Using 330 vdc output in an 18 watt build?

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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

The answer regarding the wattage and resistance value is that you can't be arbitrary about the numbers and expect positive results - math is involved.

If you're going with a dropping resistor, you apply R = V / I (resistance = volts to drop divided by current).

The wattage of that resistor is determined by P = I² R (wattage = current squared times resistance).
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geoff 1965
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

this might be of interest to you!
Q: I understand you can put a 50V reverse-biased zener diode in the center-tap of a power transformer and lower the output voltage by 50V. Exactly how does this work?
A. A zener diode is simply a diode that has a designed-in "breakdown" reverse voltage that is fairly tightly controlled. In the forward direction, it is a "normal" diode, with a voltage drop of around 0.7V. If you drive current through it backwards, it will drop a voltage equal to the zener voltage. This makes it useful as a level-shifter. Note that "regular" diodes also have a breakdown voltage, and will conduct in the reverse direction if you exceed that voltage. If you put a 50V reverse zener in the center-tap <>of the power transformer, it will conduct on the filter capacitor charging pulses and make the center-tap <>more negative than ground by 50V. Since the main rectifiers can't conduct until the voltage is higher than the voltage stored on the filter capacitor, conduction will be delayed until the voltage is effectively 50V higher than it was before you put the zener in, which in turn reduces the output voltage by 50V.

Another way to look at it is this: it makes the center-tap negative by 50V during the capacitor charging pulse. Since this is negative with respect to ground (and the output voltage), it "subtracts" 50V from the output voltage. Either way, the "real" total voltage differential between B+ and the center-tap remains the same, however, since we have level-shifted the center-tap negatively by 50V, the output voltage is lowered by 50V. The drawback is that you don't get something for nothing. In this case, the price you pay is in power dissipation. The voltage dropped across the zener multiplied by the current through the zener results in a power loss that is dissipated in the zener as heat.
Zener2.PDF
the guy who posted the PDF said he has used the diagram on the left on several amps with good effect + inexpensive! but both diagrams show diode rectifiers so you will have to check if it's suitable for a tube rectifier or use diodes.
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

Plenty of people have used zener diodes (search this site, you'll see more reference to them). You need to use heatsinks and have space to support them and airflow.

The benefit of using a zener vs. a resistor is that it doesn't add sag. However, zener diodes can be noisy to some degree, but that depends on circumstances.

Whether you use resistor(s), zener diodes, or a VVR is up to you. Whichever way you go, research and learn about it first. There's a ton of information available about it on this site alone, and more on the webs if needed.
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Re: Using 330 vdc output in an 18 watt build?

Post by Crabman »

geoff 1965 wrote:
Tue 11/06/18 12:22 am
i think you should do some more research into dropping the B+ like Josh said as well,sag resistors are relatively low in resistance usually around 100 ohms and although they do drop the B+ slightly the main purpose is to simulate the sag of a tube rectifier when you have diodes like mine.
i'm not sure what the "knock on" effect of using a much higher 180K dropper would be to be honest,it sounds easy enough to do but you might find a tried & tested method.
Hello Geoff,

Are you sure about 180K?

I don't know exactly how far we are trying to reduce the voltage here, but if we are drawing a current of say 80mA (a rough guess) that's feeding a pair of EL84's and 3 x 12AX7's and we need to drop the voltage by 80V then this would require a 1K dropping resistor (R=V/I or 80V/0.08A). The dissipation in this resistor would be about 6.4W (P=VI or 80Vx0.08A) so we would look to use a 10W resistor (absolute minimum). At least that's the way that I have been interpreting Ohm's Law.

I think it's probably rare to see this approach since most builds would look to use the appropriate PT to match the specs of the tube. Would a pair of 6V6's perhaps be a better choice?

