AVO VCM HCS fault octal and B7 bases

[JoshWard]

Hi all,
Earlier this year I was lucky enough to acquire this beautiful machine. I have wanted a valve tester for many years so when this popped up for sale at a reasonable price I had to have it, much to the upset of my credit card. Anyway, it has been working well. I did take the lid off when I first acquired it and it has clearly been restored at some stage in the not too distant past. It looks to have been a very thorough and very neat job.
Recently, I was testing some Russian KT88s I had kicking around, leftover from when I last had a valve Hi-Fi amp. I remembered having some trouble with them in the amp so I wanted to pop them onto the tester to hopefully weed out the good from the bad. I found a couple of bad ones but ever since then I have been unable to test octal or B7 bases. Instead, the valve shows a heater/screen short on the 'cold' test, turning in to a heater/cathode/screen series of shorts on the 'hot' test. I have tried this on a known good valve. Needless to say, as soon as I move to the 'test' position it trips out. Once the valve is powered down, the cathode short clears as the valve cools.
I have attempted to capture a video of this problem in action, using a known good 6V6 valve: https://youtu.be/Nb2ORNf4TLU
I have opened up the lid once more but cannot see anything obviously amiss around the bases in question (no charred resistors etc) and it is working perfectly on other valve bases (although I can't claim to have tried them all). With lots of switchbanks and wires everywhere I find the insides of this thing frankly rather terrifying so I am hoping somebody out there with much more knowledge than myself might be able to point me towards where the fault might lie?

[joebog1]

The Russian KT88's. Where they/ are they Sovtek "coke bottle " style ?.

Joe

[JoshWard]

Hi Joe,
Yes, that's right. Two different brands- Psvane and JJ. I vaguely remember the fault when using them was the heaters started to glow red and an enormous hum appearing before I quickly switched the amp off. This was a few years back, I should have binned them straight away!

[joebog1]

I have posted my story on Russian KT88's. You only had hum, I had fireworks !!. Japanse top notch amplifier, that had very weird output stage, and the "guarantee " from the seller ( and I had been dealing with him for years, but MOSTLY for NOS bottles ) and what a magnificent failure they were.
To finish the repair I had to buy TWENTY valves to find four that functioned. The feedback from that custsomer was NOT good to say the least and he never visited me again.
So "that " amplifier used a sort of Parry Cathamp output stage, EXCEPT the plates were operated at 450 volts and the screens at 520 volts. I also tried some in a guitar amp that I designed and built, they lasted about a week before blowing the fuses and overloading MY output transformer. The ONLY one of my transformer ever to fry.

Yes I am biassed!!.

Joe

[David Simpson]

Hey Josh, any chance of a picture of your "AVO HCS VCM" please.

Regards, David

PS. Managed to have a shufti at your Youtube presentation - see that it is an AVO VCM MK1 or MK2. My laptop sometimes doesn't latch onto some folk's youtube stuff, think its a McAfee thing, since I changed from Panda. DS

[David Simpson]

Further thoughts Josh - Have you fully checked the thumbwheel code against the valve's pinout ? Try meggering the heater & cathode pins for a short. The "Valve Museum" website might give a different set of pin connections than the AVO Valve Data Manual. Another good reference is the AM Ball Illiffe Valve Data Books.
Each thumbwheel indicates the electrode connected :- ie. A1, A2, S, C, etc.
As for Russian power o/p valves - much has been said over many threads. My thoughts relate to a "bargepole".
Please don't have a go at the VCM's inside - there must be an experienced Forum AVO VCM enthusiast nearby in East Anglia. Try & make contact.
Meantime, I'll look out a spare "Standardised Valve" & post it off to you, backsheesh.

Regards, David

[JoshWard]

Hello David,
Apologies, I used HCS to stand for heater cathode short. The thumbwheel code is definitely correct for the 6V6 in the video. I was able to test several other octal and B7 valves successfully before putting those pesky KT88s on there, including several 6V6s. I'm afraid I don't have a megger.
Yes, I am very wary about touching anything inside it. I would hate to make it worse!

[David Simpson]

A handy bit of kit - a Megger, Josh. Jappo/Chinko versions of Insulation Testers can be bought new on the Internet - £20 to £40 - ish. You should be able to pick up a decent 2nd hand working proper one at one of the BVWS/Retrotech/VMARS sales/auctions from a reliable seller, for less than that.

Regards, David