help oldish multimeter

[robert the violinist]

I must admit that my attention to this thread has been focused on the DMM aspect up to now and hadn't really taken on board how evil that nasty little adaptor is!

The first question that springs to mind is why would anybody want to use one?

It obviously inserts a 1Ω resistor between the anode and anode load and has SPDT switch which connects one meter lead to either the anode or the cathode whilst the other lead is permanently connected to the other side of the resistor.

The anode (or plate, if you prefer) voltage setting is a little ambiguous anyway because it is really measuring anode - cathode voltage.

So somebody measures the anode current and flicks the switch to measure the anode voltage, forgetting to change range on the DMM first ...

So, buy a new meter and start again.

Bob may say that he would never make such a mistake but, in my defence, I would refer him to his picture of the device and the setting of the DMM alongside it.

I don't know how many of these things have been sold and how many meters have been killed by people using them so i can only take Bob's device as an example - can I please make it clear that this is an attack on the device itself, it is not personally aimed at Bob!

I always insist on the negative meter lead being fitted with a crocodile clip (and have a second lead fitted with probe, if the probe end is not interchangeable). As an alternative I use probes with a spring hook so that the negative one can be clipped on a suitable earth - in this case, the earthy end of the cathode bias resistor, for example. My attention is then fully focused on the positive probe and where it goes (and it's a lot safer, too!)

Note too, that when making individual measurements, rather than just flicking a switch, the very act of removing the probe after each measurement should send an automatic prompt to the brain to consider the meter range selection.

Let's treat this as a fault finding exercise. Before applying power, measure the value of the cathode bias resistor - after all, if it has changed it could have some bearing on the fault!

With the unit powered, measure and note the anode voltage, then the cathode voltage. Using Ohms Law it will no be possible to calculate the combined anode and screen grid current, if required. However, the lion's share will be the anode current. If you really want only the anode current, measure the HT voltage and screen grid voltage to calculate the current through the grid's feed resistor and subtract it from the total.

What really niggles me about all this, though, is why?

Voltage readings in the published service data should be more than adequate for most purposes and should be available with any decent DIY design. The well known Mullard 5-10 design, for example, includes a table of voltages and also of relevance to an earlier part of this discussion, the design originally appeared in Practical Wireless only 3 years after the introduction of the AVO 8, by which time it was obviously already the instrument of choice for voltage readings.

Terry, these probes are widespread amongst thousands of guitar playing valve amp owners.   The one i have shown was bought from Bulgaria and looked better quality than the many chinese ones on ebay .
Guitar players are obsessive about tweaking their amps. I realise it can be very dangerous habit. I ve read much conflicting stuff and info on guitar site about biasing amps.
Guiarists usually want different results to audiophiles.
My amp is around 15 years old and i just want to check its working correctly still.

Today i removed the chassis and measured the voltages between pins 3, 8 and 4 to ground (the chassis via an alligator clip on the  black meter lead) and  got readings of  pin3 = 328 VDC, pin4 = 319 VDC and pin 8= 20.9 VDC.
As i understand it this sort of meas my amp is running rather `cool` compared to the usual for these class A single ended amps. ie. plate dissapation is only around 70% ,they usually run at around 80-90%.

When i had the chassis out i intended to measure the voltage drop across the cathode resistor  and the actual resistance of it but not sure which is the exact resistor .

The amp has a bias pot between pin 8 of the 6v6gt and ground (it is located on the back panel of the amp near the speaker jacks.

(when i used that bias probe alone i switch off the amp before flicking the switch on it to the other setting and putting the red meter probe in the other socket on the meter. It seemed more sensible than flcking the switch with the power on.)