Tube Amp Neophyte query

[Tonray's Ghost]

So, I know very little about electronics, save for how to make steel wool burst into flame by applying a power source. I have been watching many tube amp technician videos and it seems to me quite a common occurence is when a tube goes bad and burns out a resistor rendering the amp inoperable.

This would seem to be a nearly fatal design flaw if the tube amp for example had been in a critical piece of medical equipment, space capsule etc. In a guitar amp...just not a life or death situation...but one might think there is a way to build/design amps so a rogue tube could not take out the entire system, especially seeing that tube replacement is generally a consumer level maintenance task. This is what gives me pause in someday purchasing another tube amp down the line when I have a space appropriate for said volume. (play it loud !)

Someone educate me please.

[toomanycats]

I know very little about electronics, save for how to make steel wool burst into flame by applying a power source.

Adolescent pyrotechnic experimentation like this is why I no longer have my 70s Kenner Star Wars toys, Marx plastic army men, and G.I. Joe dolls.

[Tonray's Ghost]

I know very little about electronics, save for how to make steel wool burst into flame by applying a power source.

Adolescent pyrotechnic experimentation like this is why I no longer have my 70s Kenner Star Wars toys, Marx plastic army men, and G.I. Joe dolls.

I was actually taught the trick as a survival tool for wilderness camping. A 9 volt battery and a small wad of steel wool and you've got a campfire.

[mozz]

It's sort of done that way so the output transformer and/or the power transformer does not burn up. It's usually the screen resistor that burns up. Some builders/manu will use a 2watt or 5watt resistor there. They usually will then fuse the transformers. It's probably a cost exercise which is cheaper.

My theory is that tubes these days are more prone to short than tubes of old.

[Tonray's Ghost]

It's sort of done that way so the output transformer and/or the power transformer does not burn up. It's usually the screen resistor that burns up. Some builders/manu will use a 2watt or 5watt resistor there. They usually will then fuse the transformers. It's probably a cost exercise which is cheaper.

My theory is that tubes these days are more prone to short than tubes of old.

Would it be possible to use a fuse that would blow (and be easily replaceable) by the consumer instead of a resistor ? Again, speaking from ignorance of electronics, it would seem that might be a better design

[andrewsrea]

Let's start with probability and prevention.

Barring your amp accidentally drinking a beer (it happened to me), the little twin-triode preamp 9-pin tubes rarely short. They get microphonic (feedback), saggy, noisy (wind noises, his, etc.) and muddy when they are going bad. These, the user spends the $10 to $25 and can replace them without a technician.

I'd say I am well over 500 tube amps repaired (closer to 1K), to which less than 15 were catastrophic due to output tube (the bigger ones in the amp). This would mean an output transformer, speaker, or tube socket was no longer usable. Less than 50 had screen grid resistor, cathode resistor or control grid resistor failures. So, the occurrence is pretty low if you maintain your amp, which means buying new output tubes when you start hearing random volume changes, or the amp sounds like it has a blanket over it and fights you or you see an orange dot in the middle of the large grey metal plates inside the tube.

If the output tube is cathode biased (a.k.a.: self-biasing), or a newer automatically controlled fixed bias (like the Behringers), then the user can change their own tubes. If it is old-school fixed bias (Fender, Marshall, etc.), then you typically need a tech to install and bias the amp anyway. Any tech worth their salt will check the screen grid and control grid resistors and replace them for less than $10 each when they are in there.

The worst I've seen was an abused and not maintained Ampeg SVT (360w) bass amp, which 3 tubes on one side of the transformer shorted and the output transformer caught on fire. That cost around $400 to replace the transformer, output tubes and a few resistors back, around 2002 (that model has six 6550 tubes at $35 a pop then).

The next worst was a Vox AC30's, which took out all the screen resistors and the cathode resistor was heading to a short. Vox AC15's and AC30's do not have large enough cathode resistors and are brutal on EL84 tubes, even when not played hard. If you have one of these, you need to change the EL84 tubes annually.

Other amp brands and models will depend on how often and how hard you play. For instance, I just changed the 20-year-old Phillips USA 6L6 output tubes in my Fender Vibroverb. That amp gets played monthly on average, occasionally on '7' and pushed with an overdrive.

[mozz]

"Would it be possible to use a fuse that would blow (and be easily replaceable) by the consumer instead of a resistor ? Again, speaking from ignorance of electronics, it would seem that might be a better design"

Yes, but.............one side of that fuse may possibly go to your power supply and you could get shocked, even when turned off. So, manu aren't going to do that. I think some Marshalls have more than 1 fuse user replaceable on the chassis outside.

Also, people have a habit of putting in a bigger fuse when a small value one blows. That would make something else smoke and possibly start a fire.

There was even a practice of a fuse outside the consumer could change, say 1A. Inside was a 1.5A, hidden. So if the person swapped out a 2A-5A on the outside, it wouldn't blow but the inside would blow and chances are they would then give up working on it themselves.

