We are delighted to announce ...

What people are working on at the moment
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IslandPink
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Re: We are delighted to announce ...

Post by IslandPink » Sat Sep 28, 2019 8:17 pm

I doubt it !
Ps. the RMS voltage of the 7V transformer secondary is 7V :D
I don't think you need to invoke peak currents , it's just that the total load is at least 24VA on a 25VA toroid, if it's not in open air to cool, it will be stressed. 50% extra VA should be fine - in that case, given the poor regulation of toroids, you may be fine with 2x 6.3 secondaries.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

RhythMick
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Re: We are delighted to announce ...

Post by RhythMick » Sat Sep 28, 2019 10:13 pm

To select the TX secondary voltage required I worked on a 4v regulation loss from the peak TX voltage, so for a 6.3v RMS secondary...

6.3 x 1.414 - 4 = 4.9 ... this would be right on the edge for a 300B requiring 5v, which is why I went with a 7v. I do have a Hammond 266M12 which would give 2x 6.3v @ 3A each which should be plenty current - but it would restrict the voltage to about 4.9ish ? Is it worth me trying this ?

so for the VA required I need 7 x 1.2 x 1.6 = 13.4VA minimum... I think... Is that right?

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Paul Barker
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Re: We are delighted to announce ...

Post by Paul Barker » Sun Sep 29, 2019 9:32 am

The transformer losses are usually made up for by underwindin the primary, so at it’s rated currentand “nominal” mains voltage spec you get 6.3v. Then the better transformers would additionally provide plus voltage and minus voltage tappings on the primary to accomodate variation from “nominal” supply voltage.
I can do it right or I can do it now, but I can't do it right now.

RhythMick
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Re: We are delighted to announce ...

Post by RhythMick » Sun Sep 29, 2019 11:00 am

Of course. Rating 6.3v at 3A. At lower load the voltage will be a bit higher.

Andrew
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Re: We are delighted to announce ...

Post by Andrew » Sun Sep 29, 2019 6:12 pm

All working ok now, dare I ask?
Analogue, the lost world that lies between 0 and 1.

RhythMick
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Re: We are delighted to announce ...

Post by RhythMick » Sun Sep 29, 2019 6:18 pm

Andrew wrote:
Sun Sep 29, 2019 6:12 pm
All working ok now, dare I ask?
All stable and silent (unless I'm really determined to stick my ear next to the Lowthers). The sound is awesome. Many thanks. Definitely bring this to the next meet.

The transformers I have in now are ok but borderline. I plan to swap out next weekend.

RhythMick
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Re: We are delighted to announce ...

Post by RhythMick » Sun Oct 06, 2019 11:03 pm

Well doesn't time fly. Our new arrival is developing nicely and is already outshining its older PP sibling.

Very surprising - I only built this SE amp as a quick and easy rough build spare amp while I brought my PP up to the spec of Geoffs, and because I thought it was a shame to have the kit in a cupboard doing nothing.

I'm desperate to have more experienced ears have a listen. When's the next meet ?

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