A Guitar And Gear Support Forum

Tonray's Ghost wrote: ↑Sun Aug 04, 2024 9:18 pm I know very little about electronics, save for how to make steel wool burst into flame by applying a power source.

toomanycats wrote: ↑Mon Aug 05, 2024 6:28 am

Tonray's Ghost wrote: ↑Sun Aug 04, 2024 9:18 pm 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.

mozz wrote: ↑Mon Aug 05, 2024 12:12 pm 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.

andrewsrea wrote: ↑Mon Aug 05, 2024 8:26 pm 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.

tonebender wrote: ↑Mon Aug 05, 2024 9:03 pm 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.