What follows may get a little technical depending on a persons experiance and knowledge please feel free to respone with any questions or comments whether simple tube theory or general electrical theory, and subjective comments. As we all know tubes vs. solid state is like ying vs. yang and the like! As far as myself I don’t know everything about tubes but I know enough to design and build and usually go at it with my hair on fire. I like to go after the Extrodinary.
How I got hooked! It all started at a Chicago Audio Society Meeting Dennis Had of Cary Audio was the guest speaker, His associates brought in two 211-single-ended mono blocks my eyes got huge at the sight of all 180 lbs of of them! We were told they had just 25 watts I looked over the room and thought your going to fill the room with these? The first CD cut he played was Hotel California from Hell Freezes Over CD I sat back in my chair or was it layed back? And when the amps revealed the sound-stage width my jaw fell down and the words “All My God” came out of my mouth. Then came the bass line my mouth wide open! The next thoughts were ” This has to be the most powerfull amp that I have ever heard bar none” And I have heard some very strong solid-state amps but nothing ever captivated me like these Carys.
I’ve heard these amps equiped with 845’s at audio shows but it seems the 845 just doesn’t have the same gusto that the 211 tube has so whats the differance? The gain! Off the top of my head the 845 has a Mu of 5.3 the 211 has a Mu of 11 so the 211 has to use a 7.5 kohm primary output trans and the 845 uses a 5 kohm primary the thing of importance here is the Transconductance of the tubes times the load. I believe they are very close to each another roughly 3000 Micros. the plate resistance of the 845 is 1700 ohms and the plate resistance of the 211 is 3600 by multiplying the two you get the Mu factor or the gain of a tube this is what you use when you start to design an amp it starts from the back and works to the front. So its obvious the 845 needs a big front end push! But the 211 generates more dynamic conductance twice that of the 845 this is just part of the game but its a big part the 572 amp that I built matchs the 211 in conductance or what I like to call transit response.
This is basis or my work I learned from J.C Morrison of Fi and that is dynamic conductance is equal to transconductance x load which yields sometimes huge dynamics along with many other improvements nothing is written in stone is my motto but you have to understand theory if your going to go after the Extraodinary to any degree!
Next I will explain a fascinating point on how Audio Note could get someone to pay 60000$ for an Audio Note 17 watt 211 amp. What they did dropped my jaw!
The Audio Note amp that I wanted to talk about is the Ongaku here is a link to Positive Feed Back
http://www.positive-feedback.com/pfbackissues/0701/rochlin.ongaku.7n1.html it says that its 27 watts for a now 90,000$ I was under the impression that it was 17 watts which makes more sense to me but maybe the amp has had some changes which seems is the case. I’ll have to dig out my Sound Practices issues that I have and make sure I got the proper amp but hear is the jaw dropper for me in what they did to generate their dynamics! based on my reading recollection.
I read that they are using an output transformer that has a primary of 19kohms all silver wound of course. There are a couple of advantages to this one is very low distortion two is very high dynamic-conductance. Take the 3000 Micros this is Micro-Siemens the actual numerical value is .003 x the load and you get 57 take an average 300B amp its trans conductance is 6600 or .0066 x 2500 and you get 16.5 substantially lower. On the down side of of the Audio Note is the bandwidth is tilted down so what they do is to add a little EQ to get it flat. With a transformer like this 17 watts makes sense to me because the higher the primary impedance the less power is developed and of course its harder to get good bandwidth as well.
Your thinking what about push-pull? The same multiplying occurs hear as well but with push-pull output trannys one has to remember that the actual load for the tube is 1/4 of the primary plate to plate resistance the typical load for EL-34’s is a 4.3 Kohm primary plate to plate and 1/4 of this is 1075 ohms now take its transconductance of 10,000 Micos .01 X 1000 ohm load and you have a dynamic conductance of 10!
Ever wonder why the little push-pull EL84’s have made such a stir? I’ll tell you! Using the same formulas the EL 84 using an 8K primary its load for a single tube is 2k its transconductance is .012 X 2K and you get 24
Of course when tubes are paralled the conductance is additive.
So whats so about an 833 its Mu or amplification factor is 35 its plate resistance is 2K at 290 ma. so you know what this means its transconductance is 17,500 Micros X a 5K load produces a dynamic-conductance of 87.5 and using a 7.5k primary would put it at 130
With this Dynamic-Conductance stuff it might be easier to explain my interest over it this way we all have heard of SRPP gain stages. Well heres whats 8) about them when done well take a 12AX7 it runs on a milliamp of current it typically runs with a 100K plate resistor so to get a 1 volt siginal across this resistor you’ll need a 10 microamp current change thru the tube that # looks like this .000010 of an amp now lets go to an extream put another 12AX7 on top of the plate resistor crank the voltage up so we have the same voltage and current as we had over the single 12AX7 plate and now the load for the 12AX7 plate is the Mu of the upper tube times everything under the cathode of the upper tube which comes to 10 million ohm load of course this limited by the input capacitance of the upper tube but now instead of 10 microamps needed for the 1 volt swing you only need .1 microamps to make the same change
Next I’ll talk about what it takes to drive an 833 it might surprise you in a number of ways!
