I really must tell you this story from years gone by. From the age of around
nine, I was first introduced to valves (tubes), and at about age eleven I first clapped-eyes on an 807 tube. Now, please don't condemn me here (some people collect stamps, do train spotting etc!), but…..I was fascinated by the shape of the
807. Why, I don't really know the answer to that! I remember distinctly back then in 1961 (I am 59 now…..born February 1950) that
I wanted to build an amp with 20 of these in the output, for some strange
reason, thinking how magnificent this would look! However, I never really did it. I have built many amps over the years with just a
pair, a few with a quad, and just one with an octet (please see this article
on the CBA-225 ).
Now, some 48 years later, I have actually gotten off my arse and made a real start on it! One important factor here is the original tube data sheets. A lot of people out there might not actually be aware of this, but all tube data available….for any tube is always expected to be at the given voltages of the said data sheets. This basically means in English…….stabilized!!
Apart from very few acceptations, most Hi-Fi amps and guitar/musical instrument amps don't actually do this! The basic amp generally runs the output stage voltages considerably higher than is actually required then, when the amp is under full load conditions, the output power is now achieved as the HT supply drops to around the desired data sheet figures. In almost all cases this is fine to do without any major or serious damaging consequences. However, in reality and again in most cases, the output tubes are running unnecessarily hard under no signal conditions. Most tubes handle this quite well though without any complaints.
Now, welcome to the world of regulation! One simple way to achieve voltage regulation in high voltage tube equipment is to use a choke input filter….this is very reliable and successful but…..the choke is usually about the size of the power transformer and adds a massive amount of weight and extra size to the said equipment, which is not always practical or cost effective. This is why most amps don't have this. As we now live in the "solid-state" world this has become so much easier to do, and we can use something like a 1,600 volt / 25 amp IGBT and get the regulation we require, quite easily with a small chip to control the IGBT. This though deviates away from the tube world and, although this kind of regulation is in no way in the "audio signal path" it still nevertheless is "solid-state" at the end of the day!
Now, as it happens, around this time, my good friend John Wood was in the process of designing a regulated power supply for a Hi-Fi amp that he had also designed and was already in prototype form……sounding really brilliant! He was explaining to me about the amount of current this regulated power supply was capable of with a pair of PL509 tubes. His prototype Hi-Fi monoblock was 200 watts with four KT88’s. This being the case I asked him if he would mind me using his idea for my 1000 watt monster. John duly replied "no problem" and kindly made me the two PCB boards that I needed for the job.
When I got down to the nitty-gritty of designing this humongous monster and having decided to use John’s regulated power supplies for it. John’s plan in-fact was to use tubes (valves) for the main regulation and a chip plus a small high voltage Mosfet for the control of the said valve(s). On data sheets for the 807, and on not pushing them into AB2 (which would have required many more tubes for the drive and more complicated circuitry too), I decided to simply do the amp in AB1. Under these conditions with 750 volts plate and 300 volts screen (grid 2), 12,000 ohms plate-to-plate, fixed bias, we can expect from these data sheets 72 watts for a pair. However, under real-life test conditions with just a pair of tubes, John and I found that this was actually closer to 82 watts and seemed to be quite consistent too from various manufacturers of different tubes. There is also the added advantage that we could squeeze the plate voltage up-to 800 volts and the screens (grid 2's) up-to say 325 volts. This then means the pair of tubes could possibly make the 100 watts, even in AB1!
After all this numerous testing on just a pair of 807's at the suggested previously mentioned voltages, I accepted the fact that in AB1 the amp should really be 720 watts for 20 tubes, 820 watts is likely without any tweaking but……a 1000 watts is certainly possible with the said tweaking!! I will let you know the final results at the end of the project.
Moving back to the regulated power supply(s) issue, as previously stated these were designed by my good friend John Wood. There are quite a good few hefty tubes out there that can be used for voltage regulation but for many of them it is not always too practical. What John had ended up with was the PL509. This also includes many of it's counterparts/equivalents, PL519, PL505, 40KG6, EL509, EL519, 6KG6 etc. All these tubes have around a 500+ m/a cathode current and are readily available out there in many different variances. Having settled on this family of tubes for the job we set-out doing loads of experimenting as, if my 20 x 807's amp was expected to reach the 1000 watts, we would have needed a 750 volts supply capable of around 2.4 amps for sine-wave drive. (About two-thirds of this in reality on music power.) In actual "real-life" testing we achieved the full 2.4 amps from 4 of the PL509 tubes at 750 volts, and the supply dropped only .1 of a volt throughout all the testing……...bloody wicked!
So now having achieved all the various "real-world" testing, the plan was put into action. The final amp will have: 20 x 807's for the main output of hopefully 1000 watts (or thereabouts); 4 x PL509's for the main regulated 750 volt (plates) HT supply; 2 x EL36 (or any of their family equivalents) for the regulated screen grid (grid 2) supply (about 300 volts); and 4 x 6SN7 for the pre-amp, tone circuit, splitter and push-pull driver. Thirty tubes all-told……….absolutely cool!!
As in all other projects and repairs on the site, I have always added the relevant page to the website when all is finished and working fine. For once I thought it might be a good idea to present this latest crazy project of mine in stages……as an on-going thing until it is finally finished. I though by doing it this way it would attract a lot of continued interest in the project.
I have done all photo captions/information and comments as always.
Big thanks for dropping by and as they say, "watch this space" for Part 2!
Many thanks indeed for looking, John.