Craig Sawyers

No idea what shoulder washers for the Beta22 are like, so can offer no opinion. But if you check the thermal resistance of the 4171G you will find it is the same as for thermasil. Alumina has actually got way more thermal conductivity than thermasil, but the 4171G 2mm thick so the numbers balance out. The main thing is that if you draw things out on a piece of paper, you'll quickly see that the breakdown distance is the thickness of the washer, irrespective of the shoulder washer bush length. So although thermasil might have a breakdown potential of 1kV or more, the *actual* breakdown around the hole in the insulator is only a couple of hundred volts. The only way to make use of the native breakdown potenial is to use insulators without a hole, and use a spring clip to hold the transistor at the right pressure. The 4171G has 2mm, which is good for a couple of kV. Don't forget to use heatsink grease. The only screw technology that I know of that will take the torque needed for optimum heat transfer (1.1Nm for TO220 and 0.4Nm for TO126) is steel.

Not unless you use the right grade of stainless....

Having been down this track with the T2, what you need is an insulating bush that goes through the transistor tab, and then well into the 4171G. If it is a short bush, the transistor tab isolation is compromised. You have to draw a cross section picture to see this clearly. The ones that work well are the Aavid 7721-3PPSG. These can be a tight fit in some TO220 holes, but the bush length is 1/8" (3.18mm), which is more than enough with conductive (steel) screws and nuts. I prefer steel fixings because you can get the torque required - I tried plastic screws, but they always strip at a small fraction of the recommended TO220 package fixing torque. Future have 675 in reserve stock and 14,000 on order.

Say what? You're in Florida - or am I missing something about the Florida climate? Was in Copenhagen over the weekend visiting relatives. Minus 5C with a stiff wind straight off the sea, so was -20C with wind chill. Even the little mermaid looked frozen.

Jeez! That does not sound good, dude. Let us know how the surgery goes - sounds like plates to hold it together from what you say. Euch.

That is very helpful information. The low hfe of the 2SC4686A is certainly a concern, but I need to find a robust solution for the fact that my 2SC3675's die through second breakdown. So I either need to use a darlington configuration, or a higher current in the differential pair (by reducing the emitter resistor). Not very original, of course - but in the batteries I'm not massively concerned.

All you'll be listening to is the sound of the pot. I have no scientific rationale for why these sound just as awful as they do - tearing one apart the design principles and construction look superb. But they sound dreadful. For a more sane price than the RK50, go for a DACT - knocks the RK27 into a cocked hat.

There was a time in a land far far away that I could have understood that - but looking up Chinese Remainder Theorem made me realise that too much water had passed under the bridge to understand the first damned thing. But number theory was never one of my strong suits - I was always much too much a mathematical physicist. Back when the earth was young.

Um - nope. It means number 5 (an imperial size), 40 threads per inch, 0.25" thread length hexagonal cap head. Alas for those of us in metric countries, you need to source the right screws - the ones that go into tapped holes in the case have to absolutely right. Search the thread for European sources.

Is it me having a senior moment - or has there been a sort of mass change in people's avatars?

A couple of days ago (Sunday) went to see Meat Loaf at Birminham UK. Voice was a bit creaky, but that was probably a few decades of bodily misuse. Otherwise awesome.

That is not what I said - 60Hz transformers will always work with 50Hz. The problem arises because the primary inductance is fixed, so the primary current has a 1/f dependence. If you are trying to design a transformer that effectively utilises the core at 60Hz, you push the core with 20% more flux when using it at 50Hz. So a 50Hz transformer needs either more primary turns (to keep the NI product the same) or 20% more core. Which means that a 60Hz optimised transformer (doesn't matter whether EI or toroid) invariably hums when the primaries are connected for use on a 50Hz supply. My supplier for the T2 transformers, Paul Houlden, can and will specifically redesign and optimise the toroid for 50Hz (if he knows the spec). He also uses oversize cores (within space constraints) to reduce mechanical noise to imperceptible levels.

Future Electronics have 60 in stock. No problems with where they are supplied to. Craig

The frequency is also a problem. In the US 60Hz but in Europe 50 Hz. The lower frequency needs a different transformer optimisation. The only way to ensure trouble-free operation at 50Hz is if the transformer is specified to be dual frequency. 60Hz transformers operated at 50Hz invariably hum because the core is driven higher up the induction curve of the core.

