Weird behavior when measuring OD entrance trimmer.

[Tonegeek]

You would think measuring the resistance of the OD trimmer would be the same whether amp is off or not but in this case:
Resistance of OD entrance trimmer from wiper to ground with amp off = 13.7k
same measurement with amp on but on standby = 8.1k
same measurement with amp on, standby off = 13.7k

In each case, the resistance slowly changes when switching from one state to the other. It takes about as long as a tube warming up.

With the amp on and in standby I get 45mv on the OD trim wiper. This seems to be what is tripping up the resistance measurement. I suspect this must be an induced voltage from somewhere, but where. The HV is off. It must be from the tube filaments or less likely the relay power. It goes away though when the HV is applied, which I don't understand. I have switched the OD on and off and nothing changes except the resistance varies slightly due to the clean master being in parallel with the measurment when in OD.

It does not do this on the HRM tirim measurements. Measurements were done in circuit. The amp works just fine. Any ideas?

[heisthl]

assuming the relay is in the clean position the only source could be grid leak current from V2A not sure why it would do it in standby mode.

[Tonegeek]

I swapped V2 for a new one and got the same behavior. when there is no tube in the socket I get 0 volts. I am leaning to the heaters as causing this voltage on this wire leading to the grid. It does not seem to affect the operation of the amp. I don't get any hum until the volume is way louder than I would ever play so no worries there.

[lbradshaw]

You would think measuring the resistance of the OD trimmer would be the same whether amp is off or not but in this case:
Resistance of OD entrance trimmer from wiper to ground with amp off = 13.7k
same measurement with amp on but on standby = 8.1k
same measurement with amp on, standby off = 13.7k

I am getting the same kind of thing on my non-HRM build, which currently does not have relays. The amp seems to work fine... It is odd.

[Tonegeek]

I am getting the same kind of thing on my non-HRM build, which currently does not have relays. The amp seems to work fine... It is odd.

So it must be the tube heaters. I still don't understand though, how that would make a DC voltage. I would expect 60 cycle AC (which I have not checked for) since that is all that would be going on in that tube at the moment (being the 6V filament power).

When HV is on, then the operation of the tube must keep current from flowing in the grid circuit as i get 0 volts with HV on (as you would expect).

[Structo]

Be interesting to see a scope trace of that.

[llemtt]

assuming the relay is in the clean position the only source could be grid leak current from V2A not sure why it would do it in standby mode.

that's because the anode hasn't any voltage applied so it can't attract the electrons "leaking" into the grid

as soon as the anode gets a positive voltage all the electrons flow to the anode and very very few hit the grid

Teo

[BobW]

You would think measuring the resistance of the OD trimmer would be the same whether amp is off or not but in this case:
Resistance of OD entrance trimmer from wiper to ground with amp off = 13.7k
same measurement with amp on but on standby = 8.1k
same measurement with amp on, standby off = 13.7k

In each case, the resistance slowly changes when switching from one state to the other. It takes about as long as a tube warming up.

With all due respect, your measurements techniques are wrong.
You cannot accurately measure resistance with ANY power applied to a circuit. Resistance is actually calculated by running a very small current created by the DMM, VTVM or VOM through your circuit. The voltage drop across the internal precision resistor then determines/displays the resistance based on the voltage drop. any outside current or voltage source throws off the resistance measurement.
Also did you consider what else is connected to the trimmer when measuring with the amp off? Grid resistor and the dynamic resistance from grid to ground via the cathode, etc. To accurately measure the trimmer resistance alone, you need to take it out of the circuit or lift the trimmer ground, then measure, with power off.

[Structo]

Not to mention the cap on the other end of the trimmer that would be charging.
When I measured my wiper to ground I noticed the value changing slowly like a cap charging.
I have the pre-OD entrance network on my amp.

[BobW]

Not to mention the cap on the other end of the trimmer that would be charging.

Good point

[Tonegeek]

With all due respect, your measurements techniques are wrong.
You cannot accurately measure resistance with ANY power applied to a circuit. Resistance is actually calculated by running a very small current created by the DMM, VTVM or VOM through your circuit. The voltage drop across the internal precision resistor then determines/displays the resistance based on the voltage drop. any outside current or voltage source throws off the resistance measurement.
Also did you consider what else is connected to the trimmer when measuring with the amp off? Grid resistor and the dynamic resistance from grid to ground via the cathode, etc. To accurately measure the trimmer resistance alone, you need to take it out of the circuit or lift the trimmer ground, then measure, with power off.

Correct about the voltage, but then that's the point. Remember Standby was on, so nothing but heaters had voltage. I did not expect any voltage on the grid, but as I now know the grids are picking up some heater radiation. As far as desoldering the trimmer to acurately measure it, I am too lazy for that! I just need a relative measurement and as long as I factor for the other stuff in parallel, I am getting what I need to have repeatable settings. In the future though, I won't be taking any resistance measurements with the amp on, even in Standby.

Not to mention the cap on the other end of the trimmer that would be charging.
When I measured my wiper to ground I noticed the value changing slowly like a cap charging.
I have the pre-OD entrance network on my amp.

What cap? The only cap around is the 47p in parallel with a resistor before the tirmmer and it would be discharged through the resistor it is in parallel with.

that's because the anode hasn't any voltage applied so it can't attract the electrons "leaking" into the grid
as soon as the anode gets a positive voltage all the electrons flow to the anode and very very few hit the grid
Teo

I was coming slowly around to the same conclusion

[Structo]

Sorry, but on my amp I have a pre OD network consisting of a 220K and .05uf cap before the 100k trimmer.

[Tonegeek]

Sorry, but on my amp I have a pre OD network consisting of a 220K and .05uf cap before the 100k trimmer.

When my amp is in OD mode there is the coupling cap off V1b plate but now when I measure the OD trimmer the amp is off and defaults to clean mode so the cap would be out of the circuit.

[Structo]

Yes but if you measure the wiper of the trimmer to ground there shouldn't be anything affecting it right? Especially if you pulled V2 to do the measurement.

Also, I don't know why you would want to measure any resistance with the power on.
Sounds like a good formula to burn up a meter.

[lbradshaw]

As I said, mine does do that same thing Tonegeek's does. That said, I do measure mine with the power off.