Tuesday, 9 March 2010

Lowther DX3

Recently, I purchased a pair of second hand Lowther DX3 from eBay and they arrived over the weekend. They arrived in good condition and appeared to be in very good shape. After a cursory checked, I hooked them up to my Zen EL84 amp. This was the first time that I turned the Zen on in my new house. The first thing I noticed was that the mechanical hum that was present in the power transformer was gone! This was a good start.

One of my debates in my head has always been, is 1 watt really enough? These Lowther were certainly going to confirm or deny this. Just using my iPod as the source, the amp was near silent with no music. A very faint hiss was heard only when you were so close to the speakers that you'd almost hit the cones. This was also good confirmation. I still noticed that the EL84 were running very hot on the heaters, with 7V plus. This still needs to be fixed.

The first cue of music, was my usual mix of favourite tracks that I was very accustomed to. The sound wasn't great. But mind you, they were simply placed on my dinner table. I had no chance to build any baffles. The good news was that the speakers worked without a problem. As you'd expect, there was little or no bass, thanks for a complete lack of enclosure. Obviously more thorough review will need to be done once they are in a proper enclosure.

My first impression was that piano notes and vocals did sound very natural and real. However, I noticed that there was something that I could only say was a 'Lowther Shout'. Now, I am no expert, but there are so many variables that may have caused this. To note, these Lowthers were the older styled without the rolled whizzer cone.

More thoughts and experiences to come.

Tuesday, 2 March 2010

Different 45 Operating Points

Working out the operating points using the traditional ruler method and the RCA charts has been fun, but using TubeCAD's SE Amp CAD was not only much faster but produced far more information. The parameters are using a Tamura F5003 (Amorphous core US$600+ each transformer) with the plat at 202V biased at 35.8mA, results in an output of 1.3W and a distortion profile of; 2nd @ 1.7% and 3rd @ 0.1%). The input voltage swing required is 60Vp-p. If we wanted to maximise power without going into Class A2 (positive grid current), then the input swing would be 66V, and this results are 1.6W and a distortion profile of; 2nd @ 2.1% and 3rd @ 0.2%. According to SE CAD, %I max is 100%, %V max is 73% and %W max is 91%. Running the 45 reasonably hard. This will also be run with the Lundahl Amorphous core LL1620 or LL1623 output transformers. See the curves below.

Working on the assumption that we keep the 45 in Class A1 (i.e., no positive grid current), then the driver stage will need to deliver 23Vrms or 66Vp-p. If we assume that a 2Vrms signal is capable of being delivered by the preamp, the driver stage will need to have a gain of say 12x. This brings into contention a large range of tubes beyond the higher mu and high gm types (presently been considering C3g, D3a, 5842, etc.). Using a 1:2 transformer, either in the grid position (i.e., Lundahl LL1670) or as the interstage, would mean that the gain required by the driver itself is lowered to 6x. Opening up a range of DHT tubes. On the other hand, if a 2:1 interstage transformer was being used, then the driver stage will need a gain of 24x, BUT the impedance driving the 45 would lowered.

In one thought exercise, I might be able to get away with a 26 Tube into a 1:1 interstage transformer, and using the 1:2 step-up function of the Lundahl grid choke to provide a gain of around 15~16x. This would necessitate another Tentlabs Filament heater (to reduce hum) and probably far more attention to reducing microphonic and hum effects on the 26. Considering I already have starting a small stash of 26 tubes, this may be an interesting option. With various reports about the sound of the 26, this may be worth the effort.

At present, if using a 26 driver, the B+ for the 26 would be less then 200V, which means I can use a pair of 0C3/VR105 regulator tubes in series to provide a stable voltage for the 26. Perhaps a pair of 0D3/VR150 might be better if we were willingly to drop some voltage over a divider network. Consideration will be made to regulating the B+ for the 45 tube.