Using 330 vdc output in an 18 watt build?

[kneedeepinit]

I have built a few tube amps and am starting another one. I have used my existing transformers and then researched a suitable amp design.

I have a nice robust transformer from a vintage organ that has 330 vdc output. I would like to build an 18 watt guitar TMB but the voltages are 275 plus 275. I am concerned about my voltages being too high.

I have done some searching but have not found a direct answer so I am posting here. What is my best path forward?

[Daviedawg]

The general opinion is that you should target around 320 to 330 v dc at the plates of the EL84s. Which means a B+ more than that but not a lot.

Can you tell us any more about the transformer, such as the nominal secondary values and if it is centre tapped? Or you can look at the Valve Wizard who has a section on transformers telling you how to work out your voltages and from that you can determine if it is suitable or not.

Using a valve rectifier can drop voltage as opposed to diodes or a sag resistor to simulate a valve rectifier. So there are options to consider in an effort to make use of your salvage, which is a good thing.

Dd

[kneedeepinit]

Davie, thank you for the advice. I think my issue is that I am trying to force the transformer to match as design instead of designing for the transformer. I was planning on doing a 18 Watt Superlite TMB with ss rectifies but that is probably not the best route for this transformer.

Upon rechecking the values, here is what I have - 644 VDC with a center tap, 5.4 VDC and 6.9 VDC for the heaters.

This probably lends itself to non SS rectifier design which is fine.

[Daviedawg]

Yep that is a transformer for a valve rectifier and bigger valve output than 18 watts.

Dd

[kneedeepinit]

I think I can make it work for a conventional valve rectifier 18 watt build.

I am into recycling vintage electronics and have made a 5E3 from scratch using all vintage except the capacitors and resistors. For this build, I could use a 5Y3GT or 5U4GB rectifier tube. That should result in a usable vdc for an 18 watt build.

My questions is, which 18 watt TMB would be a good fit for this power supply setup? This is the hard part for me in the past as it took a couple of months to decide on my 5E3 build because of my intent to base the build on existing transformers and tubes.

I am thinking the Ceriatone 18 watt TMB might be a decent choice. I have 95% of the components

[JMPGuitars]

I think I can make it work for a conventional valve rectifier 18 watt build.

I am into recycling vintage electronics and have made a 5E3 from scratch using all vintage except the capacitors and resistors. For this build, I could use a 5Y3GT or 5U4GB rectifier tube. That should result in a usable vdc for an 18 watt build.

My questions is, which 18 watt TMB would be a good fit for this power supply setup? This is the hard part for me in the past as it took a couple of months to decide on my 5E3 build because of my intent to base the build on existing transformers and tubes.

I am thinking the Ceriatone 18 watt TMB might be a decent choice. I have 95% of the components

I think you're on the right track, but I would choose a build from the downloads section of this website. You'll have the least amount of headaches.

As long as your rectifier of choice gives you the correct voltage drop to get you into our happy operating range (ideal B+ is about 340VDC), any of the 18 watt variants should be fine. If you're going with an octal rectifier, you'll have some good flexibility assuming you mind any wiring differences between valve types.

[geoff 1965]

hello,
i think Dd is right on this one,rectified is going to be well over 400VDC.
just had a go with a calculator using 325-0-325 and current draw for 2 el84's and 3 12ax7's,
5U4B = 416 VDC
5Y3 = 406 VDC
GZ34 = 442 VDC
EZ81 = 422 VDC
hope this helps you
http://thesubjectmatter.com/calcptcurrent.html

[JMPGuitars]

hello,
i think Dd is right on this one,rectified is going to be well over 400VDC.
just had a go with a calculator using 325-0-325 and current draw for 2 el84's and 3 12ax7's,
5U4B = 416 VDC
5Y3 = 406 VDC
GZ34 = 442 VDC
EZ81 = 422 VDC
hope this helps you
http://thesubjectmatter.com/calcptcurrent.html

5Y3 gets him 406VDC on that calculator, and then there are methods to drop the rest of the voltage on the B+ depending on how much room he has in his chassis, and what the real life voltage actually is.

[geoff 1965]

my 100 ohm sag resistor dropped my B+ by 10 volts so i've just done some testing using a variable DC supply and a variable wirewound resistor and 180K will drop the B+ from 406VDC down to 340VDC.
bear in mind 330-0-330 will be a bit higher than 406 rectified.

[kneedeepinit]

you guys are awesome. Doing these sort of calculations is not my competence so I am grateful.

If I understand this correctly, if I added a 180k resistor between the 5YCGT tube pin 3 and the power switch, I should get a usable voltage. I am assuming this should be a 3 or 5 watt resistor. Mouser does not seem to have 180k 5 watt but they do have 3 watt

[JMPGuitars]

Not the power switch. Do some more research first. Search this site and google for voltage dropping a tube amp B+. There are multiple ways to do it before and after the rectifier tube.

There's also dropping resistor calculators and that sort of thing to help you pick a resistor value of you use a resistor(s) to do it. A VVR could work too. Other options as well.

[geoff 1965]

i have mine between the rectifier & standby switch connection,you need wirewound 5 watt or even 10 watt for a B+ dropper to be safe and fit it so air can circulate around and underneath.
try ebay they have 180K wirewounds,but the calculations are a rough guide so best thing to do is when you have the amp at the stage where you have rectified the B+ you can measure the actual voltage and we can work out the exact resistance you'll need.
good luck

[geoff 1965]

if you decide to use a sag type dropping resistor i would recommend a 10 watt wirewound,i have a 5 watt ceramic type but i've only got 360VDC passing through and you're going to have over 400! best be safe!
have a look at the superlite IIb in the downloads section it shows the type and placement.
have you got a suitable output transformer?

[kneedeepinit]

I have a number of output transformers in my collection. I will probably use the one that came out of the same donor as the power transformer and see how it sounds.

Thanks for the heads up on the 10 watt. I think that is the safest route. Most of the expensive parts of this build have been collected for free. The only thing I am spending money on will be some resistors and capacitors and a face plate so dropping a bit for a safe voltage dividing resistor is no issue.

[geoff 1965]

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.

[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).

[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.

[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.

[Crabman]

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

[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.