I'm working on a DuKAne 1A831 300 watt amplifier.
Looks like the plate transformer is shorted.
On the print below it is T1. This transformer has two output windings each at 300 volts it looks like.
The steel is 5 1/4" X 4 3/8" (standard size lamination) It appears that the thickness of the steel is about 3"
I need help in guessing its current rating to quote a replacement Transformer?
It is powering a Qty of 8, 8417s in push pull.
http://bit.ly/2b0CjSD
Looks like the plate transformer is shorted.
On the print below it is T1. This transformer has two output windings each at 300 volts it looks like.
The steel is 5 1/4" X 4 3/8" (standard size lamination) It appears that the thickness of the steel is about 3"
I need help in guessing its current rating to quote a replacement Transformer?
It is powering a Qty of 8, 8417s in push pull.
http://bit.ly/2b0CjSD
The plates take 8 x 0.15V/3.3R = 0.364A from the 600VDC, or 218W.
Screens are roughly 8 x 5mA = 40mA from the 300VDC, or 12W, maybe more.
The output filaments take 8 x 1.6A = 12.8A at 6.3VAC, or 81W.
These alone add to 321W, plus 10% internal transformer loss, so over 350W.
I'd go with at least 400W, maybe 500W line input, or around 4A at 120VAC.
Note the fuse is 8A, so 4A isn't too far off.
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a full wave doubler would have used just two rectifiers and two caps,
then you will get both 600 and 300 volts in one go...
center leg is 1 3/4 stacked to 3 inches so primary VA is 850.....
per RDH4, chapter 5, page 235....http://www.ax84.com/static/rdh4/chapte05.pdf
a rating of 300w required 468 watts input...traffo size is just about right...
i.e. 850va x 0.6 =510 watts, very close....
and if implemented using M6 cores will be top notch....
notice that this is just for the plate power, a separate traffo T2 is used for filament supplies...
this a plan i agree with, a good design trick if you ask me...
Patrick Turner also have one 300 watt amp....300W-Monobloc
then you will get both 600 and 300 volts in one go...
center leg is 1 3/4 stacked to 3 inches so primary VA is 850.....
per RDH4, chapter 5, page 235....http://www.ax84.com/static/rdh4/chapte05.pdf
a rating of 300w required 468 watts input...traffo size is just about right...
i.e. 850va x 0.6 =510 watts, very close....
and if implemented using M6 cores will be top notch....
notice that this is just for the plate power, a separate traffo T2 is used for filament supplies...
this a plan i agree with, a good design trick if you ask me...
Patrick Turner also have one 300 watt amp....300W-Monobloc
And you would also have a ton of harder to filter 60Hz ripple instead of 120Hz one.
There´s a reason voltage doublers are impopular in power amps.
Patrick Turner has a very good explanation regarding voltage doublers,
i use them all the time when building amps using TV tubes and transmitting tubes too...
powersupplies
how i wished Patrick was posting here also....
simplicity, and easier winding for power traffos because of the lower voltages and lower turns count, what is not to like?
ripple frequency.....http://www.circuitstoday.com/comparison-between-half-wave-and-full-wave-voltage-doublers
btw, vintage japanese tube amps like the Pioneer SA81 used them, the Japs thought must be good...
i use them all the time when building amps using TV tubes and transmitting tubes too...
powersupplies
how i wished Patrick was posting here also....
simplicity, and easier winding for power traffos because of the lower voltages and lower turns count, what is not to like?
ripple frequency.....http://www.circuitstoday.com/comparison-between-half-wave-and-full-wave-voltage-doublers
btw, vintage japanese tube amps like the Pioneer SA81 used them, the Japs thought must be good...
Hi Guys
There are no screen resistors in this amp, which allows maximum power from the tubes but provides no protection. Anything from 100-1k works well, with more protection with higher R value. Since Va=600V, Raa is likely about 1k6 which leads to a peak current of about 1.2A - peak output power might actually be more than 600W, so a bit higher current should be counted on. With eight 8417s one would expect the output power at clipping to be closer to 400Wrms (800Wpk) but that margin would be up to the designer to decide. Screen current can approach one-tenth of plate current depending on the tube. The 8417 has an efficient screen that draws less current than most other tubes but you still have to account for ti in the T1 replacement.
It is odd that a choke-input supply is used for both high voltages as is more typical of an earlier era. Modern amps use stacked DC supplies to produce the disparate voltages and these are easy to design. Note that in such an arrangement the lower supply has to support half the plate load plus the screen load where the upper supply only supports half the plate load. Using a full bridge for each supply the AC-DC conversion is predictable and ripple is at 100-120Hz (depending on mains frequency). In this case, no plate choke is required although a screen choke is a good idea.
Fender used a voltage-doubler supply in their PS-series amps, rated at 150, 300 and 400W. The later Super Bassman Pro-300 also uses a voltage-doubler. As others above point out, doublers use few components but have bad ripple. Stax and others used doublers for their ESL bias supplies which operate almost open-load - but since it is not truly open, there were hum issues with those old headphone energisers.
Have fun
There are no screen resistors in this amp, which allows maximum power from the tubes but provides no protection. Anything from 100-1k works well, with more protection with higher R value. Since Va=600V, Raa is likely about 1k6 which leads to a peak current of about 1.2A - peak output power might actually be more than 600W, so a bit higher current should be counted on. With eight 8417s one would expect the output power at clipping to be closer to 400Wrms (800Wpk) but that margin would be up to the designer to decide. Screen current can approach one-tenth of plate current depending on the tube. The 8417 has an efficient screen that draws less current than most other tubes but you still have to account for ti in the T1 replacement.
It is odd that a choke-input supply is used for both high voltages as is more typical of an earlier era. Modern amps use stacked DC supplies to produce the disparate voltages and these are easy to design. Note that in such an arrangement the lower supply has to support half the plate load plus the screen load where the upper supply only supports half the plate load. Using a full bridge for each supply the AC-DC conversion is predictable and ripple is at 100-120Hz (depending on mains frequency). In this case, no plate choke is required although a screen choke is a good idea.
Fender used a voltage-doubler supply in their PS-series amps, rated at 150, 300 and 400W. The later Super Bassman Pro-300 also uses a voltage-doubler. As others above point out, doublers use few components but have bad ripple. Stax and others used doublers for their ESL bias supplies which operate almost open-load - but since it is not truly open, there were hum issues with those old headphone energisers.
Have fun
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