Changing out a resistor on a 5E3- Dangerous?

[Ryguy]

I had posted a week ago about a burnt resistor in a 5E3 amp. I know the filter caps have some dealdy charge in them, but assuming I can safely wield a soldering iron, can I go in an replace that resistor by myself safely?

This is a shot of the offending resistor. (It was caused by a bad 6v6)
http://i128.photobucket.com/albums/p173/rygalli/5E3guts.jpg

[justonwo]

You better make sure you understand how to drain filter caps before you go poking around inside the amp. Even when it's unplugged, they can hold their charge for quite some time, like a big, deadly battery. Once they are drained, and you verify the voltage on all caps is 0 VDC, then there is no danger.

One other thing to consider . . . fixing your amp might not be as simple as just replacing a burnt resistor. If that resistor burned up, it most likely did so because something went wrong in your amp. So troubleshooting could be involved.

[John Phillips]

Yes, you're right to be cautious - that resistor connects the first and second filter caps, which in a 5E3 have no bleed resistors, so they will hold their charge after the amp is turned off.

Luckily there's a very simple way to drain them...

Turn the amp off.
Remove just the rectifier tube (5Y3).
Turn the amp back on (including the standby to 'play', if it has one).
Wait about a couple of minutes, or longer to be sure.
Turn the amp off again and unplug.

With no rectifier tube in, the caps can't charge, and with the tube filaments running, they will discharge through the tubes.

A small 'memory voltage' could possibly re-appear on them after a short time, but it won't be hazardous.

Replace that resistor with a 3W or (preferably) 5W ceramic-cased wirewound type, and never have to do it again... they will take the short-term overload of a blown tube for long enough for the fuse to go.

[justonwo]

John, what would cause that resistor burn up in normal operation?

[John Phillips]

Only two possible causes, given its circuit position:

A shorted power tube (screen short) - most likely, it's not uncommon and 6V6s especially seem to often fail like that. The second filter stage is where the screen supply is driven from. That resistor will have taken the brunt of it, not the screen resistor itself, because it's ten times the value. (Which is why increasing its rating to 5W and using a tougher type is a good idea, for next time.)

Or, the second filter cap is shorted - not very likely.

Nothing else is supplied by that resistor, and it can't be anything else further downstream because the next resistor (the little carbon one under the speaker jack) is OK.

[jh45gun]

Yes, you're right to be cautious - that resistor connects the first and second filter caps, which in a 5E3 have no bleed resistors, so they will hold their charge after the amp is turned off.

Luckily there's a very simple way to drain them...

Turn the amp off.
Remove just the rectifier tube (5Y3).
Turn the amp back on (including the standby to 'play', if it has one).
Wait about a couple of minutes, or longer to be sure.
Turn the amp off again and unplug.

With no rectifier tube in, the caps can't charge, and with the tube filaments running, they will discharge through the tubes.

A small 'memory voltage' could possibly re-appear on them after a short time, but it won't be hazardous.

Replace that resistor with a 3W or (preferably) 5W ceramic-cased wirewound type, and never have to do it again... they will take the short-term overload of a blown tube for long enough for the fuse to go.

John Just curious I was planning on draining my amp tube amp project I plan to work on by running a wire from ground to the positive of the caps. Will this work for any tube amp with a tube rectifier doing as you say or just this particular amp? If so I will drain mine that way. One more question after doing any mods or repairs do you have to power the amp up slowly again or can you just fire it up since it did have a recent charge in the caps?

[John Phillips]

It will work with any amp with a tube rectifier.

You don't have to power up slowly unless the caps have been drained for years (and there's even some debate about that ;)).

[Ryguy]

John thanks for the response! You helped me diagnose the problem in the first place. I am pretty certain it is a shot 6v6. Too bad, cause its and old RCA Blackplate. I will definitely follow your advice.

