Unitra W-480F: Introduction

This amplifier was manufactured in the year 1971 by Łódzkie Zakłady Radiowe Fonica (Unitra) in Poland. It is a stereo unit, constructed around one ECC82 double triode and two ECL86 triode-pentode tubes. The total power output is around 2 W, both channels driven at the same time, with a certain amount of distortion.

What makes it interesting, is the scarcity and the fact that it can easily be upgraded without major interventions. Well, the word major is actually directly proportional to how far you want to go.

Why would I invest my time and resources into such a project? Simply, because I listened to one of these around 2003 and it stuck to my heart. The schematic diagram is simple and easy to understand. In fact, it closely follows the textbook implementation for the ECC82 and ECL86 tubes. It can easily be replicated, provided that you assume the cost of the tubes. Especially since a pair of ECL86 tubes fetch a particularly high amount of money, these days.

Another possible answer to the why question could be that it's been around twenty years since I fiddled with (DIY) tube amplifiers. It can be said that I'm a bit rusty when it comes to tubes. I'd like to get back on track, since I plan to design a pair of low power monoblocks in the upcoming years. I have parts of the schematic diagram drawn on paper. I also worked out various circuit section calculations during the past years. But I haven't fully concentrated my efforts into that project, yet.

Coming back to the Unitra amplifier, I found mine on the Internet, listed for sale, on a local classifieds site. It wasn't cheap but it was more or less complete -- nevertheless, all knobs were missing. Internally it was completely original, albeit in a very bad functional and optical condition. Once it arrived, I connected it to a set of speakers and fired it up. Surprisingly, it was playing music. Only one channel, though. The other channel played very faint. And no, inverting the ECL86 tubes didn't solve the issue. So it was not an emission problem. Another weird thing was that the incandescent indicator bulb was constantly flickering for some reason.

I think this is going to be a fun project and I will remember most of the tips and tricks I used to know in the realm of tube amplifier design.

Disclaimer

The following articles are not to be treated as do-it-yourself tutorials on how to fix, restore, rebuild, or improve the unit in cause. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to launch into such an adventure.

The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.

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Unitra W-480F: Technical Data

This amplifier has the following technical characteristics.

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Unitra W-480F: Parts List

The original schematic diagram contains errors and cannot function the way it is drawn. I took the liberty to modify the schematic in such a way that I preserved the looks as intended by the designers. I renamed all the component identifiers during the process. Of course, it would've been way easier for me to redraw it in a CAD program. But I went with a different approach this time.

The parts list below includes all my improvements. However, it cannot be used to directly service W-480F amplifiers based on the original schematic. It can be used as a guideline, nonetheless.

By the way, there is no need to replace the resistors. But since this is a complete rebuild, I replaced them anyway.

Schematic Diagram

I hand-painted this schematic diagram using an old computer graphics manipulation program. I used a scan of the original schematic diagram as reference. So, before you ask: no, this is no mystery schematic from Unitra. It is my own adaptation of the original. It may look original because this was my intention. It took me a couple of hours to arrange everything and to rename the components via successive copy-paste operations.

My additions are highlighted in purple. I tried to keep these improvements to a minimum in order to preserve the simplicity of the original implementation.

This fact being clarified, let's take a look at the parts list.

Descriptions and Parts Listings

The order of the parts listed within each table is left channel first, right channel last, as per the component index depicted in the schematic diagram.

I used a couple of film capacitors for RF radiation canceling purposes in different places on the aluminum chassis. I also added a couple of ceramic capacitors for spark canceling on the mains switch.

The parts listed below in red text are not depicted in the original schematic diagram. They are provided as per my improvements to the initial circuit implementation.

The preamplifier stage is constructed around an ECC82 double triode tube. It is located on the lower right part of the aluminum chassis, near the mains switch.

The parts listed below in red text are not depicted in the original schematic diagram. They are provided as per my improvements to the initial circuit implementation.

The parts listed in blue text should normally not be replaced.

The tone control circuit is constructed around a dual RC network and it is assembled directly on the terminals of the control potentiometers.

The parts listed below in blue text should normally not be replaced.

