Greetings,
Just started working on a Sunn (Fender) 300w tube amp (6x 6550). Circuit board confirms 1998.
The owner complains that the amp periodically blows the 10A (!) fuse. I'm thinking power supply, but what could be intermittently faulting? My first idea was to replace all the electrolytic (450V) capacitors, but I would be blindly throwing $100 at the problem, not knowing the cause. I just seem to always think 'filter caps' when dealing with intermittent problems like this one where a lot of current is shorting to ground.
Any thoughts?
Just started working on a Sunn (Fender) 300w tube amp (6x 6550). Circuit board confirms 1998.
The owner complains that the amp periodically blows the 10A (!) fuse. I'm thinking power supply, but what could be intermittently faulting? My first idea was to replace all the electrolytic (450V) capacitors, but I would be blindly throwing $100 at the problem, not knowing the cause. I just seem to always think 'filter caps' when dealing with intermittent problems like this one where a lot of current is shorting to ground.
Any thoughts?
Get details on how he plays the amp.
Also, very important:
Check the speaker cabinet and cables he's using.
cable could be chewed, insulation missing, shorting inside plugs,, or run over by other amp/wheels during road-in/out.
If they short out BANG! fuse blows.
Check and see if he (or his roady) understand that you CAN'T unplug an amp speaker when its on!!!!
Go over the basic safety and operating instructions with him,
and MAKE SURE HE really understands them.
He could be playing for decades and still be electrically stupid.
You must check...
Put the amp on a scope with a known solid safe load (preferrably resistors!)
See if you see anything suspicious like spikes.
Inquire as to WHERE he is plugging it in.
IF there are air-conditioners (i.e., apt building size) kicking on and off,
or other spikes coming down the line, this could be bad news.
Does he play regularly in the same bar all the time?
Maybe their A.C. sucks.
Also, very important:
Check the speaker cabinet and cables he's using.
cable could be chewed, insulation missing, shorting inside plugs,, or run over by other amp/wheels during road-in/out.
If they short out BANG! fuse blows.
Check and see if he (or his roady) understand that you CAN'T unplug an amp speaker when its on!!!!
Go over the basic safety and operating instructions with him,
and MAKE SURE HE really understands them.
He could be playing for decades and still be electrically stupid.
You must check...
Put the amp on a scope with a known solid safe load (preferrably resistors!)
See if you see anything suspicious like spikes.
Inquire as to WHERE he is plugging it in.
IF there are air-conditioners (i.e., apt building size) kicking on and off,
or other spikes coming down the line, this could be bad news.
Does he play regularly in the same bar all the time?
Maybe their A.C. sucks.
Is this the SUnn 300T? We really need to use model numbers so we can refer to schematics.
The 10A in that is the mains fuse. You could have any number of things causing this.
Qualify the customer. What exactly does he mean by "periodically"? I have had customers come in with a blown fuse wanting to know what was wrong with the amp, when it was simply an open fuse. Some people think once in 1998 and once in 2008 is a pattern. Once a week, now that is a problem. But I sure wouldn't waste my time figuring out why an amp loses a fuse once every ten years.
Assuming it is more often than years, WHEN does it happen. Right at power switch flip? Only when standby switch is flipped? At random during play? Or at power off? In other words it worked when I turned it off, but now it won;t turn on. Those are powerful clues.
Throwing parts at a failure is like throwing sponges at a rainstorm.
FIlter caps would not be high up my list. ANything is possible, but I don't suspect them first at all.
First thing on the list is always the power tubes. They usually take out the high voltage fuse in an amp, but they can take out the mains fuse too.
Second I suspect the high voltage "flyback" diodes, the two R3000 at CR73, CR79. Those fail, they short B+ to ground.
Next on my list might be a high voltage rectifier breaking down.
Then farther down the list is an output tranformer arcing inside. Not common at all, but it happens.
