Rerurn to Romy the Cat's Site

Contemplating now how to organize biasing for 3 singe stages of my 6-chanls amp I was faced the dilemma: do I need to bias my tubes. The 6E5P/6E6P are Russian tubes and it means that are very different right from the boxes. The Milq, driver tubes, being loaded into low impedance the OPT’s primary will fluctuate current. It was not critical in the two stages design what it affect nothing but in the single-stage it is more critical. The single-stage will run:

1) S2 Fundamental Channel - 6E5P
2) S2 Midrange Channel - 6E6P
3) WaterDrop Tweeter Channel - 6E6P

From one side those channels case less about power and damping but I ma concern that with different current I will have different gain, not to mention the opportunity to overheat the best tubes. The way how the amp will be organized the single-stage channels will have real-time current monitoring and also I will have a chance to use pick the best tubes for a given channels, placing the “unacceptable” for single-stage channels to the two-stage channels. From another side it would be nice juts to have an adjustable bias to set a default current for the stages.  The problem is that I would like to do it as good as I can and I do not relay want to screw with negative bias voltage. The voltage goes right to the grid and it is very-very sensitive voltage. I presume that passing it over an attenuator might be an issue…

Then what to use.  I use 3500 Bourns and are 2W and do fine in out stage. For the input sage of the new Super Milq I would need 4 of them – a lot of necessary real-estate that would make the negative paths too long. The miniature 10 turns 1.5V wirewound are $141 each –
 
http://www.etisystems.com/mw10.asp

... a little painful and I do not know how they sound. To use trimpots is problematic as well. The Vishay Bulk Metal Foil trimmers:

http://www.goodsoundclub.com/pdf/VishayBulkMetalFoil_1280G.pdf

and the similar: http://www.vishay.com/resistors-variable/trimmers/bulk-metal-foil/

are wonderful but they are very ugly to use and in BL version (panel mounted)  and they are $38 and still have just 20K range but no to mention 9 week of thelead time…. To put there regular cement tripod is not something that I would like to do, though Dima insists that it should not be a big deal since the trimpots will be hidden behind the caps and a large 390K biasing resistor… Still they are the bias of the HF channels… and I presume that a fixed good quality resistor will do anyhow better job then anything else…

Manville I did some measurements what a single stage dose with fixed-fixed bias at 190V

Bias -4V 6E5P

1. 34mA
2. 29mA
3. 17mA
4. 26mA
5. 22mA
6. 24mA
7. 24mA
8. 21mA

Bias -4V 6E6P-DR

1. 32mA
2. 35mA
3. 25mA
4. 30mA
5. 28mA
6. 20mA
7. 17mA
8. 40mA

It does not look as it will be useable without bias regulation… :-) I just need to find a “fine way” to adjust that R10 resistor…

Rerurn to Romy the Cat's Site