Recommended method for dropping power transformer HT voltage

[beatnik]

Hi everybody

I am currently trying to get a Fisher K-10 tube spring reverb working . The power transformer needed replacing

Tried a toroidal power transformer that I had in stock from a different tube project. This transformer gives about 400V at the first power node after the rectifier

The Fisher K-10 circuit demands 270V after the rectification, so quite some voltage that needs to be dropped, about 130V

I have tried dropping the voltage using wire wound resistors. One 4.7K followed by a 10K with 22uF gives me 265V, very close to the required voltage, however as to be expected the resistors heat up quite a lot

I was wondering if there could be a more efficient way to drop the voltage from the HT of the power transformer ?

[Old Tele man]

What rectifier configuration are you using? If current draw isn't too high and some additional ripple can be tolerated, consider a simple halfwave diode configuration

[Gregg]

Check out the B+ MOSFET reducer. Easy to build, works fine, you can drop as many volts as you need and you can use the chassis as heatsink.

[mozz]

Choke input supply might help.

[Pedro Vecino]

https://www.tube-town.net/ttstore/Tr...0VA::2192.html

This is my Walking Liberty half dollar

[nickb]

Hi everybody

I am currently trying to get a Fisher K-10 tube spring reverb working . The power transformer needed replacing

Tried a toroidal power transformer that I had in stock from a different tube project. This transformer gives about 400V at the first power node after the rectifier

The Fisher K-10 circuit demands 270V after the rectification, so quite some voltage that needs to be dropped, about 130V

I have tried dropping the voltage using wire wound resistors. One 4.7K followed by a 10K with 22uF gives me 265V, very close to the required voltage, however as to be expected the resistors heat up quite a lot

I was wondering if there could be a more efficient way to drop the voltage from the HT of the power transformer ?

Any passive means will be inefficient. You'll be dropping 130V at 10mA that's 1.3W - a small number. I'd keep it simple and just change R34 to 15K 5W and call it a day.

[J M Fahey]

Hi everybody

I am currently trying to get a Fisher K-10 tube spring reverb working . The power transformer needed replacing

Tried a toroidal power transformer that I had in stock from a different tube project. This transformer gives about 400V at the first power node after the rectifier

POST-THE-SCHEMATIC

The Fisher K-10 circuit demands 270V after the rectification, so quite some voltage that needs to be dropped, about 130V

I have tried dropping the voltage using wire wound resistors. One 4.7K followed by a 10K with 22uF gives me 265V, very close to the required voltage, however as to be expected the resistors heat up quite a lot

Indeed.

I was wondering if there could be a more efficient way to drop the voltage from the HT of the power transformer ?

Short answer: no.

You are dropping 130V across 14700 ohms, so current is 8.85mA
Power is V*I=130*0.0085=1.15W

You will have exact same dissipation whether you drop those 130V across a resistor or MosFet,none will be "more efficient" than the other, and that power level is very small anyway, just over 1W .

I suggest you slightly upgrade your resistors to 10k 2W and 4k7 1W .

Dissipation will be exact same as before, but being larger resistors with more surface they will work somewhat cooler.

IF you were buying a new transformer then Pedro Vecinoīs suggestion is excellent, because rectified voltage would be straight on the mark, but as is 1 GBP worth of resistors will do the trick.

EDIT: just saw Nick's post
Yes, thatīs the way.

[beatnik]

Thanks everybody for all the inputs

15K resistance gives me the correct voltage, however the 3W resistors I have tried were running a little hot. I have now ordered a 5W and will see if that's any better

I might check out the B+ reducer at some point. Should be possible to use Mosfets also to regulate B+ ? It could be interesting for a bench power supply

[Gregg]

I didn't read carefully the first post so if we're talking preamp currents it would be better to use resistors. The B+ reducer is usually higher current solution.

[beatnik]

I had not considered before that the original filter capacitor in the unit (metal can type) is only rated at 350V

With the new power transformer there is 400V on the first capacitor section

Is the can capacitor strong enough to sustain the extra voltage or it would be more sensible to replace it with 450V rated capacitors ?

[Mike Sulzer]

I had not considered before that the original filter capacitor in the unit (metal can type) is only rated at 350V

With the new power transformer there is 400V on the first capacitor section

Is the can capacitor strong enough to sustain the extra voltage or it would be more sensible to replace it with 450V rated capacitors ?

I would not trust it. I suppose you could make the situation better by putting the resistor in the input lead of the capacitor, that is, coming from the diodes. Then there would be over voltage only when the filaments have not yet warmed up. This becomes a different kind of power supply and the ripple might be different.

Let's see if someone else can comment on this idea.

[glebert]

Sounds like it isn't right for this situation, but there is a cool circuit for fixed voltage drop that uses a zener and a BJT. I am planning on using this on a Harkte head with burned power amp so I can put in a class D amp module, but need to drop the 80V rails down below 70V.

This isn't mine, I stole it.

[Old Tele man]

You didn't "steal" it you just presented it as an illustrative "pictorial" quote (wink,wink)!

[Dave H]

I had not considered before that the original filter capacitor in the unit (metal can type) is only rated at 350V

With the new power transformer there is 400V on the first capacitor section

If it's only 10-15mA you could use a couple of 47V 5W zeners between the diodes and capacitor.

[J M Fahey]

Sounds like it isn't right for this situation, but there is a cool circuit for fixed voltage drop that uses a zener and a BJT. I am planning on using this on a Harkte head with burned power amp so I can put in a class D amp module, but need to drop the 80V rails down below 70V.

IF as I understand it you want to lower main rails to feed a power amp, please donīt.
The Zener solution, even if boosted with a transistor, works marginally well on Tube Guitar amps, typically 15 to 100W , and average running between 30 and 50W.
But here we are talking a way larger amp, say 300W or more; linear power reducer dissipation will be ludicrous.

Either get a power amp which is happy with +/- 80V or add an external step down transformer (might be a bucking type too) to reduce mains by 20% or so.