Voltage Reducing Resistors [was: current limiting resistors]

[fenderbass39uk]

Looks good Geoff well done. I'll post my findings in a few days when mine arrive.

[fenderbass39uk]

Both resistors now in circuit (they both measure 297R each B+ measured 317v initially and has dropped to 315v after 3 hours. Plate voltage is 310, cathode is 10v. AC at recto pins 1&7 is 297v. I think I'm going to order a couple of 25w/220R to try as I

20200213_145714.jpg

could use more B+. Both resistors are fairly hot but they are not yet bolted to the chassis for heat sinking effect (Where can I get silicon stuff you used as a bedding Geoff?

[geoff 1965]

got mine on ebay Colin, 2mm thick and you cut to size,sticky on both sides,something like this.

s-l1600 (5).jpg

[fenderbass39uk]

20200219_164123.jpg

Finally installed 2 x 220R power resistors and my B+ has come down from 372v to a more WEM Dominator like 327v. The resistors were getting very hot when suspended in a mid air test rig, but bolting them to the chassis with a layer of thermal pad strip has made a big difference. After 3 hours they are just warm to the touch and the voltage has remained at 327v.
(I forgot to note down the voltage on EZ81 pins 1&7 I'll check later)

[zaphod_phil]

the zener on the centre tap is tried and tested i.e. a large 25W zener which bolts to the chassis. or there is the string of 5W zeners which to be honest i would'nt trust,all it takes is one of them to fail!

No big problem if one of those zeners in the string does fail, you will just get that bit less voltage drop. The fact that you're using several zeners in series, instead of a single big phat one, is therefore giving you something of a safety factor.

The key here, in every case, is to chose zeners rated for at least double the theoretical wattage you will be dissipating in each one. If we take 70mA as being the B+ current in a typical 18W, we should multiply each zener voltage by 0.070 to find the theoretical power dissipation - and than chose zeners rated to at least twice that. I'm sure you already know that

I wouldn't use resistors here either, as the voltage drop will fluctuate depending on how hard the amp is working, while zeners will always give a solidly fixed drop. I also recommend placing any zeners between the first filter cap and the PT primary centre tap. in other places you may not get quite the voltage drop you're looking for, since zeners need to operate with clean DC current, not just choppy DC off the rectifier

[geoff 1965]

thank's and no i did'nt know those zener calculations!
i chose to experiment with the current limiters as i did'nt want any resistors/zeners between the ez81 and first cap into the OT. a resistor will add extra sag and i was'nt sure what effect a zener would have?
good pointers ZP,is the placement relevant to valve rectifiers? this triwatt has it between the rectifier and first cap but it's diode rectified.

TriwattSchematic12.pdf

[zaphod_phil]

thank's and no i did'nt know those zener calculations!
i chose to experiment with the current limiters as i didn't want any resistors/zeners between the ez81 and first cap into the OT. a resistor will add extra sag and i wasn't sure what effect a zener would have?

Zeners don't introduce any additional sag, which is one reason why I prefer to use them over resistors to reduce voltage

is the placement relevant to valve rectifiers? this triwatt has it between the rectifier and first cap but it's diode rectified.TriwattSchematic12.pdf

The same applies with solid state or vacuum state rectifiers. It's funny you mentioned the Triwatt, as that was one of my designs (although derived from Dave Reeve's original Hiwatt designs). You see an 18V zener used there, but due to its non-ideal position, I got somewhat less of a voltage drop. It was quite a few years ago, and I don't remember what the actual voltage reduction ended up being , maybe it was 12V. Anyway, that's a great example to show why the best place to use zeners is after the first filter cap(s).

[geoff 1965]

awesome response ZP! that's more material going into my "build notes" i'm glad you clarified the zener's don't add sag so i'll bear that in mind for future builds.
yeah i like the look of the triwatt,is there an amp that you have'nt been involved in!
thank's for the input of knowledge.

[fenderbass39uk]

Yes interesting stuff guys thanks for the info. Got me interested in voltage drop using resistors. Yesterday I hooked up the Dominator to a dummy 16ohm and played full volume for 5 minutes. The B+ swung between 327v down to 292v the whole time. I couldn't evaluate the sound due to the dummy load.
I'll be investigating zenors for sure in the future. And I'll try through the speaker next time her indoors goes out!

[geoff 1965]

that's strange Colin, i wonder what is causing the voltage drop? 35 volts is quite a bit,keep us posted with your findings. so far i've only tested these 25W's for stability as i'm reworking my preamp.
i know one reason why these valvepower PT's are putting out more than the 290-0-290 spec is my mains wall voltage,i've got 248VAC here.

[fenderbass39uk]

With the volume on 1/3 to 1/2 I was dropping about 18v so its relative to how hard the amp is working

[zaphod_phil]

I confess I used my admin super powers to correct the title of this thread

[geoff 1965]

update4;
strange going's on! my wall "mains" voltage keeps fluctuating between 245 to 250 VAC. i've just done a voltage test and the mains is now 250VAC which has increased the HT to 319-0-319 with 310 volts post 25W's at pins 1 & 7 of the ez81. B+ is now 366 VDC at the first cap "too much" so it look's like i'll have to invest in a variac to be accurate with my testing!
this has got me a bit paranoid now in thinking is the extra resistance of the voltage reducers affecting the HT winding? i know those 8 & 14W's got very hot so would the HT windings have been affected by the heat?

[fenderbass39uk]

My wall voltage seems to go between 242-248 all the time Geoff probably depends on the load variations in the area. As I've deliberately gone a bit lower on my B+ I'm not too concerned.........or do we need zeners!