Craig Sawyers

If you are running balanced inputs, just ground the amp to the chassis. If you are running single ended, there is a risk of hum loops. So what you do is hard ground the mains green/yellow to chassis - which you have to do in any case for safety. Then you ground the amp to the chassis via a 3W resistor in the range 1k to 5k (value not too critical), preferably non-inductive metal oxide, and keep the leads short. Professional audio gear, where roadies have to wire truck loads of gear together without hum, tends to have a three position switch - grounded, ungrounded (ie floating), or ground lift (the resistor).

Yeah - happens here across the pond too. In our case it is Parcel Force (or Parcel Farce as they are known). You're lucky if you get a note left. On one occasion, having bought from a seller on the West Coast off eBay, it was never delivered and no notes were left. 6 months later (I kid you not) I got a mail from the guy with a photo enclosed - they had sent a 50lb weight electronic instrument all the way back, by surface. I was apoplectic with rage - so much so that Farce refunded the re-shipment cost. Farce have got a monopoly with imported goods. And they behave in precisely the way that monopolys do, with apathy and zero customer service ethic. The most irritating thing is that they not only charge the tax on the goods on behalf of the government - they also charge an

Well they have allegedly sent me 34 replacements for the crap ones last Monday. They still have not got through the Christmas post backlog yet. Guilty as charged

How about from Mikado: To sit in solemn silence in a dull, dark dock, In a pestilential prison with a life-long lock Awaiting the sensation of a short, sharp shock From a cheap and chippy chopper on a big, black block. Seems kind of relevant to your hospital experience .... Or from Iolanthe: When you’re lying awake With a dismal headache, And repose is taboo’d by anxiety, I conceive you may use Any language you choose To indulge in, without impropriety;

When my mother broke her hip (it turned out to be a terminal event, alas) and we went with her to the hospital, they gave her a morphine shot. Then another. Then another, etc. By the time the pain was under control she was totally spaced out. Nuclear war could have broken out and she would not have noticed, or if she did she wouldn't have given a hoot.

Interesting. So the problem with the iSC ones could be poor packaging stressing the silicon. Mine not only have poor gain, but the collector base breakdown voltage with base open VBR(CEO) is 700V. The good ones are measured 1100V (spec is >900V).

Some C3675 measurements on a batch of 41 received from Vintage Parts. I got these through the post yesterday, and have measured the current gain at 20V, 5mA and 750V, 5mA. Minimum: 53/65 @ 20V/750V Maximum: 74/96 @ 20V/750V Majority: from 60/73 to 67/80 @ 20V/750V So why not just pick the super-high gain ones? Well, the gain enhancement at 750V has two components. First, the collector characteristics have a gradual slope, measured at 10^-6 (ie an output resistance of a megohm). That would enhance the collector current for a given fixed base current from 5mA to 5.75mA going from 20V to 750V - ie 15%. But the gain ratios measure around 22%. That is because the curves have an additional component at high voltage as the transistor approaches breakdown at >900V - a slight upward curve starting around 500V. Actual VBR(CEO) breakdown is over 900V (measured at 1100V with base open circuit, against spec of >900V) However, the very high gain transistors have a gain enhancement of 30%, which implies that their breakdown voltage is somewhat lower than their lower gain brethren. So for the batteries, where the C3675 is stressed the most, I'll be picking eight transistors with a high voltage current gain of around 75. That will put the base current at around 67uA at Ic=5mA, 740V, well within the capability of the battery drive circuit. That is the logic, at least.... If the replacement Dalbani transistors arrive today, I'll measure all those too, just to make sure they are pukka.

Wow. If you look at the pics of the C3675 I posted a few days ago you will see the same differences in the lead frame - the bad ones all have a short supporting section near the transistor body. The good news is that I've just received 41 off C3675 that are all measuring right on the money.

That sounds like a trapped nerve from unusual activity, or some similar muscular/nerve spasm. I'd try sports physio or chiropractic to get the shoulders unlocked.

