Self-made oscilloscope is missing 3 tubes - which ones?

[JoelfromHfoT]

Hello forum,

A friend recently passed away and he gave me a small oscilloscope.
I discovered it used to work with three tubes, but all three have been removed. The only tube still in there is the main oscilloscope tube.

It is a self-made oscilloscope, so there is no schematic or other documentation.
I recreated mostly of the schematic myself and I'll include it in this post.
I also measured voltages at all the tubes, maybe this helps.

Here are the translations of the schematic:
Helligkeit = Brightness
Schärfe = Sharpness
Schirmung für Oszi-Röhre = Shielding for Oscilloscope tube
Zeitablenkung = time deflection (I don't exactly know what that means, but it says so at the front.)
fein = fine

A friend of mine discovered a 6BX6 might fit in the right socket.
Maybe somebody can help me figuring it out.

Since me and this friend weren't able to find a fitting tube, it may be the best option changing the circuit the way a certain tube will fit. Unfortunately, I don't have enough knowledge to achieve that.

Thanks a lot!

(If this is not the right thread, I apologize.)

[Chrispy57]

Hi - so, are you asking for a source of a 6BX6 tube/valve? It seems to have an equivalent - EF80, and they are available at a range of prices on EBay starting at £2.99.eg
https://www.ebay.co.uk/itm/204342223...Bk9SR9y__-u4Yw

Cheers
Chris

[JoelfromHfoT]

Thanks for your advice! The biggest issue right now is finding out which tubes were in there. The 6BX6/EF80 is only one of the three.

I still can't think of a tube that would fit the schematic I shared.

[Cobaltblue]

The middle valve if I understand your drawing seems to have pins 4 and 5 connected together.
That's common when a double triode such as a 12AX7, 12AU7 or a 12AT7 is used then the other filament (CT) is pin 9.

Cheers

Mike T

[paulsherwin]

It's not going to be possible to identify the exact valve types from just looking at the base connections. For example, the ECC81, ECC82 and ECC83 all have the same connections and are equally possible candidates in a scope. The EF80, EF85, EF89, EF183 and EF184 all have the same connections.

The question is more difficult because this is a homebrew scope. Home designers are likely to use valves that they already have or which are cheap to buy, rather than the types used by commercial instrument designers.

[WME_bill]

Self made oscilloscope.
I suspect it uses a circuit taken from a magazine. I have a collection of circuits and we maybe able to identify it.
Can you tell us what is the Cathode ray tube screen diameter and type number. There is also likely to be dates on the electrolytic capacitors.
Is there a rectifier valve position as well, and presumably a stick rectifier for high voltages. Connections around the mains transformer- what ratings marked?
Perhaps a photo of the inside?
I am thinking in terms of EF80 for Y amp. EF80 or 6F33 for transitron timebase and 12AU7 for X amplifier.
Shades of CT52/84 or Cossor 1039 if you want to look at a broadly similar commercial design of 1960's. Circuits on the web. Or a Practical Wireless or Elector or Philips design?
wme_bill

[wave solder]

I have a simple Philips design here that (apart from rectifier) has a DG 7-6 tube and incorporates an EF80, ECC83 and a EF86. It has a few similarities but not the same but maybe modify the existing unit to a design like this that should work with known valves.

[JoelfromHfoT]

@Cobaltblue
Yes, indeed and that makes a lot of sense.

@paulsherwin
I didn't think about that, but yes, you are right...
My approach would be testing all of them and stick with that one that is working best, but that is going to be expensive.

@WME_bill
I was thinking the same.
The tube is entirely shielded with a big metal plate and I can't see the exact type, though I think a 12 pin socket is quite rare. However it has a diameter of approx. 6.7cm.

I can't find a date code on the Electrolyric Capacitors and I am fairly sure this wasn't done in Germany.
However I found 4 paper capacitors type "Eroid" wich could date it from the mid 60's to late 70's.
It has a built-in Siemens bridge rectifier.

Shall I try the tubes you wrote?
I'll include a few Images.

@wave solder
This would be an option if we can't find fitting tubes. Thanks for pointing it out.

Thank you all for the help!

[JoelfromHfoT]

I'm sorry, I messed up uploading the pictures.

[JoelfromHfoT]

@wave solder
Sorry, I haven't seen you already included the schematics.

[Joe_Lorenz]

Hello Joel,

judging from the photos I daresay this device is not 100% home brew but looks more like a kit. Maybe a research of 1950's to 1960's "Funkschau" magazines would be successful to find the matching schematics. The type of the crt will be printed on the conical part, so it is necessary to remove it from it's shield to be able to read it.
Please take a close look at these big Siemens electrolytics and that Siemens selenium rectifier as some of these bear a date of manufacture (printed).
Those red "Ero" capacitors, grey "Rosenthal" resistors and beige "Siemens" electroloytics were used on many (west) German middle class radio and TV sets 60 years ago.

Regards, Joe

[Ed_Dinning]

Hi Joel, there were many designs published in electronics magazines in UK and Europe as well as Philips who wrote several books on the subject and produced some kit designs.