Dave
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

hello Dave,
i simply passed 406 VDC through a wirewound 0-1meg variable resistor and measured 180K resistance for 340 VDC drop,this is without tube current draw though.i don't have your ability to do the ohms law maths! but i've been reading posts and it does'nt sound like a good idea to use a resistor in this position,you have the sag from the rectifier tube + sag from the dropping resistor which has an ill affect to the tone of the amp.
i agree with you in using 6V6's which can handle 400+ volts on the plates or even EL34's which Dd pointed out early in the post on the spec of the PT.
i could patch the variable resistor into my amp and retest if needed.
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

this is more complex and not as easy as it first seemed!
apart from sag and current draw there is also capacitance and biasing to consider.using a series dropper will probably cause too much power supply sag combined with the rectifier sag and if you used diodes instead of the tube you would have a B+ of over 440volts!
using a choke involves pre and post capacitance to adjust the voltage and sounds tricky!
using a higher value cathode resistor can help compensate high B+ but the bigger voltage drop across the resistor causes loss of power tube distortion " a big part of the 18W's character"!
the zener on the HT centre tap looks the best way so far to me unless someone has a better idea?
rather you than me! i would ask Josh to kindly post the schematic & layout of his superlite el34 and build one of them!
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

geoff 1965 wrote:
Thu 11/08/18 10:19 pm
this is more complex and not as easy as it first seemed!
apart from sag and current draw there is also capacitance and biasing to consider.using a series dropper will probably cause too much power supply sag combined with the rectifier sag and if you used diodes instead of the tube you would have a B+ of over 440volts!
using a choke involves pre and post capacitance to adjust the voltage and sounds tricky!
using a higher value cathode resistor can help compensate high B+ but the bigger voltage drop across the resistor causes loss of power tube distortion " a big part of the 18W's character"!
the zener on the HT centre tap looks the best way so far to me unless someone has a better idea?
rather you than me! i would ask Josh to kindly post the schematic & layout of his superlite el34 and build one of them!
Sorry, Josh isn't posting that. ;) It would probably be a long while before I even consider that.

I'm glad to see your research is leading you in the right direction though.
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

yeah Josh,
i'm confident now in being able to voice/tweak an amp with filtering,biasing etc but this is one area i'm unsure of and need to learn.especially ohms law and working out resistor dissipation etc,Daves earlier calculation looked like a russian recipe for roast chicken to me!!
it's obvious filtering plays a massive part,a good example is my B+ supply
$_122 (1).jpg
the first pre filtering sag is 100ohms and drops the B+ by 9 volts and needs to be at least 5W,yet the next 1K "10 x resistance" only drops 12 volts and can be 2W with capacitance and further still the phase plate resistors drop 75-80 volts off the B+ and only need to be 1W! if someone asked me to to work this out from scratch i could'nt do it.
it will be interesting to see what kneedeepinit decides to do here.
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

geoff 1965 wrote:
Fri 11/09/18 5:21 pm
the first pre filtering sag is 100ohms and drops the B+ by 9 volts and needs to be at least 5W,yet the next 1K "10 x resistance" only drops 12 volts and can be 2W with capacitance and further still the phase plate resistors drop 75-80 volts off the B+ and only need to be 1W! if someone asked me to to work this out from scratch i could'nt do it.
it will be interesting to see what kneedeepinit decides to do here.
What do each of the sections you're referring to actually do?

Look at a complete schematic (not a layout) and follow the voltage path for each part you referenced. There's a difference between an inline resistor early in the food chain and a resistor tied to a tube.
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Re: Using 330 vdc output in an 18 watt build?

Post by kneedeepinit »

Geoff, I am watching closely and learning. I purchased a 180k 5 watt resistor as per one of the earlier posts. Then I purchased a 180k 10 watt resistor as per a later post. Both will arrive in December so no hurry

Now I think I will buy a zener for the center tap as that seems to be the least problematic.

Alternatively, I can just buy the right transformer but half the fun of my projects is reusing vintage stuff instead of watching it get tossed into the dump. I am no eco nut but I don't like the disposable nature of our society now. I like to fix stuff.

I built a 5E3 from vintage main components and love it.
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

kneedeepinit wrote:
Fri 11/09/18 7:30 pm
Geoff, I am watching closely and learning. I purchased a 180k 5 watt resistor as per one of the earlier posts. Then I purchased a 180k 10 watt resistor as per a later post. Both will arrive in December so no hurry

Now I think I will buy a zener for the center tap as that seems to be the least problematic.
Do the math first. Don't just buy stuff based on our discussion. Read our posts, search the forums on this site, and search the internet for ohm's law calculators if you need it. Post back here with your thoughts / ideas / questions...then order stuff.

