AVO CT160: screen voltage question and calibration isssues

[kirin1991]

Hi all,

I am working on an AVO 160 and I read through a lot of good material published on the forum, thanks again to all the contributors.

I did preliminary checks and after checking my line voltage (230V) I set the voltage with the coarse and fine adjustment pins, then checked from the top cap connection pins the heater and anode voltages.

Anode voltages measured in AC mode, with meter connected between A1 and C, were 1.1 times the value indicated on the anode dial as expected, so I confirmed that the right tapping of the mains transformer was chosen. Heater voltages also measure within limits.

I followed the calibration procedure for the grid voltage and set 20.8V DC across RV2, and the grid voltages at the 13V and 4V mark on the dial are within the limits of the voltages indicated in the calibration manual. Then with the link closed I can't get 47.3VDC, it can only reach 46VDC, therefore the needle does not fall on the calibration mark on the meter of the instrument. I checked several components of the instrument (R1, R2, R5, R3, R4, R9) and a few others, and they all measure good, the only ones a bit high are RV2=10.44kohm and RV1=2580 ohm. I also tried to change the rectification method from tube to diode for the calibration and grid circuit, suspecting a high voltage drop in the tube, but with no success. I temporarily gave up on this and proceeded with other calibrations.

I checked the meter and it has a full scale deflection of 30uA, and the internal resistance seems correct.

I checked the current reading with a valve under test, comparing it with an external multimeter connected to the A1 link, and the reading in DC is half of the indicated one on the meter, as expected.

I checked the SET mA/v RV1 and i could see that i had 600mVDC across RV1 with the dial fully rotated, but only 450mV across R5 at the 1mA/V mark, so all the readings across R5 at the various positions of the dial were low. I tried to apply the section 6 of the calibration manual and modified the position of RV1 on the u-shaped stirrup, and now i can get the right readings at the 1, 2, 5 and 10 mA/V positions, with 520mV at the 1mA/V mark.

I then managed to calibrate two tubes with a second tube tester (Avo MKIV, I have it in Italy, now I’m currently in Denmark) and I measured an EL34 and an ECF80 to cross check the calibration. The AVO MKIV has been checked before testing the tubes, I carried out the calibration procedure of that unit with no problem.

The problem I found is that the EL34 and the pentode section of the ECF80 measure a lower current than the one I got with my reference tube tester: EL34 measures 56mA on the AVO160 instead of 64mA on the MKIV, and the ECF80 pentode measures 4.5mA on the 160 and 6.1mA on the MKIV. The triode of the ECF80, instead, seems to measure about the same current (with some wiggle room due to calibration of both grid voltages on the two tube testers). This made me suspect of the screen supply circuit, and I tried to measure the screen voltage in loaded and unloaded conditions, and I found that the voltage, in the case of EL34, drops from 127 to 104VDC before and after the tube is connected in test mode. I checked the current drawn and it’s 4.5mA, for an equivalent resistance of (127-104)/0.0045=5100 ohm.
The AC voltage before the rectifier tube of the screen supply does not drop when the load is applied, so I suspected an excessive voltage drop in the rectifier, and bypassed it with a diode, but the rectified voltage dropped of the same amount when the test of the tube was started. I tried to check the resistance between the rectifier cathode and the pin corresponding to the screen on the tube socket and there was continuity with no extra resistance, so I can’t understand why the screen voltage drops and if it’s supposed to drop, does somebody have an idea? Could this be the reason of the lower current readings?

Cheers
Francesco

[David Simpson]

Hello Francesco,
Have a look at my "very basic" CT160 functional diagram, which I've posted many times over the years.
Hopefully, you've got a copy of one of the several "Servicing Manuals" floating around the vintage radio fraternity, & also obtainable in pdf format by a Google search.
Basically, CT160's, & the AVO range of VCM's, just work on AC Mains derived Peak to Peak(P-P) voltages fed via switches from the Mains Transformer's secondary windings. So, with a valve under test - with an AC mains of 230/240V 50Hz - a Va selection of 250V puts approx. 2.8 x 250V ac on the Anode link ! So be very careful. The Screen gets 1/2 wave, unsmoothed peak pulses. Both those voltages are in phase, whilst the 1/2 wave rectified grid voltages are in anti-phase - "Grid Bias".
Importantly note - on a CT160 - the "Coarse Ia & Fine Ia" do NOT control the valve's anode current. Only -ve Vg does that once all the voltages have been set in accordance with AVO's Valve Data Manual.
Unfortunately, AVO in their wisdom, quote "RMS" values of voltages. You will need to get your head around the simple maths involving "root 2" etc.
Whilst you are just getting used to the CT160 & trying your hand at testing valves - just chose some working 2nd hand triode & pentode valves which run on low Ia. Avoid big EL's & EF's & KT's etc.
If you check back through "Search" you will find as mass of informative Threads, & some of my ramblings. On that note - I'll cease another "banging-on".

