HT transformers with extra low-I higher-V winding - what were they for ?

[GrimJosef]

From time to time a particular model of Parmeko Neptune HT transformer comes up for sale on eBay. As well as two 0V-115V mains primaries it has a 430V-0V-430V 190mA HT secondary - very appropriate for quite a wide range of mid-sized valve equipment.

But it also has a 0V-950V 5mA winding, with the 0V commoned to the centre-tap of the 430V-0V-430V one.

I also once saw another Neptune with 400V-0V-400V 85mA and 0V-1750V 1.5mA windings, again commoned at 0V.

Neither transformer had any LT heater windings.

I'm sure someone will know what this extra winding was for. If you do, could you enlighten me ? I'm just curious, that's all.

Cheers,

GJ

[Robert Gribnau]

I think it's intended for the HT of a CRT in an oscilloscope.

[Radio Wrangler]

1750v for CRT HT

950 likely the same, either half rave rectified or a cockroft-walton multiplier.

An outside possibility would be EHT for a photomultiplier or geiger tube.

CRTs usually have their grids and cathodes a long way negative so the deflection plates can be near ground or near valve HT. The negative cathode means the heater needs its own winding with EHT grade isolation. Somewhere there may be matching heater transformers with one odd well insulated winding.

David

[GrimJosef]

I wondered about a CRT, but 950V seemed like rather a low voltage. I worked on an HMV 555 stereoscope amplifier once and that had a tiny CRT which ran at some hundreds of volts, but otherwise I tend to think of them as kilovolt devices (or more than 10 kilovolts if they're in, say, a TV).

Unless there's a more plausible explanation though then perhaps that's it.

Cheers,

GJ

[trobbins]

There may have been other industrial applications - although not directly applicable to that particular Parmeko, example applications included HV testers (like RM215 from BPL and AVO where HT was up to 6kV), and megohmeters like RM175 from BPL (1050V sec), and vacuum gauge meters (like from Philips No.3 which used a Parmeko Type 5080/9A; 1450V 1.5mA 9.2kΩ).

[turretslug]

950V at not much current perhaps feeding PSU arrangements with "SenTerCel" etc. selenium stacks for something like a reflex klystron receive oscillator, those transformers must have been somewhat expensive so maybe part of ground/marine radar IF/processing/video apparatus?

Those "Neptune" etc. transformer types always struck me as a peculiar combination of robustness and fragility with their steel cases and KLG(?) ceramic-shrouded terminals!

[Ed_Dinning]

Hi GJ, I've seen quite a few transformer like this, sometimes the 1000v windinf is simply an overwind from one end of the HT (say 400v) windings. A good production technique to save a bit of wire and insulation.
As has been said, it could feed a C-W multiplier as the demand of EHT in the types of devices listed above is very small.
The good old VCR97 would run very well at 2.5KV

Ed

[Silicon]

Sonar transducers/ transmitters may require high voltages with a low average current.

[G6Tanuki]

I'm quite familiar with 'scopes and transmitter modulation-monitors whose transformers have an additional 'EHT' winding - but this was generally additionally-wired to one side of the transformer's existing centre-tapped rectifier-windings so its 'low voltage' end got an extra 350V boost.

[barrymagrec]

I have a vague memory of some flatbed recorders having an electrostatic "fix the paper" feature that used a high voltage winding on the mains transformer.

[GrimJosef]

I don't know enough about scopes to know whether a 430V-0V-430V 190mA winding (roughly what you'd need for the HT on a 30+ watt audio power amplifier) would also be appropriate for a scope. I know some of them do have a lot of valves in, but do they generally run at such a high HT voltage (maybe they do if they contain totem-pole arrangements e.g. cascodes) or sink quite so much current ?

Likewise, do flatbed recorders need a lot of HT ?