And don't necessarily assume it is a circuit failure. Nazaroo points out speaker load concerns along with connections. Heed that advice. But also inside the amp, one loose nut from a screw can be rolling around, and now and then shorts something out to blow the fuse, but rolls away as you move the amp. And a wire could be pinched somewhere and now aqnd then the inner conductor shorts to chassis. Things like that are not bad parts, but could be the problem.
The 10A in that is the mains fuse. You could have any number of things causing this.
Qualify the customer. What exactly does he mean by "periodically"? I have had customers come in with a blown fuse wanting to know what was wrong with the amp, when it was simply an open fuse. Some people think once in 1998 and once in 2008 is a pattern. Once a week, now that is a problem. But I sure wouldn't waste my time figuring out why an amp loses a fuse once every ten years.
Assuming it is more often than years, WHEN does it happen. Right at power switch flip? Only when standby switch is flipped? At random during play? Or at power off? In other words it worked when I turned it off, but now it won;t turn on. Those are powerful clues.
Throwing parts at a failure is like throwing sponges at a rainstorm.
FIlter caps would not be high up my list. ANything is possible, but I don't suspect them first at all.
First thing on the list is always the power tubes. They usually take out the high voltage fuse in an amp, but they can take out the mains fuse too.
Second I suspect the high voltage "flyback" diodes, the two R3000 at CR73, CR79. Those fail, they short B+ to ground.
Next on my list might be a high voltage rectifier breaking down.
Then farther down the list is an output tranformer arcing inside. Not common at all, but it happens.
And don't necessarily assume it is a circuit failure. Nazaroo points out speaker load concerns along with connections. Heed that advice. But also inside the amp, one loose nut from a screw can be rolling around, and now and then shorts something out to blow the fuse, but rolls away as you move the amp. And a wire could be pinched somewhere and now aqnd then the inner conductor shorts to chassis. Things like that are not bad parts, but could be the problem.
You just gave me some more ideas.
If he changes his own tubes, (maybe as an experiment) he may not have seated them properly, or there may be a loose socket contact somewhere, or dirty tube pins.
Everything Enzo said is great stuff.
But there may be a sensible order to attack.
You don't mention what you've done already.
Amps that get roaded could have loose elements inside a tube thats been banged around. When it warms up or conducts excessive current, it could make a temporary short, or when there's a vibration. The tubes should not just be 'checked' but also tested for shorts, and any element motion.
Next check things like traces on the sockets, It might not just be an obvious loose screw or piece of metal knocking about, but something light and feathery, like hair, dust or dead bugs.
The diodes are a great lead. IN fact, replace a standard bridge with two diodes in series for each original (doubles the voltage) and increase the current size too, and use balancing resistors in parallel to divide the voltage evenly.
Surge currents with on/off are a good one. Replace the simply standby with a dpdt and throw in/out a series resistor during powerup, to charge HV caps slower.
If all the tubes check out, why not power up for a quick check of voltages (HV and bias), and balance, to see if anything is unusual (for instance a short in the tranny might leave screens unbalanced.
If he changes his own tubes, (maybe as an experiment) he may not have seated them properly, or there may be a loose socket contact somewhere, or dirty tube pins.
Everything Enzo said is great stuff.
But there may be a sensible order to attack.
You don't mention what you've done already.
Amps that get roaded could have loose elements inside a tube thats been banged around. When it warms up or conducts excessive current, it could make a temporary short, or when there's a vibration. The tubes should not just be 'checked' but also tested for shorts, and any element motion.
Next check things like traces on the sockets, It might not just be an obvious loose screw or piece of metal knocking about, but something light and feathery, like hair, dust or dead bugs.
The diodes are a great lead. IN fact, replace a standard bridge with two diodes in series for each original (doubles the voltage) and increase the current size too, and use balancing resistors in parallel to divide the voltage evenly.
Surge currents with on/off are a good one. Replace the simply standby with a dpdt and throw in/out a series resistor during powerup, to charge HV caps slower.
