Amplifier Advice - UK Vintage Radio Repair and Restoration Discussion Forum

Murphy Maniac · 5th Apr 2020, 5:03 pm

To keep me occupied, I am planning on building the record player amplifier shown in the picture which was published in a hobby magazine in 1954.
My problem is that it states that the impedance of the output transformer should be about 5000 ohms with a ratio of 45:1. The only transformer I can find has a measured resistance of 550 ohms and a ratio of 45:1.
a) Would I get away with using this with the 25A6 pentode without doing any damage?
b) If it's not suitable, is there any modification that can be made to make it work?
Thanks.

M0FYA Andy · 5th Apr 2020, 5:07 pm

Don't forget that impedance and measured resistance are not the same parameter.

Andy

julie_m · 5th Apr 2020, 5:30 pm

The impedance won't be the same as the DC resistance. The units are the same, but they mean subtly different things. A resistance of 1Ω means that in a steady state, it will develop 1 volt for each ampere of current flowing. An impedance of 1Ω means that the voltage will change by 1V for each 1A change in the current. DC circuit analysis is predicated on the notion that all charged particles involved are moving at a fixed rate and there is no build-up of them anywhere. But crucially, devices such as capacitors and inductors can store and release energy, which allows them to simulate the kind of thing not allowed in DC circuits with no conservation violations; so can behave differently in response to alternating currents than direct ones.

As long as the turns ratio is correct, that's what's more important.

But there's another thing that strikes me about this circuit.

Without wishing to rain too heavily on your bonfire, I'm not really keen on the live chassis construction (and certainly not the fact that there's a DC path from one side of the mains to the audio input). You'd really be better off with something using a power transformer (and not wasting the best part of 60 Watts into a dropper resistor!)

Sideband · 5th Apr 2020, 6:39 pm

As stated, the DC resistance is completely irrelevant. Julie M explains it better than me but impedance is not the same as DC resistance. If the ratio of the transformer is correct then use it. A DC resistance of 550 ohms seems entirely reasonable and will be fine. The valve won't 'see' a resistance of 550 ohms but an impedance of 5000 ohms. Impedance is not an easy thing to understand.....Google it and you'll find plenty available.

Like Julie, I'm cautious of the live chassis technique and also the non-isolated P.U input. Far better to look for a similar circuit using a mains transformer.

vidjoman · 5th Apr 2020, 7:28 pm

If you want to use the circuit then you could add a 1 - 1 isolation transformer from the mains. Added cost but it would be safety.

5th Apr 2020, 7:29 pm

To be honest almost any valve output transformer will work, the bigger the better.

Murphy Maniac · 5th Apr 2020, 7:41 pm

Thanks for the help, I agree that the live chassis idea is dated and propose a re-think using perhaps 6X5, EF37A and 6V6 to stick with 6.3V heaters.

Paul JD · 5th Apr 2020, 8:53 pm

The 5000 ohms referred to is the reflected impedance i.e. the load that the output valve will see and will depend upon the impedance of the speaker as well as the turns ratio. The 45:1 ratio stated would need to be connected to a 2.5 ohm speaker to reflect a 5K load. If you wanted to use a 4 ohm speaker then 35:1 would be more appropriate and for an 8 ohm speaker 25:1

Npri/Nsec = √Zpri/Zsec

5th Apr 2020, 5:03 pm	#1
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Amplifier Advice To keep me occupied, I am planning on building the record player amplifier shown in the picture which was published in a hobby magazine in 1954. My problem is that it states that the impedance of the output transformer should be about 5000 ohms with a ratio of 45:1. The only transformer I can find has a measured resistance of 550 ohms and a ratio of 45:1. a) Would I get away with using this with the 25A6 pentode without doing any damage? b) If it's not suitable, is there any modification that can be made to make it work? Thanks. Attached Thumbnails __________________ Geoff. (BVWS Member)

5th Apr 2020, 5:07 pm	#2
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: Amplifier Advice Don't forget that impedance and measured resistance are not the same parameter. Andy

5th Apr 2020, 5:30 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Amplifier Advice The impedance won't be the same as the DC resistance. The units are the same, but they mean subtly different things. A resistance of 1Ω means that in a steady state, it will develop 1 volt for each ampere of current flowing. An impedance of 1Ω means that the voltage will change by 1V for each 1A change in the current. DC circuit analysis is predicated on the notion that all charged particles involved are moving at a fixed rate and there is no build-up of them anywhere. But crucially, devices such as capacitors and inductors can store and release energy, which allows them to simulate the kind of thing not allowed in DC circuits with no conservation violations; so can behave differently in response to alternating currents than direct ones. As long as the turns ratio is correct, that's what's more important. But there's another thing that strikes me about this circuit. Without wishing to rain too heavily on your bonfire, I'm not really keen on the live chassis construction (and certainly not the fact that there's a DC path from one side of the mains to the audio input). You'd really be better off with something using a power transformer (and not wasting the best part of 60 Watts into a dropper resistor!) __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

5th Apr 2020, 6:39 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Amplifier Advice As stated, the DC resistance is completely irrelevant. Julie M explains it better than me but impedance is not the same as DC resistance. If the ratio of the transformer is correct then use it. A DC resistance of 550 ohms seems entirely reasonable and will be fine. The valve won't 'see' a resistance of 550 ohms but an impedance of 5000 ohms. Impedance is not an easy thing to understand.....Google it and you'll find plenty available. Like Julie, I'm cautious of the live chassis technique and also the non-isolated P.U input. Far better to look for a similar circuit using a mains transformer. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

5th Apr 2020, 7:28 pm	#5
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Amplifier Advice If you want to use the circuit then you could add a 1 - 1 isolation transformer from the mains. Added cost but it would be safety.

5th Apr 2020, 7:29 pm	#6
Guest Guest Posts: n/a	Re: Amplifier Advice To be honest almost any valve output transformer will work, the bigger the better.

5th Apr 2020, 7:41 pm	#7
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Re: Amplifier Advice Thanks for the help, I agree that the live chassis idea is dated and propose a re-think using perhaps 6X5, EF37A and 6V6 to stick with 6.3V heaters. __________________ Geoff. (BVWS Member)

5th Apr 2020, 8:53 pm	#8
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Re: Amplifier Advice The 5000 ohms referred to is the reflected impedance i.e. the load that the output valve will see and will depend upon the impedance of the speaker as well as the turns ratio. The 45:1 ratio stated would need to be connected to a 2.5 ohm speaker to reflect a 5K load. If you wanted to use a 4 ohm speaker then 35:1 would be more appropriate and for an 8 ohm speaker 25:1 Npri/Nsec = √Zpri/Zsec