Bob Latino VTA ST-120 Tube Amp

[leonski]

Keith mentioned RAISING the B+ Voltage. this is not only with the use of a SS rectifier....But ALSO due to the fact that power line voltage has generally gone UP

for the last couple decades. Time was? 110 was the norm. Now? 117? 120? I'm out of touch. But I've heard that the 5AR4 rectifier is 'prone' to not lasting

due to this higher line voltage.

I'm told that the VTA amps now employ a transformer with windings for the different voltages possible....like 117 or 120 or whatever. The OTHER possible solution

is a 'buck' convertor in any of several configurations. Even a GOOD Variac of sufficient capactiy would work, but leave you open to someone 'cranking' it up and

damaging your gear. IF I WENT this way? I'd make certain to have a GOOD voltmeter, checked against a standard would be nice.

LINK to history of mains voltage in the USA:

www.w8ji.com/power_line_voltage.htm

I rarely measure as much as 120vac. And at times of peak demand IN SUMMER? Can go ass low as BELOW 115, at which time I start shutting stuff OFF and pulling plugs.

[audiobill]

A variac is recommended.

[leonski]

Nov 23, 2021 15:59:32 GMT -5 audiobill said:

A variac is recommended.

2 conditions:

Need a decent meter.

Keep away from 'knob turners'......especially KIDS......

MAYBE find out what max B+ should be and monitor THAT....though some are less than comfortable with 
that kind of voltage.....

[ttocs]

This is the model I use.


This is an example of some measurements taken at various line voltages supplied by this Variac into one of my M-125 amps with the bias set to 1V. The expected voltages as per the kit instructions are on the left columns, measured shown in the following columns. As per the measurements, 117V is the sweet spot for line voltage. I actually use a range of 115-117V.

[leonski]

Now THAT'S what I call DATA! EXcellent work........

ONE minor point......I see heater voltage exceeding what I'm led to believe is a safe +5% over the 6.3 'nominal'.......6.61 would seem to be 'the limit' in this regard......

I'd have to take survey of heater current and I'd just accept the datasheet value and compare with the output from the existing transformer.......

I might add that HERE in SoCal, I see Up To 120 a lot during cooler months.     That would be 'too much' based on the data.

116 to 117 is quite common during higher use hours so that is FINE.......

Still and all?   The perfectionist would keep an eye on a good meter with Variac setup.....