AVO MK4 Tube Testing - 300B

[Jerm]

Hello Everyone,

 

I am in the possession of an AVO VCM MK4 tester which belonged to my father (known as Owen Y on this forum) who passed away in 2021. I remember he was quite fond of this tester and said it was a good piece of equipment so I held onto it with the intent of learning how to use it.

 

A colleague happened to ask me to test some 300B valves and I thought it would be a good opportunity to give it a go. I have observed my father using this tester several times over the years, I have also watched the few videos on Youtube and thought I’d be able to figure it out.

 

I have two copies of the data book and they do not match, under the VCM VCM (Valve Characteristic Meter) columns, one specifies a Anode voltage of 400V and the other 500V which seems quite high? ON the other hand, data set that specifies 400V doesn't list a mA/V setting.

 

I notice that some of the tube boxes have factory values noted on them ie. 54mA/5.1MA/V. I'm assuming these are factory measurements?

 

I also asked this question on another group and there was some concern bought up about the high filament voltage of the 300B and whether that would be too much for this particular tester. Obviously I don't want to damage the tester so I am a little worried about this.

 

I'm fairly certain it is possible to test these tubes as he has some notes in a folder which show 300B test results. Interestingly, the selector switch number is noted as 264 300 000 rather than the 364 200 000 which is listed in the book.

 

Does anyone here have experience testing 300B on and AVO MK4? Any help would be appreciated.

 

Cheers,

Jeremy

Edited December 5, 2024 by Jerm
clarification of data

[xlr8or]

Just chiming in quite late on this one. I've read your other post on FB.

 

Just use the first column settings below with the 300A numbered setting configuration.

 

Heater voltage = 5v

Anode voltage = 300v

Grid voltage = -61v

Anode current = 62mA

Mutual conductance = 5.3mA/v

 

Edited December 16, 2024 by xlr8or

[Jerm]

14 hours ago, xlr8or said:

Just chiming in quite late on this one. I've read your other post on reddit.

 

Just use the first column settings below with the 300A numbered setting configuration.

 

Heater voltage = 5v

Anode voltage = 300v

Grid voltage = -61v

Anode current = 62mA

Mutual conductance = 5.3mA/v

 

 

Hi Kirk,

 

Thanks for your reply.

 

I found that exact data sheet on my travels and wondered what the distinction was between the two columns.

 

I actually already tested the tubes last week.  Ended up using Vp=350, Vg = -70, Vf = 5. The results are below, the two Angela tubes seem to measure quite low, 1,2 and 2,4 seem to be the best matched and the latter also tests the highest.

 

Do you have any thoughts on these reaults?

 

Cheers,

Jeremy

 

[xlr8or]

They all look to be low. Are any of the tubes new?

 

I would be inclined to first measure the actual heater, anode and grid voltages with a DMM to be sure those values are correct. Also, if you're using the 2nd column then the grid voltage should be set to -74v, which means the values you've shared above will even be lower.

Edited December 16, 2024 by xlr8or

[muon*]

Owen is missed here.

 

Nice to see you here, Jeremy.

[Jerm]

13 hours ago, xlr8or said:

They all look to be low. Are any of the tubes new?

 

I would be inclined to first measure the actual heater, anode and grid voltages with a DMM to be sure those values are correct. Also, if you're using the 2nd column then the grid voltage should be set to -74v, which means the values you've shared above will even be lower.

 

I'm told that these tubes are well used and could even have been damaged by a faulty amp circuit.

 

How would you suggest checking the voltages? I'm assuming I can check the heater supply by probing pins 1,4 with my DMM set to AC. Ideally it would be done with the the tube in place which makes things a bit trickier? For the Grid and Anode voltages I can measure pins 2 and 3 but what do I use as a ground reference?

 

10 hours ago, muon* said:

Owen is missed here.

 

Nice to see you here, Jeremy.

 

Hi Ian, Thanks for your kind words. I think he would be happy that I am using his beloved AVO tester, I should have made more of an effort to take him up on the training sessions

[xlr8or]

11 hours ago, Jerm said:

For the Grid and Anode voltages I can measure pins 2 and 3 but what do I use as a ground reference?

 

Any of the heater pins act as a reference to the cathode (k) with respect to ground. The diagram below should be viewed looking at the pins from the bottom side up.

 

Edited December 17, 2024 by xlr8or