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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Apr 2020, 5:03 pm | #1 |
Hexode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
Posts: 254
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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.
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Geoff. (BVWS Member) |
5th Apr 2020, 5:07 pm | #2 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: Amplifier Advice
Don't forget that impedance and measured resistance are not the same parameter.
Andy |
5th Apr 2020, 5:30 pm | #3 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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!)
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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 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,578
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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.
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There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
5th Apr 2020, 7:28 pm | #5 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
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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.
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5th Apr 2020, 7:29 pm | #6 |
Guest
Posts: n/a
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Re: Amplifier Advice
To be honest almost any valve output transformer will work, the bigger the better.
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5th Apr 2020, 7:41 pm | #7 |
Hexode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
Posts: 254
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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.
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Geoff. (BVWS Member) |
5th Apr 2020, 8:53 pm | #8 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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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 |