Using 330 vdc output in an 18 watt build?

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, Daviedawg, Graydon, zaphod_phil, rock_mumbles

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Tue 11/06/18 7:19 am

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).
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

geoff 1965
Occasional poster
Occasional poster
Posts: 326
Joined: Mon 01/04/16 3:52 pm

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

Postby geoff 1965 » Tue 11/06/18 8:13 pm

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.
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Tue 11/06/18 8:57 pm

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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

User avatar
Crabman
Unrated
Posts: 19
Joined: Thu 10/28/10 2:00 am
Location: Sudbury, Ontario

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

Postby Crabman » Wed 11/07/18 9:37 pm

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
1 x

geoff 1965
Occasional poster
Occasional poster
Posts: 326
Joined: Mon 01/04/16 3:52 pm

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

Postby geoff 1965 » Wed 11/07/18 10:50 pm

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.
0 x

geoff 1965
Occasional poster
Occasional poster
Posts: 326
Joined: Mon 01/04/16 3:52 pm

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

Postby geoff 1965 » 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!
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Fri 11/09/18 7:25 am

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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

geoff 1965
Occasional poster
Occasional poster
Posts: 326
Joined: Mon 01/04/16 3:52 pm

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

Postby geoff 1965 » Fri 11/09/18 5:21 pm

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.
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Fri 11/09/18 7:30 pm

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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

kneedeepinit
Unrated
Posts: 11
Joined: Thu 10/19/17 5:15 pm

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

Postby kneedeepinit » 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.

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.
2 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Fri 11/09/18 7:35 pm

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.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

geoff 1965
Occasional poster
Occasional poster
Posts: 326
Joined: Mon 01/04/16 3:52 pm

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

Postby geoff 1965 » Fri 11/09/18 8:43 pm

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.
0 x

kneedeepinit
Unrated
Posts: 11
Joined: Thu 10/19/17 5:15 pm

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

Postby kneedeepinit » 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?
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1041
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

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

Postby JMPGuitars » Sat 11/10/18 8:51 am

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?
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design - DolceVittoria.com
Guitars / Amps / Effects - JMPGuitars.com

kneedeepinit
Unrated
Posts: 11
Joined: Thu 10/19/17 5:15 pm

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

Postby kneedeepinit » Sat 11/10/18 9:06 am

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.
0 x


Return to “18watt Technical Discussion”

Who is online

Users browsing this forum: No registered users and 3 guests