I'm going to tread dangerously into the minefield of resistor sound, particularly relating to magnetic end caps. Cyril Bateman, a guy who donkey's years ago used to be a designer for a major capacitor company, produced a major series of articles in Electronics World on capacitor sound. He measured capacitor harmonic distortion by constructing a sub 1ppm distortion oscillator (<0.0001%) and associated notch filter followed by spectrum analyser. Right at the end of the articles, he did some measurements on resistors and pots. Apart from some very old (pre-laser) mechanically ground spiral cut carbon film resistors (0.00013%), he found that the harmonic distortion of modern, laser cut 1% metal film resistors was way below the noise floor of his measurement gear - ie <<0.00007% distortion. Regardless of magnetic or non-magnetic end caps. His only proviso is that a resistor is loaded at less than 25% of its rated power, and preferably significantly less (to reduce temperature rise above ambient). On pots, the ones producing significant distortion were multi-turn cermet, at 0.00138% distortion. Alas, there is no commonly available option for these. The way to minimise distortion is to make sure that the minimum signal voltage occurs across such a trimmer, with the majority of the value made up from fixed resistors. So if there is a percieved difference in sound quality between one resistor and another, it simply cannot be down to harmonic distortion.

I'd second that.

Good - that means that I can use up the excess of large footprint ones I bought in error for the T2

A really wierd Family Guy in which Quagmire verbally lays into Brian. Really uncomfortable watching from start to finish.

Around a long time ago, I flew into Philli domestic, which was like a regular shopping mall - really nice. I then transferred to International, which in those days was the most depressing building on God's green earth. Steps leading up to it was populated by hookers and dealers. In the hangar-like building there was something called the Oasis Bar. This was a plastic palm tree screwed to the wall, and two nailed down tables with fixed chairs. Beside it was a booze vending machine. I swear. Then they cancelled my flight - so I had to get back to domestic and catch a flight to Boston, and thence back to the UK. My first and last experience of Philli International.

That is truly awful. Like you I'd like to kick nine bells out of people like that. A dose of their own medicine meted out by a rugby front row would make them think twice about pulling a similar stunt. When my son was at University, he went through a goth phase - which meant that he was a target when out for a few beers. On the way back one night three early 20's guys (who should have known better) stood in his path and blocked it. After Rob - at 6'6" and a rock climber - advised them he'd like to move peacably home, and getting the expected "what are you going to do about is fucker", he said "OK, I'm going to count to three, and then your friends will have to take you to hospital" - One, two (missed out three) and then nutted the guy. Went down like a pack of cards with broken nose. "Anyone next?" says my son as he steps over their fallen comrade. No takers...they headed off at some speed leaving their friend where he lay. But three brave lads taking on a woman - they deserve no such considerate treatment.

Ah - those are interesting questions. Because there is a little slop in the holes in the heatsinks, I loosely attached the heatsink bracket to the circuit board with a couple of transistors with washers etc. I then assembled the case with the brackets/pcb in place, and temporarily screwed the brackets to the heatsinks and snugged up the screws. I then tightened the transistors down to lock the bracket to the PCB in the correct place. Then off with one end panel and slide the PCB and attached brackets out. Now install all the transistors, tighten down and solder. You can then take the heatsinks off the case, smear them with heatsink grease, and bolt the brackets on and tighten down. You should be able to assemble the case without difficulty after this. And yes - it was a bit of a tricky process - but keep at it with two temporarily attached transistors per heatsink, until you are happy that there is no significant gaps between the brackets and heatsinks. On the umbilicals, I twisted the heater wires and put them in a separate sleeve (I just happened to have some high temperature glass braid of the right diameter). Twisted so that the AC current would not couple to sensitive things like the bias, and in a sleeve because they are connected to -500V; possibly overkill, but I had the sleeving anyway, so what the hey.

The 10m90s is a three terminal current regulator, used here as a protective current limiting device in series with each power supply rail. The parts you are suggesting as replacements are rectifier diodes. There is no substitute to the 10m90s.

Like I said - Digikey is showing zero stock.

That thought was going around in my mind, as a discrete option (but would be messy to implement). I had no idea that such high voltage single devices were available. The gotcha is that I couldn't find stock anywhere. Digikey list them at around $6 each, but have zero stock. Mouser also list them, zero stock with a 20 week lead time with MOQ of 605 pieces.