• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Power Resistor Smoking on HF-81

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
i have had the same problem in the past removing old iron.
best thing to do is to tape it off and paint it. looks as good as removing it. and its faster :D


  • mvc-279s.jpg
    37.9 KB · Views: 110
Disabled Account
Joined 2004
Okay I inspected the amp really well with a flashlight and poked around a bit for loose connections. I found two which I would have not of found without a flashlight. I tried plugging the amp in again, but I am too scared to wait for something to smoke. I noticed that I didn't hear a sparkle from the rectifier tube which is good news. The power resistor gets very hott to the touch after 10 seconds or so. Should I try a higher watt resistor? Thanks again for your time.
Disabled Account
Joined 2004
I painted the transformers with Rustoloem Black Satin Spray Paint. The insulation looks alright to me. No more sparks from the tubes, but the power resistor still smokes alittle bit. Is it just because the resistor has gone bad? I also can't find the resistor's value which is 350 ohm should I go lower and get the 330 ohm or the the 390 ohm? Thanks for your time in this matter, Kevin.
The resistor does not smoke right away, but gets too hot to touch. I don't have access to a variac which makes the job much harder to figure out. I will try measuring the voltage across the resistor and from the transformer to see if that helps. Anymore suggestion would be great. Thanks a bunch for helping this newbie.
:att'n: This is high voltage circuitry that can be LETHAL. Make sure, especially if you're a newbie, that you've taken some time to learn how to work safely around powered-up tube power supplies and have someone nearby to call an ambulance if you fry yourself. That's not a joke. Do read the sticky thread on safety to get properly scared. Eye protection is mandatory! I've been doing this stuff for almost 40 years and I still get very nervous poking around in live amplifiers.

Don't fool around with original values of components. They are what they are for very good reason, and if you want to change something, you ought to know both what and why, and most importantly, why the original value is what it is. Best to do some thinking and systematic faultfinding- start with a schematic, then start to analyze the operation of the circuit. It will soon be apparent what experiments to do.
EeK, no variac and you're a newbie. Please please PLEASE be careful! I often forget that most people are not comfortable working around lethal voltages. I am an electrical contractor and I've had to do thing that would scare most people to death. Don't get wreckless.

It sound like you've already powered up the circuit, so we'll assume it's safe to grab measurements from. Here's the safest way to get high voltage measurements:

1. Turn the unit off.
2. Set your VOM to the highest scale.
3. Use alligator clips to attach your meter to the circuit. Make sure you're using good quality clip leads with at least 600V insulation. Make sure that the clips are not shorting out on anything. Make sure your working enviroment is clear so that nothing can catch on the clip leads.
4. Carefully energize your circuit, take your reading, and shut the power off.

Safety is the #1 most important thang when working with HV circuits. Here's a lecture I've given many times in my life:

1. If you don't feel safe, they you are not safe. I'd much rather have a pissed off customer than an injurred employee. Don't do something foolish just because the customer want you to.
2. Use your safety equipment, mainly rubber soled shoes & eye protection.
3. Turn the power off. I don't care if you have to walk clear across the building to do so.
4. If you don't understand something, ask for help.
5. The 5 P's. Prior Planning Prevents Poor Performance.

Every one of these construction guidelines transfers to electronics work. Work safe, work smart, & have fun doing so.
I have two Eico HF-81 amps at the moment. I measured the voltage with the tubes out on both of them and I think I know why I am having problems with one of them. The voltage reads 8 volts instead of 5 volts on the short red wires. The red wires that power the rectifiers are what I think the main problem is. The amp I am having problems with reads 45v and 30v on the lines while the other amp reads 35v and 25v and its lines to the rectifier tubes. Do I have a faulty power transformer? Is there a way to reduce the voltage? I will try to be as careful as possible. I have worked with tripath kits which seems much safer to work with, but the parts smd parts are so tiny that it has it drawbracks that way.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.