# Help please with mystery transformer

Discussion in 'Shock Brother's DIY Amps' started by sgotlieb, Oct 1, 2019.

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1. ### sgotliebTDPRI Member

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Hi Folks. I'm hoping that somebody on this page can help me map this transformer.

It's a mystery one from an old radio that I picked up. The radio was converted to a guitar amp but never actually worked - in fact it was downright dangerous - the way it was wired up it was sending 200v to ground.

So I pulled it apart and kept the power transformer, output transformer and tubes. It's an EF86 preamp and EL84 power tube so I'm looking at making an AC4 clone. Trouble is, this transformer is wired weird. I've mapped the resistance on the transformer but I'm struggling to work it out. Obviously too many beers today. Can somebody assist me here?

I'm interested in what points I need to wire up for the heaters and what for the high voltage, and whether the Active, Neutral and Earth were originally wired up properly. I'll use a solid state rectifier in this amp so I'm not overly concerned whether I can get 5V out of this power transformer, but I'm pretty sure I can get high voltage and 6.35V. The wire colours are deceiving - they were all soldered on by the person who originally made the guitar amp conversion which didn't work so I am dubious about all the wiring.

Can any of y'all help here?

2. ### kboldTele-AfflictedSilver Supporter

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The secondary (on RHS): 5-6 = 150R winding, 6-7 = 150R winding, 7-8 = 140R winding. 1-5-7 are earthed.
(6-8 = 140 + 150R windings)

Since trans' is out of circuit, power it up (safely) and check secondary voltages. (no touchy-touchy wires from 2-3-4)

Not sure whats happening on primary side. (2-3-4 may be centre tapped primary that allows 120V/240V selection)

Edit: Short between 1-5 may be an internal short. (This is making an assumption that LHS are primary windings, and RHS are secondaries).

Last edited: Oct 1, 2019

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One would want to know what the resistances are between 1&2, 1&3, 1&4, and 3&4.

4. ### sgotliebTDPRI Member

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Sorry, I should have mentioned that all other measurements pin to pin are giving no readings.

So, this was what I thought the wiring might be. I've turned it on and measured the voltages.

With pin 3 as active in (240V) and no connection to pin 2 (switch is off) I get the following voltage readings:

2-5, 4-5 82V
2-6, 4-6 82V
2-7, 4-7 82V
2-8, 4-8 82V
3-5 148V
3-6 148V
3-7 148V
3-8 148V

all other combinations are pretty much no reading.

Throw the switch between 2 and 3 and the transformer starts humming - quite loudly. Not unexpected - it is under no load.

1-2 140V
2-5 147V
2-6 107V
2-7 147V
2-8 400V
3-4 238V
5-6 266V
5-8 270V

Obviously my idea of what the pins are ain't quite right. I'm a bit confused here. There ain't no filament voltage here at all.

5. ### GARAGE HERMITTele-Meister

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cant see the point of the s/w between 2&3, every old transformer i've used out of old radiogram's etc only have one 240v live feed into the transformer,
are you sure the L-N-E are on the correct terminal's,

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Ime, the neutral should go straight to one end of the primary winding. The hot/active should be fused and then switched to the other primary end. That pin 3, if it is one end of the primary, should have no physical connection to any secondary. IF pin s 3&4 are the primary, they should show continuity between them. Pins 5&7 cannot be the heater filament supply if they both are earthed.

I think I am awake, but looking at this situation has me confused. Something is not correct here....or I need to finish my tea. ???

7. ### sgotliebTDPRI Member

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Okay, I've taken the active away from pin 3, directly to pin 2. So, measuring 6-8 is 520V, 6-7 and 7-8 are 260V, and 1-5 is 6.88. That's with 235V coming in to the primary. So I guess 7 is the HT centretap, 6 and 8 are the HT leads, and 1 and 5 are the filament wires. Just avoid any power in to pin 3.

I think I have it sorted.

So with 260-0-260 off the HT and using a 12AX7 and an EL84 I'll be drawing about 45mA, so that equates to about 356V coming out of the SS rectifier. A wee bit hot, so I guess I need to do some voltage dropping. Ohms law says 2.2K @ 5W should give me about 265V, which sounds about right for a 12AX7/EL84 collaboration if I want a nice dirty amp.

Of course it could also be fun to put in a switch so I can go to 1.2K dropping resistance and throw 300V at the B+, which will give me a heap of headroom for this amp, and clean it up a heap.

I'm no electronic engineer, so I don't know if the repercussions of this, but I assume dropping the B+ causes a sag situation, and it should be okay to switch from one to the other.

Let me know if this is a no-no in amp design please.

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8. ### GARAGE HERMITTele-Meister

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think the Ac4 is a single ended amp therefore no sag,

9. ### sgotliebTDPRI Member

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Okay, cool. I figured using lots of resistors would create a sag effect. I can live without the sag. I have a 5F1 with a 5UCM rectifier for that.

10. ### elpicoTele-Holic

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If you have a spare socket using the traditional ez80 or similar rectifier instead of solid state will drop your voltages. They're cheap (< \$15) and even if you don't care about making the circuit "vintage correct", tubes look nice

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