If all the tubes check out, why not power up for a quick check of voltages (HV and bias), and balance, to see if anything is unusual (for instance a short in the tranny might leave screens unbalanced.
Thanks for the replies everyone, I really dig this forum - very helpful and friendly!
I haven't done anything to the amp yet. First up I'll test the tubes...perhaps I can get them to vibrate while testing for shorts, though not sure if this is feasible. Here are some details I picked up from the owner:
- a couple years back the original 10A fuse blew after a 45 minute set
- fuse was replaced with a 15A (oh, the horror) and it worked fine for some months/years until blowing recently after 30-45 min of use.
I've told him to bring the speaker cable by so I can inspect it. I'll do some tests and report back before changing anything out (i.e. diodes).
Thanks again!!
I haven't done anything to the amp yet. First up I'll test the tubes...perhaps I can get them to vibrate while testing for shorts, though not sure if this is feasible. Here are some details I picked up from the owner:
- a couple years back the original 10A fuse blew after a 45 minute set
- fuse was replaced with a 15A (oh, the horror) and it worked fine for some months/years until blowing recently after 30-45 min of use.
I've told him to bring the speaker cable by so I can inspect it. I'll do some tests and report back before changing anything out (i.e. diodes).
Thanks again!!
Yep, the diode changeover is a mod, and only an option if you're replacing the PS bridge anyway. He may not want it (and the fee).
Also that has to be designed right. i.e., high voltage / high resistance resistors for minimal problems.
Also that has to be designed right. i.e., high voltage / high resistance resistors for minimal problems.
God there a lot of fuses on this thing. Just wound that one of the 100mA's between one pair of 6550's was toast.
No shorts in any of the tubes, but 5 of the 6550's are testing very poorly. Way below JAN minimum book values.
SHorts take many forms. You may not have a tube that reads as shorted in some teste, but that does';t mean none of them will intermittantly short internally now and then. And in particular, tube testers do not put real world loads on tubes. Kinda like idling at the garage doesn;t tell a mechanic how your car will behave pulling a trailer down the interstate.
All it takes is for one tube to have a loose bit of grid wire that vibrates and now and then touches something else.
All it takes is for one tube to have a loose bit of grid wire that vibrates and now and then touches something else.
Ya I hear ya, thanks for pitching in. Kinda hard to test a tube under that kind of stress scenario.
I'll check out the traces and HV bridges, then throw in new fuses in a give it a whirl.
Thanks!
I'll check out the traces and HV bridges, then throw in new fuses in a give it a whirl.
Thanks!
Last edited:
6550's like to arc over! I have tested many after an arc over and they will usually work, but are untrustworthy! replace them!
Haha, not an easy thing to do when 3 pairs cost over $275!! Plus the seller is looking to ditch this thing as soon as it is working. My fear is getting him to replace the tubes and still having issues with the amp shorting out.
Thankfully I have never had any problem with 6550's in my own gear. I use them a lot as a service tech for Leslie speakers, and my personal bass amp is similar to this sunn (6x 6550 vintage ampeg svt). My tube tester is a Stark 99-1 (Hickok design patent) and very reliable, I think (ballpark, of course). The 4 tubes that tested strangely weak (1000 microhmos) are already known untrustworthy - but I don't know why the values are so low - would that indicate failure inside the tube? Good 6550's have a test grid emissions between 5000 - 6000 at good spec, and usually 3000-4000 after a hard life of work...and I have never encountered readings between 0 and 1000 until working on this amp.
Thankfully I have never had any problem with 6550's in my own gear. I use them a lot as a service tech for Leslie speakers, and my personal bass amp is similar to this sunn (6x 6550 vintage ampeg svt). My tube tester is a Stark 99-1 (Hickok design patent) and very reliable, I think (ballpark, of course). The 4 tubes that tested strangely weak (1000 microhmos) are already known untrustworthy - but I don't know why the values are so low - would that indicate failure inside the tube? Good 6550's have a test grid emissions between 5000 - 6000 at good spec, and usually 3000-4000 after a hard life of work...and I have never encountered readings between 0 and 1000 until working on this amp.