[Blue Strat]

It might be worth checking your idle current. Power tubes go if you exceed their maximum idle power. 5E3s tend to operate their 6V6s at 120% or more of their maximum operating idle power (in case you were wondering, anything over 100% is a violation). It's common to increase the cathode resistor value to get things under control.

[Ryguy]

Sounds good, Mike, thanks.

[Ryguy]

BTW Mike, I had some Marconi's in there that I bought from you and they simply smoked. Unfortunately one took a hard trip down a long set of stairs...so I'll be getting some more pretty soon here.

[Bruce / Mission]

Only two possible causes, given its circuit position:
...
Nothing else is supplied by that resistor, and it can't be anything else further downstream because the next resistor (the little carbon one under the speaker jack) is OK.I agree it is just a simple screen shorted power tube but the rest of the statement is not entirely true.
The 4K7 or 5K1 screen node resistor also supplies the current for all four triode stages of the preamp.
Preamp idle current is is not very high though, maybe 5ma or 6ma, but use at least a 2 watter when replacing the burned one.
By the way, even though I very seldom see this resistor burned, it is the reason I use 2 watt metal oxide resistors in between the filter caps.
Carbon film burns open quickly but Mtl Ox doesn't catch fire at all.

[John Phillips]

Not calling you out, but since a 2W resistor does fail in that position (even a metal oxide will fail, even if it doesn't burn), why not use a 5W wirewound - which almost certainly won't - and be done with it? That way you can just change the blown tube and the fuse and get on with the show...

I should have said 'directly supplied' :) - but it still can't be a preamp fault since the 1/2W resistor to the third filter stage didn't fail.

[Bruce / Mission]

Not calling you out, but since a 2W resistor does fail in that position (even a metal oxide will fail, even if it doesn't burn), why not use a 5W wirewound - which almost certainly won't - and be done with it? That way you can just change the blown tube and the fuse and get on with the show...
Let's see what would happen if a screen and screen B+ supply shorted to the cathode or filament line (very low DCR to ground) and assume the screen node could supply a light 50ma of B+:
.05a across a 5 watt 5K resistor...
.05a^2 X 5000 = 12.5 watts! :worried
A decent 100ma PT with a 5Y3GT should have little trouble delivering 50ma across a 5K load for a long time.
The 5 watter will over heat and probably burn up too and I bet the 2a mains fuse will not blow.
It probably would blow the fuse if the plate shorted to cathode or the filament supply since there is virtually no series resistance, save the DCR of the secondaries.

I wonder if the resistor in the picture could be a 1 watt carbon film?
A small diameter axial 5 watter might fit but still troublesome for a novice builder.
I said use a 2 watter mostly because it is very difficult to get a 5 watt resistor to fit in that little space and a two watter is probably fine since you'd need a much larger wattage value to make it bullet proof anyhow.

To be honest though, I've worked on many hundreds of vintage Fender amps, been involved with over a thousand 5E3 amp builders and I honestly can't remember seeing many blown screen node resistors when they were a 2 watter and really only seen a very VERY small number of carbon comp ones burned in vintage 6V6 amps too. As a matter of fact, I've only seen a few of the 6L6 tweed amps burn that same resistor.

I've probably seen more blackface Princeton Reverb amps take out the 1000 ohm 1 watt screen node resistor then any of the tweed amps.
Replacing the 1K in a Princeton with a 2 watter is a good fix too.

[John Phillips]

That sounds reasonable. I haven't actually worked on many 5E3s (or other original Tweeds) to be honest, they're very rare over here, as are clones of them. I have fixed one with the same fault as that, and I did fit a 5W glazed-ceramic wirewound - the same type as Marshall used in the 70s for screen resistors, it went in no trouble with the legs bent in very slightly and it stood up about 3/8" from the board (good for heat dissipation too).

Those are rated for a peak overload of up to ten times the rated power for very short durations (up to 1 second I think)... which should allow the fuse to blow, particularly given that there is likely to be a plate short too.

