Craig Sawyers

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.

I've really been pasting this list thick and fast recently. But for anyone interested, here are a couple of spice models. The first is for the 2SA1413, which is a different package for the 2SA1486, and is a Sanyo model. The second is for the 2SC3675, generated by a downloaded SPICE parameter calculation spreadsheet downloaded from excel , with input data measured by me. That is essentially a dc model only, with no description of electrode capacitances, and nothing to do with inverse active mode. .MODEL 2SA1413 PNP (IS=153.22E-15 + BF=73.199 + VAF=104.34 + IKF=1.3862 + ISE=153.22E-15 + NE=1.3075 + BR=.357 + VAR=100 + IKR=3.7415 + ISC=63.966E-12 + NC=1.7723 + NK=.92315 + RB=.73481 + CJE=1.3316E-9 + VJE=.77186 + MJE=.38206 + CJC=140.65E-12 + VJC=1.1825 + MJC=.54838 + TF=5.0775E-9 + XTF=2.5439 + VTF=16.727 + ITF=193.71 + TR=30.928E-9 + NR=1 + NF=1 + TRC1=0 + TRB1=0 + XTB=1.5) .model 2SC3675 NPN (VAF=6000 IS=1.00290528476071E-14 +NF=.934004260398326 BF=150 NE=1.11055219144042 +ISE=9.61916074296322E-15 IKF=.1 +TF=2.6525846254073E-08 )

I don't know what others are seeing when they mention double posts. I'm only seeing one of each message (using IE8 ).

Latest hold up - LED's. D6,7,10,11 popped when I was in the crap C3675 phase. Replaced them with stock 5mm red LED's. But before switching on I thought I'd check the voltage. Ones specified by KG measure 1.8V at 0.5mA - which then gives the (calculated) currents on the schematic exactly. The ones I replaced them with measure 1.6V at 0.5mA. Does not seems much, but it shifts the operating point of the EL34's by 20%. So I am procuring more LED's, which puts a few days delay into the turn on. Ought to have bought a shed load more in the first place, since they are cheap. This sucker is going to be 100% according to design before it sees volts again.

3am, too much alcoholic anaesthetic. G'night and Happy New Year!

Um - pardon? I don't see double posts - you didn't do a New Year's Eve sampling while you sorted out the liquor cabinet

That is a real bummer to see 2010 out. Sorry to hear; I must admit that on the rare occasion that I've done something similar I always leave an apologetic note at least, and/or my phone number. But here in the UK, 20% of drivers have no insurance - yes, around 10 million drivers (plus) have no insurance. So there is little self-interest in such scumbags owning up. Hugely illegal of course. Police now have number plate recognition cameras in an attempt to root this out.

Right - several hours of T2 rebuild with measured semiconductors, and the beast is ready to roll again. However, I am now going to drink beer and cook. And we're out tomorrow visiting friends. So it will be Sunday when I do final checks, plug in the mains and hopefully not do this to myself.

I forgot to say - not connecting the amp ground to chassis safety ground (either hard grounding as Spritzer does, or ground lifting via a resistor as I prefer) is seriously bad in an electrostatic amp that can kill. Always connect the chassis to the green/yellow safety incoming mains in some way that cannot possibly come adrift; star washers to bare metal are manditory, as is removing paint or anodising where the chassis component bolt together to ensure the chassis is bonded to safety ground. Most commercial chassis (including all the Hammond ones I've used) omit this point entirely and just anodise or powder coat each plate so they are nicely insulated from one another, leaving it up to the user to take necessary measures to prevent death.

The problem is that Mosfet self oscillation is bewlideringly prone to both precise layout almost at the millimeter level, exactly where the bypassing is in relation to the transistor, and the gm of the transistors. There is a rather wide tolerance in gm, so perhaps an unlucky batch with high gm might be a trigger. There is an excellent Renesas apps note (simply called "Power Mos Fet Application Note" - 57 pages) which covers more Mosfet death mechanisms than I ever knew existed, parasitic self oscillation being only one. Maybe there could be some interaction with the input tubes which might explain the noisyness with some tubes that both you and Inu have mentioned. I don't have any stock of beads, so I'll go with the resistor and keep the value the same as for the J79.

Hokay - I've decided to go with 750 ohms. I have some MRS25 in 750 ohms. These are truly tiny 600mW resistors, and will fit vertically between the hole in the board and the gate of the K216's with no difficulty. Measured one on my ancient (but super accurate) HP R-X bridge, and even at 150MHz it still meausures 710 ohms in parallel with 0.3pF - so pretty much an ideal resistor.

Here's a question for Inu and Kevin. Is there any indication of the power Mosfets oscillating? Or bursts of oscillation at certain points in real signal? Reason I mention it, and I should have recalled this from power amp designs, they have a horrid tendency to burst into high frequency self oscillation. There was one generation of Trace Elliott bass guitar amp in which the power mosfets physically exploded through this effect (although at 1kW output there was LOTS of energy driving them). I spotted that on the K216's on my curve tracer, but the patch leads were around 2 inches long to the test fixture and certainly had an effect. In the T2 design, there is a 750 ohm resistor in series with the J79 gates (and KG has it tight up to the gate, which is exactly right), which is a classic way to stop this (the old grid stopper idea), and kind of suggests that it was introduced to stop self oscillation of the J79 in the orignal circuit layout. But there is no such protection on the K216's. So the question is - do we need to do anything? Either a resistor in series with the K216's gate or a ferrite bead on the gate lead?

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).