Simple question.........how many watts does my vta st-70 consume?

Hello

I am thinking of using an isolation transformer rated at 900VA........for my system.......vta st-70 how many watts does my amp consume?

Will this rating of isolation tranformer be appropriately sized?

Thanks David

your 900 watt transformer just might work.....

audiobill wrote: your 900 watt transformer just might work.....

I know my amp out puts 35 watts aside so does it also use this much?

Thanks David

islanddave wrote:Hello

I am thinking of using an isolation transformer rated at 900VA........for my system.......vta st-70 how many watts does my amp consume?

Will this rating of isolation tranformer be appropriately sized?

Thanks David

Just a guess : 250w when full power, 120 when idle

If needed i can measure mine.

peterh wrote:

islanddave wrote:Hello

I am thinking of using an isolation transformer rated at 900VA........for my system.......vta st-70 how many watts does my amp consume?

Will this rating of isolation tranformer be appropriately sized?

Thanks David

Just a guess : 250w when full power, 120 when idle

If needed i can measure mine.

How would i measure it.............I have a multi-meter?

islanddave wrote:

peterh wrote:

islanddave wrote:Hello

I am thinking of using an isolation transformer rated at 900VA........for my system.......vta st-70 how many watts does my amp consume?

Will this rating of isolation tranformer be appropriately sized?

Thanks David

Just a guess : 250w when full power, 120 when idle

If needed i can measure mine.

How would i measure it.............I have a multi-meter?

You need an energy meter , like this :
https://www.kjell.com/se/sortiment/el-verktyg/el-produkter/starkstrom/energimatare/luxorparts-energimatare-p50003

it measures current and voltage, takes current delay ( whatever you call it) and
presents the actual power consumption.
In addition it can integrate the consumption and multiply with the rate.

My vta70 consumes 167 - 170w at idle

Short answer is yes your 900va transformer will easily work with your VTA ST-70 without breathing hard.

Can I ask why you'd want to use an isolation transformer? The main power transformer in the STxx does this for you.

Also be aware that the voltage out of the isolation transformer will be slightly different to the line voltage. Given you are looking at a 900VA version it may be higer than you'd like at the load you are pulling. All depends upon the type and quality of transformer used.

My ST-120 pulls about 3 amps (360 watts); a pair of M-125s will draw about 4.5 amps. All in, not bad. You could do worse than that if you left a few lights on overnight. I read somewhere, though, that tube amplifiers leave a huge greenie "footprint" because their sonic output is about 3 percent of energy consumed -- the rest goes to heat and light.

pichacker wrote:Can I ask why you'd want to use an isolation transformer? The main power transformer in the STxx does this for you.

Also be aware that the voltage out of the isolation transformer will be slightly different to the line voltage. Given you are looking at a 900VA version it may be higer than you'd like at the load you are pulling. All depends upon the type and quality of transformer used.

Excellent point! I was using a 100va isolation xformer to power an AA5 the other day. I had to use a variac between the xformer and the wall outlet due to the output of the xformer being about 5 volts higher than the wall. So, 125 instead of 120 with the load of the radio attached.

pichacker wrote:Can I ask why you'd want to use an isolation transformer? The main power transformer in the STxx does this for you.

Also be aware that the voltage out of the isolation transformer will be slightly different to the line voltage. Given you are looking at a 900VA version it may be higer than you'd like at the load you are pulling. All depends upon the type and quality of transformer used.

Stand-alone isolation transformers (not medical grade) are often bi-directional. They will very slightly raise voltage one way, and very slightly lower it the other.

Medical grade/instrument-grade devices are one-way and deliver as closely as possible output voltage to match input voltage. It pays to check.

islanddave wrote:Hello

I am thinking of using an isolation transformer rated at 900VA........for my system.......vta st-70 how many watts does my amp consume?

Will this rating of isolation tranformer be appropriately sized?

Thanks David

It can be calculated. Which works with most tube equipment.

a) Losses: The typical three-transformer stereo tube amp has internal losses of between 6% and 12%. VTA amps have a larger power-transformer than Vintage (OEM) amps, and so will have slightly very slightly larger losses balanced by a lower operating temperature. But, will likely be at the lower end of the range given their quality. Instrument amps will push up nearer the 12%, sometimes more.

b) Tubes: An EL34 will dissipate about 36 watts.
A small-signal tube will be somewhere between 6 and 7 watts.
A 5AR4 is about 10 watts.

(4 x 36) + (3 x 7) +10 = 175 Calculated watts. Add losses, and +/- 190 watts is fair-enough. Your 900=watt transformer will be fine.

MOST ST70s have a silk-screened label on the rear panel that tell you it uses 175 watts of power, I did see one photo once that said 190 watts.

tubes4hifi wrote:MOST ST70s have a silk-screened label on the rear panel that tell you it uses 175 watts of power, I did see one photo once that said 190 watts.

My OEM device has 190 watts on the nameplate. And uses about 180 or so on the meter. There is some dither based on the input signal and volume, but that is very, very small. It has all-Siemens EL34 output tubes.

https://www.dropbox.com/s/2120l5s2wbjkjrg/IMG_20170212_152635735%5B1%5D.jpg?dl=0

PeterW,
an oldie but a goodie, and some nice cookies too!