The LeyMarcAVO DC-VCM

[David Simpson]

Folks, just a few preliminary thought for fully DC-ing my much altered CT160. The cabinet from a Marconi 995A/2, a Marconi TF1041C VVM, two lovely Leybold DC PSU's, and two nice wee 200uA Ernest Turner mc meters donated by my pal GM8 AGM - Martin, all lend themselves to building a DC VCM. Just the valvebase unit from my CT160 will be used via 9 way c/o yaxley sw, and a lovely big 10pin Jones plug/skt recently aquired from a friendly VMARS contact - John Roper. Additional Va HT can be supplied from my home-brew 900V electrolytic reformer/psu.
A "Sussex" would be my ultimate dream, and I'm full of admiration for it's designer & folk who have gone on to build them. However, with my mind-set stuck in the 60's, I though that an AVO/Marconi/Leybold DC VCM would make a worthwhile wintertime project. God, I've been horrendously busy outside since springtime.

Regards, David

[Dekatron]

Hi David,

I think you might get into problems with the Leybold power supplies as those are probably un-regulated and only controlled by a large potentiometer for the output voltage. They usually have a quite small smoothing capacitor too which means that you have large ripple on the output voltage. The voltage is also dependant on how much current you draw as the resistance in the circuit is quite high, if I remember correctly the potentiometers used have a resistance from 4.7K up to 10K.

I have a similar type of power supply manufactured by Nordstedts here in Sweden and it only uses a dual 10uF/450V capacitor after the bridge rectifier for the high voltage output and a small choke which means I have something like 10-20V peak to peak ripple depending on how much current I draw. One more thing with my power supply is that it was designed for 220V and is not adjustable for 230V (or any other mains voltage), not a big problem for the high voltage outputs but the 6.3V windings get something like 7.5V.

[David Simpson]

Thank you Martin for your reply. However, surely you didn't expect me to just bung the PSU's into the empty 995A/2 cabinet & hope for the best. The Leybolds will be stripped down and their circuitry much added to by capacitive & inductive filtering, and the nominal 6.3V heater o/p's will be rectified, smoothed and carefully monitored. As will all voltages & currents. The Leybold's HT o/p's will not be the sole source of Va or Vs. Well smoothed external high HT, with greater current capacity, will be available at the flick of a switch.
Obviously, these are early days and it will be many months before construction commences. The picture just showed the very basic gubbins of the project. Much much more time will be spent carefully chosing additional components, switches, etc. That is why I clearly stated that I was in the "preliminary thoughts" stage just now.
However, helpful & constructive advice will always be very appreciated. One such - would be regarding me chosing the TF1041C for measurement purposes as opposed to the AVO VVM. Of which I have little knowledge? But asthetically, it would lend itself to an AVO CT160 ish project.


Regards, David

[Dekatron]

Well, you never know, I sometimes miss the most obvious things myself so I thought it would be a good idea to just point it out.

[cannissolis]

seems a pity to strip those beautifully made Leybolds when loose components can be picked up at jumbles

[David Simpson]

Right enough, Cannissolis, but sadly up this far north - there are very few junk sales. Believe me, I have been looking for a while since last year. But junk sales are just that - someone else's junk.
I've had the Leybolds for about 3 years, one was in perfect nick, but the other required it's Siemens selenium stack rectifier changing for a modern encapsulated bridge. As it happens, just yesterday, I stripped down one of them. Despite their robust construction and reliable & controlable o/p's - the intercomponent wiring was poor. Soldering was a bit blob-ish, and the sequence of colouring of the wiring was completely random. ("Earth" was "Red"!). So much for German workmanship. On the other hand, dissmantling the Marconi VVM was a dream. After 40 years, I'd foregotten just how well they were made, and easily parted into a seperate PSU & front CU sub-units for servicing. This bodes well, as these sub-units fit perfectly into the TF 995A/2's empty cabinet.
The Leybolds' chassis' are just too bulky to be incorporated inside the cabinet. So will remout their mains t/f's, hefty chokes, electrolytics, rectifiers, etc on a new chassis. Thus leaving plenty of room for additional circuitry, Their big w/w pots will be mounted on a new front panel.
Even if all goes pear-shaped, CT160-wise, I'll still have a jolly decent combined multi o/p PSU/VVM which will lend itself to many future valve based projects, I hope.

Regards, David

[cannissolis]

Well it sounds like a fun project at any rate, didn't realise you were stripping the vvm as well, eek! ah well I have stripped my fair share of test gear over the years. I always feel guilty when I do though.

