BobW wrote:
I'll admit, I've been an EE since the mid 70s, but based on what you've stated, I have no idea what you are seeking, compression or more headroom at a lower bedroom volume?
Hi Bob:
Yea, kinda both. The conversation seems to be going where I had intended, which is; "what are the problems and solutions to the little amp big sound equation". Some things I understand very well other things I'm noodleing as we go. These guys understand TONE. That's what you didn't get in college and I haven't yet learned out of the tube manual.
When I began to investigate the idea my first thought was to use the Dumble pre section and plug it into a single ended 6C4. Well, that ain't going to work because of the relatively huge voltage swing of the Dumble pre. Then I thought of an attenuator ahead of the output section. Then whats the point of all that gain in the pre? As you know, the whole maryann needs to be designed as one.
That's what this discussion is about. Where it's gonna go, is anybodies guess.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Ok, how about this:
What happens if we use a pre amp tube with less voltage multiplication? Like an AT7 or AY7. If we are getting the pre amp distortion by pushing the pre tube why not push a tube that will yield less output voltage anyway?
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
When you do the math, you will find that without making drastic measures, it's really hard to vary the gain of a tube stage a huge amount without making big sacrifices in the kind of tone you get. Now, with a preamp like a Dumble preamp or whatever preamp to provide a large voltage swing to drive the grids of power tubes, of course you have to put in enough gain stages so that you drive it hard enough to either a) get enough drive to achieve the desired clean output power or b) get enough drive to achieve the desired power tube distortion. Putting in a resistor-divider to attenuate the preamp signal before it gets to the power tubes is a perfectly acceptable way to adapt a preamp to work with different power amp design.
However, much of the purpose of the gain stages may not be *just* gain. There is tone shaping going on, making use of different cathode resistors and coupling caps and that kind of thing to induce certain types of compression or low-cut, mid emphasis, whatever it is you are doing, within the confines of a gain stage. So you might even have a tube stage in there just for tone shaping (such as a cathode follower to drive a tone stack is a good example, it's not about voltage swing, just tone shaping).
I don't know a lot about Dumble amp tone and design but from hearing them on recordings, it sounds to me to be mostly about preamp distortion and midrange tone shaping, which kind of negates the need for a small-wattage amp anyway. Just build one with a push-pull pair of 6V6's and perhaps you're golden for low-volume D-tone. The small-wattage amp endeavor is all about power tube distortion, as far as I can tell. When you do that then you have to consider every link in the chain and how it's going to develop distortion, not just the preamp. So that includes the PI and the power tubes and the OT and the speaker, etc. AND the preamp if you have enough gain in there to overdrive a preamp stage.
That said, I can say that the breakup/tone kind of character of my Serena in "blues" mode (one gain stage only, vol/tone, PI, power tubes) is very similar to my 18W trem channel which is the same basic gain toplogy. The 18W has more clean range on the vol knob which is indicative of the higher drive characteristic of the EL84s but it's not night and day. It's kind of close, really. So I don't really think that using smaller power tubes really dramatically changes the way you look at gain in the preamp. To get them to behave the same would take maybe a 5:1 or 10:1 voltage divider (7-10dB atten).
I think the keys to "big" sounding small amps are like this:
1. Full-size speaker/cab ... you get a Champ cabinet with an 8" speaker and it'll sound chinsy no matter what you put into it.
2. push-pull with full voltage preamp, 250+ plates, full-on long-tail PI. With a 3-stage preamp (with TMB) or 2-stage without VT preamp, you will get a lot of distortoin in the PI so skipping this step or making SE amp is a recipe for "sounds like a Champ"
3. careful shaping of the bottom end and top end to reduce flubbiness or strident distortion tone (bandwidth limiting, tone shaping).
The first two are easy, the third is not nearly so easy because it's not obvious at all. This is mostly because everything the guitar amp contributes to the tone is defined as "distortion", and each element of the circuit contributes to this whole. Since you hear it all blended and compounded together, it might be tough to figure what the source of the distortion "elements" are, or where to fix/tune them.
However, much of the purpose of the gain stages may not be *just* gain. There is tone shaping going on
Josh is correct, it's not all about gain, it's mostly about tone shaping and increasing harmonic gain otherwise the 2 additional stages wouldn't be required to drive the PI.