The power amplifier is build around two ECL86 triode-pentode tubes and it occupies the top center section of the aluminum chassis

The parts listed below in red text are not depicted in the original schematic diagram. They are provided as per my improvements to the initial circuit implementation.

The parts listed in blue text should normally not be replaced.

The power supply is based on a simplistic design, built around two SPS-6B-250-C100 paralleled selenium bridge rectifiers and a couple of filtering capacitors. There are also two voltage dropping resistors and a couple more capacitors.

The parts listed below in red text are not depicted in the original schematic diagram. They are provided as per my improvements to the initial circuit implementation.

The parts listed in blue text should normally not be replaced.

Again, this parts list along with the specified component identifiers, can only be used with the schematic diagram that I provided.

Tube amplifiers generally have few electronic components. And this one, being so simple, makes no exception. However, these amplifiers use high voltages which are lethal. Extra care should be taken while working with high voltage. Always discharge electrolytic capacitors in the first place.

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Unitra W-480F: Restoration

I must say this is my second tube amplifier that I restored so far. The first one was a Romanian built Doina Delta guitar amplifier that I never had the chance to completely finish; I sold it as it was. Sure, if radio receivers count, I could add that I also partially restored an Electromagnetica branded unit some 15 years ago. You can imagine that I was pretty excited to work on the W-480F.

Sourcing parts posed no real issue as there is nothing exotic in this implementation, except for the selenium bridge rectifiers. Even if those have a limited lifespan, I never intended to replace them. I went with Dale precision metal film resistors. The first voltage dropping power resistor is still a Dale part, albeit from the RW Military series of wirewound resistors. For the signal path, I chose Illinois Capacitor film parts and Cornell Dubilier silver mica capacitors. Wherever I could, I kept the original Miflex capacitors. Cathode decoupling electrolytic capacitors are made by Sprague and are of type 30D -- yes, they're still produced. High voltage capacitors are made by Nichicon, BT series. The heat shrink tubes are all produced by 3M.

I tried to keep the quality high with each and every part involved in this amplifier. These are already pretty scarce and my restored unit will be unique.

The tubes are a bit pricey, though. ECL86 are not particularly easy to find. ECC82 double triodes are readily available in different price ranges. And it can be replaced with ECC83 that will provide slightly more preamplifier gain. I thought initially to modify the chassis and include an extra ECC83 between the power tubes and to replace the two ECL86 parts with EL84 tubes. But that would've meant a different schematic, thus resulting in a different amplifier. I will save the EL84 tubes for a different implementation.

I worked in no particular order. Thus, the pictures will not reflect a natural progress from power supply section to preamplifier, passing through the tone control circuit, and ending with the power amplifier. Due to the reflectiveness of the polished aluminum chassis, the unit is difficult to photograph.

My updates to the original schematic are listed below along with other improvements. The schematic diagram updates are highlighted in purple. Thus, I:

1. Added two 100 nF / 250 V input capacitors connected directly from the 5-pin DIN input connector to the grids of the two triodes in the ECC82 tube. These will protect the audio source in the unlikely event of a catastrophic ECC82 tube failure. In this case, 60 Vdc (or more) might find their way into the audio source output stage. This is no joke as I saw this behavior live back in 2004 or so. A friend build a powerful tube amplifier without input coupling capacitors. In his case, he accidentally made a short-circuit with his screwdriver while performing final adjustments to the amplifier. The audio source was a (then very expensive) 128 Mb MP3 portable player which burst in flames in a split second. The fire self-extinguished in less than two seconds. But the audio device was truly dead.
2. Added two spark canceling ceramic capacitors to the mains power switch. I chose this method to protect the original switch, which is, otherwise, no longer available. Of course it can be replaced with a modern equivalent. But I want to preserve the original aspect of the amplifier as much as I can.
3. Added a CLC power supply ripple filter based on a 1.5 H / 56 Ω reactor choke rated for a maximum of 200 mA DC voltage. The Hammond 156R reactor choke is small enough to fit on the aluminum chassis, without looking out of place. Low power supply ripple is a must for single-ended class A amplifiers.
4. Split the common cathode biasing circuit used for both ECL86 tubes into two individual circuits. Now each tube has its own resistor and capacitor network.
5. Added two 10 nF / 250 V polyester film capacitors in two different places in order to suppress RF radiation and randomly spawned AC currents between the ground bus and the aluminum chassis.
6. Used appropriate thickness stranded copper wires for component connections.
7. Used tightly twisted stranded copper wires for filament voltage transport.
8. Used beeswax coated cotton thread for sewing wires together. This ensures a clean and tidy look instead of a ratsnest mess.
9. Replaced the 6.3 V incandescent bulb with a 12 V rated one in order to prolong its lifespan and ensure a lower power, orange tinted, indicator light at night.

