Triode Dick's Page Bill ...A brand new KT88 push pull am plifier... N ice to m eet you: Bill is the nam e... The old KT88 goes back six years. A lthough the listening results are still very good, I see to m any points that are qualified for im provem ent. S hortly put: It is possible to build an even considerable better am plifier, technical and sounding, w ith the sam e m eans, but w ith six m ore years of experience put into it. If I look at the costs, then and now , it looks like if the new am plifier can be build less expensive. Y ou can say w hatever you w ant, but that’s quite unique now adays … They have to be tw o firm little am ps, good for about, say 50 and 100 W atts. “W hat do you say? Tw o?” Y es, there’s also a brother of Bill, nam ed: ‘Bob’. Could it be a little bit m ore? That’s the general idea behind these tube am plifiers. Bill has tw o KT88 pow er tubes a channel, all build on one chassis. Bob is getting double that num ber of tubes and is configured as a m ono block. The evolution: • M ore pow er. E ven sw itched in triode m ode, there’s still 25 w atts left. U ltra-linear sw itching w ill double that figure. • The pow er supply is m ore firm and m ore stable. Both channels w ill have their ow n rectifying, capacitors and chokes. • The output transform ers are specially m ade for this project and clearly im proved: a w ide bandw idth and over-dim ensioned for a very deep low reproduction. • Easier and foolproof to build. • Parts are easy to get, even the cabinet! • There is enough room (in the am ount of voltage am plification) to apply feedback, if that’s w hat your speakers w ant. • The am plifier stays stable as a rock at capacitive loads, w hat electrostats w ill do jubilate….. W hy Bill and Bob?... I get on a regular basis questions of m en ( unfortunately, is w hat I call for years, but you have to find fem ale DIY ’s w ith a m agnifying glass), w ho sim ply w on’t have enough to a single ended 300B am plifier, but nevertheless still w ant to drive their “m ore juice appreciating favorite speaker system ” w ith a tube am plifier. A noble thought, w hen you ask m e…. A lthough I w ill not claim to be entirely im partial….. There are tw o w ays to go in tube-audio land: A n am plifier w ith a sm all but o so fine pow er output, driving speakers w ith a higher than average output in dB, w hich you have to like how ever. These kind of speakers have often their singularities and typical characteristics. If that is w hat you w ant, the choice for an appropriate am plifier is clear. A big horn speaker has enough to be supplied w ith a couple of m illi-w atts, to reach an ‘ear-blow ing’ sound pressure. The second w ay to go, is using a m ore general applied loudspeaker system , w hich indeed has often a som ew hat less sensitivity, but also adds m ore often a not easy to handle im pedance. (just look at the num ber of speaker system s using a double w oofer, w hich, in 9 tim es to 10, produces a loudspeaker im pedance of som ew here around the 4 Ohm , w ith exceptions m uch low er than that). A specified sensitivity of f.i. 90 dB/w /m looks very sym pathetic, but if assum ed that there w ill be a load of 8 Ohm , there w ill be effectively 87 tiny little decibels left, if it’s about a 4 Ohm system . The am plifier m ust deliver at halve im pedance tw ice as m uch current. That m eans tow ards a tube am plifier that if you connect your speakers at a 4 Ohm tap of your output transform ers, it w ill cost you half your output voltage. It is the one or the other.… To give your m usic m ore ‘S lam m ’ you need a tube am p w ith m ore m uscle. That’s w here Bill and Bob com e in focus. I invented Bill for the less w ealthy m an, w ho needs m ore pow er, but don’t w ant to use a solid state am plifier. Y ou cannot buy a fabric am plifier w ith a little bit of quality for a few bucks, but that’s no new s. But if you can build a m ore than w orthy KT88 push pull am plifier for 7 a 800 euros w hich can w ithstand the challenge against the tw o, four or m ore tim es expensive fabric am plifiers, it w ill be very interesting.. That’s a reachable goal w ith som e savings. But isn’t it difficult to build a tube am plifier? Do I have to search the w hole w orld for parts? A ll that fuzz w ith the custom s, im port fees, high delivery costs etc? The answ er is a big: ‘N o’… A nother designing goal of Bill is, that this am plifier could be build in a easy w ay, w ith contem porary com ponents, w hich are for sale everyw here in our country. A nd it succeeded very w ell. Designing an am plifier you have to start at the transform er set. Y ou look at the tubes you w ill use and begin to philosophize, calculate and w eigh.. In this case, w here w e are talking about push pull am plifiers, there w ill be looked at the type of phase splitter that w ill be used, how the end stage w ill be configured; class A , class A B, if I use an extra tube in the pre-stage, how I w ill build the pow er supply etc. etc… The setup of the end stage w ill be a single pair of KT88’s or a quartet. I w ill go for a setup in class A B, w here the biggest part is functioning in class A . That m eans that the am plifier w ill operate on a higher voltage level as the previous KT88PP am plifier. The pow er supply m ust also be better as the last m entioned. W e’ve gathered som e know ledge the last couple of years … W hen I designed the old KT88 am p, there w as no Duncan, w ith his beautiful PS U designer softw are, from w hich you can sim ulate an alm ost perfect pow er supply, if you bother to take som e tim e and use it w ell. W hat’s im portant is: you can study the behavior at changing loads. For exam ple how fast the voltage is recovering w ithout strange variations, after a high current dem and for a short period. Because of this, w e can assem ble a cooperative team of capacitors and chokes. In the early days, it w as m ore or less a guess. N ot that it produced bad am plifiers, but a clearly better setup is now in reach. That’s w hy you see different com binations of Caps and Chokes in m y later am ps. In the pow er supplies of the older designs, I first looked at the shelf w hat parts I had. A subtle m usical pow erhouse... For Bill and Bob I w ant lots of pow er, but m ore than that, I w ant m usical quality w ith a low frequency reproduction that stands. A tube is not per definition the m inor of a S olid S tate am p in the low er regions. The tube can produce a striking, beautiful precise, low frequency reproduction. Y ou do need som e OT’s w ith enough iron and a m assive pow er supply w hich can deliver the pow er dem ands A nd som etim es a feedback, if a speaker is dem anding that. Don’t be to difficult about that. If you need to de-couple som e dB’s for a better sound reproduction and w ith that a better total reproduction, there is nothing against that. W here som ething is against is: putting m any dB’s feedback in the circuit, for no reason, w ith the phrase: ‘W hen it’s not a bad thing, it w on’t harm anything also’. But that it does and so w e w on’t. Y ou can m ake your choice here if you do or don’t need a feedback. Key is, that the base, w ithout negative feedback, m ust produce a fantastic am plifier. N egative feedback is no bandage or drug, but, to say it in a trendy w ay: a usable tool.. ‘Bob’ w ill be a different chapter. It consist of tw o large m ono blocks, for w hich a pair of fat (and so expensive) C-core OT’s are specially w ounded.