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marshall 18 watt tremolo psu question
marshall 18 watt tremolo psu question
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Old 7th February 2019, 05:01 PM   #21
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 7th February 2019, 10:09 PM   #22
PRR is offline PRR  United States
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Use any heavy-duty 250V switch.

Typically same-as your Power switch.

Yes, 350V is "over rating". But a "250V" switch is expected to stand LARGE surges and currents and arcing. Vacuum-cleaner motors, incandescent lamp filaments, etc. The secondary of a PT is not an "infinite source" and doesn't abuse the switch nearly as much as most loads the switch is designed for. Fender and Marshall used 250V switches for many decades, few failures.
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Old 8th February 2019, 06:06 AM   #23
jazbo8 is offline jazbo8
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marshall 18 watt tremolo psu question
@intensate - multiple threads on the same topic have been merged.
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Old 9th February 2019, 02:11 AM   #24
thoglette is offline thoglette  Australia
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Default Diodes are cheap insurance

Quote:
Originally Posted by Osvaldo de Banfield View Post
Diodes in the anode of EZ81 have no sense. You would remove them.
Rectifiers vacuum tube/valves has been working pretty fine much time ago than ss diodes was born.
Except that we beat the heck out of valves in most (western) guitar amps and they fail far more often than in "normal" usage.

Indeed the very first mod in Premier guitar's Immortal Amp series is exactly this: diodes in series with your rectifier.
Click the image to open in full size.
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Old 12th February 2019, 08:44 PM   #25
intensate is offline intensate  United Kingdom
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Default output transformer turns ratio /impedance?

Hello all

The amp design I am working on,I used a doner chassis that came complete with the mains transformer and output transformer along with ec83 pre and el84 output in push pull

It was a diy build from way back when

The output transformer I assumed was a match and has 2 output impedance's of which I wanted to find out if they were say 4 and 8 ohm etc


I put 4.7ac into 1 of the sec pairs and got 189vac on pri
so 189/4.7 =40.21 turns ratio
40.21x40.21 =1616 then x 4 ohms = 6464 or 8 ohms 12928

the 2nd pair 192/4.7 =40.85
40.85x40.85=1668 x 4 ohms =6672
and 8ohms 53376

I am confused as this doesn't match what push pull el84 need and I am not so confident with my maths either

On measuring the pri there are 3 wires ,1 being center tap.I measured across the 2 and not the center tap ,did this make the readings I have ?

also my multi meter ac settings are 600 and 200vac only?

Thanks all
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Old 12th February 2019, 10:25 PM   #26
PRR is offline PRR  United States
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Quote:
Originally Posted by intensate View Post
...the 2nd pair 192/4.7 =40.85
40.85x40.85=1668 x 4 ohms =6672
and 8ohms 53376
...not so confident with my maths either...
My figures say 13,350; 13k not 53k.

All the rest looks OK.

Are you sure the "4.7VAC" stays at 4.7V when connected to a low-impedance winding? Did you measure that with the transformer connected?

Two windings making 189V and 192V is very odd. You expect them to be the same, or very different, not 2% different.
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Old 12th February 2019, 10:28 PM   #27
rayma is offline rayma  United States
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Quote:
Originally Posted by intensate View Post
It needs to switch 350v dc at about 100ma
You won't find one, so switch the AC only.
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Old 13th February 2019, 12:43 AM   #28
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by rayma View Post
You won't find one, so switch the AC only.
Indeed - you need to do your switching up stream of the rectifier.

Switching DC power above 12V is a specialist activity: in AC any arc-over shuts off every half cycle. In DC the arcs never have to stop.

I have a 200W amp on the bench with 70V rails; fried output transistors and speaker protection relays welded shut.

Even 12V will melt a switch if it's badly designed. Ask anyone with a '70s British car with electric windows or sunroof.
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Old 13th February 2019, 07:27 AM   #29
intensate is offline intensate  United Kingdom
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Hi PRR I measured the 4.7 with no load and not connected to the op transformer thanks I will try it loaded with op transformer
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Old 13th February 2019, 07:03 PM   #30
intensate is offline intensate  United Kingdom
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Hi PRR

I re did the mesurements under load
Under load the transformer reads 4.6 v ac and I re did the volt readings and it seems both secondarys give an output of 190.2 vac on pri

so its not clear to me which ones are 4ohm and 8 ohm?

thanks for any help
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