Dropping mains voltage

[famousmockingbird]

I have a small tube preamp that requires 100v mains input. It pulls 1/4 of an amp @100v. Instead of using it with a variac I need to mount something inside the chassis to do the job.

What are some of the best ways to do this? I know a 100 ohm resistor will work but that is probably the worst way to do it. It will drop too much during start up due to the heater inrush current. Not to mention lots of heat or I need a large resistor like 20w.

I was thinking of using a 24v isolation transformer. I can fit a 12VA in there.

L1 is the secondary and L2 is the primary of the isolation transformer. Seems to work in theory. Is there anything I should know? Is there a better method?

 

[kevinkr]

Wire a 15V - 20V transformer in series with the mains to the pre-amp to buck the mains voltage. A 0.5A transformer should be adequate based on the measured consumption.

 

[famousmockingbird]

Wire a 15V - 20V transformer in series with the mains to the pre-amp to buck the mains voltage. A 0.5A transformer should be adequate based on the measured consumption.

Hey Kevin, I believe the method you propose is what I have in the screen capture. L2 is the primary and L1 is the secondary. Or are you talking about something completely different?

 

[Eli Duttman]

A bucking transformer is an excellent method. Another possibility to get the ultimate B+ voltage right, from "120" VAC mains, is using a large forward drop rectifier, like the 5Y3, and a critical current choke I/P filter.

A bucking transformer is (IMO) simpler.

 

[famousmockingbird]

Hi Eli, I believe we are all on the same page here. The "bucking" transformer seems like the way to go.

Secondary goes in series with the preamp transformer and the primary is connected to mains. Just got to get the correct phase or else it will boost.

 

[legendre]

I will second and third the bucking xfmr. It's really the better way to go, particularly as compared to using a power resistor. BTW you never mentioned what your mains supply voltage is - it looks like you're in the USA, so I'd assume 117-120V average?

Seems like a 16-20V xfmr would be the ticket.

 

[jcx]

autotransformer divider connection may be better in that it also reduces the buck xfmr magnetizing current by running its primary on the reduced line V too
although you should check your numbers - the divider ratio is slightly different

 

[famousmockingbird]

The line here is 125v

 

[legendre]

The line here is 125v

If you have a regular 125V line, then you should be just ducky with a 24V bucking xfmr. 24V is nice, as it's a standard industrial control voltage, and parts should be easy to find.

As others note, rate it 2X the total requirement of the load (so 500mA) and you should be just fine. Far better than a resistance or even a different rectifier - as either of the latter has much more effect on the Zout of the supply.

 

[AquaTarkus]

So, going by legendre's suggestion, you connect the 25V secondary of the AC transformer in series with the AC primary of your preamp's power transformer, in reverse phase so that it subtracted

Sent from my phone with Tapatalk. Please excuse any typpos.

 

[PRR]

I'm not sure what's so bad about a resistor. A preamp is a pretty steady load. Prolonged start from cold heaters could be a benefit. 6 Watts is not a huge waste. If waste really mattered you'd get an opamp (oh, now I will be ejected for naughty-word). In winter the heat is a bonus (cold here tonight).

The "problem" I see is that "1/4 Amp" is maybe nominal or guesswork? Then you may be juggling 100, 115, 130 Ohm resistors to hit your target. (Get a 20-bag of 10r 1W and series them all, short-out until you find 100V, then buy that size in 20W.)

 

[famousmockingbird]

I measured 260mA at 100v when it was powered by a variac. This was quiescent condition. The unit is fitted with a 1amp fuse.

 

[legendre]

So, going by legendre's suggestion, you connect the 25V secondary of the AC transformer in series with the AC primary of your preamp's power transformer, in reverse phase so that it subtracted

Yup, exactly. If connected in-phase, the bucking xfmr will +raise+ the B+ output - if wired anti-phase, it +reduces+ the B+ output. The latter is what we want, in this case.

If you're unsure of the phasing, just hook it up one way, bring it up on a variac and see if the output is higher or lower than you'd expect. If it's higher, reverse the connections - if it's lower, then you're good! It's a Good Idea to use a variac for this, as you might over-voltage something if you mis-wire and send it full voltage.