Bulletproofing an amp?

[guitarmike2107]

ok .. why are we shooting at our amps anyway... this must be an American thing

[selloutrr]

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.

look at the Fender Sunn Model T reissue schematic.

[Zippy]

ok .. why are we shooting at our amps anyway... this must be an American thing

How 'bout "idiot proofing"?

The problem with that approach is that some people take it as a challenge to be a better idiot.

[Gibsonman63]

You quickly get to the point of cost vs. benefit.

If you are making thousands of amp, just make sure they are safe when they fail, and they can go back to GC and swap it out for a brand new one mass produced jewel.

The kind of amps being built by the guys here are generally not sold to novices and a little educaton of your consumer can go a long way. Look at the amps that KF and HAD built. They definitely don't follow the belt and suspender mentality and failures are rare.

[Zippy]

OK, back to the intent of the thread, that is, to consider modifications that may improve the robustness of an amplifier and protect against likely failure scenarios commensurate with using tubes:

1) The slow fail protection of a tube rectifier by inserting SS diodes in series (ala R.G. Keen).

2) MOV across power mains.

3) Current inrush limiter.

Anyone for fusing beyond the mains (ie CT or HT)?

[Zippy]

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

I used to have a truly bulletproof guitar - the good ol' Dan Armstrong Plexiglas model. Heavy, toneless, transparent - everything you need in a guitar.

[Structo]

I've been looking around at various amps and their fusing schemes.

Marshalls use HT fuses a lot.

It seems that for 50w amps a 500ma HT fuse is the norm and in a 100w amp a 1 amp fuse is used.

This is for 120vac operation.

[Richie]

Bulletproofing an amp??? Ship it through UPS, that will let you know if its bulletproof!!

[Andy Le Blanc]

old school solder techniques, toward reliability, goes a long way

the type of fasteners used to hold it together

electronically, heater voltage, heater to cathode ratings are more important
than first glance, the value of the first cap the rectifier sees...

you can fuse and diode it up, but it won't do a thing for you if you didn't mind
the design center values, they are there for reason, if you push the envelope
you risk running into reliability issues, DIY builders usually design backwards
You start with a clone and start making assumptions...

I had a mil/JAN PA come thru once, the solder and dress were immaculate
it had one leaky coupling cap in the power side, not bad since it was assembled
in 1952, all the caps were the bathtub case, you can still source them I think
they all were chassis mounted, and the component board with resistors
with lock nuts, all the hard ware was quality, the dress was encased in
braided steel tubing, steel nuts and bolts, robust... cathode bias

quality in... quality out, a conservative rugged design

one fuse...

tubes have sockets for a reason, they MUST be replaced.
chose designs that are kind to the tubes, invest in quality parts
and dress the circuit to a high standard, proven reliability.

[ChrisM]

I've been looking around at various amps and their fusing schemes.

Marshalls use HT fuses a lot.

It seems that for 50w amps a 500ma HT fuse is the norm and in a 100w amp a 1 amp fuse is used.

This is for 120vac operation.

Main's voltage doesn't matter

Fuse value will depend where it is located, what current the circuit is drawing and how much of a margin the fuse is designed for before it blows.

[selloutrr]

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

I used to have a truly bulletproof guitar - the good ol' Dan Armstrong Plexiglas model. Heavy, toneless, transparent - everything you need in a guitar.

I still have a bass and guitar. In the early 90's I had a Plexiglass Fender P-bass. It had no tone but damn it looked cool. the only real problem was I couldn't play it naked


As for getting back on topic.

Do a google search or visit small amp builder sights and compile a list of the extra features they add to make it bigger better faster more. I'm not 100% sure what you are trying to achive in this thread since you can do every possible MOD you come across and an amp can and will still fail. IMHO, the more you put in a circuit the more possiblity you introduce for failure. Keep it simple and use good parts and it's going to far out last your lifetime. Look at a fender champ it's simple and they still run, the only thing that goes wrong is lack of care or scheduled service.
Tubes and transistors have a life span, as do caps, and to some extent resistors. all of these facts equal a ticking time bomb you can prolong failure but you can't provent it.

If you really wanted to bullet proof an amplifier educate the user.

[selloutrr]

Loss of speaker load protection

On the output jack using a mono shunting tip jack solder in two 125ohm 20+ watt resistors in series to ground. In the event the speaker is not plugged in or comes unplugged while the amp is turned on. This will give the OT a load buying you enough time to power down before you cause damage to your amp.

[Structo]

I've been looking around at various amps and their fusing schemes.

Marshalls use HT fuses a lot.

It seems that for 50w amps a 500ma HT fuse is the norm and in a 100w amp a 1 amp fuse is used.

This is for 120vac operation.

Main's voltage doesn't matter

Fuse value will depend where it is located, what current the circuit is drawing and how much of a margin the fuse is designed for before it blows.

Could you clarify that Chris?

Fuses have voltage ratings.
I realize that it is the current limit that determines when it blows but why do amps that have international power transformers in them have different specs for what fuse to use depending on what the mains voltages is?

When I look at amps there is one rating for 220vac and another for 120vac.

Basically the 220vac fuse is half the value of 120vac.

So say on a 100w amp powered by 120vac, the fuse is generally a 4 or 5 amp slo blo.



For example, the Marshall 1923 amp states on the back panel that for 230vac operation it needs a 1.6 amp fuse.
For 120vac operation it needs a 3 amp fuse.

On the HT fuse it doesn't matter because the voltage there is the same regardless of the mains voltage.

Check it out
http://marshallamps.com/downloads/files ... %20Eng.pdf

[Zippy]

On the HT fuse it doesn't matter because the voltage there is the same regardless of the mains voltage.

You are saying the same thing - that the fuse rating for the HT does not require knowledge of the mains voltage.

[joCCe]

I've been looking around at various amps and their fusing schemes.

Marshalls use HT fuses a lot.

It seems that for 50w amps a 500ma HT fuse is the norm and in a 100w amp a 1 amp fuse is used.

This is for 120vac operation.

Main's voltage doesn't matter

Fuse value will depend where it is located, what current the circuit is drawing and how much of a margin the fuse is designed for before it blows.

Could you clarify that Chris?

Fuses have voltage ratings.
I realize that it is the current limit that determines when it blows but why do amps that have international power transformers in them have different specs for what fuse to use depending on what the mains voltages is?

When I look at amps there is one rating for 220vac and another for 120vac.

Basically the 220vac fuse is double the value of 120vac.

So say on a 100w amp powered by 120vac, the fuse is generally a 4 or 5 amp slo blo.

The same amp would have double that, so 8-10 amp slo blo.

For example, the Marshall 1923 amp states on the back panel that for 230vac operation it needs a 6 amp fuse.
For 120vac operation it needs a 3 amp fuse.

On the HT fuse it doesn't matter because the voltage there is the same regardless of the mains voltage.

Check it out
http://marshallamps.com/downloads/files ... %20Eng.pdf

A 100W light bulb on 120VAC will draw: 100W/120V = 0.83A
A 100W light bulb on 240VAC will draw: 100W/240V = 0.42A

So, you will downsize the fuse if the volts go up.

Here is a good explanation: http://rikravado.hubpages.com/hub/Watt- ... -and-Volts

Joakim