Ampeg SVT 5Pro Schematic

[Bassboy49]

OK, sorry Jazz P Bass, I missed your suggestion about checking around the tube for a drop in voltage. I did that today and could not find any drop in voltage. The intial voltage spike, when I turn the amp on, appears to be around 210 and then after a few seconds, the tube lead drops to around 180. That appears to be consistent on the three leads that have voltage.
I don't think this is a problem, but, on the tube PCB there are two solder points that do not have anything soldered to them, one comes from "resistor"(?) 132A and the other doesn't appear to go anywhere. It's marked OPT4A. Is something missing here (broken off?)? There is another one of these marked OPT2A on the clean channel PCB and it does not have any visible component soldered to it either.
All the PCB's had some stuff that I cleaned off around solder points. When I put my testor around these areas where the crud was, (checking for continuity) I found some "bleeding" through the crud. The crud looked like hardened caramel. Quite a bit around the power transistors and the voltage regulators.
Finally, I have checked Radio Shack and they do not carry a 7815 here, they have only 7805 and 7812. I'll check with my guy at the music store tomorrow. Thanks again, I think I'm getting there.

[Grant_More]

Hi Guys,

Have a look here for a debug of this issue: YouTube - Ampeg SVT-5 Pro Problem

I recon two fixes are possible:

1. (Easy) increase the size of the heatsink on IC4.

2. (Hard) adjust the turn-on temperature of the fan by adjusting the control circuit.

I'm going to try 1, I'll let you know how I get on.

Cheers,
Grant

[Bassboy49]

Thanks, Grant

Your video was very educational. I'm presuming that when you suggest increasing the size of the heat sink on IC4, you mean replacing the "doo-hickey" that slips onto IC4 with a larger one? Awaiting your experiment results. Thanks again.

[Grant_More]

Hey Jeremy,

"you mean replacing the "doo-hickey" that slips onto IC4".

Yes, that's exactly it. The slip-on thing is the heatsink and its job is to get the heat out from IC4 and transfer it to the air inside the amp case, cooling IC4 in the process of doing so.

I think the Ampeg design team have calculated the heatsink requirement without taking into account a couple of important factors and have hence fitted a heatsink which has too high a thermal resistance (i.e. is not good enough at getting the heat out of IC4, causing it to overheat in still air (when the fan’s not running)).

Let me explain.

We know that IC4 is getting too hot with the stock heatsink in still air (no fan) so we need to decrease what's called the thermal resistance between the silicon chip die (the working electronic bits) inside IC4 (which we call the "junction") and the free air inside the amp case (which we call the "ambient”).

We can calculate the thermal resistance of heatsink we need in still air (no fan) based on the power which is being drawn through IC4 and the maximum junction temperature of IC4.

The maximum operating junction temperature of IC4 is 125degC as per the manufacturer datasheet. At some point above a junction temperature of 125degC, IC4 will sense an overheat and shut down. If we keep the junction temperature of IC4 at 125degC or less, it will never shut down and our problem is solved. Great!

Let’s assume the maximum ambient temperature (Ta(max)) inside the amp case is 40degC when we turn our amp on. At this heatsink temperature the fan will most probably be running, but to be safe let’s assume this is the turn-on temperature of the fan and it’s possible it won’t run until this heatsink temperature is reached.

This means that the maximum allowable temperature (Tja(max))difference between Tj(max) and Ta(max) is 85degC in still air (no fan). So, with maximum power dissipated in IC4, we’ve got to rate our heatsink to ensure that we can get rid of enough heat to ensure this temperature difference is never exceeded.

How much power is dissipated in IC4? Well, we can consider IC4 to have a maximum rated current of 500mA and the voltage difference of 22-15V, so we’ve got 500mA running through IC4 with a 7V voltage difference. That’s 3.5W of power dissipated in IC4.

We’ve got 3.5W dissipated in IC4 and a maximum Tja(max) of 85degC, hence our thermal resistance between junction and ambient (this is formally called Theta JA, but we’ll call it Rja here to keep it simple) can be a maximum of 85/3.5 = 24.3degC/W.

Now, there are several thermally resistive interfaces between the junction and the ambient and we must consider the thermal resistance of all of them when calculating the heatsink required. The interfaces here are:

1. Interface of junction to case (Rjc)
2. Interface of case to heatsink (Rch)
3. Interface of heatsink to ambient (the air) (Rha)

From the manufacturer’s datasheet for IC4, the interface of junction to case has a thermal resistance (Rjc) of 5degC/W.

The next one (case to heatsink) is debatable. Ampeg have been cheap here and haven’t applied any thermal transfer compound between IC4 and the heatsink and hence I think this is a non-trivial value. I’d guess 5degC/W without the compound. In any case, we’re going to apply heat transfer compount to the new heatsink, so let’s be conservative and assume a 1decC/W thermal resistance here with the compound applied.

Finally, we have the thermal resistance of the heatsink, which is what we’re trying to calculate.

So, Rja = Rjc + Rch + Rha. We want to know Rha, the thermal resistance of our heatsink to ambient.

Rearranging gives us:

Rha = Rja – Rjc – Rch.

Filling in the numbers gives us:

Rha = 24.3 – 5 – 1
Rha = 18.3degC/W

This is the magic number we need for our heatsink thermal performance, assuming we use thermal transfer compound.

Interestingly, if we don’t use thermal transfer compound, using my “5degC/W without the compound” assumption above, the heatsink would need to have a Rha thermal resistance of

Rha = 24.3 – 5 – 5
Rha = 14.3degC/W

Guess what, the heatsink which Ampeg have used has a thermal resistance (Rha) of 21decC/W (see Farnell part number 171-0625). Too high a thermal resistance – IC4 may overheat under the conditions described! ;0)

I think they’ve forgotten to take into account the thermal resistance of the IC4 case and the interface between the case and the heatsink. Hey, everyone makes mistakes – it’s such a great sounding amp, I don’t mind doing their heatsink maths for them! =0)

The great news is that Farnell make a heatsink which has an identical footprint to the stock heatsink, but is longer and has a thermal resistance of 16degC/w. See Farnell part number 171-0628.

I think we can pop the old heatsink off (it just slides off once the glue is released, cut it with a knife) and pop the new heatsink back on, of course with a light smear of thermal transfer compound (it’s white stuff – from any electronics store or from Farnell part number 317-950. A very light smear is all you need, don’t glug it on or it will act as an insulator and increase the thermal resistance.

So, if I’m right here – we have a sub £5 ($10) fix which will take ten minutes and hopefully completely fix our amps.

I’ve got a couple of these heatsinks on order from Farnell due Tuesday, I’ll let you know how I get on.

Cheers,
Grant

[Grant_More]

Hey Guys,

Cross-posting here. I think we've managed to wrap this one up over on post t17092. Take a look.

Cheers,
Grant