Marvel

Rick, The thermistor in Shannon's ST-35 is on the 115 volt side of the power supply tranny, so it works on the whole circuit. That was the point of my quote from Rod Elliott in Australia. Even the ps tranny will get a spike at turn on because of the magnetic field being built in the coils. Very quickly, but it is there. Plus what hits the diodes can be far beyond the rating of the diodes. This contradicts Craig's earlier post. I'm NOT knockin' his work, it is exceptional, but the current limiter is CHEAP, and helps protect more that the ps caps. Perhaps, instead of the standby switch with the resistor we were talking about earlier on the center tap of the high voltage output you could just use an IRCL there. Then you would gain the benefit of a 'soft start' on the output tubes. I'll email Shannon and ask him about the idea. Bruce

Tom, Is that the 210 without the wings on it? Cool! Bruce

As is a tunsten filament in a light bulb. The resistance is very low at turn on, and quicly goes up. If not, it would burn out very quickly. Bruce

When your monster (or not so monster) power amplifier is switched on, the initial current drawn from the mains is many times that even at full power. There are two main reasons for this, as follows ... * Transformers will draw a very heavy current at switch on, until the magnetic flux has stabilised. The effect is worst when power is applied as the AC voltage passes through zero, and is minimised if power is applied at the peak of the AC waveform. This is exactly the opposite to what you might expect * At power on, the filter capacitors are completely discharged, and act as a short circuit for a brief (but possibly destructive) period These phenomena are well known to manufacturers of very high power amps used in PA and industrial applications, but "soft start" circuits are not commonly used in consumer equipment. Anyone who has a large power amp - especially one that uses a toroidal transformer - will have noticed a momentary dimming of the lights when the amp is powered up. The current drawn is so high that other equipment is affected. This high inrush current (as it is known) is stressful on many components in your amp, especially ... * Fuses - these must be slow-blow, or nuisance fuse blowing will be common * Transformer - the massive current stresses the windings mechanically and electrically. It is not uncommon to hear a diminishing mechanical buzz as the chassis reacts to the magnetic stress * Bridge rectifier - this must handle a current way beyond the normal, because it is forced to try to charge empty filter capacitors, which look like a short circuit until a respectable voltage has been reached * Capacitors - the inrush current is many times the ripple current rating of the caps, and stresses the internal electrical connections According to this, a SS rectified amp will have more of a problem, as they are instant on and the caps used for filtering aren't charged at the moment of turn on. This contradicts an earlier statement. I'm still trying to figure this out too. I pulled this off of a site that has a design for an active ICL (using current sensing, resistors and a relay, etc.). The single IRCL chips are way simpler to integrate into the design, as they just go in series on one leg of your AC mains. Bruce

The TL Audio stuff is also pricey, but maybe not quite as high as the Manley gear. The 56 input VTC runs about 10,000 pounds (made in England). Still a lot of U.S. dollars. But they make smaller units too. I've written for prices, but haven't heard back yet. Bruce

Michael, Replacement drivers for Advents: http://www.simplyspeakers.com/12diaphragms.htm Not the cheapest, but more the real deal. Bruce

All depends on the money you want to spend. These from TL Audio are very nice, and new: http://www.tlaudio.co.uk/tlaudio/docs/products/m3.shtml The pic is the high end model, but the link is for a smaller rack mount version. Did I mention they use tubes? Oh yeah! Bruce

Jim, You can't fool me. That's a Japanese fellow at home with his horns. How about these Goldmund Apologues?

