Fender HD Deluxe -Update on using different dropping resistors

[Mars Amp Repair]

OK folks,
I had another HD Deluxe in for service & decided to experiment with a higher value of dropping resistor for the + - 16V supplies.

I had mentioned before that I really didn't believe that the circuitry on the 16V supplies drew enough current to ever fan out the supply as much current as they designed the zeners to operate at with the 470ohm dropping resistors. Especially, as it appears that the supplies only feed the relays & the few op-amps in the preamp.

Well I have a bunch of nice 750ohm 7watt cement resistors that have built in stand-offs & decided to give it a try.

I monitored the +16v and -16v supplies while switching the channels & such. I also lowered the ac line volage to a bit below 100Vac & did the same.

As suspected the zeners never broke their zenering voltage (for lack of the correct term) & held just fine. The resistors drawing less current as well and being of a higher wattage only just got warm.

I have attached digit pix. Looks like it was made that way. glen

[Amp Kat]

Well will it be enough though because you went from 102 ma's dissipating 4.8 watts to 64 ma's at about 2.8 watts so you reduced your heat but cut the current in half basically. I didn't catch the problem of the amp but as long as it works ok and doesn't wreak havok with the TLO-72's you may be ok. Did you check the outputs of the op-amps and see how close they are from their nominal levels ?

[jrfrond]

You guys DO know that Fender solved this issue and issued a TSB years ago, right?

[Enzo]

Glen, it isn't that they won't zene at the lower current, it is an issue of how well they will regulate. Zeners don't have a hard crisp right angle curve. The real test is what happens to the 15v rails when high signal levels and power outputs are flowing through the amp. Are they stable or do they wiggle around?

[Mars Amp Repair]

to answer JRfrond, Fender's solution of reducing the dropping resistors to 33ohms has been pretty much universally discounted by most techs.

If the dropping resistors were feeding a resistive load, then the voltage across the 330ohm replacement would drop thereby dropping the power dissipated by them.

In the case of feeding zeners, the voltage across the 330ohm remains the same & the current the zeners are sinking gets higher as well as the current through the resistors increases. Hence, the resistors still get too hot & the zeners are now sinking much more current.

Part 2 from Enzo; Yes I did put the amp through it's paces & the zener voltages remain steady with in about 200mv even at line voltage of ~100vac. I just couldn't imagine that a couple of Pre-amp opamps & 2 small relays could possible draw anywhere near the current those dropping resistors were feeding to the zeners as to swamp out the supply.

Now I'll qualify this whole thing & say this is the first one I've tried this on. We all know there always seems to be an exception out there & I certainly can't try the amp in all situations it will be exposed to. But that being said, initial indications are this works fine.

glen