Restoring point to point tube amplifiers is pretty easy. But if raising the standards regarding component placement and avoiding noise by closely following proven rules, the job increases in difficulty exponentially. It took me around two weeks to complete this project.

General Considerations

I should begin by saying that working on this amplifier presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for more than 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.

Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also, I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also, I only use parts that are suitable as replacements in various sections of the amplifier, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.

Flux fumes are extremely toxic and should be avoided at all costs.

Initial Overview

Here is the initial situation. There is a layer of filth on each and every component. In addition, there is a lot of dust caught in some kind of waxy substance. It looks like some kind of fur and it smells bad.

I am used to dealing with various degrees of dirt during my restorations. But this amplifier competes for the first place. The king of dirt, in other words.

Complete Disassembly

In the beginning, I wanted to do a light restoration, but after assessing the initial situation, I just decided to strip everything down. Completely.

Fifty years of dirt looks absolutely disgusting. The fine dust appears to be embedded within the aluminum micro-structure as it doesn't wipe off with the bare finger.

There is sticky dirt even on the dual selenium bridge rectifiers. The exposed connection wires are all corroded.

There were some riveted parts which I removed as well by drilling the brass rivets. Then, I cleaned the aluminum chassis with acetone and isopropyl alcohol. This is the best I could do. The chassis is now completely clean, but the years of dirt left some ugly visible scars.

I had better luck cleaning the central aluminum part that houses the tube sockets.

I thoroughly cleaned all of the mechanical parts and it seems it took me ages to finalize them. Once I did, I realized that I cannot use the old machine screws as they were all rusty. Thus, I went for all new screws, washers, and nuts.

Let's move on.

Mains Power Switch Restoration

The power switch is switching both live and neutral lines that are going to the mains transformer. Since these kind of switches are most probably no longer available as original replacement parts, I decided to restore it.

I disassembled the switch and cleaned it with a chemical solution and then I washed it in the ultrasonic bath. This must be one of the most unique and complex hand assembled switches that I serviced.

It literally screams hand built. Finally, all small parts are cleaned. Some are already assembled on the central mobile piece.

I added two ceramic capacitors for spark canceling purposes. Any electric arc that appears between the internal mobile armatures and the static electrodes is shunted through the capacitors upon switching from OFF to ON and vice versa.

The new Vishay modern ceramic disc capacitor are rated 10 nF / 2 kV and I mounted them on opposite sides of the switch assembly. You can only see the reflection of the second capacitor as it's not directly visible under the switch.

The switch has some play in the central pivoting axle. I inspected the construction and it appears this is by design. At least, I didn't find any simple way to get rid of the play. The feeling is weird and the switch gives the impression that it is about to collapse. Considering the multitude of individual small mechanical parts, it is very robust.

Preamplifier Rebuild

A stereo preamplifier built with only six resistors and four capacitors. What could be simpler than this? The initial circuit, of course. Why? Because there were only two capacitors. I decided to install two additional input coupling film capacitors, rated 100 nF / 250 V, for obvious reasons.

The original preamplifier implementation reveals a cheap build: thin filament supply wires, unshielded audio signal cables, and short-circuit prone terminals. The left-right channel balance control circuit has an interesting design: a potentiometer connected between each anode of the two triodes of the ECC82 tube. Based on the cursor position, the audio signal flows to ground, through a capacitor.