I was seriously interested in their ST70 amp. Mostly because of my real ST-70 I suppose. Erik, I know that Shawn has a current limiter in his spec for his DIY 35 amp, but it has a solid state PS. He also had comments on his forums about the use if a switch on the center tap being dangerous. Maybe because of the ss power supply. Here are a couple of ways to get around the spike they were having (from the DIY tube forum - I make no claim to these ideas): 1. I tried wiring the 1M resistor across the standby switch today. So far, so good. I do not have a scope so I wouldn't see any really fast transients, but what I do see - is that the voltage at B+ rises from 0 to 6V in about 15 seconds after power switch is turned on and stays there. Each leg of the transformer secodary measured to ground is about 340 VAC - the ground reference is there. 2. Cathode Switching The most common "new" stand-by method is called "cathode switching". This is simply a ground-interrupt switch in series with the cathode circuit of the output stage. To reduce the switch transient, and more importantly, to avoid the risk of cathode stripping, a resistance is wired across the switch contact. Any value from 4K7-2W to 10K-2W is suitable. This is equally applicable to both cathode bias and fixed bias amplifiers, with any type of output tube. O'Connor, K. 1996 Principles of Power.Power Press Publishing. p3-25. Later, Bruce

That would depend on your sound card. If it is an SB, they are notoriously fragile. A bummer anyway. I had to feed my soundcard (one of them anyway) into a line level mixer, just to buffer it to feed the inputs on my ST-70. Otherwise it sounded awful. Bruce

How much for the Ives? Am I on the right track or am I just loco? [] I think I sold all my Ampex gear.

I agree. You wouldn't know he's a Phd. would you? Although, Jeff does a good job too. [] Bruce

Picky, A decent used tube pre should work well with the NAD 2200. And a tube pre would require almost zero upkeep, where you would have to keep an eye on a tube power section. Sorta. I don't pay much attention to the ST-70, but I do check it now and then. Also, I think the HIIs listed around $1100 U.S. Bruce

I have gathered different material over the past couple of years, and wanted to pass this one on. It was written by a PhD guy, who is working in digital high tech stuff, but who also works on tube equipment designs. He has the attached pdf on his website, so I don't feel awkward about posting it. Please note that he works more on guitar amps, but this is still very useful. He had a company called Engineering Musician Design, but it is defunct as he has moved on to other work. I have some other material he published where he was stepping through the design of a guitar amp, a section at a time. His current site is here: http://www.eecs.umich.edu/~mmccorq/ This and another tube paper are listed under research. Hope some of you may find it useful. Bruce Intro_to_tubes.pdf

He should send them to me. They would go great with my Merlin I'm sure.

D-Man, For a while Sound Valves had the surplus parts for the Dynacos. Then they rolled their own. Their ST-70 was a nice looking amp, and not too highly priced. But they are now defunct. Bruce

Never get rid of amps. You are just supposed to get more! []

I can't believe how huge those output transformers are on the SA100. Those are all sweet looking pieces of gear. Bruce

Jon, I understand your concern with upping the wattage rating on the resistors. The problem is the originals are too small so they tend to break down quicker over time. Replacing them shouldn't be too hard at all. If the traces on the board are still good, you can clean them off and replace the resistors as mentioned above, leaving the leads a bit longer so they mount away from the board. If there is room and you are careful, you could mount the resistors on a heatsink, to cool them even more. Believe me, it will work wonders. Bruce

Here's the pic. I forgot to scale it down.

I also used the Curcio under chassis ps board. I did replace the power tranny with one from Ned at Triode Electronics. The attached pic shows how massive it is compared to the original. Ned's is on the left, original on the right. It still gets pretty hot, but we have TVA power here and the voltage pretty much stays at 125 VAC. Bruce

tpg, So, does your mom understand now, that there was something wrong with it? Looks nice. His designs are great. Bruce

Craig can probably (make that definitely!) give you more info on the biasing of the KT88s. The plate current rating and the bias current aren't the same. The .02 mfd cap can stay the same value. The heaters on the KT88s pull a bit more current than the EL34. If you are using the original PS tranny, you may find it warms up even more than before. I would think it would be quite a stress on the PS to use KT88s. Whose upgrade driver board did you go with? I am tempted to replace the Curcio board I put in last spring, mostly to see what difference there would be. It sounds okay. Way better than the original, but there are better designs around. Bruce

Terry, Does that mean you aren't enjoying that little rascal yet? Sounds so sweet. Bruce

Many of them are made with a silicone rubber compound, which will resist the high heat of the tubes. I had one of the 7199s in my old Dynaco that would start to whine at me every so often. I would tweak the chassis and it would stop for another hour or so. But they were going on 25+ years. Bruce