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?
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?
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?
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?
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.
Best resistor for the plate would be wirewound.
The resistors looks something like this:
http://www.yksd.com/distanceedcours...r/Chapter11/11-02Images/26ResistorUnknown.jpg
Could it be the resistor type making noise?
http://www.yksd.com/distanceedcours...r/Chapter11/11-02Images/26ResistorUnknown.jpg
Could it be the resistor type making noise?
I will try CCS loading at some point, but I will try to learn as much as possible from the simple approch....
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.
Wire wound are typically good for this application.
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?
Tapping a finger on the table besides the amp can also be clearly heard....is the normal for tubes?
- Status
- Not open for further replies.