If the screen resistor did happen to blow from a shorted tube, i think you would smell it burning.

[Tonray's Ghost]

Let's start with probability and prevention.

Barring your amp accidentally drinking a beer (it happened to me), the little twin-triode preamp 9-pin tubes rarely short. They get microphonic (feedback), saggy, noisy (wind noises, his, etc.) and muddy when they are going bad. These, the user spends the $10 to $25 and can replace them without a technician.

I'd say I am well over 500 tube amps repaired (closer to 1K), to which less than 15 were catastrophic due to output tube (the bigger ones in the amp). This would mean an output transformer, speaker, or tube socket was no longer usable. Less than 50 had screen grid resistor, cathode resistor or control grid resistor failures. So, the occurrence is pretty low if you maintain your amp, which means buying new output tubes when you start hearing random volume changes, or the amp sounds like it has a blanket over it and fights you or you see an orange dot in the middle of the large grey metal plates inside the tube.

If the output tube is cathode biased (a.k.a.: self-biasing), or a newer automatically controlled fixed bias (like the Behringers), then the user can change their own tubes. If it is old-school fixed bias (Fender, Marshall, etc.), then you typically need a tech to install and bias the amp anyway. Any tech worth their salt will check the screen grid and control grid resistors and replace them for less than $10 each when they are in there.

The worst I've seen was an abused and not maintained Ampeg SVT (360w) bass amp, which 3 tubes on one side of the transformer shorted and the output transformer caught on fire. That cost around $400 to replace the transformer, output tubes and a few resistors back, around 2002 (that model has six 6550 tubes at $35 a pop then).

The next worst was a Vox AC30's, which took out all the screen resistors and the cathode resistor was heading to a short. Vox AC15's and AC30's do not have large enough cathode resistors and are brutal on EL84 tubes, even when not played hard. If you have one of these, you need to change the EL84 tubes annually.

Other amp brands and models will depend on how often and how hard you play. For instance, I just changed the 20-year-old Phillips USA 6L6 output tubes in my Fender Vibroverb. That amp gets played monthly on average, occasionally on '7' and pushed with an overdrive.

Thanks for edu-ma-cating me...makles sense that of course only the bad cases end up on youtube tech channels and the % in total is extremely small out of the total amp in service population. Now let's see if that Super-reverb can fit behind the GF's makeup desk in the master bedroom.

[tonebender]

I've used tube amps for 3 decades with 1 failure during a show. The transformer blew in my Delta Blues. I still remember, we counted off to play Mustang Sally and I had no sound. I ran out to my van, retrieved my Vox Night Train head, sat it on top of the DB and ran a speaker cable I had made special with the two slider connects on one end and a 1/4" jack on the other to the speaker in the DB. I was up and running in time for my solo in the outro. I always carry a backup amp, well maybe not every time but probably all but a few over the years. Especially if it is too far from home to run get another. I have been lucky enough to be able to play most gigs within 10-12 miles from home. These days my Fender SCXD combo is the backup and easy to take. My primary is a Gibson Goldtone 15W combo. Pretty sure I will never use a SS state amp again for gigging. I can see using one around the house for practice but for gigging it tube only for me. I have had several other tube amp failures at home with a bad component or bad solder job failing, but altogether still only a few maybe 5-6. I have never had a tube failure that I remember.

[andrewsrea]

I've used tube amps for 3 decades with 1 failure during a show. The transformer blew in my Delta Blues. I still remember, we counted off to play Mustang Sally and I had no sound. I ran out to my van, retrieved my Vox Night Train head, sat it on top of the DB and ran a speaker cable I had made special with the two slider connects on one end and a 1/4" jack on the other to the speaker in the DB. I was up and running in time for my solo in the outro. I always carry a backup amp, well maybe not every time but probably all but a few over the years. Especially if it is too far from home to run get another. I have been lucky enough to be able to play most gigs within 10-12 miles from home. These days my Fender SCXD combo is the backup and easy to take. My primary is a Gibson Goldtone 15W combo. Pretty sure I will never use a SS state amp again for gigging. I can see using one around the house for practice but for gigging it tube only for me. I have had several other tube amp failures at home with a bad component or bad solder job failing, but altogether still only a few maybe 5-6. I have never had a tube failure that I remember.

I was asked to fill in on rhythm guitar on the spot at a gig, where the guy got drunk after setting up. I showed up to see my friends in the band and they asked me to fil in, using his equipment which included an 1970's all tube Peavey combo (like the Deuce, but with a tube preamp). Luckily, I had a Crate Power Block 100w lunchbox amp in my car, because a patron got bumped and spilled cold beer all over the Peavey's hot tubes in the middle of a song! It made a horrible bang and took out the bar's stage circuit breaker. Always good to have a backup!

[tonebender]

Redundancy is the key to success, just ask an astronaut.