Voice Of Music 1450 Tube Amp

[CJ]

carefully measure primary current after warm up , maybe crank a little guitar while watching the meter,

 

[PRR]

> maybe crank a little guitar

This amp is single-ended, can't get out of class A, so the power demand will hardly change with signal.

Get a SVT, or a Bogen 300, and yeah the consumption goes up significantly when you beat on it.

I don't think it is even worth putting a line meter on it. The amp is sub-Ampere. The line cord is 13 Amps or better. Pick an in-between number, on the low side of the spread. 1A, 2A, whatever is handy. 0.5A-0.6A may be fine in 230V land.

 

[abbey road d enfer]

Transformer size needs 1.2 increase in core area for 50 Hz

5 pts extra credit: why does that not correspond to a transformer that is 1.2 times bigger?

Since 50Hz increases the number of turns by the same 1.2 factor and the current is the same, the wire gauge does not change, resulting also in more copper.
In the 80's I was the Cetec-Vega distributor in France. Their receivers were plagued with untolerable hum; they had just asked their winder to  double the primary turns. They didn't have a 50Hz power source. They tested their receivers on 240/60.
They had to find a winder that understood the issue and had the test gear. The final xfmr was not 1.2 times bigger, but maybe 10% bigger, and they had to use an expensive core.

 

[CJ]

answer to 5 pt bonus question: if you need to increase the size of the core area by a factor of 1.2 for 50 Hz operation, then the lam size has to increase by 1.2^.5  as area = side x side

so the sq root of 1.2 is 1.095 so the stack for say a 1" x 1"  core goes to 1.1" x 1.1" instead of 1.2" x 1.2"

so the core needs to be 1.1 times bigger as the flux formula uses cm^2 .

the correct way to fuse that amp is a follows:  unplug amp, short secondary taps with alligator clip,

attach ammeter to primary, pwr amp up for 5 seconds, don't worry, the sec will will not cook,

fuse value should be a bit under short circuit test.
most power transformers have hefty wire for the primary, so if the primary shorts, it will trip a 10 amp fuse,

but a short on the secondary side might not trip the fuse as the turns ratio will lower the short current on the sec side, and the high DCR of the secondary might also play a part in this.

i over fused a git amp and left it on by mistake. when i came home, the apartment was filled with smoke as the fuse did not blow due to the secondary shorting out. could have been bad.
cheap ass hammond xfmr, 

slightly OT, here is a blurb on VAR and Weight>

 

[CJ]

lets work out on some formulas for 50 and 60 Hz and see what happens,  sounds exciting, right? 

Core Loss: (from book page below)

(  60 Hz / 50 Hz ) ^ 1.71-1.36 = 1.2^ 0.35 = 1.066 

so core loss will decrease by a factor of 1.066 when using a 50 Hz xfmr at 60 Hz,

or in reverse, using a xfmr at 50 Hz instead of 60 will increase core loss by about 6.6%

Power: proportional to frequency should vary as follows:

P > f ^0.2

compare 50 and 60 Hz: 

P = 50 Hz ^0.2 = 2.186      P = 60 Hz ^0.2 = 2.268 

so using the xfmr at 50 instead of 60 Hz will drop it's power rating to 96.4% = you lose 3.6% pwr capacity.

Weight:  how much more weight do i need for 50 Hz to keep ratings the same?

W> f ^ -0.15

W = 50 Hz ^-0.15 = 0.556  W = 60 Hz -0.15 = .541

so the 50 Hz xfmr needs an additional wt of 556/541 = 1.0277 or 2.77% more weight to maintain the same ratings,

Summary:

When using a 60 Hz xfmr at 50 Hz, core loss goes up 6.6%,  power rating  drops by 3.6%  , and weight should go up by 2.77% to compensate for the 10 Hz difference.

All in all not a big deal since power transformers are usually over designed to supply more VA than needed and run at low flux values way below the saturation value to keep radiated energy down and also to limit inrush current, to limit the steady state exciting current into the primary, or even the audible noise from the core.  So you may run a tad bit hotter and you may have a bit more noise, other than that plug it in and lets jam! 

 

[JohnRoberts]

The simpler way to think about the difference between 50Hz and 60Hz iron is to think about the magnetic flux (energy) that must be passed with every cycle of the mains. The rectified output mainly draws current at the peaks of the mains waveform (+ and - voltage peaks for full wave rectified). So a 50Hz transformer must pass its total average energy during 100 (50x2) discrete pulses. The 60Hz transformer has the same energy spread over 120 (60x2) pulses so needs 5/6ths as much iron and copper, all else equal.

In large scale manufacturing transformers are not over designed so 50 hz and 60Hz markets got different transformers. These days universal switching supplies probably cover both markets for SKUs that can support the extra cost of the fancy PS, while HF switching supplies ultimately save cost and size with smaller magnetics. (Back in my time the switchers were still too expensive to use widely).

JR

 

[musika]

[

most power transformers have hefty wire for the primary, so if the primary shorts, it will trip a 10 amp fuse,]most power transformers have hefty wire for the primary, so if the primary shorts, it will trip a 10 amp fuse

Used a 1A.  It's sounding great and plenty loud.

And, speaking of loud, just finished this book:

https://www.amazon.com/Play-Loud-History-Revolution-Electric/dp/038554099X

Great read - highly recommended!