HD Deluxe & deville + and - supplies

[Mars Amp Repair]

hey folks,
I know it's been addressed on this forum about not changing the 470 ohm resistors to 330ohm (or visa versa) on the HD Deluxe supplies.

The HD Deville has 330ohm resistors which reduces the power hence the heat produced & also has 5 watt zeners to be able to take the additional current. Does anyone have any reason why this approach could not be implemented in the Deville? I seen none.

I have been stading off the 470ohm resistors with collars on the leads in the HD Deluxe's, but feel modifying the ckt to emmulate the Deville ckt produces a better solution...thanx, glen

[Rick Erickson]

The HD Deville has 330ohm resistors which reduces the power hence the heat produced & also has 5 watt zeners to be able to take the additional current.

If you change the 470 Ohm resistors to 330 Ohms the power dissipated across the resistor, and heat generated by the resistor will increase - not decrease. Simple Ohms Law declares that if the voltage drop remains constant and the resistance decreases then current AND power will increase. [I=E/R] [P=ExI]
The Fender HR amps that were designed to use the 330 Ohm resistors have a lower feed voltage to the resistors, hence less voltage drop across them. That's the reason they run cooler. A look at the schematics for the original and the revised amps will reveal this dirty little secret. If you put 330 Ohm resistors in the version designed for 470 Ohm resistors you will cause more heat & pc board damage than if you left it alone.

RE

[Don Moose]

The Fender HR amps that were designed to use the 330 Ohm resistors have a lower feed voltage to the resistors,

So what is the original feed voltage? Would a 7815/7915 pair deal effectively?
Maybe keeping the series resistors - that would limit dissipation in the linear regs.

[Enzo]

Remember those two resistrors serve zeners, so the voltage across the resistors will be the same no matter what value they are. Well within reasonable limits, a 2 ohm resistor might change things some...

[Mars Amp Repair]

SHOOT!
I forgot the zeners hold the 'grounded' end of the resistors at a constant voltage...my BAHHHHD!
Then I'm thinking that the actual current draw on the + & - 15V supplies is probably negligible as I think it only feeds the op amps & perhaps those tiny relays.

I guess the next approach might be to increase the value of these dropping resistors to a point that will not allow the supplied voltage to the zeners to drop below (or even close) to the zener thresh hold voltage with a full load, but in supplying less current to the zeners and consequently consuming less power & hence, getting less hot!

I've got a bunch of 750ohm 7watt resistors with flat metal pc mount standoffs that might just do the trick.

I guess keep using 470 ohm resistors but hike the wattage to 7watts might work, too. It depends on what kind of trouble it would be to find some.

As I mentioned I have been standing the 470 ohm resistors off the board with little metal standoffs. I scrape the printed traces & extend the soldered leads about 1/8" on the traces. It seems to dissipate some of the heat..certainly more than the original fittings.

Has anyone experimented with higher value dropping resistors? I'll have to try it. I agree that adding the 15V reg IC's would probably work. Definitely a better solution than the brute force design so many of these amps have. glen

[Enzo]

I have not tried larger values, but I do remove the coating from the traces and lay down the resistor lead wire on top of the trace as far as I can, like a half in or so. Run a bead of solder along that. That acts as a small heat sink and helps dissipate the heat into a larger trace area.

Raising the resistors off the board a little ways allows more lead length exposed to act as heat radiator and allows more air around the parts. A little glob of silicone betwixt them adds support.

[Alex R]

I have a roll of lead-free high melting point solder; if anything has melted a solder joint I turn up the iron and repair it with that, as well as getting more air around it etc. Then no doubt the board will catch fire before the joint fails...

I suppose the ultimate solution without a new transformer would be a pair of alu-clad resistors bolted either side of a heatsink. V-regs would need heatsinking too but they could be fed a lower voltage if you could change the tranny (has to be more than 2v above your target, I think I recall)

Mars Amp Repair: I was half way through a post before I remembered about the zeners, you're not alone. But I think that the voltage would want to drop below the zener threshold pretty early on the curve if you started bringing up the value of the resistors, as the drop across them started to eat into the above-threshold voltage the zeners need to keep a steady 15v.

[Enzo]

The zeners need enough current to zene.

Are we confusing solutions together? I thought the point of a three leg vreg was to eliminate the need for large dropping resistors. If you are going to mount heavy resistors off board anyway, why not leave the zeners? They regulate well enough for this application, and at this point the resistor is no longer an issue. You wouldn't need a heat sink with the proper resistor. Those Dale and similar aluminum resistors will dissipate just fine on their own at this level. They have their own mounting holes, and the chassis wall is fine for the purpose.

[Mars Amp Repair]

Yeah I agee Enzo,
But what I'd be looking for from a repair shop trying to make a living, would be a relatively inexpensive & relatively quick & dirty effective solution. I'd have to look into what input volages the 3 legged regulators can take, but I think that could be a decent inexpensive & relatively quick solution to this issue too. You could easily drill 3 holes in the pcb with a pin vise type device & mount the regulator. The time consuming part would be if you needed to mount it to the chassis for heat sinking. Adding a small heat sink would also be a decent solution, but you'd have to find a source for cheap heat sinks designed to mount on a pcb. Then there's securing the sink to the pcb.

Anyway I don't particularly like experimenting with a solution on a customer's gear. I'd need an amp that I could hold on to for a while to be certain there wasn't something I was missing.

I think I'm with Enzo, find a solution with the components that the circuit was originally designed for by just changing values. I'll try the higher value dropping resistors on my next unit & let you know.

I think because the supply voltage is so high to begin with as well as the current draw on the circuit being so low, there won't be much of a chance to have the zeners stop zenering (that a word?!)

glen

[Alex R]

Enzo's Dale resistors bolted to the chassis on flying wires idea sounds quick 'n dirty to me.