After stripping everything out and cleaning the tube sockets in the ultrasonic bath, I started the assembly process. First, I created a ground bus from 1.5 mm thickness solid copper wire. It connects the ECL86 sockets to the ECC82 socket and it goes further in the tone control area.

I created the wiring harness for the filaments from 1 mm thickness stranded copper wire. I used a power screwdriver to tightly twist the wires together. Some transparent heat shrink tube helped to strengthen the harness.

Soldering the resistors quickly brought up all the memories about how long it takes to shape and insulate the terminals for a clean build.

I added the input capacitors and the shielded audio signal cables. The preamplifier power supply filtering electrolytic capacitors are visible in the background.

All components installed and all wires soldered.

And a view from a different angle.

Up next, a detailed view on the ECC82 tube, made by Tungsram.

This was the first circuit section that I restored within this W-480F unit. I enjoyed working on it and it set the pace and tempo for the entire restoration job.

Tone Control Rebuild

This is a classic implementation of a bass and treble tone control circuit.

First, I carefully removed the potentiometers along with all parts involved in the tone control RC networks. Next, I cleaned everything and started rebuilding the tone control with quality parts.

It's worth mentioning that the dual Telpod potentiometers were coated with some kind of clear protective lacquer. Although it proved reactive with acetone, I decided I didn't want to temper with that. Thus, I decided co clean the dust with a soapy water solution.

I severed one Miflex film capacitor terminal upon removing the old components from the tone control circuit. It simply snapped off, without applying any considerable force. I found a pair of 2.2 nF / 100 V vintage WIMA FKS capacitors in the spare parts box. These are polyester film capacitors, housed in blue rectangular plastic packages. While technically fit for the purpose, they don't exactly follow the optical design of this amplifier. But they are good as a temporary solution, until I will replace them with axial capacitors.

A detailed view on the local star ground connection of all four signal cables.

Overall, this was an easy job. Too bad I couldn't make use of the original 2.2 nF / 100 V parts polyester. Thus, let's advance to the next section.

Power Amplifier Rebuild

The power amplifier is constructed around two ECL86 vacuum tubes, for stereo signal amplification There is only a handful of passive components involved in its implementation. Good thing is that there are only two film capacitors in the signal path.

Some initial cost cutting measures included the presence of a single cathode biasing resistor and bypass capacitor for both tubes. While it was common back then to design such circuits, these days, components are cheap and the design can easily be improved by adding individual cathode biasing components for both power tubes.

Another weird thing is the 15 Ω output impedance. However, my tests revealed that a 6 Ω speaker system can easily be driven by these output transformers. Initially, I wanted to change the transformers but after some listening around, I completely changed my mind. The old transformers are here to stay.

Here is the original circuit. Same lame wiring and short-circuit prone terminals. A true ratsnest.

After reassembling the tube sockets and connecting them to the copper ground bus, I soldered the resistors to ground on the filament supply rails.

Making visible progress. Somehow, I like point-to-point component routing. But it is insanely time consuming if aiming for a clean build. I reused the old Miflex polyester capacitors.

I replaced the two 15 pF capacitors located within the negative feedback loop with high quality Cornell Dubilier silver mica capacitors. For the record, I measured the old 15 pF parts on the HP 4276A LCZ Meter: one gave a reading of exactly 15 pF at 20 kHz and the other one around 18 pF at the same frequency.

I soldered all passive components and connected all signal and power wires. Every local ground connection goes to a local star point on the ground copper wire bus.

And a view from above. The new Sprague 30D series electrolytic capacitors integrate very well, assuring a period correct look.

A detailed view on one of the local star grounding points. This particular point also constitutes the main connection to the negative rail of the dual selenium bridge rectifier assembly.

This section of the amplifier was the most laborious to work on. Due to space constraints, it was a bit hard to solder everything in place. I made a very good component placement plan, which I closely followed. Once wiring came into the picture, it was almost impossible to access several soldering points of this circuit.

Power Supply Rebuild

These are the dual selenium bridge rectifiers, mounted on the aluminum chassis.

Next, I added copper wires coated with clear heat shrink tube. The star ground connection points will be on the two screws on the left side of the dual rectifier assembly.

