Bulletproofing an amp?

[Zippy]

Following the thread "Need an opinion on warranty"...

http://ampgarage.com/forum/viewtopic.php?t=16030

What is the current state-of-the-art regarding making an amp more robust and self-protected?

How many fuses and where? Other forms of protection such as MOVs, diode strings, etc.?

What do you use?

How much is enough?

Are there any negative sonic issues to come from using too much protection?

Thanks.

[selloutrr]

don't over think this.

"DANGER - NO USER SERVICEABLE PARTS TAKE TO A QUALIFIED SERVICE TECHNICIAN"

If they are dumb enough to poke around it's population control.

Anything you do will in some way effect the tone. I guess it would depend on the circuit design the modification you are adding, where in the circuit it goes, and what materials the components are made out of.

I can't give you a simple answer.

Keep in mind, anything you have access to on the outside of the amp will be fiddled with before it's taken to a service tech. Fuses are be dangerous to both the amp and the operator. I've seen my share of welded fuse holders, from people blowing fuses so they shove it full of something that can conduct of way over spec'd amperage. only in turn to cause a larger problem deeper in the circuit.

making your amp out of quality components, not over taxing your iron, a bias section that is solid, and proper ventilation with a 3 prong ground cord and master power fuse it's about all you need for a strong life long amplifier.

you can add safety mod's until the cows come home but if the amp wasn't built well to begin with it's going to fail.

[diagrammatiks]

this is a pretty good series by rg keen.

http://www.premierguitar.com/Magazine/I ... r_Mod.aspx

with most of the components now available amps are usually a bit more fail-safe then ever before.

the weakspot is still the transformers...and the only way to fix that is to pump money into them.

[VacuumVoodoo]

Make them idiot proof. I mean an idiot shouldn't even be able to find the ON/OFF switch on the amp, not even mentioneing the fuses.
[IMG100]http://i26.photobucket.com/albums/c149/ ... a/0029.gif[/img]

[Structo]

Amps with tube rectifiers seem more prone to problems when that tube fails or shorts out.

So adding the diodes as in RG Keene's article is cheap insurance.

How about protecting the output transformer as well?
I believe Ampeg and Ken Fischer used diodes on the output tubes as well.


Let's face it, depending on light bulbs to power our guitar amplifiers is archaic and primitive.
But dang, they sure do sound good when everything is working as it should.

Gaz's thread also made me think about this.
So far I am the only person crazy enough to own something I built.

But I'm seriously thinking about adding at least a HT fuse to my 100w ODS clone.

I'm still researching the best location for that fuse.

[Darkbluemurder]

Amps with tube rectifiers seem more prone to problems when that tube fails or shorts out.

It seems I am the only one who never had a tube rectifier fail but had two shorted SS rectifiers. So much on the myth that SS rectifiers are more reliable.

[selloutrr]

I've only seen two failed rectifiers come across the bench. Never experienced it in an amp I personally own. I too I have fixed several SS rectifiers with fried diodes.
I'd say it's a toss up. The weak link in both designs are component choice. The rec tubes were both new production and the diodes were to small and running at close to 90%. If you use a quality socket and tube you are probably good same with an over spec diode. Like anything mechanical the key to reliable use is regularly scheduled maintenance.

[Zippy]

What about fusing beyond just the main?

MOV?

[ToneMerc]

What about fusing beyond just the main?

MOV?

Jim, I install MOV's in just about everything I build.

TM

[Structo]

MOV's are fine to use to curtail spikes or transient voltages that may damage sensitive components. I just use a surge protector power conditioner and plug my amps into that.

The other type of component that works well is a Thermistor.
Also known as a inrush current limiter.
You place these in the primary of a power transformer to allow a slow start up.
These can take the place of a standby switch.
But most tube guys like a standby switch to use as a mute or to put the amp on standby while they take a break and to have the amp ready to go when they come back to it.

Some Hi Fi guys like to use them on audio amps because they can just flip on the power switch and have the amp warm up slowly.

These work by having a negative temperature coefficient.
As they heat up the resistance goes down.
So the voltage is low at the beginning of power up then rises to maximum.

For example, a CL-40 is a 6 ampere device.
At room temperature it has 5 ohms resistance.
Once warmed up it has .11 ohms.

[img:250:179]http://i3.ebayimg.com/04/i/001/35/95/7854_35.JPG[/img]

In theory they work great but as discussed in that forum there is one circumstance where they aren't good to have.

This is when you have run the amp for a while then turned it off.
Then without letting it cool down you turn it back on.
Now the MOV is already hot and will let a inrush of current pass.

But as long as you are aware of that limitation you can avoid that scenario.

[renshen1957]

Following the thread "Need an opinion on warranty"...

http://ampgarage.com/forum/viewtopic.php?t=16030

What is the current state-of-the-art regarding making an amp more robust and self-protected?

How many fuses and where? Other forms of protection such as MOVs, diode strings, etc.?

What do you use?

How much is enough?

Are there any negative sonic issues to come from using too much protection?

Thanks.

Hi Zippy,

Much of what is added to an amp such as fuses on every secondary winding, additional internal mains fuse, etc have been added by the respected nation's safety organizations for various reasons, fiats, etc, but not to protect the amp's circuitry. The idea is that it increases safety (debatable) such as Sweden's elmination of standby switches or another country's insistance on safety ground's wires must have yellow and green stripes.

As to diode strings, diodes can and will short out, B+ short will blow a fuse if the HT are fused or if lucky the mains fuse, if not a variety of scenarios can happen, mostof these can be expensive. Whether 3 diodes would all fail at the same time and the same side is open to debate, however there is the possiblity of failure over time.

MOVs are designed to conduct significant power for very short durations (such as lightning hitting a power line), but MOVs typically do not have the capacity to conduct sustained energy. A varistor provides no equipment protection from inrush current surges (during startup), from overcurrent (created by a short circuit), or from voltage sags; it neither senses nor affects such events.

MOVs fail in time, one reason to replace your surge protectors on your computer and TV set every year or two, the MOVs wear out of time.

R B Keen's article is nothing new, the Bullet Proofing your rectifier was to my memory first written up (under a paragraph heading) in Dave Funk's Tube Amp Workbook Vol 1. 1996 which complied and arranged magzine articles which dates back earlier.

Amps are generally abused as appliances, pushed beyond their limits, etc. Tubes as electrical mechanical components wear out or fail in time. Many amp designs have tubes intentionally past theorectical limits. You cannot add a safety component to deal with every possible parameter of failure. You can build updated or super stock version of classic designs to remove the engineer mindset of 60 years ago. Examples the screen resistors in a AC30, the cathode resistor in some cathode biased Ampeg bass amps, the bias supply in vintage Hiwatts, etc.



Best Regards,

Steve

[Travst]

So far I am the only person crazy enough to own something I built.

That's not exactly true, Tom.

[Cliff Schecht]

Aspen Pittman's book mentions SS protection diodes on tube rectifiers even back in the mid 80's when his first version of the GT bible came out.

[selloutrr]

I'd think kevlar would be the most effective way to bullet proof an amplifier. Just saying it would work.

[Ian444]

A simple flashing red LED on the front of the amp to indicate an out of tolerance difference in cathode currents between 2 tubes of a 2-tube PP amp would be nice. Anyone got a simple schem? Just an LED warning to say all is not quite right, take me to the tech.