Regards, David

[Craig Sawyers]

A good thing to get your head around is the patent that covers the design. Attached.

Craig

[kirin1991]

Hi David,

Thanks, yes, I have read a lot of material and understood the AC nature of the machine, and I have read in many posts that when measuring the voltages stated in the calibration manual it's necessary to be sure of the behaviour of the multimeter in use. I even made an experiment myself with my DMM and tested a circuit of known AC RMS voltage, which I connected first to a half wave rectifier and then to a full wave rectifier, and I could confirm that my DMM in DC mode reads mean voltage, so 0.9*RMS in full wave rectifier and 0.5*0.9*RMS in half wave rectifier mode.

I am under the impression that the 20.8V stated in the calibration manual are meant to be DC mean voltages, but please correct me if I'm wrong.

The main doubt that I have is about the screen voltage after the rectifier, that seems to drop when the valve is under test. Is the drop normal, can anybody confirm?

[ms660]

Originally Posted by kirin1991

I am under the impression that the 20.8V stated in the calibration manual are meant to be DC mean voltages, but please correct me if I'm wrong.

That's right, DC mean.

Lawrence.

[David Simpson]

I've PM'd Francesco some more detailed info. All of which has been banged-on about on this Forum over the years by me. So won't boor folk again.
What he needs is a DC Standardised Valve. I've none left, having given away about 20 odd over the years - 6AQ5's & 6AU5's. They're much better than the pokey wee CV451/491's which AVO recommend. Now, with the advent of a number of folk building or buying ROETEST's & Sussex's in recent times - perhaps someone else could take up the dc standardised valve banner ?
I've a wee handful of NOS pentodes with similar Ia's to the above. So might DC standardise & Gm graph them over the winter months. But, this God awful weather has scuppered a number of urgent smallholding jobs, and the shooting season has started. So they take priority.

Regards, David

[kirin1991]

Thanks David, it would be very nice to have a second reference tube.

I have some updates:

I started measuring the screen circuit, and I found out that the waveform I can see from the oscilloscope is very deformed, and the voltage changes when I change the anode voltage!

I then disconnected the cathode of the rectifier tube that sources the screen voltage, and I can indeed see a clean half-wave rectified voltage when I check the pin of the rectifier, but on the other side (on the rotary switch) I can see an AC voltage that varies with the selected anode voltage. It goes up to 120VRMS when the anode selector is on 400V and goes basically down to zero when the anode selector is at the lowest setting.

Could it be a problem with the rotary switch itself or you have other suggestions? I can send some pictures from the oscilloscope if necessary to clarify the situation.

[kirin1991]

Hi all, back with some new info.
In my quest of trying to make the anode and screen circuits independent I tried installing two rectifying diodes in series with the two wires that go the SH1 of the rotary electrode selector (see attached picture).

By doing it I found out that finally changing the anode voltage seems to not influence anymore the screen voltage, but then I had a strange surprise.
I checked the anode voltage from the anode top cap connector with an oscilloscope and found that I still have a big full wave AC waveform at the pin (about 220VAC), even if I move the anode voltage selector to the lowest setting.

I managed to trace back the source of this AC voltage to the 0-50V floating taps of the main high voltage transformer, where I can measure the following: I see 50VAC if I go with a tester and put the tester leads between 0 and 50, but if I put one test lead to ground and the other to 0 and 50, I get respectively 215VAC and 265VAC. This is confirmed by the reading of the oscilloscope.
I know it's a floating tap but is it supposed to be normal? Do I have a busted transformer winding?
I can see no extra AC voltage in the other floating 50AC winding that supplies the negative voltage for the grid and set mA/V circuit.

Do you think I can test connecting an external transformer with a 50V secondary to the circuit instead of the 0-50V taps of the original transformer to see what happens?