Does anyone know the reason for Mesa Boogie 20/20's 10 330 OHM resistors in series?

[jmaf]

Hi, I received a question from a customer that I couldn't answer: why does Mesa Boogie 20/20 have 10 330 OHM resistors in series without pi-filter caps in between them? Why not a single 3k3 resistor?

Here's the schematic:

Does anyone know why it was designed like that? Thanks very much for any ideas, I was puzzled by this and couldn't figure out a logical reason.

[Alex/Tubewonder]

That resistor chain dissipates ca 1.2W. You could expect a 3.6K 4W resistor there.
However, each resistor in the chain dissipates 0.12W. They put 10 1/2W resistors because they are CHEAPER than a 4W resistor and give you a 1:4 power dissipation margin.

[jmaf]

That resistor chain dissipates ca 1.2W. You could expect a 3.6K 4W resistor there.
However, each resistor in the chain dissipates 0.12W. They put 10 1/2W resistors because they are CHEAPER than a 4W resistor and give you a 1:4 power dissipation margin.

Hi Alex, thanks for the reply.

I could imagine a low cost amp industry doing this, but I didn't think Mesa Boogie would.

[Alex/Tubewonder]

Hi Alex, thanks for the reply.

I could imagine a low cost amp industry doing this, but I didn't think Mesa Boogie would.

No, they easily save a few hundred of $$$ per batch of amps, then they buy tubes in bulk for a few $ a piece which they then sell for tens of $ to you for replacements.
They make a couple thousand $$$ by putting in a chain of 10 cheap resistors instead of one power resistor. They are in business of maximizing profit. It's us small boutique amp builders who are the charity segment of amp industry. But we have more fun....

[jmaf]

No, they easily save a few hundred of $$$ per batch of amps, then they buy tubes in bulk for a few $ a piece which they then sell for tens of $ to you for replacements.
They make a couple thousand $$$ by putting in a chain of 10 cheap resistors instead of one power resistor. They are in business of maximizing profit. It's us small boutique amp builders who are the charity segment of amp industry. But we have more fun....

Yeah, we definitely have more fun... Soldering 10 identical resistors to save pennies is not my idea of having a good time.

[Enzo]

It also spreads the heat. Remember the little Studio 22 Mesa amp? Had that 1500 ohm 2 watt resistor that liked to get so hot as to unsolder itself? THis arrangement won;t do that.

A 5w resistor costs then maybe 20 cents. The half watt resistors cost maybe half a cent each. SO they would save about 20 cents per amp. The wave solder machine doesn;t care how many resistors it solders.

[jmaf]

It also spreads the heat. Remember the little Studio 22 Mesa amp? Had that 1500 ohm 2 watt resistor that liked to get so hot as to unsolder itself? THis arrangement won;t do that.

A 5w resistor costs then maybe 20 cents. The half watt resistors cost maybe half a cent each. SO they would save about 20 cents per amp. The wave solder machine doesn;t care how many resistors it solders.

This sounds more mesa-boogie-like to me than the financial explanation alone. The individual voltage drops and power dissipation are divided by 10x, thus heat dissipation is too.

[daz]

Yeah, i would not assume they would do that to save pennies, even if it comes to a few hundred dollars saved on a thousand amps. I mean, there are other ways to save much more and i don't see them going cheap on those things. Doesn't make sense. That scenario is reserved for the mass producers, and they don't just do it in one place, they do it everywhere they can find in the amp. Mesa could go a tad cheaper on pots for example and probably save 100 times what they would on that resistor theory.

[Alex/Tubewonder]

So what's wrong with saving a buck by implementing a technically sound solution? BTW the amount of dissipated power is the same, it only dissipates over a larger area thus the resistors run cooler but its contribution to temperature rise inside the chassis is still the same. But the PCB won't get scorched which is the main reason behind this. Pennies saved are an added benefit.

[Enzo]

Exactly, the chassis won;t be any cooler, but the resistors won't drop off the board from molten solder.

[jmaf]

Thanks to all who replied. Appreciate the expert advice.