The old dual 50 uF / 250 V electrolytic capacitor occupied a large amount of space between the ECC82 tube and the dual rectifier assembly. Upon removing it, I immediately thought about using the remaining hole as a pass-through for the power supply wires. Thus, I installed a rubber grommet of type G1155, made by Heyco. The head diameter is 22.2 mm, inner diameter is 12.7 mm and the overall height is 6.4 mm.

Wiring in progress. For a vintage look, I chose to sew the wires together using various techniques.

Next, I installed the Hammond 156R reactor choke. This is the best place I could find for it. I mounted it on two small rounded brass standers of around 50 mm in height.

Finally, I soldered all power cables to the mains transformer.

These are the 22 uF / 450 V electrolytic capacitors used for further power supply ripple filtering. I did some round shaped bends that I used as guides for the wires. Then it was easy to solder everything together.

Due to obvious reasons, I replaced the original dual 50 uF / 250 V electrolytic capacitor with two individual radial capacitors. Since there is no immediate way to mount these on the aluminum chassis, I decided to make my own clamp out of a thin sheet of metal.

The metal piece was initially a 40 mm diameter round clamp, intended for pipework. I used a Dremel tool to cut a section of it. Then, I polished the edges with a high speed sandpaper disk. First, I measured 35 mm on each future circular arc that will hold a capacitor. Second, I measured a 10 mm area for the center fixture point. Next, I bent the two 35 mm pieces in the vertical plane, using the vise.

I made a 3 mm diameter hole in the center of the fixture section.

After a couple of minutes of thinking about a way to bend the metal in order to shape the circular arcs, I came up with the idea to use a wood screw to fix the U-shaped metal part into a piece of oak wood. Then, I used a 16 mm diameter tube to manually shape each arc.

This is the first time I'm making a clamp and I'm happy it looks as it was factory made. It literally took me half an hour from start to finish. If I were to make another one, I reckon I can do it in under ten minutes.

Here are the 47 uF / 450 V power supply ripple filter electrolytic capacitors, secured by the new clamp. All wires are connected and nicely sewn together. I mounted the first voltage dropping resistor in air so that it can better dissipate the heat. Interestingly, the resistors heats up to only around 40 °C or so.

Funnily enough, my hand and the mobile phone are visible in the getter deposit reflection of the ECC82 tube. Tube amplifiers are hard to photograph due to reflections; the camera auto-focus function is totally puzzled.

Let's try to see things from a different angle.

That's it for the power supply section.

Chassis

An overview of the current state of the amplifier.

This is the rear panel with the 5-pin DIN input connector, the speaker connectors, the mains power plug, and the fuse compartment. All clean and shiny. I tied the cables together with beeswax coated cotton thread.

Let's see things in details.

The 10 nF polyester film capacitor is shunting RF radiation that might be caught up by the signal cable shielding mesh.

And the other 10 nF film capacitor that shunts stray AC currents wandering around the grounding bus and the aluminum chassis.

Next, a detailed view of one of the ECL86 power tubes.

I connected the output transformers with color coded wires.

Details on the filament power supply wires. I used the same gauge tightly twisted stranded wires for the incandescent bulb. Untwisted, the cables are (very) flexible. But while twisted, with a little bit of persuasion, they will retain their routed shape.

The 12 V incandescent bulb lights up mellow.

In the end, I am grateful that the designers of this amplifier engineered a good layout. It was easy for me to bring improvements to this design, without many alterations, fifty years after its introduction.

Final Overview

We've reached the end of this journey. This is the completely rebuilt amplifier.

This concludes the electrical rebuild of this Unitra W-480F unit.

Reflections

Sometimes I wonder where we had lost the simplicity of these classic class A audio amplifiers. Their sound is very good, the complexity is very low, and the serviceability is very high. On the other hand, power consumption is not their strong point. But I'd rather make a concession on the electricity bill, rather than spend a couple of hours listening to a harsh sounding audio device.

This amplifier might not sound like a Sansui AU-20000, but it is a true little performer, nonetheless.

Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

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