Craig Sawyers

Well first off - don't believe for one minute that my build of the T2 was without problems! But the shoulder washers wasn't one of them. The only thing to watch is that these are one thou larger in bush diameter than a regular TO220 bush. So they go into some transistors real easy, and in others they are pretty tight (technically an interference fit) - but they do go in with some persuasion - depends on where in the tolerance band the bushes and transistor holes lie.

It is the Aavid 7721-3PPSG. Bush length is 3.18mm (1/8"). Wall thickness is 0.36mm (14 thou) and the material is 40% glass filled polyphenylene suphide, which has a dielectric strength of 385V/thou - so 14 thou = 5.4kV. I've used these exclusively on the T2, and we're talking 50 devices mounted to grounded heatsinks this way, without any difficulty.

I've never found a plastic screw (and I tried a few different types on my T2 build) that will take anything like the recommended torque for a TO220 (around 1.1Nm, 8-10 inlb). The best reference for semiconductor mounting I've come across is Onsemi's AN1040D http://www.onsemi.com/pub/Collateral/AN1040-D.PDF . I ended up with a long Aavid bush that went most of the way through the 2mm alumina insulator, and then used steel fastenings.

Whatever you do, ask them about masking the areas where the fixing bolts go so all the chassis components electrically connect together. You really need that for safety, and at the very least not to feel that tell-tale tingling feeling when you touch a floating piece of chassis. You *can* remove the coating where the screws are afterwards (Dremel), but any coating worth a damn is tough, and this is a pain in the ass given how many fixing bolts there are. They may ask you to do this, in which case you need to find out what sort of tape they recommend to do the masking. I discovered this gotcha decades ago after I bought a second had copper vapour laser (at work). All the laser head chassis components were thick powder coated, and not electrically connected. There was so much high power interference getting out the chassis seams that I got an rf burn. I was not impressed.

Yup. Solid rules those Yeah - I had a couple. Which weren't really enough for setting up a T2, so I went on an eBay raid. So now got 4 8060A's, an 8024B and an 87V. Put 1mm jacks on the test points on the tube side of the board and made up some 4mm to 1mm test leads. So I can set the batteries up by just taking the top plate off and plugging straight in. Put a ground wire on the board so another pair of meters go from ground to the stax jack. Switching those between DC and AC allows the noise voltage to be measured durning final battery tweaking.

Interesting comment, Inu. Could you expand on that please?

Here's something interesting. The BH seems to take quite a while after turning on to come on song - around an hour or maybe more. But the T2 wakes up very fast - a minute or two after the HT relay clicks in and it struts its stuff. Have any of the others with completed beasts noticed this?

That is a classic. If you haven't done that at least once, you haven't lived! Depending on the Fluke, you are likely only to have fried the input protection. There are a couple standard fuses, a couple of fusible resistors (which you have to get from Fluke), four thermistors (cheap standard parts) and some transistors wired as diodes. Some or all may have gone to heaven. You can tell from the way I rattled that off without looking at a schematic that I've been where you are now. The lesson I've learnt the hard way on several occasions is don't test stuff when tired.

Result! Happened twice to me, once with a pen and once with an HP calculator. Bought replacements - at which point the original escaped from the bowels of the cabin. I'm sure there are black holes whose purpose is to screw with your mind. It is where all my odd socks and allen keys go.

I haven't read any Lovecraft for a very long time. That is my wake up call to read or re-read a few of his very disturbing books.

Yes - A bit like Hichhiker's Guide "Every time I press one of these black controls, labelled in black on a black background, a little black light lights up black to let me know I've done it. " The only relief is the glow of the tubes and the awesome lignum vitae knob

Just for the record, there were three mods to the power supply and amp: (1) The power supply mod developed by Kevin and Inu to make it stable (2) Adding precautionary 750 ohm resistors in series with the gates of all K216's (3) Adding 5pF 1kV caps (I used ceramic, Inu used mica) across the 100k feedback resistors. That was also an Inu mod. (1) and (3) are essential IMHO. (2) is discretionary - these resistors were not present in the original from examination of the photos. It has already been said - this sucker runs real hot. Once it has been on for a few hours the heatsinks, front panel and top plate are toasty.

