Introduction

Build and update history

Initial build - February 2005

Tubes changed - June 2005

Silver wiring - August 2005

Django added - September 2005

Tube sockets - March 2006

Schottky diodes - June 2006

I used the Cree 5A 1200V Schottky diodes (CSD-05120A from Parts ConneXion) to replace the original ones. Certainly an improvement as the sound became more focused and clearer, with voices and individual instruments easier to follow. Percussion had more impact too. Note that on the Crees the metal plate on the cover is live so it is necessary to heatshrink them.

Power tube resistors - June 2006

The Chinese RX21 10R and 220R 4W resistors were replaced with Mills 10R and 220R 5W (wire wound non-inductive). The sound became a bit warmer and silences much quieter but there was a loss in the highest frequencies. So I changed the power tube signal resistors from Kiwame 1K 2W (carbon film) to Vishay 1K 1W (VTA52, bulk metal foil). This made a huge difference as it became much clearer, but now a lot of the warmth of the music had gone. Some high frequencies sounded quite sharp and the upper midrange had a thin, metallic feel. After a few days I changed the Vishays for Audio Note 1K 1W tantalum resistors. These were much better, with the right level of detail and no harshness. A rounder, fuller sound than the Vishays and far more enjoyable while still being more detailed than the Kiwames.

Bias pots and jacks - June 2006

Brian provided some new black test jacks and upgraded bias pots (0-22K in 5 full turns instead of less than one turn on the old pots). The screws were different sizes to the old ones and there were no pins for the bias PCB so I replaced the PCB with an offset tag board attached to the transformer cover screw (if you do this be careful not to attach any wires to the mounting leg tag!) and the position of the pots and jacks were swapped as they fit the holes better that way. The chassis needed a bit of filing for the plastic sleeve of the jacks to fit. I also wired it so that turning the adjustment screw clockwise increases the measured voltage, which I find more intuitive than the normal Ella's decreasing voltage. (See the Diagrams page for the new layout).

Screen diodes - July 2006

Using information provided by Dennis Grimwood I used my old power supply IN5407 diodes on the power tubes circuit. These go between the output transformer and the screen grid resistor and block out any AC current on the screen. Rather than try soldering the diodes to the tube sockets I used another pair of offset boards attached to the transformer cover screw. The sound was now a bit smoother and had more "presence", with perhaps more midrange detail. Here is a summary of how they work: Forum message. The Diagrams page shows how to fit them in the Ella.

Faceplate - August 2006

Resistors - August 2006

Coupling capacitors - November 2006

After months of thinking about it I decided to try changing the coupling capacitors. It took months because this wasn't a cheap change - I replaced my Obbligato 0.1uF 630V caps for 0.1uF 600V V-Caps from VH Audio, then at $40 each. The sound improved, mainly in the midrange (which I found clearer) and the dynamics (far more impact). The bass became a bit rounder and perhaps the treble was more controlled too. They are better than the Obbligatos but do seem to take a long time to burn in, and of course are far more expensive.

Bridge rectifier - April 2007

I realised there was a single diode (IN4007) on the negative bias circuit that was performing half wave rectification, when full wave rectification should make the current in this circuit smoother. I tried using a cheap bridge rectifer (DB107) and found that the music sounded more powerful but with a bit of added harshness in higher frequencies, probably caused by diode switching noise. So I replaced the DB107 with four Cree 1A 600V Schottky diodes (CSD-01060A). Brian did a simulation and found the DC ripple reduced even further if the 220uF capacitors were replaced with 470uF, so I did this too using the same type as the originals (Rubycon YK, 100V). I think this was one of the better changes I've done - vocals are smoother, there is more detail in the high frequencies and the sound seems much "fuller".

Negative Bias bypass capacitors - August 2007

Dennis saw my bridge rectifier change and suggested that I also add bypass capacitors on the negative bias circuit. This circuit is supposed to be DC only, providing a steady bias to the power tube grids. But AC can get into the circuit too, for example the signal itself via the 100K resistor and 22K potentiometers. The rectifier (or diode) will then convert that AC into DC and add it back to the bias supply, causing instability. By adding non-polarised capacitors at the output of the bias supply at the PCB any AC can be drained to ground without affecting the DC at all. I used Solen 0.47uF 630V caps, although anything up to 1uF and rated for 100V is fine.

Tubes changed - September 2007

Output transformers - October 2007

The newer Ella uses improved output transformers (Japanese "perfect lay" 5K, 75W, Z11 core) so I decided to try them out on my Ella. Wiring them in was easy enough; the problem was physically fitting them. The cores are much bigger so I had to do quite a bit of filing of the brackets, particularly rounding off the top back section so the transformer covers would fit. And I also had to drill some holes in the chassis. The result is a clearer sound, especially in the lower frequencies and also improvements in the dynamics.

Zobel network - April 2008

Active Bias System - June 2008

I had some doubts about adding a circuit board that I couldn't fix myself if it ever failed, but it's easy to install so would be easy to swap back to the original manual bias system if necessary. And this module is completely analog so at least there is no digital noise added. First I changed the resistor on the boards from the supplied 3K3 to 7K5 and wired it in parallel to one of the existing 22K bias pots. This gave me a variable bias up to 60mA per tube rather than the fixed bias current it defaults to. Then I had to move the bridge rectifier I had added so that the boards would fit. After that installing them was quite easy as I could use a bracket to mount them off the tube sockets. The only problem now is that I have a hole where the removed 22K bias pot used to be, but at least that lets me see one of the blue LEDs on each board. Having already improved the negative bias supply with the rectifier and capacitors the additional improvement in sound wasn't that huge, but there did seem to be a little more detail and improved bass. The module certainly does what it's supposed to - the bias goes to the level I have chosen and stays there regardless of any change in amp temperature. Before, the bias could change quite a lot from when it was first turned on to after a few hours use, as well as changes from day to day, presumably due to fluctuating mains power supply voltages.

Tube sockets - December 2008

After all the previous changes the wiring around the power tubes had become quite messy. Also since the socket pins could move slightly I found that after changing tubes I sometimes broke a connection. So I decided to replace the sockets with the teflon ones where the pins are fixed and to take the opportunity to rearrange the wiring. In particular I used pin 6 for the 100K negative bias resistor so that I could remove the tag board I had added in June 2006. While doing this change I also upgraded the 0.47uF capacitors on the 6SN7 sockets to the Obbligato premium film ones.