Hot Rod deVille or deLuxe: 330 or 470 ohm for low voltage resistors?

[rockman627]

I've read in a few places that it is better to have 330 ohm resistors for r78 and r79 instead of the 470 ohm resistors that Fender uses. It seems to me right off the bat that the 16v supply will change, and it does. I did this change and it is now reading 34v. Is that really better? Won't components expecting 16v be stressed?

[Helmholtz]

Schematic?

[Greg Robinson]

Where have you read that? 330Ω were used in some earlier revisions which caused problems with scorching circuit boards and leads desoldering themselves, so subsequent revisions increased the value to 470Ω. Even still, this is often still a problem area for overheating, and it is best to raise these resistors away from the circuit board for better heat dissipation.
These resistors only need to provide enough current for the zeners to shunt regulate plus the current for the opamps. Decreasing their value will only result in extra dissipation in both the resistors and zeners (which should also be raised above the board for better cooling), and a minor increase in the opamp rail voltage which is of no benefit.

[rockman627]

https://bustedgear.com/images/schematics/HotRod.pdf
Frequently Asked Questions.pdf Justin Holton; differences between deville and deluxe.

[Greg Robinson]

Well, Justin, whoever he is, provides no justification or explanation as to why the 330Ω resistors are "better" (how? why?) than 470Ω. And in the schematic you linked you'll see that revision A had 330Ω, while revision B has 470Ω. So I'd say there's nothing to that assertion.
I'm sure others will be along to echo my experiences. This is a common point of failure in the Hot Rod series, even with the improved 470Ω resistors. Any further improvements from that would be to space the resistors and zeners off the board for better cooling.

[Greg Robinson]

Anyway, to provide some further information, the 330/470Ω resistors drop the +/-48V rails down to +/-16V zener regulated rails. That's 32V in either case. Ohm's law gives us P=V²/R or 32²/330=~3.1W or 32²/470=~2.2W. Both resistor values are 5W. It's generally considered bad practise for resistors to statically dissipate over half their rating, as that causes their temperate to be excessive and risks burning circuit board material, which is exactly what happens here, hence the design revision.

[Helmholtz]

Your point is very clear, same voltage drop across lower resistance means higher dissipation and resistor temperature.

[Mick Bailey]

Almost every one of these amps I've worked on has scorched areas under the resistors and Zeners. Some have serious damage. I've always installed 470R resistors where 330R were originally fitted and if there's significant damage to the board I install alloy power resistors (15W) on a small piece of aluminium plate and run fly leads to the nearest sound connection on the main PCB, as sometimes the damage is so severe as to prevent use of the existing resistor locations. The Zeners get mounted off the board by about 1/4".

[g1]

As far as the schematic goes, one is for the Deville, and one is for the Deluxe. They have different LV voltages to drop, 48V vs 33V.
So they use different value resistors to drop different amounts of voltage. The author of the article did not seem to analyze that part of the circuit much, and missed that, as well as the differences in the bias circuit (one uses a doubler).
So it has nothing to do with 'better', but rather different circuit requirements.

Aside from that, I don't disagree with the idea that the 470's are a good idea for the Deville's as well.

[rockman627]

Thanks for clearing that up guys. When I first got into guitar amp work, I got a lot of my info from books by Dan Torres,Gerald Weber and various websites .As I'm gaining more experience I'm questioning some of my initial impressions.

[mozz]

"I install alloy power resistors (15W) on a small piece of aluminium plate and run fly leads to the nearest sound connection on the main PCB"

Exactly. I do this with even the Blues Jr.