Univox 1501 Fried Resistors

[Trout]

Welp, I finally got my garage sale amp on the bench for internal inspection prior to a re-tube and testing and discovered exactly why my gal got it for $5.00 bucks!

Internally its actually very clean and tidy except a couple fried resistors! See Pics
Pic1
Pic2 Pic3
Pic4 schematic of 6l6 version
Pic5

The schematic of the 6550 model I have can not be located on the web. But the 6L6 version utilizes the same circuit board and is fairly similar.

I also noted a cap that looks to me as if its slightly swollen. Ive seen this on some lower end motherboards before, But never in an Amp.

My Questions simple(Kinda)
Noting the location of the 100ohm resistors. What would be the typical reason for these to smoke?
Bad Tubes? Perhaps overloaded output?
Shouldnt a fuse blow before this serious of an event?

Ok Thats 3 questions

As far as I can tell,, Everything else seems to be in order.
Gene

 

[Giaime]

Hello! Good to se you (and your 5$ amp) over here!!!

About those resistors, they're clearly connected to pin3.

From the photo2, I can say that the big blue wires at pin 2 and 7 are heaters, pin 8 and those black wires are the cathode, pin 5 and the yellow wire is the control grid, those red wires at pin4 are the screen supply... and those burned resistors are at the anode pin. I can't say why those are there: it would have much more sense if the resistor goes to the screen and the wires to the anode. I can't tell a purpose for this resistor...

Excuse me: I just re-looked to the 6L6 schematics and see that those resistors are indeed there on the plates of the first couple of tubes... maybe they're plate stoppers for oscillation, but I'm not sure because the power tubes already have grid stoppers... who knows? I would remove them... or substitute with higher rating resistors: 2W seems a bit too little... probably they've burned when someone put some hotter 6550 in there, that drowe much more current and made the resistor go to resistor's heven

Don't know their purpose...

It looks so tidy and clean! Change that cap if you see something strange on it... it would let the magic smoke out if it is really leaky!

Hope you find your answers!

 

[Trout]

A while Back a member had a rough drawing of the output stage on this amp, After about an hr trying to remember who, I finally found it here Stoo on weird 6550 opt stage

Might shed some light on those particular resistors.
After further inspection I havent found anything else questionable inside the chassis.

Hey, If some nimrod stuck 6L6's in those holes Think that might of done it? I have no previous owner info on this amp, But I know back in the 70's 6L6's used to be on every music shops shelf Hey,, haha,, they look the same right?? pass the bong WHOAAaa Dude that was awesome!

As far as changing the cap/caps, Absolutely being changed. I have had several computer motherboards with swollen caps not vented and cause boot failures. Of course a motherboards halt on errors, Amps Dont.
Swollen means potentially shorted and in my book, smokes next! Especially on this type of application.

Now I have to figure out which bathroom I left my parts express catalog in
Gene

 

[Enzo]

Resistors burn from excess current flowing through them. In the case of resistors around power tubes like this, it is almost always due to tube failure. Other than a shorted output transformer, what else can cause excess current to flow?

Why didn't a fuse blow? What fuse? The mains fuse? Hardly. Resistors burn up when their power rating is exceeded. In the case of a 1 watt or 2 watt resistor, all you need do is run more than 1 or 2 watts through them and they will burn up.

The mains fuse in the amp is a 3 amp part. That means the amp has to use more than 360 watts - at 120VAC mains - before the fuse blows. Watts are watts, and they don't care which side of the transformer they are on, so if it only takes let us say 5 watts to burn up a resistor in there, it is not even going to make the fuse warm.

With a tube failure, more is happening than just the resistor, but that 650 volt B+ is about five times the maains voltage, so current mirrors about a factor of five. That is to say a 1 amp load on the B+ equates to about a 5 amp draw from the mains. So roughly a 600ma draw from B+ should pop the 3A fuse. Looking at the 100 ohms, plus the resistance of the transformer, another 100 for discussion purpose. A dead short tube puts about 200 ohms across the 650v. That would be over 3 amps from the B+. Since the transformer is rated at 400ma, it will sag considerably lower under such a load. But the 100 ohm resistor, even if a 2 watt one, needs only about 140ma to be at 2 watts, and that is well within the capability of the power supply.

SO the resistor can easily burn up without popping the fuse. You could put a 100ma fuse in series with each tube if you like, that would save the resistors.

 

[Trout]

Resistors burn from excess current flowing through them. In the case of resistors around power tubes like this, it is almost always due to tube failure. Other than a shorted output transformer, what else can cause excess current to flow?

Why didn't a fuse blow? What fuse? The mains fuse? Hardly. Resistors burn up when their power rating is exceeded. In the case of a 1 watt or 2 watt resistor, all you need do is run more than 1 or 2 watts through them and they will burn up.

The mains fuse in the amp is a 3 amp part. That means the amp has to use more than 360 watts - at 120VAC mains - before the fuse blows. Watts are watts, and they don't care which side of the transformer they are on, so if it only takes let us say 5 watts to burn up a resistor in there, it is not even going to make the fuse warm.

With a tube failure, more is happening than just the resistor, but that 650 volt B+ is about five times the maains voltage, so current mirrors about a factor of five. That is to say a 1 amp load on the B+ equates to about a 5 amp draw from the mains. So roughly a 600ma draw from B+ should pop the 3A fuse. Looking at the 100 ohms, plus the resistance of the transformer, another 100 for discussion purpose. A dead short tube puts about 200 ohms across the 650v. That would be over 3 amps from the B+. Since the transformer is rated at 400ma, it will sag considerably lower under such a load. But the 100 ohm resistor, even if a 2 watt one, needs only about 140ma to be at 2 watts, and that is well within the capability of the power supply.

SO the resistor can easily burn up without popping the fuse. You could put a 100ma fuse in series with each tube if you like, that would save the resistors.

OK Understood

This is one of those times that I wish the original tubes were still in place, I bet they could shed some light on this.

Now, Providing I use a decent tube, Am i better off sticking to the 2watt resistors there? Better to fry those instead of something else? Or go 5 watt?

Providing I can get her going, Its future life will be spent more or less practice and occasional recording, I doubt it will be gigged but
It will see moderately high output now and then.
Gene

 

[Giaime]

Well I'd go to robust 2W metal film resistors: I won't go higher because it's good that those resistors act as fuses against the risk of a shorted tube, so in case of faliure they will protect the power transformer.

Absolutely change that cap.

Good luck!

 

[Frank Berry]

That bad capacitor is probably in the bias supply.

My guess is that the cap shorted, the amplifier ran with no bias on the output stage and this caused the tubes to draw too much current and ... the resistors popped. It's possible that the output transformer may also be bad.

Make sure your negative bias supply is operating before you put new output tubes in the amplifier.

 

[Trout]

That bad capacitor is probably in the bias supply.

My guess is that the cap shorted, the amplifier ran with no bias on the output stage and this caused the tubes to draw too much current and ... the resistors popped. It's possible that the output transformer may also be bad.

Make sure your negative bias supply is operating before you put new output tubes in the amplifier.

Will Do,

Im hoping the tubes arrive this coming week, I think they must be coming Via snail mail. Its been over a week.

Gene

 

[axehandlejack]

i realise that this thread is old! there may be some free ones i'll post if i run across them again when im looking for my u65 one with clipping instead of tremolo!

http://www.musicparts.com/products.asp?Company=Univox