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B+ problem?

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kianbach

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Joined 2005
I've built an amp but when I fired it up the heater voltages were spot on:5v rectifier(5Z3),2.5v 2A3,6.3v 6sl7.B+ on the other hand is 200v down.I measured the voltage on the tranny 380-0-380 no problem.Same going into rectifier.Measured opt connection to B+.It reads 215v and the HT wiring to rectifier is hot enough to melt solder.Any suggestions as I'm a newbie and lost????????
Its the Morrison Micro circuit I'm building.
 
K

kianbach

Member
Joined 2005
SY said:
OK, let's take it step by step. What is the B+ supposed to be and how much current is the circuit supposed to draw?

If you've got a link to the schematic, that will help us walk you through the process.
Click to expand...


Here's the diagram
 

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SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
OK, without going through the circuit in detail, one can see that the total draw from B+ will be on the order of 50mA (it might be 40, it might be 80, but we're just approximating). OK, so that means, as far as the power suply is concerened, the load looks like whatever resistance it takes to conduct 50mA (0.05A) at 400V. Pull Ohm's Law out of our mental toolbox: the equivalent resistance is voltage divided by current. So R = V/I = 400/0.050 = 8,000 ohms. So an 8,000 ohm resistor will load down the supply properly (nearest standard values are 8.2K or 7.5K).

Now, we put Ohm's Law back neatly and pull out Ohm's Power Law, power = voltage times current. In this case, P = 400 x 0.05 = 20W. So... disconnect the B+ feed from the circuit, attach an 8.2K (or 7.5K) 25watt power resistor across the power supply output. Check the voltage. If it's 400V like it's supposed to be, the power supply is fine and you need to start troubleshooting the amplifier. Contrariwise, if the voltage is low, then the power supply has a wiring error or a bad part.

Either way, you've cut the problem in half.
 
K

kianbach

Member
Joined 2005
no hum pot for minimising the hum
Click to expand...

If you look closely at the photo you'll see there are 2 humpots between the input and output valves.The 2 big caps between the input alves are the bypass caps for the 2A3 cathode resistors which are connected to these humpots.measured hum is 3mV on each channel.DC filament supply for the input valves consisting of a bridge rectifier and a pair of 22000uF caps with a dropper between them.Totally silent :)
 
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