I visited a number of web sites today, One that I have enjoyed and had alot of laugh’s is this one this guy has a Masters Degree in Electrical Engineering by the way can’t say he doesn’t like to have fun in this group of photos he tries to see how tuff the 833 really is note the plexiglass shield in front of the tube for safety for that just in case bang!
I found this photo of a western electric 212D laying on its side next to a 300B I’ve held one of these in my hands once! makes an 845 look tiny.
Here is a schematic of this guys 212D amp all western electric note the 310 at the front end its like having a western electric model 91 driving the 212D what simplicity!
Here is a picture of Bob Danielaks 833 amp that has a pair of 833’s on a chasiss supply is on another also these are the schematics for both.
This the most that I can find written or talked about DIY for the 833 another just as powerful Russian tube which is gaining popularity is the GM70 which is talked about in this forum group.
I have posted this link in the past the Wavac 833 1.3 review from Positive-Feedback but for conveniance I’ll put It here. A point I want to make is that one can do a DIY project and make it look good maybe not like the Wavac here but certainly better looks than say Danielaks 833 for a high WAF (Wife Acceptance Factor) a terminology borrowed from Sound Practices Magazine.
This an earlier review of the Wavac before the 1.3 version I like the pictures and note the Grand Karma loudspekers!
Driving the 833
Its no easy task and the guys at www.diyaudio.com have made it clear the driver stage will make or break the sound of an 833.
So what’s the problem? The problem is the 833’s input capacitance developed from a thing called miller effect that is the grids capacitance X its gain my 572 amp ends up with 50 pf still needing a low impedance driver. The 833 with its massive gain is a whopping 300 pf, Six times greater!
There are any number of ways to drive the 833 transformer coupling seems to be the most common using a step-down interstage transformer perhaps the easiest route. But then you’re dealing with an unknown if your doing a proto-type and have limited funds. One method maybe to use a hardy cathode follower that is choke loaded (that is under the cathode) an advantage is the 833 maybe able to be direct coupled to the cathode follower. I also like the interstage-tranny for another point in that you don’t have to use a cathode follower.
What the 6EM7 does for the 572-amp! The 6EM7 can do for the 833!
The 6EM7 is a dissimilar duo triode the first half can be thought of as a 6SL7 the second half can almost be thought of a 2A3 its plate resistance is just 750 ohms actually lower than a 2A3 it has a Mu of 5.4 and a trans conductance of 7200 Micros a plate dissipation of 10 watts. I have used the same set of 6EM7 tubes in the 572’s for at least 5 yrs now they are still at 100% they are very hardy indeed!
How it works for the 572!
There’s a 39K resistor at the input of my 572, which makes perfect sense to me because the input capacitance represents a value of close to 160k ohms knowing that. A resistor value of 39k is an acceptable input value-loading resistor. When driving an input one likes to use something with at least a 10 times lower output impedance. The actual input load for my 572 is the 39k and the 12k in parallel under the cathode of the 6EM7, which works out to be 9.2k. Not an easy load to drive! But the output impedance of the 6EM7 is 140 ohms! And the follower is running 20 ma of current to source the 572 when its grid starts conducting current.
What has to be done for the 833!
Here’s my thinking! And I have certainly done a lot on this for an 833 amp! There are lots of options some very worthwhile to look into! Some which can generate some killer dynamics in the front end of an 833 amp but I think its wise to take what is working and use it in the simplest approach for starters!
One could take the 6EM7 and run 6 of the 6EM7’s in parallel driving the 833 (at a fairly low cost) due to the 833’s 6-fold increase in input capacitance after all the 6EM7 is doing a great job with the 572. Supplying some 33 volts to the grid of the 572. The amps input sensitivity is 750 millivolts in for 10 watts out!
Where the 572 develops a gain of 6.6 and 223 volts on a 5 k primary which produces its 10 watts out the 833 develops a gain of 25 into a 5k primary at 20 watts it needs to produce 316 volts on the primary.
This is relatively easy for it, as it needs just over 12 volts to do so! Using the six 6EM7’s paralleled the amp would need only 330 millivolts for its 20 watts out!
I think a project like this should use two supplies one for the front end and one for the 833.
I would estimate a cost of building an 833 amp like this at 2000$ depending on tube rectification or solid state rectification a far cry from some 70000$ for the Wavac for a taste of 833 magic!
I also might add if just an 833 had it its filament supply and its B+ with an output transformer one could drive an 833 off a 16 ohm tap with 10 watts giving it the 12.5 volts it needs to make 20 watts out in Class A2.
I’ll do some further research on pricing, and look at alternative designs maybe the 2000$ price can come down at least the 28 lb output transformers that I use are still available 250$ each.
I found some new output transformers allowing for fewer parts and a large savings in the supply at least a 600$ savings. This amp at 650 Volts and 100 ma. bias at 0 grid Volts with the 10K primary would produce 16 watts
After Ploting my own curves for the full blown 833 I have concluded that the amplifier that I would build
Class A2 would produce at least 35 watts with a B+of 650V and 300 ma. Bias at +25 volts this is great news as it certainly will drive most speakers rated at 90 db very loudly.