Using 330 vdc output in an 18 watt build?

[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!

[JMPGuitars]

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.

[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.

[JMPGuitars]

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.

[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.

[JMPGuitars]

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. 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.

[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.

[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?

[JMPGuitars]

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?

[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.

[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?

[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

[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.

[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.


https://ibb.co/ghp5LA

does anyone have feedback on these options?

[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?

[kneedeepinit]

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?

Geoff, the PT came from a Conn organ that had a large number of tubes. I do not recall the model. Other than the voltages, what parameters would you like me to measure?

[geoff 1965]

if it had a large number of valves then it should be okay,i was thinking of the 6.3volt heater supply mainly.1 el84 & 1 12ax7 combined will draw just over 1 amp so ideally you want a minimum 3 amp supply.

[kneedeepinit]

if it had a large number of valves then it should be okay,i was thinking of the 6.3volt heater supply mainly.1 el84 & 1 12ax7 combined will draw just over 1 amp so ideally you want a minimum 3 amp supply.

ok thanks

this PT is a monster, very heavy, almost too large for ease of mounting.

because I have a number of amps already and this is just a recreational build, I am okay with buying some parts and then swapping them to get the proper voltage. I think I am going to go the route of the smaller zener with a transistor as this should get me close on the first try.

Geoff, you raised a possible issue with the cost of the variable resistor. From what I have seen in my limited research, it was not a 50 watt heavy duty type, just a regular trim pot which are quite cheap. Am I making a mistake on that assumption?

[geoff 1965]

not sure! but i would think in a heatsink situation it would have to be wirewound.
i'm suprised nobody who has actually done this B+ reduction has contributed to the post yet!
yeah i think the zener2 method with the transistor looks the best so far,it's from aiken amps and is tried & tested.
having the heatsink looks the safest way,if my sag resistor fails i'll only have an extra 10 volts going into the el84's,if you're zener fails you'll have to book a seat on the next space shuttle to retrieve them!!

[crgfrench]

Sorry, Josh isn't posting that. It would probably be a long while before I even consider that.

Hi Josh -- not to ask for any proprietary trade secrets here, but viz. your EL34 Superlite, I am curious if you have a demo of it we could see/hear (youtube?)

and just generally is your design for it all tube or do you also include SS like a MOSFET?

Best,

Craig