AVO MkIV VALVE CHARACTERISTIC METER

[Alec_Smith]

Hi Everybody!

I`ve been collecting radio sets for a while now, but until recently I have never been able to test valves. I have just purchased a serviced avo mkIV and find the controls very straight forward, but I cannot work out in my head how to set the negative grid volts switch.
I have working instructions, but reading it is almost like understanding ancient Greek! lol. It explains the switch positions, but I cannot work out by example what the settings here should read for valves under test.
Two examples I wish to cover are the valves ECC83 and 6SN7GT.
Can someone please help me understand the negative grid positions that I should employ for these examples which should help me determine those settings for other valves?
I feel really stupid asking, but no-one has been there to show me. The Avo was purchased directly from the widow of an ex-radio engineer who sadly passed away a few months ago. Unfortunately she cannot help.
Any help gratefully appreciated!

Alec.

[RobinBirch]

Ok,
THis is from memory as my VCM Mk 4 is in store at the moment and I haven't done this for a while.

Looking in the book an ECC82 has a roller switch setting of 741 226 413.

So, following through the set-up instructions:

1) Circuit selector to check C, electrode selector to A1
2) Meter switch to 100 on the Ia scale
3) Backing Off controls fully clockwise
4) Heater Volts to 6
5) Anode Volts to 250
6) Grid Volts to 6 (I can't remember if the grid volt setting is on two switches or not. If it is on two switches then set the range 0-10 and put the other one on 6).
7) Screen volts doesn't matter
8) Set the roller selector
9) Leakage to ~ and let it all warm up (no valve in place) and set the pointer to ~ using the SET~ switch
10) Put valve in
11) Fully rotate leakage switch and check continuity at H and insulation at all other places.
12) Circuit selector to H and check leakage between heater/cathode and all other electrodes.
13) Circuit selector to C/H and check leakage between Heater and Cathode with valve hot
14) Circuit selector to TEST, this should give you the anode current. Reduce the meter switch setting if it goes over scale. If the protective relay acts and everything is set up ok then valve is probably scrap.
15) Use backing off control to reduce meter to 0, reduce the Meter Switch setting to do this as it will give a more accurate 0. Meter Switch to mA/V if valve ok then needle in green band.
16) Repeat 14 and 15 with the Electrode Selector set to A2.

Now then. Does this help or not. The most confusing bit will be, if it has a two switch gid volts setter, setting the range and then the value on the two switches.

Let me know what else you need and I'll try and do this from memory.

Regards

Robin

[Alec_Smith]

Hi Robin,

Thanks for your prompt reply.
The issue I am having is with the Grid Volt switch.
It has two scales, one white and the other orange.
The scales are marked:
0 - 20 (white)
0 - 5.0 (orange)

To the right of the scale is a selector switch which is marked:

0 - 5, 0 - 5 +10, 0 - 5 + 15, and 0 - 5 + 20. (Orange)
The next setting (rotated on same switch but in white)
0 to 20 + 20, 0 to 20 + 40, 0 to 20 + 60, and 0 to 20 + 80.

It is this area that my brain is having problems with these settings.
How would I apply these settings for say an ECC83 and a 6SN7GT?
Also what other pointers do you have in understanding these settings?
I am grateful very much for your time and trouble in assisting me.

Alec

[ALANS ANITAS]

Do you have a copy of the AVO valve data book? This should give you the correct settings for the controls. As it is 40 years since used one of these machines, I cannot remember these settings. Try searching this website for AVO VCM, look on Ebay, do a Google Search, or if you only want to test a few types,make a request here. Several dealers exist in this country who specialise in valve testers, but AVO do not seem to have exported them. They can test many thousand types of valves.


Regards

ALAN

[Simon Gittins]

Alec,

As well as the switch, there is a silver coloured ring outside it that is the fine control (this moves the scales you describe).

For ECC83, the negative grid voltage is 2. This is obtained by setting the switch fully anticlockwise to the "0 to 5" position and moving the fine control until "2" is showing on the orange outer scale.

The only point I would add to Robin's instructions is that for the ECC83 and similar valves with identical sections, the "normal/unlimited" switch needs to be set to "unlimited".

Let us know how you get on.

Regards,
Simon.

[Alec_Smith]

Hi Everyone,
Thanks for your replies. I do have the handbook instruction manual and data book, as I have said the problem is understanding the graduated values given as described earlier in my post.
For the ECC83 the negative grid volt setting is given as: 2 and I now understand that as you describe. But say if I am using a 6SN7GT the setting is given as 8, so how would I have to set the control to read that correctly?
I don`t understand the multiple settings such as 0 - 5 + 10 and so forth.
Sorry to be a pain, but my problem is how you calculate for higher settings.
There are many valves that require a higher setting than the straight forward 0 - 5 another example would be WD119 which has a higher setting of 19, so what settings would I need to achieve that figure?
I think if you can guide me on those points I may then crack it.
Thanks for bearing with me.

Alec

[Station X]

The fine control has two ranges, 0 to 5 and 0 to 20.

The selector switch settings have two parts. The first part tells you whether to use the 0 to 5 or 0-20 scale on the fine control. The second part tells you what voltage to add to the fine control reading.

For example to get 55 volts:-

Set the selector switch to "0 - 20 +40"
Set the fine control to 15 volts on the 0 to 20 scale.

40 +15 = 55

[Alec_Smith]

Hi All,
Thanks for your help in this matter - the penny has finally dropped! Doh!!
I will try again tomorrow.
Many thanks to you all!

Alec

[Alec_Smith]

Hi again, everyone!
I have tested now a range of valves and am impressed with the results following your help. Just one small `itsy bitsy` question.
I want to get the gm reading for each valve using the backing off controls.
I can`t seem to get this to work. Can anyone tell me the exact procedure using the backing off controls for the Avo MKIV VCM?

Alec

[Simon Gittins]

Hi Alec,

Pleased to hear you're getting results.

The comparative "goodness" method is probably the easiest one to try first, this gives you a reading on the Replace/Good scale. The Set ma/V control (two parts, like the grid control) is set before starting the test (e.g. 1.6 for ECC83).

Starting from the point at which you are measuring the anode current, turn the Backing Off control until the reading is close to zero, the Meter Switch needs to be moved, step-by-step to the lower ranges adjusting the Backing Off control each time. When you get to the lowest range (2.5mA) the Fine Backing Off control can be used to set the reading to zero. Now just move the Meter Switch one more step to the mA/V position and read the result on the Replace/Good scale.

I hope this helps.

Simon.

[RobinBirch]

Hi Alec,
It occurs to me that if the controls, especially the backing off and similar variable ones do not do exactly as they are supposed to do then the set needs a it of alignment. How to do this is detailed in the maintenance manual and isn't difficult.

I don't know where my one is at the moment but I am sure someone can point you in the right place.

I have to say that if you can get this set up properly and yourself reasonably well versed in its use then it is one of the best all round characteristic measurement/valve testing boxes around.

Cheers

Robin