Checklist to observe before applying the mains

[Sir Trefor]

When it come to working on tube amps, what should the general practice be to check before powering on? I just replaced the big electrolytics in the PS section of a Heathkit, and the fuse blew when I powered up- There's a lot of things that I could have done, but what should I check?

The fuse blew instantly, as though there were a short circuit. I put a new fuse in, plucked out the rectifier and power was sustained- all of the tubes lit up, and all of my power transformer voltages are correct.
So; Rectifier IN=short, Rectifier OUT=sustained.

Problem is obviously in the PS circuit somewhere, but I made every effort to follow the schematic, so now I have to troubleshoot; and I'm not sure what to look for.

The original rectifier was what blew out in the first place; I plugged a new one in, but everything started to get very hot; VERY hot- untouchable. One of the electrolytics was sizzling, another started smoking- so I went and replaced the lot of em'

I thought that the caps were the source of the problem, but maybe I'm wrong?

Please help if you can.

PS- I am astute at working with high voltages- coming from the electric motor trade. But tubes are still new to me; I am but an amateur in the learning process.

[chrish]

Have you got a solid state or valve rectifier?

I am very new to valves, so if that is what you have, I cannot help.

If solid state, sounds like you may have a polarity issue. Have you got a one piece bridge rectifier or one you made from individual components? Check the polarity of the outputs, you might have wired it backwards.

My last chip amp was built with the help of my father, an electronics engineer. Normally I build the amp from power supply to amp unit, checking each stage along the way very carefully. As I assumed my dad knew what he was doing, we just wired it up and turned it on. Polarity was wrong. Blew all my power supply caps and the power supply regulators and the amp chips! Not fun, but a valuable lesson!

[Wavebourn]

When I breadboard tube amplifiers I usually:


1. Make a chassis

2. Bolt everything I need

3. Connect power socket, switch, fuse, primaries of the power transformer,

4. Switch on and check AC voltages on secondaries.

5. Unplug. Wire PSU including rectifiers, voltage regulators, etc...

6. Turn on and test voltages

7. Turn off. Unplug. Discharge capacitors using resistors if no bleeding resistors exist.

8. Wire the rest.

9. Connect dummy loads with speakers through 100 Ohm resistors in parallel. Fight against parasitics. Tweak parameters...

10. Connect speakers, signal source, enjoy the music...

I understand that you are probably on the stage 6 and got some errors such as wrong polarity of caps or a bridge. The tranny should be connected to "~" leads of the bridge. "+" lead should be connected to the "+" lead of the capacitor, "-" from the bridge should be connected to the "-" of the capacitor.

[kevinkr]

Hi Sir Trefor,
Sounds like you might have a bad rectifier tube since the fuse blew immediately at power up and didn't with it removed. It is also possible that you have wired a cap backwards, particularly if some small amount of time passes before the fuse blows.

Usually I do a careful visual inspection, probe key points with the dvm to make sure there are no shorts, and use a variac and/or ballast lamp to bring up the device I am testing.

What I most strongly recommend is the use of a ballast lamp in series with the primary of your power transformer during the trouble shooting phase. This will prevent fried power transformers at the minimum and in a lot of cases prevent a lot of other damage as well. At its simplest this can just be a hacked extension cord with a lamp socket wired in series with the hot conductor. You can use a 100W bulb to limit current to a safe value in the event of a short. Higher and lower wattage bulbs can be used depending on the normal operating current of the device you are testing. Usually the voltage will be quite low, but you should be able to measure some % of normal operating voltage. The bulb will illuminate to full brightness in the event of a short and limit current in the primary to the current required to light the bulb. (Slightly less than 0.9A@120V)

Take advantage of friends with electronic backgrounds when available and you are stumped, very often they will notice things you may have missed repeatedly.

Make sure to discharge supply capacitors prior to working on equipment, and always check to make sure they are discharged whether or not there is a bleeder resistor installed. (They can fail)


Slightly off topic safety recommendations:
Protect yourself and other equipment with a ground fault interrupter (GFI/GFCI) that powers everything on the bench. Note that the GFI will not protect you from contact incidents with anything other than the mains voltage.

Make sure the chair you sit in when working on any HV circuit is not conductive! (Exact opposite of good esd practices) I use a wooden chair.

No bare feet..

Anti-static mats and ground planes are nice when working on low voltage electronics, but not a good idea around high voltages, and make sure that you cannot come in contact easily or accidentally with grounded bogie objects (excluding the d.u.t which most likely will be and should be grounded) while probing high voltages.

It's a really good idea to have someone else around when working with tube circuits in the event of an accident.

If possible set up probe points in HV or very high current circuits prior to applying power - this does require a couple of meters to be practical.

Insulate all but the very end of probe tips.



