Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 19, 2019, 03:00:26 am
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I was looking to a pair of transforsers I've
one is 220V primary 380V secondary (50VA) no Center Tap
the other is 220V primary 12V secondary (50VA)
380V (AC) rectified with SS FWB will result in a B+ of around 535V (DC) @ around 90mA
535V is a high voltage and I was thinking to a way to tame it a bit
If I use a vacuum tube rectifier the internal resistance of the tube will lower a bit the voltage, but if I want to use the 12V transformer for heaters ....
Is possible to use a pair of EZ81 tubes connecting the heaters in series ? (and, may be also the diodes in series to lower a bit more voltage)
Thanks
Franco
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Is possible to use a pair of EZ81 tubes connecting the heaters in series ? (and, may be also the diodes in series to lower a bit more voltage)
Yes, that's possible. But since your PT has no center tap, it won't work in your case.
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Ciao Steve
This is the Klemt Echolette arrangement (diodes in parallel on alternate tube and heaters in parallel for 6.3v supply)
so I know this is feasible
(https://i.imgur.com/kVlfYl4.jpg)
I've think is not obligatory to connect the rectifier diodes in alternate mode
I think they can be used in paralleled on each tube
If the series heater arrangement is possble I've think to this
(the SS diodes connected to the vacuum diodes are there to increase the max voltage affordable)
But I don't require to increase the max current
(https://i.imgur.com/e528HMY.jpg)
The second option with the two EZ81 in series is what I was thinking as this will increase the max voltage affordable and drop more voltage than the parallel arrangement
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BTW I don't know of any 12.6v Heater Rectifier Tube, is it existing ? (just curious)
Franco
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12X4 is the US-number 12V rectifier, good to about 70mA of DC.
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Thanks PRR
Franco
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You also probably should use balancing resistors to ensure an even voltage across each rectifier filament
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Mhhh
Tubes will be of the same kind, do you think heaters will have so different values to require to be balanced ?
How do you suggest to proceed to decide resistors values ?
Thanks
Franco
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Mhhh
Tubes will be of the same kind, do you think heaters will have so different values to require to be balanced ?
How do you suggest to proceed to decide resistors values ?
Thanks
Franco
While the same current goes through both filaments, the voltage across each filament might differ if the supply isn't regulated, and you'd want to keep each filament within the designed operating voltage for better performance.
As to what size, big enough to not steal needed current away from the filaments, but not too big that it doesn't function well as a shunt regulator, so somewhere around 100R each.
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The second option with the two EZ81 in series is what I was thinking as this will increase the max voltage affordable and drop more voltage than the parallel arrangement
That's how I would do it. Tube filaments will be fine without any resistors.
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Thanks Tubeswell & Steve
.... in my idea I'm feeding the series heater, theorically, with a bit lower voltage, 12v instead of 12.6v
to be sure about the voltage I think it must be measured on the circuit (the 12v transformer is a 50VA) that at the moment is not planned at all, because the question was posed to know if it will be possible the series arrangement of the rectifiers, as to reuse the 380V AC PT
Franco
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The critical instant will be the startup surge when the filaments are cold. If the filaments are uneven resistance, you will get more voltage across one than the other, which might or might not harm the tube.
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Tubeswell I understand what you mean
but I'm a bit confused about shunt regulation on this application
which is the way to connect the resistors
a resistor in parallel with each tube filament like when we put resistors in parallel with each e-cap connected in series ?
or a single resistor is to be connected between the junction of the two filaments and ground ?
I tried to find info on Merlin's books, but seems he consider to use resistors in parallel with filaments only if the tubes has a different current consumption
(https://i.imgur.com/qwzRg8W.jpg)
Franco
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I'd just put a resistor across each filament.
A 'shunt regulator' is simply a regulator that shunts current ( in which the regulator output voltage is connected to the voltage that you want to regulate). The current through the regulator is typically large enough to ensure that any changes in current in the circuit being regulated do not have an overwhelming effect on the regulator's nominal output voltage. However, with simple resistors, the 'shunt' function is obviously crude and won't regulate perfectly, because any change in current in the circuit being regulated will change the current (and hence the voltage) in the shunt stabiliser. But in this case, all you're using the resistors for is to ensure an even voltage on both filaments. So it's less like a 'shunt regulator' and more like a voltage divider.
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Thanks Tubeswell
Franco