If you look on the worldradiotv site there ar many magazines that can be read on line and have details of these circuits, see what you can find.
If you have no success, PM me after the 1st of April when I will be back home with access to my library and may be able to find the circuit for you.

I can also let you have some used/ tested valves if you are still after them then

Cheers, Ed

[JoelfromHfoT]

@Joe_Lorenz

Hello,
Thanks for the information!
I was also thinking about a kit, wich would make sense considering he cut out the front to fit a switch.
Though, if this was a kit, wouldn't there be a manufacturer printed on the case like Heathkit did it?

Well, I think I will try getting that shielding off and I will Take a look at the "Funkschau" Magazines. Furthermore I will retry finding the date.

@Ed_Dinning
That is very nice of you, thank you very much!
I will also Take a look at this site as well.

When I found something new, I'll post it here.

[stuarth]

It may be possible to get clues from the wiring diagram and pin voltages shown in the first post, but there are inconsistencies between the two (it is very difficult to trace out a circuit with no knowledge of what it’s supposed to be!).

However, here’s a few observations.

The left hand valve seems to have 7.4V AC between pins 4 & 5, conventional heater pins for B9A, so doesn’t narrow the search much. The voltage diagram shows pins 1 and 6 of that valve have 630V (presumably the unloaded HT voltage) on them, this implies that either the valve is a pentode with anode and screen grid on pins 1 & 6, eg an EF86, or it is a double triode with the two anodes on pins 1 & 6, eg an ECC85 or 88. However, the wiring diagram shows pins 1 & 6 connected to ground, not consistent with the voltage shown.

The other two valves show pins 4 & 5 connected together, this implies ECC81/82/83 types, I don’t know of and other common B9A valves where these pins would be tied. These 3 valves have the same pinout, so as Paul says, you can’t at this stage decide which.

On the voltage diagram, the middle valve has the presumed heater pins (4,5 & 9) at 315V, half the HT voltage. You would do this to reduce heater cathode voltage if you were running with a high voltage on the cathodes. Maybe this valve is used as cathode followers driving the CRT deflection plates, it would explain the lack of high voltages on the CRT base. Pins 1 & 6 are the anodes of these valves, so are at HT, and HT on pin 2, a grid, could be because that grid has a DC path to another valve anode.

The voltages on the right hand valves don’t make sense in this analysis, I’m afraid, the voltage chart seems to show full HT on the grid pins and 0V on the anodes and cathodes.

The above analysis may be complete nonsense, but may help working through the wiring towards a circuit diagram.

Stuart

[JoelfromHfoT]

Thanks for sharing your thoughts!
The inconsistencies that you discribed mark the point where my friend and I were stuck.

There may be some faults somewhere in the circuit, but finding them without a schematic is nearly impossible, isn't it?

This is also why I'm thinking about modifying the circuit to a way it does work with an ECC, though this is not as easy as it first sounded apperantly.

[Diabolical Artificer]

The left hand valve seems to have 7.4V AC between pins 4 & 5, conventional heater pins for B9A

I'd say more likely to be B9E, B9A valves are 12.6v heater pins 4.5 & 9, B9E are heater pins 4 & 5, pin 9 screen. That suggests an ECC88. Loads of scopes of this era used ECC88's, ECF80's for the TB & Y amps.

That big gold cap in pic two looks dodgy as do some of the other HV paper caps. I'd check all the electrolytic's and especially the EHT caps, they may need re-forming or replacing. Also check any high value resistors 500k & above. The Siemans bridge rectifier also needs a look at. Check all the power supply rails for AC ripple over 100mV.

Andy.

[JoelfromHfoT]

Yes, as soon as all tubes are plugged in, the heating voltage will drop to approx. 6.3V.

I was planning on checking all the capacitors and other parts as soon as I know the tubes, but maybe the other way around would make more sense?

I also got the tube out: it is a VALVO DG7-31. With this information I will look at magazines and so on.

[JoelfromHfoT]

Just a quick update: The "Funkschau" magazine did use the DG7-31, but I discovered one major issue: Many magazines are not completely or not at all documents.
Chances I can find the article covering this oscilloscope are quite low...

[Joe_Lorenz]

Joel,

there have been many DIY projects involving DG7-31 as it had been introduced by Philips as "ideal for low-voltage low cost oscilloscopes!". See this leaflet, please:

http://www.r-type.org/pdfs/dg7-31-32.pdf

And there was a book by Werner Diefenbach (one of many books he wrote) where such a project was discussed.

BTW: One of your photos shows a multi point connector. What does it connect to?

Regards, Joe

[JoelfromHfoT]

I feel like this is a bottomless pit.
The chances that I can find the right book/magazine showing this oscilloscope are very low.
The multipoint connector was proably added later and it is wired to the CRT and the other tubes. There was a little white box on the back of the oscilloscope that connected to this connector. You can plug in a Headphone cable into the white box but I can't tell what it does.