Substituting a valve output transformer - UK Vintage Radio Repair and Restoration Discussion Forum

'LIVEWIRE?' · 2nd Sep 2014, 10:20 am

The primary of the o/p tx in a Roberts P5A on which I'm working is o/c. I have one or two other o/p transformers around, including one from a Ferranti AC/DC radio whose o/p stage is a UCL82. Could I use the Transformer from this set to replace the Roberts one? Checking the valve data for both, the relevant figures seem to be as follows:-
EL33 Ra 7000 ohms, Pout 4.5w, Ia 36mA. UCL82 Ra5600 ohms Pout 3.5w Ia 35mA. To me, these figures seem sufficiently close enough to suggest that the 'UCL82' O/P Transformer would be OK. What do other members think?

paulsherwin · 2nd Sep 2014, 10:26 am

It'll be fine. Output transformer turns ratios are a lot less critical than some purists would have you believe. Just check that any replacement can handle the current, and increase the cathode resistor to reduce this if necessary.

2nd Sep 2014, 2:13 pm

I concur with Paul, almost anything will work, remember that one of the sales criteria was power output so the 'correct' transformer would give this. Unless you want a really loud set it will be OK.

'LIVEWIRE?' · 2nd Sep 2014, 7:56 pm

Having checked the output tx in question, there are two slight problems, both of which may be solvable. First, the Ekco/Ferranti Tx has a Feedback winding, whilst the Roberts doesn't. In the circuit for the EKco, the 'col' end of the volume control is returned to ground via the FB winding, so I could either modify the Roberts circuit to do the same ( I know this reduces gain!) or leave the winding unconnected, which presumably won't matter.
The second thing is that the Ekco transformer is larger than the Roberts one, and only has one fixing hole in it's clamp, the other side of which is a flat lug which fits through a slot in the chassis, This won't be easy to overcome. I checked a Bush VHF42 transformer, which might be easier to fit, although it, too, is bigger than the original, but I'm puzzled as to why this Tx has two high resistance windings, both of which read about 400 ohms. I should have a circuit diagram of the Bush set somewhere, but can't find it!

Herald1360 · 2nd Sep 2014, 9:30 pm

If there's physical space for the bigger TX, drill a hole in its second lug and mount it on a suitable ally plate using countersunk screws. Drill the plate to suit the holes in the chassis.

If the chassis holes end up under the TX, use the countersunk screw trick in reverse. It may help to leave the screwheads slightly proud so that whichever ones have their nuts tightened last have their heads held firmly

2nd Sep 2014, 10:20 am	#1
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Substituting a valve output transformer The primary of the o/p tx in a Roberts P5A on which I'm working is o/c. I have one or two other o/p transformers around, including one from a Ferranti AC/DC radio whose o/p stage is a UCL82. Could I use the Transformer from this set to replace the Roberts one? Checking the valve data for both, the relevant figures seem to be as follows:- EL33 Ra 7000 ohms, Pout 4.5w, Ia 36mA. UCL82 Ra5600 ohms Pout 3.5w Ia 35mA. To me, these figures seem sufficiently close enough to suggest that the 'UCL82' O/P Transformer would be OK. What do other members think?

2nd Sep 2014, 10:26 am	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Substituting a valve output transformer It'll be fine. Output transformer turns ratios are a lot less critical than some purists would have you believe. Just check that any replacement can handle the current, and increase the cathode resistor to reduce this if necessary.

2nd Sep 2014, 2:13 pm	#3
Guest Guest Posts: n/a	Re: Substituting a valve output transformer I concur with Paul, almost anything will work, remember that one of the sales criteria was power output so the 'correct' transformer would give this. Unless you want a really loud set it will be OK.

2nd Sep 2014, 7:56 pm	#4
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Substituting a valve output transformer Having checked the output tx in question, there are two slight problems, both of which may be solvable. First, the Ekco/Ferranti Tx has a Feedback winding, whilst the Roberts doesn't. In the circuit for the EKco, the 'col' end of the volume control is returned to ground via the FB winding, so I could either modify the Roberts circuit to do the same ( I know this reduces gain!) or leave the winding unconnected, which presumably won't matter. The second thing is that the Ekco transformer is larger than the Roberts one, and only has one fixing hole in it's clamp, the other side of which is a flat lug which fits through a slot in the chassis, This won't be easy to overcome. I checked a Bush VHF42 transformer, which might be easier to fit, although it, too, is bigger than the original, but I'm puzzled as to why this Tx has two high resistance windings, both of which read about 400 ohms. I should have a circuit diagram of the Bush set somewhere, but can't find it!

2nd Sep 2014, 9:30 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Substituting a valve output transformer If there's physical space for the bigger TX, drill a hole in its second lug and mount it on a suitable ally plate using countersunk screws. Drill the plate to suit the holes in the chassis. If the chassis holes end up under the TX, use the countersunk screw trick in reverse. It may help to leave the screwheads slightly proud so that whichever ones have their nuts tightened last have their heads held firmly __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O