Buying a Zener, you need to know the voltage, and you need to get the heat sink for it.
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

i like your approach to using/fixing things and Josh is right about buying components,we are still at a discussion stage,wait until you have decided exactly what amp you are going to build and the specs needed.
have you seen the 20W's in the downloads? similar to the 18's but have a higher B+ so might be worth considering.
just an after thought, can you remember how many tubes were in the amp the PT is from? i'm thinking of the amps of the 6.3V heater supply.
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Re: Using 330 vdc output in an 18 watt build?

Post by kneedeepinit »

https://ibb.co/hw4raA

this is what I have in mind. The heat sink is not an issue but the diode is.

My problem is that I cannot find a 50v 20watt zener diode. Mouser does not seem to have one. Can someone find a link or a product number for a suitable diode?
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

kneedeepinit wrote:
Sat 11/10/18 8:06 am
https://ibb.co/hw4raA

this is what I have in mind. The heat sink is not an issue but the diode is.

My problem is that I cannot find a 50v 20watt zener diode. Mouser does not seem to have one. Can someone find a link or a product number for a suitable diode?
Did you do math to get to those values?

Assuming the values are correct, it's pretty easy: https://www.newark.com/w/c/semiconducto ... ion-pd=50w

You can use a higher voltage zener, and then use a different rectifier tube if needed to drop less voltage.

Keep in mind that the wattage rating that you calculate is a minimum, a higher value is fine and often preferred. I tried Mouser, their site is messed up right now. Newark is a good place to compare to, sometimes their prices are better.

Are you in Europe, is that why you drew a 1A fuse?
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Re: Using 330 vdc output in an 18 watt build?

Post by kneedeepinit »

Did you do math to get to those values?

Assuming the values are correct, it's pretty easy: https://www.newark.com/w/c/semiconducto ... ion-pd=50w

You can use a higher voltage zener, and then use a different rectifier tube if needed to drop less voltage.

Keep in mind that the wattage rating that you calculate is a minimum, a higher value is fine and often preferred. I tried Mouser, their site is messed up right now. Newark is a good place to compare to, sometimes their prices are better.

Are you in Europe, is that why you drew a 1A fuse?
[/quote]

I don't trust my math skills for this calculation so I was basing it on previous posts from people who are competent at the math.

Thank you for the link. I am not sure why mouser couldn't come up with those options. I don't see an 50v zener but I do see a 62v. I can try this with a 5u4GB recitifer and think I will be close.
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

struggling to find an axial lead 20W,there's plenty of stud mount but remember this has to be reverse biased.
you might have to use a 1W zener with a NPN power transistor like the one on the zener2 pdf file.
has the transformer got any numbers stamped on?
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Re: Using 330 vdc output in an 18 watt build?

Post by JMPGuitars »

I mentioned the whole reading this site thing, but I think I should link you a couple answers. ;)

Here's a warning regarding the CT method:
viewtopic.php?f=4&t=25111&p=240614&hilit=zener#p240614

viewtopic.php?f=4&t=25065&p=240352&hilit=zener#p240341

search.php?st=0&sk=t&sd=d&sr=posts&keyw ... 4&start=15
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

wise words as usual from Curtiss! yeah that would be my main concern using either zener or resistor is if they fail!
my other concern is we still have'nt established the full spec of that PT,with those voltages it could easily be from a fender champ type which has 70ma HT supply and 2 amp heater supply which is no good for a 18W.
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Re: Using 330 vdc output in an 18 watt build?

Post by kneedeepinit »

so I have narrowed it down to two options which have a history of working and not affecting the sonics of the amp.


the first option is a series of zeners on the CT. The advantage of this design is that it is simple. I can economically change out the zeners once the units is in fabrication in order to end up with the desired B+


https://ibb.co/dnmo7q

the second is to use a low power zener with a variable resistor. The advantage is that I can vary the output voltage until I get exactly what I want.

Image
https://ibb.co/ghp5LA

does anyone have feedback on these options?
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Re: Using 330 vdc output in an 18 watt build?

Post by geoff 1965 »

have you calculated what's needed like Josh said? very important! this area is going to be subjected to a lot of heat. i.e. the zener2 pdf uses a low value zener because the transistor is the heatsink.if you use a string like the first method but then remove any to get the voltage you're going to reduce the overall wattage and there is no heatsink.
the second method looks okay and the variable resistor would act as a heatsink but they are expensive!
just put my mind at rest with the PT! you've said it's robust and from an organ so i presume it had a push/pull power section not single ended?
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