I am sure lots of people can add to this list, but hopefully you find this useful and not too paranoid..

Have fun!

[hd38]

In "short" (no joke inside !)

- use a variac/auto-Xformer, always. Rise slowly, monitor
- differential protection, always (mains supply rail w breaker available)
- left hand in pocket when you take readings on line
- check for caps b/w live or neutral and chassis, remove if any
(applies for instance to old "all supplies" vintage radios)
- remove all tubes, check voltages
- then insert rectifier, recheck voltages
- whenever all is correct, replace the tubes
- be patient, keep cool, breathe !

Hervé
Grenoble

[Miles Prower]

Originally posted by Sir Trefor
When it come to working on tube amps, what should the general practice be to check before powering on?

I dunnow. Never heard of any "general practice", just what I do.

If using a recycled power xfmr, I always inspect carefully, and make sure the thing works right before using it. Regardless of whether it's solid state or hollow state, complete the power supply first. After completion, but before power up, check all connections, then check 'em a couple more times. Occasionally, I've caught mistakes before they went up in smoke and flames. It's also a real good idea to use the resistance function of a DMM (power off, of course!) to see what resistance the PS is looking into. If it says "0R" then it's probably not a real swell idea to power up just yet.

Next, I wire a lightbulb in series with the primary. This gives you a good idea if something isn't right, since the bulb shouldn't light up brightly (except for a second or two as the filter capacitors charge up). That way, you just might prevent some inconvenient poofage.

It's also a good idea to test each stage as it's completed. That way, if something's not right, you know where to look. Again, make certain that everything's wired the way it should be: all electrolytics properly polarized, all connections going to the right socket pins.

The original rectifier was what blew out in the first place; I plugged a new one in, but everything started to get very hot; VERY hot- untouchable. One of the electrolytics was sizzling, another started smoking- so I went and replaced the lot of em'

That's not a good thing. If you got the polarities of the electrolytics wrong, that's what can happen whether it's a low voltage, high current SS PS or a high voltage, low current hollow state PS.

[anatech]

Hi Sir Trefor,
Did you use brand new capacitors, or those "NOS" bombs?

If your fuse blew immediately, I would suspect that your rectifier got damaged. Try another. I generally use a variac to power things up. In your case I might pull the rectifier and power up the heaters. Then I'd use a variable high voltage supply to get the rest working. This is fairly safe and smokeless.

-Chris

[hd38]

Yes !

I completely agree with the expression "NOS bombs"

Energy stored varies as square of voltage, which is HIGH
Electrolytics are ageing even in the drawer and they need to be reformed gently before use, you may find how at the following link or ask Google with "reforming capacitors" :

Reforming capacitors

[Sir Trefor]

Wow, I am surprised and grateful at the response to my question!

Did you use brand new capacitors, or those "NOS" bombs?

No, the old multi-section supply "cans" were replaced with new Sprague Atoms- all voltages correct.

Sounds like you might have a bad rectifier tube since the fuse blew immediately at power up and didn't with it removed. It is also possible that you have wired a cap backwards, particularly if some small amount of time passes before the fuse blows.

Polarity of the caps has been double checked- it is correct. The Rectifier tube may be the problem; as it is an old, used 5AR4. It "was working" before being taken out of service, but, then again...

One thing that I did NOT mention was that I lost the schematic I drew as I took everything apart- I was putting it back together the way I "remembered" it. But, I found a true schematic at the public library(Sam's photofact), and I am currently wiring it properly; I had made a few mistakes- and the supply caps were not properly connected to ground.

What I most strongly recommend is the use of a ballast lamp in series with the primary of your power transformer during the trouble shooting phase.

THIS is a great suggestion; I think I even have a socket out in the garage...

- use a variac/auto-Xformer, always. Rise slowly, monitor
- differential protection, always (mains supply rail w breaker available)
- left hand in pocket when you take readings on line
- check for caps b/w live or neutral and chassis, remove if any
(applies for instance to old "all supplies" vintage radios)
- remove all tubes, check voltages
- then insert rectifier, recheck voltages
- whenever all is correct, replace the tubes
- be patient, keep cool, breathe !

Excellent advice, thank you.

Thanks to everybody for what should be common sense. I'll be sure to let you know how it goes when I'm finished.

PS-Might I use a rheostat in place of the variac?

[anatech]

Hi Sir Trefor,

PS-Might I use a rheostat in place of the variac?

Yes, but it will be difficult to use.

If you do any kind of work like this you should pick up a variac off of ebay. A 2A model will be fine for most of your work. If you find a 5A model, great. 10 A models are getting large and heavy.

An AC voltmeter and a suitable current meter on the output will be valuable. You'll be very glad you did this, try it.

-Chris