Rerurn to Romy the Cat's Site


In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: A bug in negative supply chain on channel BPosted by Romy the Cat on: 1/1/2019
 anthony wrote:
Hi Romy,

Further to my earlier message, I've measured the bias voltages for the driver tubes in all Channels.

A -4.3V not adjustable
B -13.82V not adjustable
C -4.8V not adjustable
D -4.2V (ran out of potentiometer travel...may have to adjust a resistor)
E uses fixed cathode bias...not measured this round
F -4.3V (adjustable with pot.)

Looks like something is going on with A/B/C.  B is the worst and is the 6E5P that is not conducting (has 400V at the anode).  I've not installed the crossover components in this channel yet (the 12.1K resistor and 0.043uF cap to ground) but perhaps I should install something even though the crossover point will probably change...I'd not have thought they would make a difference at this stage.


Anthony, it is OK. The voltages at all channels are near OK (with exception of C). The bias voltages are irrelevant as all that you care is to get near 200V on plates. It is depending from the tubes you got. It might be 3.5V of 4.5, it is truly irrelevant. You do not want to be at 1.5V as some of other types tubes do as it will be flooded by input voltage and you will clip the driver stage. This amp has no feedback, so no voltage injection into cathode is applied. In our case we have 6E5P that has a relatively high bias and with over 3.5V you will hardly be able to drive the first tube out of A1 operation. 
 
Your channel B bias is 13.82V wish is the problem. The tube is closed with such high bias. As you will bring the bias doe to near 4V the tube will open and the plate current will flow. You nned to debug your negative supply chain on channel B

Rerurn to Romy the Cat's Site