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Crate V18 with no PSU, got an idea
Crate V18 with no PSU, got an idea
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Old 14th August 2018, 11:30 PM   #11
Vincebus Riveruptum is offline Vincebus Riveruptum  Chile
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Crate V18 with no PSU, got an idea
Quote:
Originally Posted by Gnobuddy View Post
I tried the same thing many years ago, when I was still a boy. I had no success either. I could not wind the coil as neatly by hand, nor stuff the laminations as tightly. And the sharp metal edges cut my fingers, so I left some blood behind in the (badly) wound transformer.

-Gnobuddy



LOL
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Old 23rd August 2018, 12:36 AM   #12
Vincebus Riveruptum is offline Vincebus Riveruptum  Chile
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Crate V18 with no PSU, got an idea
Past week was for getting the materials and a planning everything for the transformer

Making the transformer was a painful experience. First despite the difficulty to coil the thing by hand, noticed the space was limited again even with 24 AWG(was AWG 20 or so before) so were no room for heaters and the 12-0-12 for the solid state stuff, had to use a separate transformer for that purpose. At the end the tranny got super thick and the wires where almost touching the iron in between, looking like ****, i call it the potato transformer lol.

Also, something i had not mentioned, before finishing it got wrong voltages, my calculations where not precise enough and had 203v at the beginning... ran out of wire and had to improvise a bit, good thing i had another tranny with 24 awg inside and cannibalized it. After trial and error, added more turns until i get close to 230 -240v, got 233v at the end, close enough.

The separate transformer was easier though, i had everything at home and took an afternoon to make it, picked some 12v 3A transformer and rewinded it for the heater supply the 12-0-12, however the voltages were a bit off again. i got 7.2V for the heaters(center tapped), i know this is way off, any suggestion to fix it whitout dissasembling it? I was thinking of shottky diodes and chopping of some voltage.

Yet i cant test anything until i get one EL84, also i have to make the pcb for the rectifier and the filtering. also i have to plan how to fit the stuff inside, but that job will be for the next week.

After all im happy because it was the hardest step, the rest is going to be much easier. Thanks for all the suggestions guys

Gotta upload some pics
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Old 23rd August 2018, 12:35 PM   #13
Printer2 is offline Printer2  Canada
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Those are unloaded voltages, right?
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Old 23rd August 2018, 03:21 PM   #14
Vincebus Riveruptum is offline Vincebus Riveruptum  Chile
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Crate V18 with no PSU, got an idea
Quote:
Originally Posted by Printer2 View Post
Those are unloaded voltages, right?

yep
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Old 11th September 2019, 06:02 PM   #15
mgee1960 is offline mgee1960
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I have a crate V-33 with a fulty power supply board, I experienced a power surge off mu Variac could I convert this amp power to a traditional (old school-pre 1980's) power supply? I have pics of the board & schematics
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Old 11th September 2019, 07:30 PM   #16
mgee1960 is offline mgee1960
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Quote:
Originally Posted by mgee1960 View Post
I have a crate V-33 with a faulty power supply board, I experienced a power surge off my Variac could I convert this amp power to a traditional (old school-pre 1980's) power supply? I have pics of the board & schematics

Last edited by mgee1960; 11th September 2019 at 07:41 PM.
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Old 14th September 2019, 05:59 AM   #17
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Vincebus Riveruptum View Post
I got 7.2V for the heaters(center tapped), i know this is way off, any suggestion to fix it without dissasembling it? I was thinking of shottky diodes and chopping of some voltage.
I would avoid diodes, they will add a lot of noise to the heater current because of the sharp-cornered current and voltage waveforms.

But the solution is very simple - you can use a small power resistor - it's easy, effective, and cheap.

As an example, let's say your 7.2V drops to 6.8V under load, and let's say your heaters draw 2 amps. You need to go from 6.8V to 6.3V, so you need to drop 0.5 volts. Ohms law says R = V/I = 0.5V/2A = 0.25 ohms.

So you would insert a 0.27 ohm or 0.22 ohm (nearest standard value) resistor ins series with the heater power, and that should get you the right heater voltage.

Keep in mind that the heater current may not be exactly what the factory says, and it will change slightly when you add the resistors, so you may have to re-calculate and try out the resistors a couple of times before you find just the right values.

If you are picky, you can divide the resistor into two equal values, and put one in each of the heater wires, so that the centre-tap remains accurately centred (and not slightly imbalanced by having the additional resistor on one side). In our example, you could use two 0.12 ohm resistors (divide 0.27
ohms by two, and then use the nearest standard value.)

The only remaining question is how big (power handling) the resistors should be. Power dissipation is current * current * voltage. In our example, that is 2A*2A*0.27 watts, which works out to 1.08 watts. You could use a 2 watt resistor, or a somewhat bigger one to add more safety margin. A 5W resistor would run quite cool.

If you split the resistor into two equal ones of half the resistance, the power dissipated in each one is also halved - only 0.54 watts each. So in this case you could use two 1W resistors (or better, two 2W resistors.)

I recently found an old Hammond power transformer with a 6.3V heater winding - but it put out 7.5V RMS according to my DMM! This is a common problem with older transformers in North America, because the mains AC voltage has increased quite a bit in the last fifty-odd years.

I fixed the problem exactly the same way I just described, with a small resistor in series.


-Gnobuddy
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Old 14th September 2019, 06:10 AM   #18
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by mgee1960 View Post
...Could I convert this amp power to a traditional (old school-pre 1980's) power supply?
You certainly could. The amp cares only about the voltages that are applied to its wires...it doesn't care how that voltage is generated, whether old-school power supply, modern switching supply, or a large box full of lemons with wires stuck into them.

The only real issues are that the old-school power supply will weigh five to ten times as much as the switching supply, and take up more room. I'm assuming you're okay with the weight increase, and that there is plenty of room inside the amp case to acommodate the larger power supply you're going to build.

The HT (B+) and heater power require nothing different from thousands of classic guitar amp circuits. The only thing a little different is you need to provide +15V, 0V, and -15V as well; I would suggest using a separate small transformer for this, or even a pair of small switching supplies (a very simple solution that reduces the amount of stuff you have to build.)

Alternatively, a 15-0-15 (AC, RMS volts) transformer feeding a bridge rectifier and filter cap will generate somewhere around +/-20 volts DC, which you can regulate down to +/- 15V with a couple of 3-terminal voltage regulator chips. Or you can use a 12-0-12 ACV RMS transformer, which should generate something close to +/- 16 volts DC, and you can drop that a bit more with another RC filter stage, avoiding the need for voltage regulator chips.


-Gnobuddy
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Old 14th September 2019, 03:48 PM   #19
mgee1960 is offline mgee1960
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So where can I find a diy to build? Thanks
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Old 14th September 2019, 05:41 PM   #20
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by mgee1960 View Post
So where can I find a diy to build? Thanks
There are only a few components in an old-school power supply: AC inlet, switch, fuse, power transformer, a couple of silicon diodes, a filter capacitor. If you have previous experience building tube amps, the power supply itself is pretty easy to build - but remember that safety is all-important. Both your own safety while building and testing the power supply, and the safety of anyone else who might use the amp in the future.

So, if you have no previous experience building electronics, a high-voltage power supply is not the place to start. There are lethal voltages involved.

If that is the case, why not talk to your local tube amp tech? He/she should be able to put together a power supply for your amp.


-Gnobuddy
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