That sounds lousy. Kick-ass antibiotics to kill off the lung rot? Reks - you sir are going through the mill big time. Hope it all works out. In no particular order - I learned recently from the latest John Irving novel (Last Night in Twisted River) how to deal with chopping boards. Kettle of boiling water run over board in sink. Scrape with a suitable implement (be very scared at the ooze that comes off). Continue with boiling water and scraper until clean, then re-oil. And finally just got back from an excellent stage play with Samatha Bond in an Oscar Wilde play called The Perfect Husband at the Vaudeville Theatre in the Strand, London. Matinee, finished at 5:30pm with the evening performance scheduled for 7:30pm. Enjoying a good bottle of Chateau Musar with Mrs S, and multiplexing with checking out a new batch of semiconductors for repair of my T2 clone.

Yes - all of the duff C3675's were from Dalbani. A couple of years ago I bought a batch from them that are perfect - and luckily the two in my BH, the four in the T2 PSU, and all the board mounted ones on the T2 amp are from that batch. But the additional 34 that I ordered from them in the Summer were Chinese iSC brand junk - and all but four of the heatsink mounted ones were from that batch. Dalbani are replacing them without question with Sanyo ones, but I've chased my tail for quite a while tracking down the root cause of my woes, and had to buy quite a few more K216's and 100V zeners in the process. All the other silicon from Dalbani has been just fine. Well, not quite - some spare (Sanyo) J79's had the leads hanging on by a thread where they enter the body of the device. But again these were replaced with warp speed. If you haven't soldered yours in yet, shove them in the post and I'll verify them on the tracer for you (Sweden to UK is not too far).

I was brought up in a house in the North East of England in the late 50's to early 60's with just a single fire in one room. In bad winters my bedroom got so cold that a layer of ice would form on the glass of water on my bedside table. No kidding. The bathroom was like an ice box - you certainly did not linger on the john (or the netty in my local dielect).

<grin>. The Tek 575 transistor curve tracer has precisely two (germanium) power transistors in the step amplifier. Every other active device is a tube. Not totally suitable for the transistors we're concerned with - maximum base step is 0.2V (so not high enough for the FETs) and the basic instrument will only do 200V sweep. The mod-122C version goes to 400V sweep, which is still not enough for the high voltage bipolars in the T2. The only two golden oldie Tek tracers that will do the job on T2 semis this are the 577 and (if you have deep pockets - which you need for tackling the T2 in the first place) the 576. I'm using a 577 for my measurements.

Jeez - is there no end to the junk semiconductor hell we seem to be going through on this build? Even the brand name seems to be counterfeited judging by Justin's Sanyo-stamped C3675's that have a beta of 4 (spec = 30 minimum). I've just decided to pull the four J79's too and verify them on the curve tracer. Tomorrow. In 35+ years of electronics design and build I've only ever had one out of spec device until now - and that was a 74S-something in which the transition times were out of spec and was generating a few nanoseconds duration glitch. In 1982.

With a bit of luck my batches of C3675's will arrive tomorrow. To prepare for that, I've pulled all the suspect 3675's and checked them on the curve tracer. Two from the batteries were shorted. All the others on the heatsink survived, but had beta between 6 and 15 and had premature breakdown at 550V to 650V. All but one of the eight K216's were also dead. So the heatsinks now only have four high quality C3675's, the A1486's and the J79's. Still need to pull the dead 100V zeners and dead LED's. I hope that has extricated all the demons. All the board mounted 3675's are high quality ones.

Yeah - that is really excellent service! The key about the expansion issue is that wood is not isotropic. Along the grain there is hardly any change. But perpendicular to the grain you can expect a movement of 3% or more depending on changes in humidity. Think of wood as a bundle of drinking straws - there can be very little movement along the bundle, but rather a lot across the bundle as you squeeze. The legs of this stand are along the grain direction, so doesn't change much. But the bit of the shelf that fits in the notches is across the grain, and will swell and contract as humidity changes. Not instantly, you understand - but it will certainly follow seasonal changes. The only way to equalise out the anisotropy is to make plywood, where alternating thin layers of wood are arranged with alternate layer's grain directions perpendicular. That is the basic approach needed when veneering (at least in modern times) - a core material has cross-banding layers glued to it. The veneer goes on top of the cross-banding.