Wife came home to the new walkman on the kitched table [ATTACH=CONFIG]4210[/ATTACH]

Omygod. I'm speechless and breathless - I've really heard nothing to compare with this. I could witter on about speed and clarity, and the tendency to listen to things far too loud because there is zero listening fatigue. Looks guys - I have a BH, and bloody good it is too. But KG's recreation of the T2 is simply in another league. OK - the right channel has noticeably hiss, but that is because I need to frig with the battery voltage a bit on that channel. What I need to do is stop listening for long enough to pull the lid off again. Mind you - it served up a bit of a fright. I'd had the thing on/off/on/off etc as I was messing around trying various cures for the 650kHz problem with no mishaps. Put all the covers on, heaters came up, but when the HT came on the fuse blew. WTF??? Tore covers back off to find the gremlin. There wasn't one - the 2AT fuse I had in the IEC was clearly not man for the inrush job (6.3A in there now) - but it waited until I'd put the gadzillions of screws in before it gave up the ghost.

At the moment a used Edicron quad that I had lying around that still measures pretty good. I've a good set of original Mullards that will probably end up in there, or I might get another set of Winged C (I've a set of those in the BH, and they sound very good indeed) and keep the Mullards for Sunday Best. I've got a set of Pearl tube coolers too - evening out variation in the envelope temperature and reducing it double tube life - something that was researched by Mullard etc back in the 50's. 6DJ8's are Mullard's pilfered from a wrecked Tektronix scope that measure right on the button.

WHOOOO! Squared! Second channel now operating. Adjusting those pots is pretty trivial having worked out what they actually do - and having halfway decent silicon in there. I'm going to award myself a run, some lunch, and then put the front panel on and have a listen

WHOOOOOO! First channel operating!!! RV1 and RV2 do exactly what the simulation and spreadsheet says, as is my suggested way of adjustment. Outputs sitting at -4.5V and batteries at 743V. I can probably get the output offset lower than that, but I'll now concentrate on the other channel for a while.

Way to go! That is another corner turned

Wierd factoid - I've missed every episode of this. But the general opinion (including critical acclaim in the UK) is so high that I have just got to get a set of DVD's of this and catch up.

I've done both. Both are active - all the battery does is apply a 740V DC level shift; from an ac point of view the battery is short circuited via C3.

Ah - that is interesting, Inu. I've added 750 ohms to mine as a precaution; we were clearly thinking along the same lines. Incidentally, I put a spreadsheet of battery voltage against RV1 and RV2 position from a spice simulation here http://www.tech-enterprise.com/tekstuff/Batteryvoltage.xlsx

"Snow" and "Mojave" are not words I would associate in the same sentence. Very wierd.

Equipped with my shiney new Spice models, I've had another go at the battery, this time with the bottom current source around Q30 modelled too. I've been investigating what RV1 and RV2 do in preparation for turning the thing back on in the coming days. RV1 does nothing much over quite a lot of its range. The model I have of the 2SK246 seems to have an Idss of 10mA, so turning RV1 *anticlockwise* initially has the battery completely off and at maximum voltage (790V or so) for the first 6.5k of its travel. The battery voltage then falls at 34V per turn of the pot. In the model, 740V is reached at 6.9k. This would appear to be the "coarse" control. RV2 is essentially linear at 3.7V per turn (ie around ten times more sensitive than RV1), and would appear to be the "fine" control. It operates over a total range of 90V. Clockwise reduces the voltage. So the strategy seems to be to set RV1 and RV2 to mid travel. Adjust RV1 to get withing striking distance of 740V (+/- 25V or so) and then fine trim with RV2. At least that is what the modelling says. I'll try it out in the coming days and report back.

Oh boy. I know precisely how you feel, and how difficult it is to break into a modern car. Worst for me was around ten years ago, in January, in a very remote car park. I'd been out for a trail run, and got back drenched with sweat, opened car and tossed keys on seat. Closed car door to go round and get dry kit out of the boot (trunk) and the central locking randomly fired, locking my keys in and leaving me getting progressively frozen standing outside. I'll not bore you with the tale of woe that ensued.