[David Simpson]

Yes, Cannissolis(Its a nice Forum name, but do you have a simpler given name ? I keep thinking I'm living in Cyprus again, afaristo).
Right enough, the TF1041C easily strips down(as designed by Marconi for servicing & repair) to it's bare essentials. I had thought about sticking with the AVO name & using their VVM, but know very little about it. I'm guessing its not so easily stripable.
A nice piece of 3/16" satinized aluminium has made a fine base for mounting the heavy chokes & mains t/f's out of the Leybolds. Lovely chokes in fact, 15H, 100mA rated. Also the two big Siemens 50/50uF Electrolytics have enjoyed a long stint on my homebrew reformer/High HT PSU at their working voltage of 450V and display minimal leakage. Will probably mount them on a nice big piece of 3/16" paxolin, which will be also used for tag-mounting components. An old vintage tag strip also lends itself for component mounting.
I was again dissapointed with Leybold for just taking the leads straight out of their t/f's & choke's windings, without the secure benifit of tag-strips. So have fitted some wee ones.
'Fraid this'll be a long project. Our syndicate's pheasant poults arrived yesterday. So will need a few weeks of molly-codling. At home - still heavily into grass cutting, and fruit picking is a burden.

Regards, David

[David Simpson]

Further progress, thanks to prolonged rain this last couple of days.
Have decided to forego an internal selectable DC heater supply. Already have a super modern 3channel 30v/2A Bench DC PSU with accurate voltage & current displays. Would've meant taking up valuable needed space inside the ex Marconi 995A/2 cabinet.
Both Leybold's HT PSU's rectifying circuitry now installed using SKB2/08's as bridges to replace the old tubular Selenium rectifier bridges. The two big 50/50uf capacitors along with the 15H chokes give lovely ripple-free 380V max for Va & Vs.
The Leybolds T/F had two nominal 6.3V windings each. So after a bit of jiggery-pokery to select the correct phasing, make a jolly good selectable 0 to -30V 4way switchable grid voltage when fed to a wee BY164 bridge.Two 100uF electrolytics & a 10H choke ensure ripple-free DC. A 10K pot gives finite selectability over the 4 ranges.
Still plenty to do. Eventually will connect to the CT160's Valvebase unit via a hefty Jones plug and a multi-wafer C/O switch. This is very much a "suck it & see" project. All stages are carefully monitored current & voltage-wise with AVO 8's or 9's. 5 seperate fuses protect all i/p & o/p voltages.

Regards, David

[Ed_Dinning]

Hi David, to save head scratching on winding phasing, you could have used a voltage multiplier for the bias supply as the current is so low.

Ed

[David Simpson]

Right enough Ed, that was plan B, should phasing have proved too much for my grey cells.
I'm still penciling the optimum layout for switchgear on the plywood CU template. Two thumping great 3.3" dia 4K pots which Leybold used for their HT o/p take up a fair bit of space. The two Turner mc meters fit in quite nicely. Just leaves a few toggle switches & wafer switch assemblies to work out. A while back I aquired, buck-sheesh,a nice NOS kit of RS"Maka" wafer switch components. Wafers, switch assemblies, shafts, spacers, etc - the whole caboodle. Guess that'll involve a lot more head scratching.


Regards, David

[Ed_Dinning]

Hi David, the RS kits were fine and quite easy to use; beware of bbm and mbb types. Note that most of these were LV only and there was a separate special wafer for mains use. Screens were also supplied with the kit for RF use.
The best kit, which I'm still searching for is the Plessey one. This was used professionally and would allow you to build all types of switches, including "shorting" poles for use in coil packs. It included a small pair of eyelet pliers to close the hollow rivets used to mount the switch contacts.
I have a lot of the Plessey bits but I'm still searching for the rivet pliers.,

Ed

[David Simpson]

Hello again Ed,
I had anticipated the max working voltages & currents being a bit higher than the standard wafer's rating. So had planned on doubling-up wafers for each function, and also only using every other set of contacts to avoid any making/breaking problems. Not that switching with wafer switches under high Va/Vs/Ia conditions is anticipated. My gash box produced a nice h/duty 2 pole mains en/g relay which I'll use for applying Va & Vs after all preliminary electrode voltages have been set up. This relay will only be operated for short periods via a n/o spring-loaded push button.
I'm not planning any sort of CT160ish direct reading of Ia/Vg. AVO expects CT446 users to use their brains slightly when testing for a'. So why not use the same similar proceedure for Gm. Only takes a few seconds.
Way in the future, should this DC only hybrid work ok, possibly I'll think of a signal test facility. On the lines of a VCM163. The nice wee osc unit out of a scrap CT446 I have might lend itself for this function.