Glad you're getting a good resolution in the end, but I suspect that it is a design problem at the root. The verticals are long grain, which hardly moves at all with changes in humidity. The shelves are short grain (vertically), which changes dimesion quite a lot with change in humidity. Having a quick look, assuming a generic hard wood and that the shelves are 2" thick, and that the moisture content goes from 8% to 5% (typical between summer and winter, air conditioned or not etc) gives a dimension change of 16 thou (0.4mm). So a snug fit as manufactured at 5% can lead to an impossible interference fit at 8%. These effects are real. Large woodworking benches have to constructed to try to prevent the wood from splitting between winter and summer (for example the end caps are on sliding splines and fixed by a single bolt - ie not glued. End grain/long grain problem). I finished building mine in the winter, and by high summer some of the joints had moved by way more than a millimeter. Now that it is winter, they have all closed up again. And this timber is 4" thick, quarter sawn beech. Same thing with fitting drawers - you leave around 1.5mm clearance at the top if you build them in the summer, so that when the wood expands in the winter it does't jam. If you build in the winter you can cut the clearance down. If the drawer really jams as humidity comes up, there is absolutely nothing you can do until humidity drops again, it will be stuck fast. So if Arnold builds yours up at high humidity, the shelves will be slightly loose in the summer. If he builds at low humidity, there will be no way on god's green earth that you will be able to dissassemble them when the humidity comes up - they'll be jammed hard

Top Gear (second part of the Christmas pair). Fairly average I thought. Then the Dr Who Christmas episode. Fairly average I thought, although Michael Gambon is always a pleasure to watch. Earlier, Inception on pay-per-view. Excellent.

I know it is FET's rather than tubes, but Erno Borbely used this scheme in many of his products, detailed in "JFETS: THE NEW FRONTIERS,PART 2", Audio Electronics 6/99, 16-20. He did not use cc loads on the top drains, but has a cc load on the long tailed sources. I downloaded it originally from Erno's site, but he has now shut down the business and taken his site down.

Sounds like a great day (apart from your sister-in-law's troubles!). Sure beats the hell out of frozen up burst heating pipes, which happened just before Christmas. Bonehead house builder had run unlagged and uninsulated pipes above our entrance porch - so thermally essentially outside. We've been way sub-zero for the last two weeks. When they thawed a little one day when the temperature crept above freezing, water came out of the split pipes under full heating pump pressure - it was a truly impressive sight. Got everything isolated and turned off while phoning the plumber, who got there half an hour later. By then I had pulled the closing panel off, saw the exposed pipes, and was having fantasies about finding the builder and laying about him with a length of two-by-four. The good news is that the burst was *outside* the house, so there was no damage to the house at all. Now properly pipe lagged and joists insulated above and below with high grade lagging. It ain't gonna freeze again.

The alternative, used on old Krell power amps (and many high power pro amps) is to have a power resistor in series with the transformer which is shorted out by a relay after a second or two. Various delay circuits are used, either discrete component or 555 based. It has the advantage, as compared with a thermistor, that the impedance in series with the transformer winding is kept to a minimum.

Interesting - I was planning on doing precisely the same, having got a ton of Mullard 6DJ8's from an old and sad 535A. They all measure right on the money, so it will be interesting to see if I get a similar noise problem. Perhaps I'm missing something, but it seems a differential pair cascode with constant current loads

Kevin - any insight from Stax on this? Is there any indication that they selected 6DJ8's, or that the screening can on the original T2 played a critical role? Now 22:40 in the UK, got a belly full of excellent food and booze - so I'm retiring to a place of horizontal repose.

Fantastic news! Is that number 3? KG, Inu and now Blubliss? I'd definitely be #4 if the XXXXing C3675's had arrived before a 4-day holiday. But they didn't, so I'll have to slum it with a BH for a while longer

I just love the endless variety on these lists. There is whole musical subset that operate at a level I could only aspire to. I did look at that exercise linked above, and it brought back the grind of learning the clarinet. Mind you, having mastered a bunch of exercises, it never failed to amaze me that a horrible sequence of notes in a real piece would magically just fall under the fingers, with one part of the brain thinking "I wonder where that came from?". Of course it was buried somewhere in an exercise and had got hard wired into the wetware.