Mystery Output Tranformer Measurements

[Rontone]

I have an OT with 2 Primary connections labelled A1-A2 and T1-T2, also a CT, Its Elstone brand but cant find data about this one, only datasheets for a different type

The secondary has no numbering or lettering marks, seems to have two outputs series linked, unlikely to be 2x 2 ohm coils series linked to 4 ohms, right? So I'm presuming its either 2x 4ohm linked to 8ohm or 8's linked to 16,

The amp had valves missing, but was push pull, inputs for radio and LP, an EF86 first, then a ECC type driver and no powers tubes, but 9 pin sockets, so I'm guessing its a pair of EL84's



So with 1V on the secondary, A1 - A2 measures 21.58V, so an 8 ohm load = 3725 ohm on the Primary, 16 ohm load = 7450 ohm Primary, does that sound right for EL84?

The other primary tap T1-T2 measures 1V sec. = 9.24V pri. so quite a low ratio, 8 ohm = 680, 16 ohm = 1360, is it possibly there to link primary coils together to create different values?

[Rontone]

I've tested all the Primary connections, and it seems to be setup as in the picture, I think T1 and T2 are taps, they both measure very close to 75 ohm each from the CT, but A1 and A2 measure quite different ranges, so it looks as if it can be setup in a few different ways

So I will have to set it up with power in the other tapped modes A1-T2 and A2-T1 too and check what ratios come out, its looks as if this could fit various builds...

On another note, just reading Inside Tube Amps by Dan Torres, a section on general secondary readings, commonly 8 ohm taps read as 0.7 on a meter and 16 can be around 1.0, which these secondaries both measure, so I believe its intended for 8 and 16 ohm use

[tubeswell]

OT with 2 Primary connections labelled A1-A2 and T1-T2, also a CT ... amp was push pull... powers tubes, 9 pin sockets

Could well have been EL84 or some pentode with same pin-out in miniature 9-pin envelope. (You didn't confirm the socket pin connections in the amp, nor the method of bias, cathode resistor value(s) etc) 

1V on the secondary, A1 - A2 measures 21.58V, so an 8 ohm load = 3725 ohm on the Primary, 16 ohm load = 7450 ohm Primary, does that sound right for EL84?

Yes


Seeing as how the taps* come off the 18% point on each half of the primary, they can't be added up with the A1-A2 measurement. Maybe this OT was connected in UL mode? Is there a separate screen supply in the amp?


*Edit: when I first responded, I called these taps based on your hand-drawn schematic, but now I’m not sure that is correct- see my further response below.

[Rontone]

Thanks for looking over that, I dismantled this one for the chassis, the poor thing was really bent on the back, so removed the rear edge and made a steel plate for a new rear, the transformers are a bonus and thought they may be useful

The power tube pinout is hard to see in the picture I have here, before dismantling, I can see pin 7 to the OT, so thats anode, pin 2 has a 4k7 resistor to a 0.1uf poly cap, pin 3 cant quite see but looks like its hooked to the + side of a 50uf 25v electrolytic, 4+5 are heaters, 6+8 unused, lastly pin 9 has a 470R resistor to one of the larger voltage caps

[trobbins]

If your meter can accurately read say 100mV, then it is often easier to connect mains frequency 12Vac to one half-primary (A1-CT), and then measure A2-CT voltage, and T2-CT, and each secondary winding section.  For completeness its best to also drive the A2-CT winding and measure A1-CT and T1-CT to confirm they measure the same voltages as the other side.  That reduces your measurement errors and makes the impedance ratio more accurate.  The impedance ratio may end up closer to the 8k PP datasheet nominal level for UL operation with 43% TR UL taps for EL84/6BQ5 PP.

The initial measurement of T1-T2 / A1-A2 = 43% turns ratio, which aligns with a typical UL OPT primary winding, and the datasheet UL setup.

[Rontone]

Maybe this OT was connected in UL mode? Is there a separate screen supply in the amp?

The OT was only connected to pin 7 of the output tubes and the CT went to the main capacitor can, here's a few pics, you can see how wobbly the rear panel is, the bulgin mains socket is nearly touching the chassis and the mains board!

Is g2 or g3 the screen on an EL84?

I just came across this interesting read while looking, good to know, to always doublecheck unconnected pins and hook up points https://wgsusa.com/blog/when-working-el846bq5-tube-amps-you-must-know-about-pins-one-and-two

[sluckey]

Is g2 or g3 the screen on an EL84?

G2 is the screen on ANY tube.

[tubeswell]

I’m not sure your hand-drawn schematic tallies with the photo you posted of the OT. In the photo, there appear to be 2 lots of three terminals on one side of the OT ( and 4 terminals on the other side). Do the 2 sets of three terminals display any DC-continuity between each other on your R-meter? (If not, they’re 2 separate windings, and your hand-drawn schematic can’t be right)

[PRR]

> G2 is the screen on ANY tube.

Except the space-charge tubes used in 2 years of car radios, never in serious stage amps.

[Rontone]

Do the 2 sets of three terminals display any DC-continuity between each other on your R-meter?

Yes, there is with all connections and combinations on the primary side, they are labelled on the primary side, but not on the secondary

CT-T1= 75.6
CT-T2= 75.4

T1-A1= 237.1
T2-A2= 102.7

CT-A1= 312.6
CT-A2= 178.0

Then T1-A2 and T2-A1 both measure too and add up with the above

So the taps are very matched, but the outer windings seem to be out from each other, could this affect its use with the centre tap, or if used in UL mode,

There is no continuity between pri. and sec. or the outer case, could a bad winding on one side between CT and the outer account for a large difference, perhaps this is a sign of damage

[trobbins]

No issue with different primary winding section DC resistances - that is just an artefact of a simple cheap manufacture and is typical of most output transformers - the only time that is ever a concern is for the high end of hi-fi output transformers.

[Rontone]

Thanks for all the help with this guys, I've been reading up a bit more,

The little amp this OT and the PT came from were someones pride and joy when they built it, it would be nice to reconstruct it into something lovely,

The Power Transformer has a 300v centre tapped 100mA secondary, a Rectifier heater connection for 6.3v or 5v, and a separate 6.3v CT heater connection,

They are of the right Marshall/Vox era, I like the look of the sluckey dual-lite, pair of Mullard EL84's with this OT, I have a few EF86's and ECC83 Mullards, would just need a 5Y3

It specifies 315v, would it be a problem using 300v?