Tiny Terror Biasing

[Ben_W]

I will soon be purchasing a TT for home practicing, as my 100W half stack is just too loud to practice with.

Now my main question, is that lots of people seem to be saying if you purchase one, to check the biasing of the amp. Is there a certain time-zone, when they all started getting better? Or is it just pot luck? The main reason, is that i may be able to get a good deal on one second-hand, but will get a new one if I will end up needing to pay obscene money to get a tech to fix the valves.


Cheers,


Ben.

[sjwebb90]

I owned a Made In Korea TT when they first rolled off the production line.

The bias is set by 120 Ohm resistor for both 15W and 7W modes. I measured the actually bias when running in the 15W & 7W modes and it was running too hot in 15W mode and too cool in the 7 Watt mode.
Ideally with EL84 valves there should be 12Watts on the anode plate.  On my model there was co-incidently 15Watts on the anode plates in the 15W mode and around 8Watts on the anode plates in the 7 Watt mode

If run in the 15watt mode  the valves life expectancy would have been be a lot less.


1) I corrected this by first replacing the 15w / 7W switch with a DPDT switch variant.
    http://www.flickr.com/photos/66957213@N00/2627824665/in/photostream/
   
2) Replaced the Bias resistor with a 160 Ohm resistor (150 Ohm and 10 Ohm in series) for 15Watt operation.
    http://www.flickr.com/photos/66957213@N00/2627824975/

3)Then using the DPDT switch, connect a 68 Ohm resistor in parallel with the 160 Ohm resistor for the 7W mode
   http://www.flickr.com/photos/66957213@N00/2628642862/in/photostream/


As stated above, this refers to an early model, maybe Orange have addressed this issue in the later models.

Only real way to know is get someone to check the biasing.

I also seem to remember too that certain Made In China variants made between certain times had dodgy transformers and were part of a recall.

Cheers
SJ

[jpfamps]

Getting a tech to change the biasing resistor should not cost much. 1 hour's labour tops.

Many amps run EL84s very hot. In fact the 12 W anode dissipation seems to be regarded as a minimum target by most manufacturers. The highest dissipation I've measured in a current production amp is 21W!! I don't recommend this.

Obviously for reliability, reducing the plate dissipation to sensible level will help.

Actually 70% of maximum dissipation is recommended value for class AB amps, although EL84s do seem to survive when biased at full plate dissipation in most amps.

Whether you should change the biasing at the lower power setting is a moot point. Advocates of "power scaling" (which the low power setting effectively is) would argue that you should reduce the anode dissipation at the lower power setting to maintain the operating class of the amp. Others might argue that you should keep the dissipation the same (or even increase it)  i.e. bias the amp more towards and probably into class A at the lower power setting.

Personally, as long as you are not compromising the reliability of the amp, I would go with whatever biasing solution you think sounds best.

As far as I know, all the "dodgy transformer" models were recalled by Orange, although you can contact Orange with the serial number and they will tell you if this was a problem with your amp.

Incidentally, the recalls were due to the transformers falling foul of a change in EU safety regulations rather than defective manufacture.

[Ben_W]

Thanks guys, I will try and get the serial number and ask Orange, just to make sure about the components, but I will go and try it, and have a look. That was the main thing I was worried about, was that the dissopation would be incredibly high, and i'd get through valves like guitar strings.