman USB interface card:

AD5 = Anode current

AD6 = LED

AD7 = Tune position

AD8 = Load position

Otherwise see Fig. 5. for some key details about measurements.

 

Figure 5: Bypass capacitors


RF Protection

The Velleman K8061 USB interface card is not designed to operate in environments where radio frequencies are present in high volumes. Naturally the interface card is placed on the low voltage side of the cabinet, not where the tank circuit is, and all cables that go from the low voltage side to the high voltage side do so through bypass capacitors. That is, however, not nearly enough protection for the interface card. Yes, some glitches did happen during the early tests. The earliest problem was solved by adding a 0,22 uH bypass capacitor from ground to +12VDC pin and another one between the terminals of the USB interface connector (Fig. 5).

 

Figure 6: Shielded cable

Then I added shields to all cables that go to the USB interface card. Basically, I wrapped aluminum folio over the cables and then a bare copper wire over that. The copper wire I grounded to the amplifier schassis. Then I added tape all over it so that it doesn't short circuit anything undesirable (Fig. 6). I addition, I added snap-on-toroids basically everywhere. There are some GM3SEK grid and screen power supply related wires that I have not protected in any specific way as this particular circuit is designed to work inside amplifier's RF environment.

 

Figure 7: Tank circuit

 

Figure 8: AC power controls


Summary

The Last Dinosaur Amplifier is a computer controlled tube amplifier that is based on well known designs and the Velleman K8061 USB interface card. I never really counted the cost, as the project begun with already existing spare parts. It took considerably more time that I ever imagined, mainly because of some quite basic mistakes like trying to first use too small a cabinet. The end result is quite sturdy.

I am grateful to OH2RA, OH5KW, and OH6LI for the advices that made this amplifier possible.

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