help oldish multimeter

[Old Sparky]

Robert - if the bias probe you have is similar to one I have, you are correct in thinking that your meter should be set to a mA range. My probe connects in the cathode circuit of the output valve, making measurement of the cathode current very easy. I seldom use it, but it's a very useful tool when I do. However I note that your probe is marked Plate (i.e. Anode) Voltage, so it would be wise to check to which pins the two leads are connected, and what function the switch performs, presumably to switch the test leads in or out as needed.

[robert the violinist]

Thanks, the switch on the probe is marked plate voltage on one side and plate current on the other. So should i assume measure both using the switch but  on different settings on the meter.
My amp as a bias pot on the back of the chassis , adjustable with a screwdriver , what exactly does turning it change?
Its a classA single ended amp. I assume it has a cathode resistor somewhere . Ive read about calculating  dissapation but that seems to involve (besides knowing plate current and voltage) measuring the cathode resistor and voltage drop across it. So im a bit confused . I want to understand it as i may change the valve from a 6v6gt to a 6k6 to reduce power and see how it affects the sound.

[Old Sparky]

When I looked again at your Bias Probe, Robert I saw what that switch does. I'd be inclined to measure both the plate (anode) voltage and current, particularly if I was thinking of changing the valve. Assuming the amplifier is working as designed, the V. and I. figures you get should be close to what the designers intended. With a pentode valve, the cathode current is the sum of the anode and screen grid currents, the latter only being a few milliamps. Measuring the voltage across the cathode resistor will tell you, using Ohm's Law, what the anode + screen current is. The bias pot. is provided so that the bias Current can be set to the correct value, and will be useful if you change the o/p valve. I don't have the pin-outs for either the 6V6GT or 6K8 valves to hand, but if they differ, that bias probe will not be useable with the substitute valve unless it is rewired.

[ppppenguin]

6V6 and 6K6 (not 6K8 which is a completely different valve) have the same pinout.

[robert the violinist]

Old Sparky thanks , i`ll try and understand it.  I used to be a chemist so we tended to ignore physics

Jeffrey`s correct 6k6 not 6k8.   6K6 will roughly half the power of the amp i think.

[robert the violinist]

Old Sparky thanks , i`ll try and understand it.  I used to be a chemist so we tended to ignore physics

Jeffrey`s correct 6k6 not 6k8.   6K6 will roughly half the power of the amp i think.

Update, i tried the new extech meter , followed instruction and got these readings for plate dcV and mA. Readings stable and meter display not varying much at all.
They seem a bit low to me but unsure.

Ive attached a section from the amp build manual. Its written by Barry Vyse who i believe worked for Marconi-Osram andthis is one of his amps or constructed in one of his amp workshops.

[Old Sparky]

Thanks for pointing out my error re the alternative valve type number, Jeffrey. I 'should have gone to Specsavers!' My mistake makes what I wrote about pinouts and the bias probe in post # 33 irrelevant!

[Terry]

Terry i wont be using that meter, its the extech im using. that photo was just to show  that the unshrouded banana plugs wouldnt fit that particularly meter.

With respect, that doesn't affect my comments about the voltage setting as shown in that picture.

As to whether the plugs will fit or not, we would need to know the diameter of the hole and the plug body. It looked to me as if they would fit and, as the sockets look to be well recessed, there would be no possibility of being able to touch the pins when mated.

[Terry]

Terry i wont be using that meter, its the extech im using. that photo was just to show  that the unshrouded banana plugs wouldnt fit that particularly meter.

With respect, that doesn't affect my comments about the voltage setting as shown in that picture.

As to whether the plugs will fit or not, we would need to know the diameter of the hole and the plug body. It looked to me as if they would fit and, as the sockets look to be well recessed, there would be no possibility of being able to touch the pins when mated.

[robert the violinist]

Terry,  Yes i totally agree with your point  on safety and the meter setting.
The plugs do fit the extech as shown in the post #36. I apparently read the ma reading on the wrong setting as the probe has a 1ohm resistor or something inside it  and your supposed to read it direct from the mv setting on the meter with the probe switch set to plate current.  So this read 27  instead of the 7.88 i got initially . I believe the plate voltage is low though ad should be in the 350-450v region. Im trying to learn how to do this ,so any advice at all is welcome including any safety tips.

Bob