speaker substitution

[mckayprod]

I'm working on a friend's Marshall AVT50 combo.  2 main problems: The speaker is bad (a Celestion G12-412 AVT, 4 ohm), and the cooling fan bearings are shot and it makes a lot of noise. My 2-pronged solution was to swap in a random 8-ohm 12-inch driver from an old organ, and disconnect the fan.  The speaker sounds surprisingly good.  My question is, with the new 8-ohm load, the power output from the chip drops from 50 watts to 25-ish, right?  That also reduces the heat it dissipates, and maybe makes the fan unnecessary, right?  Did I miss anything?

[J M Fahey]

Close 
Power will probably drop to around 35W , no big deal if the new speaker is good, but *do*  replace the fan.

In other amps there is a significative heat dissipation built in and the fan just "improves" it, but in many of latest Marshall amps, they use CPU rated heat sinks (read totally inadequate on their own)  and NEED  that fan.
But it will be a cheap one, it's a PC type.
Measure its size, read the label (it will probably be 12 or 24V DC) and start googling.

[mckayprod]

Thanks for the prompt reply; this is a great forum.  I have the fan ordered.  It's VERY small (40mm square X 10mm) but it's in there for a reason, right?  Hypothetically, what would happen if I put a 50W, 8-ohm resistor in parallel with the speaker?  It seems to me that the chip would be back to rated power and the new speaker would have some power-handling help.  I wouldn't do that since the amp sounds terrific right now, but it's a thought experiment.

[Enzo]

If you replace a speaker load with one of twice the impedance, as in replace a 4 ohm with an 8 ohm, the power draw is essentially cut in half.  So 50 watts to 25 watts is close enough for discussion.   What possible use would that parallel resistor have?  If you put it there to get the overall load back down to 4 ohm, sure the 50 watts comes back, but half of it would be dissipated in the resistor, leaving, you guessed it, the same 25 watts through the 8 ohm speaker.  So your choice is to let the amp produce 25 watts, all into the speaker, or to have the amp working twice as hard to put the same 25 watts into the speaker.


And DO replace the fan.  It is faulty reasoning to decide that since the amp won't be putting out much more than half power that the fan becomes unnecessary.  That assumes the fan only matters at maximum power.  The heat is generated at lower power levels as well, and will accumulate unless it is blown away.   It is like putting a pot of water on the stove.  On high it boils relatively soon, while on medium it takes longer to boil.   But it still boils.

[DrGonz78]

Yeah I had one of those AVT50 heads on my bench not too long ago. The main problem was the two fans were making a horrible noise at power up. I for one do not like to replace things until have tried to fix them. I agree that just replacing the fans is a very reasonable idea, but I had to go the way of salvaging. Taking the sticker off the back of the fans reveals the mechanical parts, but don't booger up the sticker part. I used some good ole 3 in 1 oil to lube up the fans. Put them all back together and presto they were silent. I was amazed especially because at first I thought the amp wasn't even on. Check of the fans and they were a spinning round.

I have been using the same approach in my many desktop computer fans around house lately. 3 in 1 oil is a great thing to have around the shop. Recently I had an old computer with a noisy fan on the video card. Fixed that right up wih only 1 drop of the stuff.  :dbtu:

[Enzo]

Oh, I agree, I usually try lube, the bearings have dried out and they chatter.

Though I do disagree with 3in1 oil.  In my mind that stuff turns to gum over time.  I always have light mineral oil on hand.  Or even the lube in Caig Faderlube is a light oil.  Even Deoxit has some lube in it.

But yeah, peel the label and a drop on the bearing.  voila.

[DrGonz78]

Though I do disagree with 3in1 oil.  In my mind that stuff turns to gum over time.  I always have light mineral oil on hand.  Or even the lube in Caig Faderlube is a light oil.  Even Deoxit has some lube in it.

I will definitely start trying out other products like the light mineral oils etc. I really like Caig Faderlube and it is my go to these days on pots. Thanks for the heads up Enzo.  :tu:

[J M Fahey]

I use a drop of medical grade vaseline,bought at the Pharmacy.

As neutral a product as can be, never oxidizes so it never turns to gum.

[mckayprod]

Like I say, this is a great forum.  Enzo, you're obviously right and I'm a dork about the parallel resistor. I can live with 25-35 watts as long as the speaker can.  And, the idea of fixing a cooling fan pretty much blows my mind (no pun intended).  I always thought of them as disposable components (they're certainly cheap enough).  Thanks for the good ideas.