Getting back to the main topic:
This is a schematic of one channel of the amp. Please note the component designators (R1, C2 etc) are wrong, but the values, 47E, 1,5K, BC550C I believe are correct.
The voltages are helpful if troubleshooting a faulty board or you want to get an idea; these voltages
will vary from board to board, due to component tolerances etc.
Also, the components themselves will change their characteristics slightly with temperature change, for example expect to see the voltages measured in the circuit to change from when the amp is first turned on, from being cold, to when the amp has been runing after an hour or so -- this is normal.
As an example, the Metal Film resistors you get from the likes of Mouser or Digi-Key can have a temperature coefficient (TempCo) variation from 50ppm to over 300ppm depending on make, model and design. Resistors like the expensive VISHAY Bulk Metal® Foil Precision Resistors, are super stable and super precise, typically available in 0.1% and 0.5% tolerances with a TempCo ± 8 ppm/°C (– 55°C to + 125°C) and ± 4 ppm/°C (0°C to + 60°C). If you're interested check out the VISHAY website, look for: Bulk Metal® Foil Technology Industrial Precision Resistors, VSR Series - this is what one of my mates used in his amp; just to be different. (VSR4, 47E or 47 Ohm and VSR4, 1,5k Ohm, as well as all the other values in the circuit. For the 10 Ohm Emitter resistors of the output stage he used: two VSR6 20 Ohm resistors in parallel for each 10 Ohm resistor position) Along with the other parts in the amp, it sound pretty cool.
These voltages were measured on an original Lehmann Black Cube Linear:
If you stick in different transistors, the voltages will fluctuate and change with the gain of those devices etc, if the voltages you measure are way out, may be more than +/-50%; I seriously think something is wrong with the circuit.
Avoid using an old Analogue Style Meter (Ones that have a needle that moves on a fixed display) when measuring voltages around the BC560C and BC550C transistors. The internal resistance of these types of meters will affect the operating point of the circuit, also known as "loading the circuit" and can give false readings, or readings that seem there might be a fault. I have a FLUKE 189 meter that I cherish, but a FLUKE 110 or 112 is just as good - I personally like FLUKE meters a lot, But any good DVM (Digital Volt Meter) is OK, like HP, SANWA etc...Please don't take this as a beat up on analogue meters - they have there place too...it's just sometimes some tools work better than others...
For what it's worth, the transistors mounted on the Lehmann board were:
- ON SEMI Brand: BC550C
- ON SEMI Brand: BC560C
- FAIRCHILD: BD139-16
- ST: BD140-16
I've used these voltages as a guide, kind of reference point with the mods we were doing. If you get really different voltages and think the amp is working fine, I would be really interested to know!
))
