thanks for the help!
my main interest is determining whether the power transformer is compromised or not, whether the amount of heat it's dissipating is normal, or an indication that it has sustained damage... and overall, how high of a PT temp would be sustainable. i'm wondering what most folks regard as a safe temperature range - at what temperature would you start considering replacement, what temperature would you definitely replace it at, etc.
i'm also wondering if there are any other more direct methods of isolating and testing the transformer to evaluate it thoroughly without putting the whole amp at risk. my best method is using ohm's law to calculate a large power resistor for each secondary winding that will simulate the actual load, and to load it with those and track voltage and temperature over time - for 480V B+ and roughly 120mA bias current, i'd use a 4K 100W+, and for the filaments at 6.3V/5.4A, i'd use a 1.2R 50W+, and expect them to both get very hot. just a bit of a pain to get the right values.
besides the high temperature and voltages, and the known history of a sustained filament short, i haven't observed or measured anything that directly indicates issues with the PT - the voltages are in proportion to each other and rock steady, and power supply noise is very low - but the temp and higher than expected voltages, and known history of a failure state, are enough to raise my concern.
measuring primary current is a great idea, thanks! input is supposed to be 260W, so i may just stick a 0.1R 5W in the primary temporarily to measure the voltage across it, which should be about 0.21V if it's correct. i may do the same for the filament winding, as i'm more concerned about it as that's where the shorts were - current should be 5.4A, so i should see a 0.54V drop across a 0.1R.
the 90W output is RMS, sorry i should have specified. the roughly 5% increase from the 85W on the schematic is about what i'd expect with these higher voltages.
i agree that the original bias voltage resulting in lower plate current, would probably result in lower transformer temperature overall, but i'm concerned with how high that would push the B+ (480V) and filament voltage (6.9VAC). i'm mainly concerned with filament voltage that high reducing tube lifespans, and the high B+ is mostly of concern for the power tube screens which have a 450V max (and are only 2V away from the plate voltage), and for the reverb driver tube and transformer, which are tapped off the main B+ and were already pushed far past the 330V max for the 12AT7 plate by design.
for whatever reason, the amp is behaving like an amp from the 60's that was designed for more like 115-117VAC or lower primary voltage... usually i find amps from the 90's and 00's to have the voltages run more like maybe 2-3% above schematic values at the most, but this nearly 10% is more similar to what i'd see from 60's or even 70's amps.
i agree that having the variac isn't ideal from my perspective, but the idea in reducing primary voltage, is that it would allow me to bring the bias back to the original cooler setting, reducing PT temperature without the supply voltages getting too high as a result - basically just bringing both voltage and current back to schematic values. safest bet would be to replace the power transformer, but i'd be loathe to recommend that most expensive option without clear indication that it is compromised.
i've presented the owner with multiple options to address this issue, including a buck transformer, a few more thermistors in the primary, zeners to drop the B+ and metal oxide power resistors to drop the filament voltage... as well as the variac, and power transformer replacement/upgrade. while i'm still evaluating whether or not the power transformer is compromised and should be replaced either way, he's expressed he likes the variac idea most, as it's the least invasive, and allows for him to run the amp cooler most of the time, but also bring it back to the original voltage whenever he wants. i have cautioned him that it's possible for the variac to put out even higher primary voltage than the wall depending on the setting, and when he gets it, i'll be taking measurements with it hooked up to the amp, to determine the absolute highest setting on the dial he should ever use.
my recommendation is the buck transformer, and i've explained it would be doing exactly the same thing as the variac, but he'd rather make as few direct mods to the circuit as possible.
Topic: 65 Twin Reverb RI - power transformer temperature (Read 952 times)