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June 20, 2014

INTRODUCTION: This unit potentially has practical value to me since it is both an AC and a DC Valve-Voltmeter, also as an AC meter at 100MHz plus, it has a much wider bandwidth than my AC only HP 400H (4MHz). (By the way, Valve Voltmeter because I am of English origin and I think that valves are SO much more interesting than toobs, you see.) I have had this VVM for quite a while and it was one of those projects that I flirted with and then set aside, again and again. The reasons for this were 1, it did not work properly and 2, I could not understand the circuit; I finally re-drew the DC amplifier and meter circuit for clarity and I have included this diagram in the theory section below.

CONDITION: It is in excellent condition with tubes that are effectively free from microphonics and balance perfectly, and the balance once set, stays put. However, I could never get it to read accurately and consistently. I had removed the meter cover and gently blown on the needle, it seemed completely free however, I finally realised that it was in fact sticky. The disturbing force due to blowing is quite a bit stronger than that due to the available magnetomotive force! Since the meter is quite well sealed, I felt that the problem was not likely to be particles in the gap so I gingerly backed off the top pivot. Sure enough, the meter cleared and now does move under electrical stimulus freely and can be reversed at any point without sticking. I cannot offer any explanation as to why the pivots were dragging other than perhaps corrosion. Having said that, I do wonder at the quality of the (Simpson) movement; The mass balance of the needle is very poor despite the presence of balance weights, and it can only be calibrated and used either flat or vertical. I set it up flat because I will use it on a bench, not sitting on a shelf. The picture below shows the large mechanical zero error due to the mass unbalance:


DESCRIPTION: It is an extremely neat unit, and typically for military equipment is enclosed in a grey aluminium case. (By the way, I do have the knobs from the terminals, I simply forgot to replace them before taking the picture.) Both the AC and DC probes are present however, other than the power cord and spares fuses and lamps, none of the other clips and cords are present. Here it is showing the probe compartments open:


The AC input at the terminals is routed via the rectifier probe which is stored on a mounting clip that connects to the probe tip and probe grounding ring. To make accurate measurements above 100MHz, it is necessary to connect the source signal ground directly to the probe ground ring and test tip directly. Even at 40MHz, the leads should not be longer than 3 or 4 inches, according to the manual. The probe tip and ground ring with their respective cradle contacts are both shown in the picture below. The ground wire should not be more than at the most 4″ long. This means that the source ground and test points must be close together. One problem was that the tip of the AC probe was shorting onto the case, you can see where the paint has been scratched. I managed to adjust the cradle a bit but it is still close:


Here is the spares compartment:


And here is what is contains:


And finally, before we get in deeper, here is the inside, note the two spare knobs and spare rectifier on the left by the pots:



AC and DC voltage ranges, 1.2, 3, 12, 30, 120, DC only, 300.

Input loading, DC 30MΩ, that is useful for me!

AC, 5MΩ shunted by 70pF at the panel terminals or 5pF at the AC probe tip.  The manual has a graph showing that the resistive component of the loading falls (the load increases) with frequency, two points from the graph being 5MΩ at 10KHz and 90,000KΩ at 100MHz. A good scope with a top quality X10 probe can do better, typically 10M shunted by 2pF. However, when this unit was produced, scopes that could match the bandwidth of this meter existed only in engineer’s dreams! (Engineers are weird like that, I KNOW!) Also, this unit is much more handy than a lab grade scope of the era in any case.

Claimed Accuracy:

DC, all ranges 3% FSD.

AC, 10 to 50Hz, 5% with correction curve

50Hz to 100MHz, 4% without correction

50MHz to 150MHz, 3% with correction curve

150MHz to 300MHz, 8% with correction curve

Observed accuracy:

The meter meets the % FSD specification on the AC and DC ranges. I made at least 3 spot checks on each range after some tweaking. It is worth noting that as is normal for meters, the specification is given as % FSD; The % of actual values is often quite large but within 5% except for the actual value at 1V on the 3VDC range which is -6.25%, this translates to -2.08% FSD. Most DC errors were on the low side which suggests that I might be able to set the calibration better. Having said that, the AC errors were a mix of high and low so to actually accomplish better calibration would be very tricky and most likely not stable, so I will not attempt to “improve” it.

It is essential to carefully zero the meter with the input terminals shorted on each AC range, you cannot simply switch from range to range. The zero variation on the DC ranges is negligible. This is very likely due to diode contact potential about which I say more in the theory section below.

I checked the frequency response. It is claimed to be flat over the range 100Hz to 100MHz which if true is excellent, for the period of this meter at least. I checked this aspect out using a HP 8601A generator terminated into a 50Ω through BNC with the AC probe and ground ring connected right at the termination, on the 3VAC range I observed a 1dB increase up to 97MHz increasing to +1.5dB at 110 MHz which is the generator limit. Clearly there is a resonance somewhere in the signal path yet I am impressed. This meter is better than it seemed to me at first, second and third blush. This is the connection arrangement to the signal generator:

TS-375A/U HF Test Connection to Generator

Overall, this seems to be a capable instrument, at least in the context of its vintage. However, it is essential confirm to that it is reading accurately in the range you desire to use by connecting a known accurate meter in parallel with it and testing it using a DC or true sine AC supply. This is not as silly as it sounds because the point of this meter is the extremely light load it applies to the circuit under test and its wide bandwidth. Your DVM probably cannot match it in these aspects.



