My Design Philosophy
Wow, philosophy... as if I know what that is :) This is just a title that looked interesting and allows me to summarize my thoughts on amplifier design on a single page. During my projects I have come to follow certain ideas, as they have proven themselves valuable in my experience. Not everyone has to agree, to each his own...
The tubes I use:
Many people will notice I don't use anything but triodes, no pentodes or tetrodes can be found in any of my schematics. I have tried several types in the beginning but always found that a good triode outperforms multi-grid tubes. I've never looked back since. The triodes that I use are also almost exclusively directly heated, only where the remedy is a greater evil than the illness do I use indirectly heated tubes. Again, experience has taught me that DHTs offer a much more beautiful and natural sound, more direct, than IHTs. I am not saying any DHT sounds better than any IHT, some DHTs are down-right crappy, some IHTs sound absolutely wonderful !
Most people agree that a single-ended DHT output tube is THE way to go. DHT just rules! Kicks ass... but then they plug in an ECC83 to drive the whole thing :( I've seen hundreds of schematics using the most bizarre and hard to find DHT output tubes, but almost 90% use horrible tubes like the ECC83 and ECC82 to drive those babies. Why not go for a DHT driver, and even more crazy: a DHT gain stage. There are numerous (relatively) cheap DHTs out there that can do any job that a ECC83 or 82 can do. If you think a DHT output tube opens up the sound, just wait until you hear it with a good DHT driver stage!!!
As to the low Rp / low mu idea, It is a very plausible one and I have found that I agree in general. A low Rp tube usually means low mu, so the terms describe the same tube. A low Rp tube tends to sound better, but this (I think) has more to do with the OPT. As the lower Rp means the OPT will have a lower ratio between primary and secondary there will be fewer windings and better coupling. This means less capacitive coupling between windings and lower losses. Taking this idea at it's theoretical value, it would imply that a 6C33 with 80-100 ohms would sound better than a 45 with 1800-1900 ohms, or dare I say a 10Y with 5K impedance. But I do not find this to be true at all, I actually love the 10 and RS241 tubes which both have impedances of over 3500 ohms. Making a good OPT for these tubes (10-15K primary) is indeed more difficult, but I think it is well worth the effort.
The loads I use:
A really nice topic as many people have their own distinct opinion about this. I'm seeing a lot of circuits like C4S, mu-stage, SRPP etc for loading tubes. RC-coupling seems to be out of style as it is not as fancy as any of the other circuits. People seem to like adding a handful of extra components such as solid state devices or even a pentode. All these circuits try to give a high impedance load while offering only a small DC resistance factor. All very nice, but why not just use an inductor. Not many people use choke loading or IT-loading these days. The reasons I hear are that this is an expensive and heavy solution. People don't seem to mind spending big bucks for NOS tubes or components, but that plate choke is just out of the question. I feel many people tend to go head over heels for musical quality but get sidetracked along the way. We wanted to avoid solid state and pentodes, yet they are being used as a compromise to get rid of some of the obstacles that are encountered (not enough gain out of the tube etc).
I've used SRPP and current sources but keep falling back on either RC or LC loading of the tubes. Although I prefer inductive loading, it can't always be implemented properly. A tube with a high plate impedance requires a very large induction value (several hundreds of Henries). Basically tubes with an Rp of over 8-10K are best loaded with a resistor as the choke will quickly exceed 300H to get any kind of bass out of it. The high inductance means huge numbers of turns. This is turn means high DC resistance or a huge core or capacitive losses. There just is a limit to how high you can go with inductive loading. When you DO have tubes with Rp's of less than 8K, an inductor (choke, IT or OPT) is just the best thing that can happen. Low Rdc, huge AC impedance, maximum gain, openness, smooth, sweet, lovely, round, airy as can be. Yes they are big, yes they cost more than a few resistors, but this is what tubes are all about.
The fun I have:
Designing amps can be very frustrating as I am overwhelmed by the different types of tubes that I have access to and the limited amount of time and patience that I have :) DIY should be fun, but I find that sometimes I drive myself nuts over my next design. I've never built anything that was the same as the design I first came up with. Sometimes I even think of quitting this hobby all together and just buying something off the shelf.
But then what is this hobby all about!?!?!?! FUN, right?! Sometimes you just have to let go of all the anal-retentive tight-ass audiophile DIY'er macho bullshit and just have some plain old fun!!! Get whacked out of your skull and make something that just looks cool and does nothing at all except dissipate 500 watts and make your hair stand up. No silver wire and other voodoo to get those electrons to swirl to the left instead of to the right. Just make the tube light up blue or white and use that really funky transmitting tube; just because it looks sooooooooo cool !!!