12BH7A in common cathode

[Skorpio]

Hi,

I am building on a little testproject with tubes. It is ment to drive the Firstwatt F4 amplifier, so gain must be in the area of 20dB and the output swing about 14Vrms.

Today I am using a BoZ single MOSFET common source amplifier and I would like to compare this topology directly with a tube stage as this:

One 12BH7A tube, common cathode stage, +300V B+. With a decoupled cathode resistor gain should be just about 20dB...what I am missing is optimum bias and Plate resistor.

My calculations say the following:

Rplate=10K, Rcathode=270R, Ip=15mA, Vplate=150V
Rplate=15K, Rcathode=470R, Ip=10mA, Vplate=150V

what will be best conditions?

[Skorpio]

I have now the version with 15K plate resistor working and it is performing very well indeed. But there is still work to do:

I have two 12BH7A tubes, and only one of them is free from hum. The other have hum/noise in one of the sections. Is this a bad tube?

Most, it not all hum has been removed with different layout test. All is done on a wood plate, so no chassis is present. I use AC heater with 6.3V. But there are som noise present in both channels. Like hiss from a cassette recorder without Dolby...

I am used to SS circuits, where you are done when the hum issues are solved, but this seems different. The PSU is a simple CRC after 4x1N4007 diodes. As this is hum free i thought is was all needed...

Could this noise be a PSU problem? Or is it the 12BH7A tubes? Or is it just something that is normal in tube circuits?

[SGregory]

What kind of anode resistor (RPlate) did you use? I used a set up with a CCS, 7mA and 100Vplate 2.7V cathode and had no hiss.

Best resistor for the plate would be wirewound.

[Skorpio]

The resistors looks something like this:

http://www.yksd.com/distanceedcourse...torUnknown.jpg

Could it be the resistor type making noise?

[Skorpio]

I will try CCS loading at some point, but I will try to learn as much as possible from the simple approch....

[SGregory]

a 12bh7 is not the quitest tube around, however, the type of load resistor can contribute to the noise as well.

Wire wound are typically good for this application.

[Skorpio]

I have found tha the tube is extreamely sensetive to mechanical surrounds. Tapping on it with a match sounds like hammering on waterpipes with heavy tools (in the speakers)!

Tapping a finger on the table besides the amp can also be clearly heard....is the normal for tubes?

[Arnulf]

Yes, it is normal for tubes (with real grids, grid as in length of wire wound into a grid, not a solid piece of mesh or rod or anything like that) to be microphonic, though some tubes are far more susceptible to this than others.