Unusual SE Transformer - Which is the real purpose ?

[kagliostro]

Looking for a PP transformer for a pair of EL95 I recovered, from an old Grundig stereo radio a pair of SE transformers used with an ELL80 tube (the ELL80 tube is the equivalent of 2 x EL95 tubes in the same envelope)

The transformer seems to have an UL connection, but as far I can understand it isn't

The original speaker has an impedance of 4.7Ohm and connected to the transformer gives me a reflected impedance of 11.5Kohm on the large winding and 17.6Ohm impedance on the small winding intake

B+ is connected on the middle intake, the opposite side of the winding with high impedance to the plate and the opposite side of the winding with very low impedance (via a resistor) to the grid (see attached Transformer Schematic)

More, the grids of the two pentodes are join, and this is furtherly confusing me (see the attached Grundig Schematic)

Which is the reason for such architecture ?

Many Thanks

K

[kagliostro]

I think I got BINGO !

That portion of the TU is used as a build-in choke

it was a system used in old apparatus

so I think I can give a try and use also this part of the TU though the shematic I can have

didn't use such architecture

K

[DummyLoad]

used by also by RCA. have a pair of the OT here somewhere in a pile, someplace, safe, i think...

PRR commented on a thread some time ago; it's a hum-bucking technique, but yes, also, i think it_kind_of_is_a_choke.

--pete

[PRR]

> an ELL80 tube

Very rare and possibly valuable.

[kagliostro]

Very rare and possibly valuable

Yes, right, it is so also from this side of the pond 

Unfortunately nor one of my two tube tester has the data on the manual to test such a tube

I've two old italian tube tester, one is a Chinaglia mod.560



and the other is a better one, an Unaohm GB25

(very similar to this unit)


the problem may be has a solution, the manual of the Unaohm tester has data to test the el95 tubes, so, I'm not confident with this thing, but I'm sure that studing around the problem I can find the way to test also ell80 tubes

may be an adaptor is required or I must find how to transcode the rotary switch combination to obtain what I want

Unaohm GB 25 A Manual - Schematic - Test Data

http://www.mediafire.com/view/?wv3hz4d8588l1zj


http://www.filedropper.com/manualeprovavalvoleunaohmgb25aedatidiprovagb25-gb26-gb37

K

[PRR]

> transcode the rotary switch combination

Manual, page 27, shows connection for 6AU6. Not your tube, but you can see what the knobs do.

The knobs P1..P10 are the socket pin-holes 1 to 10 (not all sockets have all 10 holes).

The machine supplies six (seven) signals:

0 - no connection
1- P
2- G2
3- G1
4- K
5- F1
6- F2

(They *really* should have written p G2 G1 K etc on the switch positions.)

Look at 6AU6 connections page 27. Knob P1 (pin 1) is turned to position 3 which connects to the G1 signal inside the tester. Yes, Pin 1 on 6AU6 is G1. Knob P2 is turned to position 4 which is K (and G3), P3 is on 4 which is F1, P4 at 6 which is F2, P5 at 2 which is Plate, etc.

So for a strange tube:

Put all pins on No-Connection (position 0).

Now switch to 6.3V(??) heater power and put your two Heater pins to 5 and 6. Put a AC voltmeter in the heater holes and verify 6 or 7V. Put the tube in and verify it glows.

With the tube data, pick one side of a twin-tube, carefully work out for each pin which P G2 G1 K etc connection is.

Since you "know" this tube is a dual of a listed single tube, use the Vg1 Vg2 Vp settings for the known tube. (If you didn't have an equivalent, you would guess a "similar" type; most radio IF/RF pentodes can use Vg1 Vg2 Vp settings for 6AU6 but give somewhat different Gm result; most "power pentodes" can try 6V6 settings though EL84 may need lower G1 to get good current and Gm.)

Run the shorts-test CortoCircuiti.

Run the emissions and Gm test, compare with data sheet.

Now figure the pins for the other half, change the pin-knobs, check shorts, read data.

[kagliostro]

PRR sometimes occurs to me that you are the good brother of Braniac

I do not know how to thank you enough 

---

So it will be interesting and very useful labeling the position of the rotary switch with the connection is acted (Plate - G2 - G1 - Catode - Heater ....)

That is really very convenient as to understand in a more easy way what you are doing when performing tests

---

The manual has the data setting for the el95 tube and the ell80 is exactly a pair of el95 tubes in one bottle

so, using the trick you give me about setting the rotary switch and the el95 data for the other knob settings

I think the Tube Tester can read exactly the specific data of the tube

Many Thanks Again

 

K

[HotBluePlates]

I think I got BINGO !

That portion of the TU is used as a build-in choke

You're right! I couldn't see that until you posted it.

B+ connects in the middle tap, and flows towards the bottom where there is a 2kΩ resistor and another filter cap. B+ also flows in the other direction towards the plate.

Screen current will be much less than plate current, but the opposite directions of travel means the screen current will slightly offset the idle plate current, reducing the tendency of the OT to saturate.

In the end, it's as though each tube's screen has its own choke.

[kagliostro]

At the begin the junction between the two 2kΩ resistors was confusing me

the presence of the electrolytic capacitor helped me to get an idea about what was happening

K

[Steve_P]

> an ELL80 tube

Very rare and possibly valuable.

Used in Magnatone T-32s and 22s. pricey.

[kagliostro]

Used in Magnatone T-32s and 22s

I didn't know

Many Thanks for sharing the info

K