Essentially, it is a DC amplifier and meter. A good DC amplifier is not a trivial proposition because of drift. Most VVMs rely on large amounts of degenerative feedback to stabilize the circuit, however this can result in problems with HF oscillation. Perhaps that is why the HP 400H mentioned above is limited to 4MHz. This design relies on both regenerative and degenerative feedback that I will attempt to explain with the aid of the manual. (The HP 425 Micro-Volt/Ammeter uses a light chopper that produces a rough squarewave that is proportional to the DC current and in this way, completely removes drift. This is followed by an amplifier having a passband at the chop frequency that suppresses other frequencies (including 60Hz) heavily. In fact the chop frequency is chosen not to be harmonically related to line frequency to further ensure no hum on the extremely fragile signals. The amplified chop is then demodulated, rectified and applied to the meter. I think this exemplifies that DC amplifiers are truly non-trivial!)

Because it is a DC device, AC currents must be rectified and this takes place in the AC probe. The result is a peak voltage and this is corrected such that the resulting DC voltage presented to the DC amplifier is equal to the RMS voltage, if the AC current is truly sinusoidal*. This correction is applied by R-111 in TS-375/U and fixed R-133 plus variable R-134 in TS-375A/U. If the waveform is other than sinusoidal, a form or crest factor must be applied. The AC rectifier is right at the probe tip so that the AC signal does not have to travel along a wire and this is how the wide bandwidth is realised.

* The GR 1201-C oscillator that I was using to test and calibrate the AC ranges with has a slight clip on the positive peak (I don’t like GR equipment other than their bridges, heresy I know) and this was sufficient distortion to confound my efforts. The HP DVM I used as a reference would indicate the true RMS of this signal but the VVM would not. I ended up using a HP 205AG that produces a clean waveform with enough available amplitude to test the 120V range.

First here is the simplified circuit:

VTVM TS-375 A:U DC Amplifier Simplified Schematic

The screen cross-coupling provides the regenerative paths while the DC plate to grid offsets, (coupling batteries in the diagram) provide the degenerative paths and the output terminals. The plate resistors and the tubes comprise the 4 arms of a bridge and so you can think of the meter as being across the bridge from one plate/plate resistor node to the other plate/plate resistor node, via the coupling batteries. This visualisation may help you to see the bridge.

Degenerative feedback brings stability, any error is returned to the input as a cancellation voltage. However there is a limit to the degree of degenerative feedback possible with a conventional amplifier due to the limits of the single stage amplification. In this design, the additional gain needed for stability is obtained by regeneration, permitting a higher level of degenerative feedback and thus higher voltage stability in this single stage circuit than would otherwise be possible. Quoting from the manual “By this method a gain ratio greater than the actual gain of the amplifier tubes proper can be realized.”VTVM TS-375 A:U DC Amplifier Schematic

Now to the actual circuit: The “batteries” have been replaced by gas voltage stabiliser tubes, the internal resistance of the tubes being low enough to act effectively as batteries in this circuit. The strike currents for the stabiliser tubes is provided by the keep-alive resistors, R-106 and R-107, each being 100k. These terminate at the negative supply lead. The keep alive currents are too great for each plate circuit so the positive end of the strike tubes are fed via V-103 cathode followers from the + supply lead, the cathode followers being driven by each screen grid. In this way, the practical circuit emulates the simplified circuit. The resistor R-108 is included in the cathode circuit to allow for the voltage dropped across R-106 and R-107.

I found the circuit as drawn difficult to follow so I re-drew it thus:

VTVM TS-375 A:U DC Amplifier Schematic Redrawn

The regenerative (positive) feedback paths are obvious, R-104 limits the regeneration to a prescribed level. What was not obvious to me was the degenerative (negative) paths and the meter circuit so my re-draw show how the meter is connected to each plate/plate resistor node of the bridge via the screen CFs and the gas tubes. The gas tube keep alive resistors are effectively in parallel (Thevenin) with the plate load resistors. The degenerative feedback paths are shown in blue and red. Taking the blue first, the path is plate to ground then via the input voltage, to the input grid. Since the whole circuit is floating, the ground connection does not affect the operation. The red feedback path includes the meter which has a negligible resistance of 1K. Any grid current is orders of magnitude too small to cause sensible deflections of the meter which is driven by the current due to the unbalancing of the plate voltages.

(At this point a question arose for me: Why isn’t the meter simply connected directly across the plates? The answer is that the resistance value of the meter path would reduce the regenerative feedback too much. As it is, the CFs isolate the R-104 regeneration path from the meter path. It is a clever circuit.)

At the point of optimum adjustment, the screen-grid circuit is critically regenerated and on the point of self-sustaining oscillation if the degenerative paths to the control grids were disconnected. The manual suggests thinking of the circuit as an amplifier having infinite gain due to the critical regeneration which is completely degenerated by external feedback paths (the blue and red paths); thus the stability improvement possible due to degenerative feedback around any amplifier is in this case, carried to the limit.

At this point of critical regeneration, the screen grids do the actual work of unbalancing the bridge to produce an output (proportional to the input) and the control grids simply serve to initiate the unbalancing action. For any steady value of input, the potential of each control grid with respect to its cathode is the same and the potential difference across the grids is zero. The value of zero grid voltage excursion is that it entirely removes the effect of the curved grid voltage, plate current relationship and the amplifier is strictly linear in its input to output relationship. As I said, it is a clever circuit!

I mentioned diode contact potential earlier: A hot cathode diode will develop a negative potential on the plate with no ac input, this potential is termed the “contact potential” of the plate wrt the cathode. The diode in the probe is compensated by adding a similar diode to the red side grid circuit. I suspect that the very slight change of DC resistance across the compensating diode as the range is changed may account for the need to re-zero the balance every time the ac range is changed.






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  1. Bruce Baur permalink

    Another great article! Looks like this meter was made to take a beating and keep on ticking.. BTW, under spec’s you state input impedance 5meg, 70uf, not 70pf.

  2. Richard- I will soon have a TS-376 A/U. Would you happen to know where to find a service manual for it?

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