Metropoulos Forum

Hello everyone, I am having a problem with low bias voltage on a 100 watt build. With a 27K bias range resistor I barely got 17Vdc on pin 5 of the PT's. I put in a 10K and got the following results:
Wall outlet: 125Vac
Bias tap: 49Vac
Voltage drop across the 10K: 2.5Vac
After rectification: 20.5Vdc

The diode reads good on my meter so I am wondering what is going on. I have to tell everyone that this is a kit I am building for someone. It comes from a certain company who makes excellent speakers, but as I remember from some previous posts, their kits are not held in the highest esteem on this board. I am using a Classic Tone OT, and a Mercury 10H choke.It is a Chinese PT and I have uploaded a pdf of the specs. The schematic for the kit itself shows the bias being tapped off of the HT winding, but I used the bias tap. Here is a pic of the amp with one of my PTP boards installed: And one of the bias supply: Thanks in advance for the help and suggestions.

Ivabiggun

I built a Weber kit and used what I would a "Fender Style" bias circuit, IIRC the tap had about 75 vAc

Normal bias voltage on a proper Marshall 100w PT (or correct spec clone) is 90-100vac

Thank you for the replies. I thought we didn't mention Weber on this board. I guess I will have to rebuild the circuit according to the Weber schematic and tap of off the HT. RussB, that is a nice job on the bias layout.

Ivabiggun

Ivabiggun wrote:Thank you for the replies. I thought we didn't mention Weber on this board. I guess I will have to rebuild the circuit according to the Weber schematic and tap of off the HT. RussB, that is a nice job on the bias layout.

Ivabiggun

Probably because it seems to be a form of masochism do make their amps when there are Ceriatone and Valvestorm/Metro and Marstran which are much better products. IMHO!

I've been trying to build a bias circuit and I built one yesterday using your schematic. It works fine. I've been getting ~82Vac so I guess that's ok. And this probably means there's some shorting or lose connection at your end which is responsible for the drop.