4 amp slo blow vs 3 amp slo blow fuse

[TerryD]

The Ampeg B25B (if that is what it is) calls for a 4 amp fuse right on the back.  The schematic for a B25b has 3amp.


My friend had a 3 amp fuse in the back.  Would that cause the fuse to blow...just because it was not the 4 amp slo blow that the amp called for??

[92Volts]

I'd trust the schematic more than the label on the back. Especially because 4 amps is an outrageously high value and I can't imagine why this (55 watt output) amp would pull anywhere NEAR 4 amps (480 watts at 120v). Let's do some half-assed math:

55 watts output at 50% output stage efficiency: 110w
Heaters 6.3v*(0.9a*2 for output tubes + 0.3a*3 for preamp tubes) + 5v*2a for rectifier: 27w

137w of power. That's just over 1A at 120v. My math is half-assed but definitely in the right ballpark. If I home-built this amp I'd put a 2A fuse in it.

Companies seem to shy away from "nuisance" fuse trips (which could piss off lots of customers) at the cost of not protecting the amp as effectively in the event of a serious problem.

So the answer to the second part of your question is yes, too small of a fuse will blow more easily, maybe for no reason. But I don't think 3A is too small here. And if it did, this is better than an error in the other direction. After all fuses are sacrificed to save other, more expensive components!

[sluckey]

Can't trust the schematic because it has not been proven that this is in fact a B25B. So I'd use the factory label beside the fuse holder.

[davidwpack]

Both of my B25-BS have 3A ext. fuses.

[tubeswell]

FWIW, Ampeg's 1972 and 1974 versions of the B25B schematic (linked below).



http://el34world.com/charts/Schematics/files/ampeg/Ampeg_b25b.pdf


http://el34world.com/charts/Schematics/files/ampeg/Ampeg_B25B_.pdf

[TerryD]

As always, Thanks everyone.


A fuse is a fuse right...it will look like a fuse.   For the life of me I see only one fuse on this thing and thats the ext. fuse.  The "4 amps" is etched in the metal along with the tube designations and is not just a label.  So perhaps its not a B25B.  But then does anyone have a guess what it might be?  Here's the tube configuration  6k11 12ax7 12ax7 6cg7 12dw7 12ax7 12au7  7027 7027....but inside the first tube is a labeled 6k11/6q11

[sluckey]

The amp is a V2.

     http://sluckeyamps.com/misc/ampeg_v2.jpg

[2deaf]

. . .  4 amp slo blow that the amp called for??

Does the backplate actually say "slo blow", or does it just say "4 Amps"?  I mean, the schematics never spec slo blo.  Maybe the fuse is considerably larger than necessary because it isn't slo blo.

[PRR]

If you have not replaced the fuse yet, bring it up (4A fuse) on a Lamp Limiter, 100-150 Watt Incandescent (or halogen) lamp.

The V2 is a VT40 which is a big amplifier. A 4A fuse seems generous, the 3A should be OK, but a VT40/V2 this age can have significant problems beyond a mere fuse. My VT40 burned its power transformer, due to a 7027 that was OK cold but shorted when hot; also the filter caps are old-old-old and can suck everything down.

It won't play full-throat with a 150W lamp limiter, but it should idle at 70%-90% of the indicated heater and B+ voltages. (Note that the B+ will be over 600V on todays lines.)

[trobbins]

My VT40 burned its power transformer, due to a 7027 that was OK cold but shorted when hot

Sounds like a good precedent for all to check if their amps could do with a secondary side fuse of appropriate value.

[TerryD]

on the schematic it says 4 amp but then by it's side it says 2amp*  Would that equal 4 amp regular but a 2amp slo blow?

[PRR]

> by it's side it says 2amp*

Read the NOTES!!

I once found a lame joke in the notes. So always read them.

In this case, "*" refers to 240V models.

[TerryD]

oh jeez


Whats the trick to replacing these capacitor cans? Fenders are so easy you can't screw them up, but I don't really know what I'm doing here.  The diode metal (what do you call the wires on both sides of caps resisters etc....never had to communicate this before..leads??) connectors are rusty also?  Will exterior rust affect things electronically?  Thanks

[PRR]

When I looked into my VT40, the professional path to replacing the can-caps was to abandon them in place and get a "cap board" which held snap-caps and fit under the deck.

I had re-PT-ed this one at lower B+ and the old caps seemed fine at the lower voltage, and my retirement was in sight, so I let it be.

[TerryD]

OK  I put the 4 amp slo blow fuse in against all sane advice.  It seems to be working fine.  It's got all sovtek tubes so I think it's been worked on...pretty sure actually.  I cleaned the pots etc.


Here is the million dollar hypothetical question.  He had a 3 volt slo blow in there.  He also had the speaker ohms on the 8ohm setting but running into a set of 4 speakers.  Could that eventually blow the fuse????  Think about it...huh?  It looks to me as if the fuse took some time to eventually blow...if that's possible.


Could moisture in the amp have blown the fuse?


Can I hope against hope?

[PRR]

A V2/VT40 with a too-low load impedance can make a LOT of power. More than the tubes OR fuse can stand. Murphy's Law says the $90 tubes will blow to protect the 20 cent fuse. Sometimes Murphy is wrong.

Teach him how to set the impedance properly.

[TerryD]

Excellent!