If the screens are partly fried,
then they won't conduct as well, even though they short-test ok.
Lets say you had a small arc or screen melt-down.
Now there is less screen to attract electrons.
Sometimes you just have to admit tubes are damaged,
or that your tube-tester was on the wrong setting ... Doh!
Last edited:
well like I said...or in other words, I fully trust my tester. I know testers are a bit wishy-washy and should be taken with a light hearted approach. I have calibrated my unit and I know the values I get are within reason - especially since I can switch between good and and bad tubes and get consistent results for every tube. But alas...
So if you use 6550s a lot elsewhere, do you have six of them you could install for testing? Worn out tubes that you know work are good enough for this.
They don;t have to be matched, we are not going for tone at the moment, we are looking for what blows fuses. Mismatched tubes won;t blow fuses.
You have the two bias test points, they are simple 1 ohm resistors in the cathode lead of the two sides output tubes. You could monitor there to see if one side or the other or both spike when fuses blow.
A word about tube testers. They can point out bad tubes - this one is shorted, this one has no gain, etc. But what they can;t do is guarantee a good tube. They can tell you that at idle, the tube has transconductance and doesn't seem to be shorted. But it can't tell you that the tube won;t have intermittant characteristics.
Like servicing a car. They can connect it to the computer readouts and diagnose all manner of stuff, they can balance the tires and wheels, they can inspect the brakes and pads and wheel bearings. Tons of things they can do in the garage. But they can;t tell you if the car will shimmy bad at 70 miles per hour, or that it will stall once every couple weeks.
We need to eliminate the tubes as the problem or not.
They don;t have to be matched, we are not going for tone at the moment, we are looking for what blows fuses. Mismatched tubes won;t blow fuses.
You have the two bias test points, they are simple 1 ohm resistors in the cathode lead of the two sides output tubes. You could monitor there to see if one side or the other or both spike when fuses blow.
A word about tube testers. They can point out bad tubes - this one is shorted, this one has no gain, etc. But what they can;t do is guarantee a good tube. They can tell you that at idle, the tube has transconductance and doesn't seem to be shorted. But it can't tell you that the tube won;t have intermittant characteristics.
Like servicing a car. They can connect it to the computer readouts and diagnose all manner of stuff, they can balance the tires and wheels, they can inspect the brakes and pads and wheel bearings. Tons of things they can do in the garage. But they can;t tell you if the car will shimmy bad at 70 miles per hour, or that it will stall once every couple weeks.
We need to eliminate the tubes as the problem or not.
I hear ya, good stuff!
Unfortunately I have only one test worthy 6550. I fear to use my coveted good tubes for a test scenario that may do harm...(either brand new stock Tung-Sol's, NOS GE's, or those installed in my gigging amp which I don't want to mess with). Let me see what I can dig up tomorrow... perhaps the local store will have some beaters they can send my way.
Unfortunately I have only one test worthy 6550. I fear to use my coveted good tubes for a test scenario that may do harm...(either brand new stock Tung-Sol's, NOS GE's, or those installed in my gigging amp which I don't want to mess with). Let me see what I can dig up tomorrow... perhaps the local store will have some beaters they can send my way.
Two new pairs of EH 6550's, a new 10A ps fuse and 100ma fuse and the amp is running. Plays fine. Its been cooking for about 30 mins and hasn't blown yet... So strange! The local store tested the old tubes on a few different modern testers and the results were in the same range as my tester (ya!). Two decent tubes, two worn tubes, and two that were completely worn through, giving oddly low emission results. No shorts though. Anyway, just thought I'd put this one to rest (I hope!)...I'll let the amp cook for another hour tonight.
Thanks for all the help!
adam
Thanks for all the help!
adam
- Status
- Not open for further replies.