I just have a strong objection to using resistors as sort-of fuses. If the primary fuse isn't sufficient to protect the internal components (suitably rated) from that sort of fault, you need a B+ fuse as well, IMO. I don't believe a well-designed amp should ever blow anything other than tubes and fuses.

[jpervin]

Yes, you're right to be cautious - that resistor connects the first and second filter caps, which in a 5E3 have no bleed resistors, so they will hold their charge after the amp is turned off.

Luckily there's a very simple way to drain them...

Turn the amp off.
Remove just the rectifier tube (5Y3).
Turn the amp back on (including the standby to 'play', if it has one).
Wait about a couple of minutes, or longer to be sure.
Turn the amp off again and unplug.

With no rectifier tube in, the caps can't charge, and with the tube filaments running, they will discharge through the tubes.

A small 'memory voltage' could possibly re-appear on them after a short time, but it won't be hazardous.

Replace that resistor with a 3W or (preferably) 5W ceramic-cased wirewound type, and never have to do it again... they will take the short-term overload of a blown tube for long enough for the fuse to go.

John, you mentioned that this method of cap draining will work with any amp with a rectifier tube. Will it also work on an amp where there is no standby switch, such as a Champ?

[Bruce / Mission]

Vacuum tube are thermionic...

http://en.wikipedia.org/wiki/Thermionic_emission

When hot, they will conduct so if you leave the amp on and unplug it, the hot tubes will continue to draw idle current from the filter caps until there is virtually none left or the tube(s) is not hot enough anymore to draw current...
it makes no difference if it is a solid state or hollow state rectifier.
The trick as John mentioned is to make sure you let the tubes get hot before you try this method of draining the filter caps or it will not work.
This is especially critical with solid state rectifiers because they turn on instantly and charge the filter caps with huge amounts of high voltage before any of the other tubes can get hot enough to conduct.
In the solid state scenario and a quick flip of the on-off switch with the amp ready to play, you have a good chance of fully charging the filter caps with nowhere to go or bleed off because the tubes are not even warm...
The caps will charge up to PEAK ripple voltage, ie., 1.41 times the average AC output of the power tranny secondary.... then if you grab onto the business end of a high capacitance filter cap, they will knock you on your fanny and you can wake up in a puddle of your own pee. :messedup

[Hacksaw]

The caps will charge up to PEAK ripple voltage, ie., 1.41 times the average AC output of the power tranny secondary.... then if you grab onto the business end of a high capacitance filter cap, they will knock you on your fanny and you can wake up in a puddle of your own pee. :messedup

if your lucky enough to wake up in your own pee... I read on this too much probably.

from the CDC website..

Effects of Electrical Current* on the Body
1 milliampJust a faint tingle.
5 milliampsSlight shock felt. Disturbing, but not painful. Most people can "let go." However, strong involuntary movements can cause injuries.

6-25 milliamps (women)† 9-30 milliamps (men)Painful shock. Muscular control is lost. This is the range where "freezing currents" start. It may not be possible to "let go."

50-150 milliamps Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.

1,000-4,300 milliamps (1-4.3 amps)Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.

10,000 milliamps (10 amps)Cardiac arrest and severe burns occur. Death is probable.

15,000 milliamps (15 amps) Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!

*Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.
†Differences in muscle and fat content affect the severity of shock.

[John Phillips]

The other important variable is the length of time you're exposed to the current.

CE regulations for circuit-breakers specify 30mS; 100mS is thought to be the upper end of the safe limit I think. Most fuses (even fast-blow) don't blow anywhere near that fast, and slow-blow - deliberately designed not to blow quickly obviously - can take over 1 second... so even though the amp fuse may be as low as 1A and is never usually much more than about 5A, it offers very little protection from electrocution. That's not its purpose - it's to stop fires and damage to the amp, and to prevent it overloading the power cord, which typically has far less capacity than the wall circuit in the house and the breaker.