DIY HiFi Forum

Posted by bcherry (M) on June 17, 2008 at 17:36:57

In Reply to: RE: Ella mods: Active Bias System posted by Goneill on June 15, 2008 at 20:28:09

yesterday we received our first new-style Ella with the ABS installed. The ABS locks down the bias and it's nice to see whether cold/hot/in between, all tube pairs are 100% balanced. since it only takes one or 2 milliamps of imbalance to seriously impact on the performance of the transformer (push pull Transformers have no air gap and cannot tolerate DC current), we now enjoy the full potential of the amplifier at all times.

a little more:
The fixed resistor on the ABS is intentional to make installation as simple and easy as possible, just as the screw terminals are, as well as the very concept.

The concept:

Take an existing fixed bias amp, select the correct resistor for it, fit with minimal changes and forget about bias. Very result oriented. No mucking about.

For those that dislike screw terminals and absolutely want to solder, the screw terminals are easily unsoldered. However these connections have absolutely minimal impact on sonics (unlike most connectors in amplifiers and tons less than tube sockets) and most people can strip a wire and place it in a screw terminal.

Equally, it is possible to desolder the LED’s and replace each by external LED’s on long wires, but with already 8 or 9 wires to each PCB, adding another 8 would have been overkill for most DIY’ers, so simplicity meant putting the LED’s on the PCB. Anyway, you only have to worry if they stay red and in future the matching PSU board will take care to shut the amp down and light a “sumpting wrong” light in case of red-light, so, no worries.

If using a Pot & Resistor combo to adjust the Bias it is recommended to take the two holes on the PCB holding the resistor to attach the wires in the usual fashion and wire the resistor directly to the adjustable resistor terminals.

In addition it may be worthwhile considering a series resistor to apply a lower limit, as adjusting the Bias to 0mA usually has little value.

Eg. in Goneill’s case: use an adjustment range of 30 – 60mA, using a 2K4 (2.4k) resistor in series with the combination of the 22k adjustable resistor paralleled with a fixed 3K9 (3.9k), which allows an extremely precise adjustment of the current over the sensible usable range, with the lowest resistance 2.4KOhm (I = 29mA) to 5.7KOhm (I = 60mA).

For fancy stuff use a series resistor plus pot in parallel with suitable resistor as here:

     R s           R p
---^^^^^^----+---^^^^^^---+---
             |          ^ |
             |        /   |
             +---^^^/^^---+
             |    /
             +---+  R adj

To calculate:

Rs = R from table for minimum current desired

Rp//Radj = (Rmax – Rs) where Rmax from the table for maximum current desired

For the parallel combo we can solve a complex equation but it is probably easier using a generic on-line parallel resistor calculator like this:

http://www.electronics2000.co.uk/calc/calcrsp.php

So, for Goneill’s example:

I min = 30mA
I max = 60mA

R min = 2370 - nearest easy standard 2400R/2K4/2.4k
R max = 5620

Rp // R adj = 5620 -2400 = 3220

Put the 22000 Ohm (22k) into the R1 field for the 2 resistor parallel calculator

Enter the total value (3220)

Press calculate R2 (in our notation Rp)

Result 3.772 Kohm, nearest preferred value 3K9/3900Ohm/3.9k

Again, for simplicity all this was not put on the PCB, as there are so many tubes and possible ranges that it is hard to come up with a single setup that fits all and is flexible. The worst case could have been two adjustable resistors on the PCB (to set lower and upper limits or current without external R) added to the (optional) external pot.

For the more advanced DIY’er the needed changes are not difficult, for others the ability to have a simple resistor take of all might be godsend.

Finally, yes, it is all analog. One can make a computer in a digital way or an analog way.

Before Russia gave in and copied IBM’s digital computers (in the 1960, before IC’s were invented) they used to do cutting edge analog and hybrid (as in analog for speed, digital add on for final precision) computers, that had impressive performance.

The ABS uses basically analog computing circuits based on this sadly forgotten technique, using the “always just right” paradigm compared to the digital principle of “always around 1/2LSB wrong”… ;-)

regards
Engineering Resource Team
DIYHFS
Take control! DIY.

Edits: 06/17/08

• RE: Ella mods: Active Bias System - Goneill 23:04:55 06/17/08 (1)
  • RE: Ella mods: Active Bias System - OzMikeH 06:29:02 06/25/08 (0)

Post a Followup:

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ DIY HiFi Forum ]
[ Comment ] [ Edit ] [ Delete ] [ Copyright Warning! Click for Details ]

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2008, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---