Tube Transformers – Output, Filament and Power
Tube Transformers – Why you may stick to solid state.
Many more DIYers are building thermionic tube amplifiers now than ten years back, possibly through hype or possibly nostalgia. They certainly have their place and in one area where they seem to be getting mass appeal are pre-amplifiers, headphone amplifiers and low power audio use in bi-amping. My own experience suggests that solid state is superior for bass reproduction where upwards of hundreds of watts are required to drive sometimes very inefficient bass bins. I do not believe that heavy sounding bass requires a tube amplifier although if you have the money then why not go whole hog and build a 2 x 100W tube amplifier. For a whole lot less, a 15W push pull stereo amplifier and 2 x 100W transistor amplifier will give you all the fidelity you need for bi-amping purposes. Some things for the novice to remember however is not just based on budget constraints – a 6W tube amplifier is going to cost you more than a 100W transistor amplifier. What are the implications?
Unless you are buying the entire kit from a renowned manufacturer which must include chassis, transformers, potentiometers and tubes etc remember that in a lot of cases what you read is based on nostalgia. Thermionic tubes do sound different just as you will gather that certain transistors sound better than others in a specific application. After handing over your hard earned cash remember that the heart of the beast is the audio or output transformer. A poor quality output transformer may discourage you from future projects due to poor frequency response, distortion or low power. Have you done a comparison between a fully fledged tube amplifier i.e. from front end to output, and a hybrid – tube preamplifier and solid state output stage. This dispenses with the output transformer which for most design engineers is the pitfall. (*see note below). If you are still keen then some ‘hi-fi’ die hards prefer to build their own output transformers – it is not difficult but very time-consuming. Good quality transformers use interleaved windings – i.e. packed primary, secondary, primary etc. The lower the primary Z (impedance) the more tubes you will need to parallel (as in transistors feeding two Ohm loads). Datasheets will give you the transformer impedance required otherwise the supplier will know.
(*Note: Certain vintage enthusiasts believe that the output tranny actually makes the audio more musical, in other words, better not worse. A very good output transformer is worth it’s weight in gold).
Tube amplifier design: Stick to Tagboard
The best tube amplifiers are not necessarily built on PCB – they are point to point which will require tagboard. Although I come from the marine industry which used a lot of tube gear still in the 80s plus PCB usage, these were designed and built for extreme weather conditions. Wires did still corrode and drop off. Modern wiring with the dreaded ROHS soldering techniques is not compulsory. In fact it’s a waste of time. Stick to tried and tested techniques.
Output Transformers – Counterfeit and poor quality
Output transformers can be purchased on eBay. Check reseller credentials and don’t say you haven’t been warned – counterfeit is rife. Quality output transformers are expensive. Stick to known brands of tube – there are some very good Chinese tubes on the market but I note that the ‘h-fi’ technologists still veer towards the Russian tubes. Better still, buy locally from someone whom can assist. Tubes do not usually come with a return policy – what you buy you keep. We don’t need to explain the rationale around this. Likewise tube transformers.
Mains transformers – fortunately here there’s many ways to skin a cat. I prefer EI transformers (laminated) in linear supplies to switched mode. This is a matter of choice. Machine wound toroidals have become popular as the pricing has decreased but in many countries they are harder to get than the old laminated variety – shipping can cost you whatever you win on an import. Reverse connecting transformers – connecting secondary to primary is one way to get 220V a.c. output although it costs you space. For experimental setups this is often a solution. Otherwise, if you are into designing with SMPS, use 220/110V -> 12V transformer, rectify, filter and feed into a home brewed switcher. I find this the cheapest solution. 500VA transformers are cheap to come by. (I have used UPS transformers which come at the most economical price = free). Many of them are 14V or 28V. I am advocating this because most of us are wary of designing switchers to run off +350V mains. Tube circuits do not draw huge amounts of current which allows thinner copper wire to be used which is easier to wind. If you are into looking at SMPS design, a good starting (and ending) point is from my personal favourite, Rod Elliott: http://sound.westhost.com/project89.htm – this project can easily be modified to get the high tension and heater supplies for moderate power circuits. More on heater supplies below. +EHT or +B is often derived from voltage doublers or triplers for low current supplies. This is ideal for a pre-amplifier.
Tube amplifiers use dangerous voltages so do not attempt a build or repair without following the proper safety guidelines – in all cases this means switch off, discharge and test. Rod Elliott’s SMPS may have a low input voltage but current draw will be very high, easy enough to cause severe burns if you are not careful.
Another circuit, designed for ham use, is that found here: http://w5jgv.com/hv-ps1/ – this power supply is not suitable for the newcomer.There will be a lot more questions than answers – stick to the one above for experimentation because the input side is low voltage.
Tube Transformers – Heater / filament voltages
Modern technology has given us the advantage of cheap low voltage regulators with exceptional current handling capability. Tube heaters draw masses of current and using transformers like that used in UPS, lend themselves well for heater voltage control through DC regulation. Using a 7805 regulator with the common terminal lifted by 1.4V (using two silicon diodes in series, forward biased) will elevate the output voltage to about 6.4V which is more than adequate for a 6.3Vheater supply. However, +5V DC should not hinder tube performance that much, in fact it will enhance tube life by running at lower than optimal. It is not advised to go below this voltage – this reduces thermionic emission to the extent where the tube will no longer operate as designed. It will not damage the tube!
Heater filaments can be coupled in series. Stick to the linear regulation, it is cheap and will offer a huge benefit: Over voltage will not only cause a short lifespan, it may kill the tube. Tubes are expensive.
OTL or Output Transformerless
The gungho of all audio. With the output transformer out of the picture you may save hundreds of $$$ but be careful of the pitfalls. Firstly, besides what the hype tells you they are often more expensive than amplifiers with output transformers. They run several tubes in parallel. The circuitry is highly tuned – they do not like impedances south of their rated, possibly 4 Ohms. Read http://davidberning.com/products/zh230 for a great review.
Bottom line is that if you cannot afford the price of the amplifier you won’t be able to afford the loudspeakers. Although they may be for the high end technophile audio guru, you will need the program material and you will need the proper auditorium. I have heard OTL and SETs and would be hard pressed to tell you that you are getting bang for your buck. They are for the extreme audio enthusiasts out there with very fat wallets.
My own experiences with tubes, whether it be radar, sonar, high powered SSB all proved one thing: Tubes are very resilient. Running any tube exceeding rated voltages is not a good thing. Although CRT rejuvinators may supply excessive voltage to the heater, they were never that successful. A low emission device needs to be trashed properly (not put into the bin). In most cases tubes cannot be repaired although I have used a capacitor to discharge through an inter-grid shot which cleared the problem. (CRT). Tubes make phenomenal voltage amplifiers yet can be the only solution in high energy circuits. In fact, in many circuits today only thermionic tubes can be used. They may look different but they still work the same.
Tube transformers may still cost an arm and a leg but hopefully this article may assist you in finding cheaper alternatives.