Regards, David

[Ed_Dinning]

Hi David, the usual way with the min Maka Switches was to use them to switch relays that were handling significant power. The mech seemed reasonably robust though.

Ed

[David Simpson]

Hello again Ed,
There are min wafers in the kit, but thankfully most are standard size which I'll use. The RS catalogue rates them at 300V. However, I do have a small handfull of the more sustantial old paxolin Plessey,Cossor,& AVO types.
Whichever type I use, I'll test them all first with my thumping great modernish Megger SL, just to be on the safe side.
Now that you've got me thinking, wafer switch-wise, I'm sure I've some ceramic wafer switches somewhere. Possibly out of an old Sailor SSB Tx. Must have another delve into several old boxes of gubbins.

Regards, David

[David Simpson]

"Belay that last pipe",(A Naval expression I aquired whilst a member of a joint RAF - NAVY Shackleton AEW Squadron), regarding using RS Wafer Switches.
A recent aquisition of a donor AVO VCM MK2(See thread) will provide a selection of true AVO VCM wafer switches. Plus it's Valvebase Unit will fit nicely on top of the ex Marconi TF995A Cabinet. Thus removing the necessity of running a multi-cored cable via Jones Plugs to my much-altered CT160's Valvebase Panel.
So in fact the title of this thread should really now be VCM MK2/TF995A/TF1041C/Leybold DC'd VCM. I'm not the ostentious sort of person to call it a Simpson DC VCM, as a number of VR friends & Forum folk have given such helpful advice & encouragement. A suggested abreviated title would be most welcome.
I'm still planning on using a separate regulated DC PSU for Heater supplies. Also will continue to use the CT160 normally for basic valve testing. This project will be used for testing Triodes up to big Pentodes which are required for specific renovation &/or homebrew projects. Also Ia/Vg graph plotting under DC conditions. Future thoughts on testing valves under signal conditions are being considered. But one step at a time, just for now.

Regards, David

[David Simpson]

Further progress, but slow.
The top of the recently aquired VCM MK2(?)'s cabinet now fits nicely on top of the Marconi cabinet. And the Valvebase Unit now fits snugly into that. A lot of drilling & making of hinge brackets has made it partially raisable - to access proposed circuitry underneath,(shunts & multipliers etc for the 100uA meters). The meters will be fitted into the sloping phenolic resin board. So will some of the switchgear. The old VCM's black handles are now fitted to the bottom cabinet.
The phenolic resin board smells just like paxolin, but is as hard as nails to drill & file.

Regards, David

[David Simpson]

Moderators - Now that this project no longer involves a CT160, and lacking any alternative naming suggestions - prahaps you would please consider altering the title to - LEYMARCAVO DC-VCM (Short for Leybold, Marconi, AVO - the main donor parts).

Regards, David

[David Simpson]

Further Progress :-
Final wiring up of circuitry in view. Glad I made a plywood control panel first - such a layout is prone to alteration. GM8 AGM - Martin has kindly donated a nice piece of 2mm aluminium which will need cutting out using the ply as template.
Lovely Lavell 500uA meter fitted nicely - to function as Vg Meter. The two Turner meters are for Ia & Is. Marconi VVM now fits snugly in its new home.
Must give more thought to testing valves in their amplifier mode. Either by using the VVM in it's AC function, or by external means. Will just concentrate on Ia/Vg under pure DC conditions just now. Under-used terminal strips and spare space can be used later for modifications. Any suggestions ?

Regards, David

[David Simpson]

Advice appreciated :-
Will soon need to fit some type of back-dials for wafer switches & pots, plus labels for toggle switches. Dont fancy permantly sticking on dymotape or writing on adhesive paper labeling with felt-tip pens etc.
Some sort of nice etching on a decent bit of slim laminate. Are there kits available ? Could use Lettraset I suppose, if its still made, but its a sod to use.
The new aluminium faceplate drilled out & sprayed - now looks a treat. The big Anode & Screen HT pots out of the Leybold PSU's do get quite warm. So have fitted a deflector plate & a th/st mains switch for safety. However, I dont expect to be drawing heavy Ia/Is currents for prolonged periods.

Regards, David