Bias readings----Marshall jcm2000 series

[Valvehead]

Interesting discrepancy...

Biasing a DSL / TSL Marshall --- using the bias points on the rear VERSUS the actual OT readings.

As an example, i have a dsl on the bench and it reads about 80 MV a side using the bias points

However, flipping the amp over and shunting the OT i get 60 ma and 68 ma . (yes , milliamps)

Just curious about your thoughts on this-- why is the reading so far off ? (every 2000 ive tested does this)

and what do you personally trust?

Myself? i will take the measurement off the OT .

Comments ?

[Enzo]

Take one of your plate wires off and insert the current meter in SERIES with it. If I recall correctly, and that is always a crapshoot, those OTs have a fairly low DC resistance - 15 ohms maybe - and that is in parallel with your meter. It could possible affect the readings.

And for that matter, stick a bias probe under the tube, and see how well that agrees with other methods.

Ever have an amp that will measure current with the shunt method on one side of the transformer but not the other? Or won't take a reading until you reverse the meter leads? Just evidence that the meter does interact with the circuit. Sorta like the common thing where measuring voltages on a LTP PI tube gives you something like 70v on the cathode and 50v on the grids, both taken from ground. Obviously there is not 20v of bias on the grid or the tube would shut down. When measuring from CATHODE to grid, you get a very reasonable volt or so. This is classic meter loading of circuits.

I don't know this to be the case here, but it is what I suspect. Seems unlikely that Marshall would consistently build a line of amps that all measure 20-25% high on current.

I assume you are taking bias readings from the center pin of the three, and not from chassis?

WE tend to think of our test instruments as disinterested sideline bystanders, quietly observing. But they are actually involved, and interact with the circuit a lot more than people think. And a scope is no more innocent. If my scope and my meter both have a 10 meg impedance, they will both load the circuit down similarly.

[Valvehead]

damn...you are good enzo....

on this amp , im checking MV on one dmm --that is on the 3 pin bias deal on the back = jcm2000 series.

Then--Im later checking mA with a different dmm - shunt ( yes the first meter doesnt read amps anymore--want to guess why? you know the answer

and yes--ONE amp in particular that wont read one side of the OT is the Peavey 5150 / 6505


But yes--all the marshall 2000 (what ive seen) read 10-20 ma off from what is seen at the "test points" which are in mv--- but isnt that amp equipped with the 1 ohm resistor to ground for this ? Or my meter is freakin nuts...........?

[Enzo]

Yes, the Marshall has 1 ohm resistors. And while they may not be dead on 1 ohm, I bet they are not off 15-20%, and especially not all of them. SO yes, mv = ma.

I don't think your meter is nuts, just has certain characteristics we cannot always ignore. That is why I suggested the tests I did. Very simple to pop a wire off an OT terminal and clip the current meter inseries, just takes a couple seconds. How does that compare to the shunt reading?

And then trying with a bias probe offers another way to take readings two ways to compare. One through the test points, the other through the bias probe. That can be done both at the same time.

Your meter will be a voltmeter in parallel with a 1 ohm resistor at the test points. But your meter will be an ammeter in parallel with a transformer winding when shunting. WHy expect the two very different process to react the same way?

And remember also that the center test pin is not chassis ground. They sometimes are connected by a 10 ohm resistor and some diodes and a cap. Even on the amps with a jumper instead, the path from that pin down through the circuit board and to the chassis has a resistance. When measuring 480v on the B+, we can ignore that fraction of an ohm. But when we are measuring something at 1 ohm, we can't ignore a half an ohm through all that.


Oh geez, what is wrong with me. I forgot to mention screen current. When you go in there, measure voltage across one of those 1000 ohm screen resistors. Since they are 1000 ohms - and they will probably be close enough, if they are not dead on, they will read in the ballpark - one VOLT equals 1 milliamp. In shunt readings you are mesuring plate current only. In cathode current you are seeing plate current and screen current added together. So 10v across the 1k resistor would mean 10ma through the cathode. And that would pop your 68ma right up close to the 80ma. It will be interesting - at least to me - how much of your differential is from screens and how much from the meter business.

[g1]

One more thing to add: we assume that our "shunt" is actually a short, this is not always the case. You could check the resistance of your current meter with your other meter. They are usually a very low resistance but there are exceptions. For example, I have a Meterman ($200, not a real cheapo) that has .1 ohm resistance when measuring amps but is actually 10 ohms when measuring milliamps, definately enough to throw off shunt method biasing. Took me quite awhile to figure that one out as I was getting the same kind of discrepancies as you are. Also, the temperature of the OT will have a very slight effect on it's resistance.

[Valvehead]

the meter in series with a plate shows 71ma.

the volts at the screen res is 465 and 468

[Valvehead]

G-1 , my "ma meter" measures 2 ohms in that mode.

[g1]

You seem to have 2 factors in play here. As Enzo pointed out, you need to subtract your screen currents from your cathode currents. You need to measure the voltage across the screen resistors (one probe to each end).
Also your current meter "shunt" is not low enough for these low resistance OT's. You have 2 ohms in parallel with 15? ohms so there will still be some current bypassing your meter. This is why your current reads different in series with the plates than it does in parallel with the OT windings.

[Enzo]

As g-one said, it is the voltage across the screen resistor we want, not the voltage to ground. The voltage across that 1000 ohms will tell you the screen current, which is added to the plate current throough the cathode.