Cheers,

GJ

[Ed_Dinning]

Hi GJ, the higher the EHT on a scope the higher the deflecting voltage needs to be (for older scopes without PDA).
Typical non PDA scopes needed a mA or 2 for the tube current and divider chain; PDA normally need sub mA

No idea about flat bed recorders, would depend on the amps used I suppose and writing speed. I doubt much would be needed for ES hold down

Ed

[GrimJosef]

Thanks Ed. Actually I may be in the process of answering my own question. In post #34 here https://www.vintage-radio.net/forum/...=153491&page=2 Craig posted the specs of the mains transformer used in some Tek 58x series scopes. Adding up the products of voltage and current for the five HT windings on that transformer I get to a total of 250VA. Working out the VA figure for my 430V-0V-430V 0.19A winding is a bit more tricky, but it's clearly very much less than the Tek transformer is providing. It seems valve scopes can be power-hungry beasts.

Cheers,

GJ

[Radio Wrangler]

There are so many possibilities that we'll never know unless another source of info on these parts opens up. There are many sorts of complex devices with a basic CRT for display.

The HT winging voltage suggests valves. Big ones, or lots of small ones. The lack of heater windings might counter this suggestion, or else the bobbin for the core size was full, and a companion LT transformer is used.

For most typical valves there is a ratio of heater power to useable output power, or mA of output current for rectifiers. This efficiency ratio got improved in stages by successive improvements in cathode technology. Honking great transmitting valves used the other escape route - extremely high anode voltage.

If the application had used a lot of small signal valves only, the HT winding would be less to avoid losses in HT droppers to get down to their comfort level of HT.

So, this HT level suggests that something needs appreciable power via fairly powerful valves, dominating the HT consumption, and any small signal processing would be simple enough for the HT needs to be met via droppers.

Apart from the mystery winding, this transformer would be a good fit for a nice audio amplifier.... And except for being made to very expensive standards.

I've now got this mental picture of soldiers gathered round a hifi setup with CRT VU meters. MIL-spec music!

David

[GrimJosef]

Thanks David. Actually these particular 430V-0V-430V-950V transformers seem quite common. I've done a bit more research and more than half a dozen have sold on eBay over the last 3 months. I have one which I picked up (not from eBay) last year.

They carry quite a few ID numbers on their serial plates. I was going to say that these numbers aren't very informative, but in fact they probably are - the stumbling block is my ignorance of what they mean.

Attached is a pic of the serial plate of mine. The numbers are

Type: 2778
Ref: 10K/16868
Model: 6000/73
Serial: D.8173

The Type, Ref and Model numbers seem to be common to all examples of these transformers but the serial numbers can vary. I've seen ones with E.1354 and F.6274 (more than once), for example, so maybe these relate to production batch numbers ?

They seem to be associated with a similar heater transformer, which is marked 2777, 10K/16867, 6000/65, F.6275. The primaries on this are, again, two 0-115V and the secondaries are 0V-4V(4.4A)-6.3V(2.2A)-17.85V(1mA), 0V-5V(3A), 0-6.3V(0.9A), 0-6.3V(0.15A), 0V-2V(1.5A) and 0V-4V(1.5A). So quite a mixed bunch !

There also seems to be an 8H, 190mA choke marked 893, 10C/18705, 6000/57, F.6276.

Cheers,

GJ

[Radio Wrangler]

If the CRT guesses are on the money, then one of those heater windings is likely extra-well insulated, to be run at cathode potential 1-2kV negative.

In CRT instruments, it's usual to run deflection plates near ground or approaching valve HT for easy direct-coupled drive.

Cathodes rarely rum much more than 2kV negative. CRTs running more total kV usually have a PDA running high positive voltage. So onesy twosy negative cathode volts is common to slow basic CRTs and to fire-breathing stuff from Tek and HP with teens of kV on PDAs.

But if this is mains freq HT, then it'll be all there is. It's unlikely there is an inverter for PDA, because one of those is normally wound for the cathode volts and maybe a bit of overwind into a multiplier for the end voltage.

David

[G6Tanuki]

10K/16868 looks like a Military stores-code; I wonder if these transformers are out of some military RADAR display?


A-Ha!! My Google-fu is working well; 10K/16868 shows up as the HT transformer for the Hartley Electromotives Model 13A Oscilloscope!

The 10K/16867 is the LT transformer.

Here's the curcuit of the PSU showing both transformers.

[GrimJosef]

Congratulations, and thanks !

Cheers,

GJ