Ask Amp Man: Playing It Cool with a Hot Rod Deluxe
Jeff addresses reverb dropouts and volume drops or fluctuations potentially caused by failure of a Fender Hot Rod Deluxe's low-voltage power supply.
I’m sure you’ve heard it all before: channel-switching problems, the reverb drops in and out, unwanted distortion, and volume fluctuations in a Fender Hot Rod Deluxe. I bought mine used about three years ago and enjoyed two glorious, trouble-free years of music making. Then all the aforementioned problems started to happen. Plus the jewel light flickers and randomly goes dim.
The internet forums—bet you love those—point to a pair of 5-watt 470 Ω resistors. From what I can tell, the general consensus is to replace those with 330 Ω resistors and keep them off of the board with silicone.
Okay, I did all of that. These resistors heat up like crazy, supposedly wreaking all kinds of havoc, such as cold solder joints and burned PC boards. I can attest to the PC board. Now I understand that while the 330 Ω resistors solve some problems, they heat up even more. I know that you’ve got the answer because you have a column in the best magazine around!
Thanks for reading and for your kind words. Per your assumption, yes, I’ve heard it and seen it all before. Okay, probably not all, but definitely this. The two resistors, designated R85 and R86 in the “Blues” series and R78 and R79 in the “Hot Rod” series, are part of the low-voltage power supply in these Fender amps.
While these Deluxe models are functionally tube amps, there is some degree of solid-state in the signal path. Not like modeling, profiling, or sampling types of amps, but in a much more basic way. All of the clean, crunch, and overdriven tones in these amps are generated by the good ol’ vacuum tube, but there are a couple of places where a solid-state device is used.
One of these locations is the effects loop. Here, an op amp is used to buffer the signal prior to the effects loop send and another is used as a recovery preamp for the effects loop return. Another area is the reverb circuit: An op amp is used to generate the send signal to the reverb tank, and another handles the recovery of the reverb signal from the tank. Also, switching ICs (integrated circuits) control the amp’s FETs (field-effect transistors) and relays.
The reason I mention all this is that these solid-state devices are powered by the amplifier’s low-voltage power supply. And guess what? The low-voltage power supply is derived through the two resistors in question. So yes, issues like reverb dropouts and volume drops or fluctuations are all potentially caused by failure of the low-voltage power supply. Let’s take a look at the potential causes for this failure and what I’d recommend doing to remedy it.
The two resistors in question. They can heat up and cause solder joint failure, wreaking havoc on the amp.
It’s true that the final low-voltage outputs, +16VDC and -16VDC, come through these resistors. In a typical scenario, when you use a resistor to drop voltage—such as a plate resistor in a tube circuit—the smaller the resistance value, the less voltage drop across it. Think of a decreasing resistor value as coming closer to a piece of wire. A short piece of wire will have virtually no voltage drop.
The original resistor values in your amp were 470 Ω, and you mentioned a suggestion to replace them with 330 Ω resistors. Conventional wisdom dictates that with less voltage drop across the resistors, they should actually run at a cooler temperature because any voltage that’s dropped across the resistor is converted into heat.
But you say that the 330 Ω resistors actually run hotter, and here’s why: These resistors do not immediately or directly feed the low-voltage circuit. The power supply uses two Zener diodes on its output. These diodes “clamp” the output voltage at a particular point—in this case, 16 volts. According to the schematic, the “input” voltage on the original 470 Ω resistors is 48 volts. The output voltage is clamped at 16 volts. The voltage drop across each resistor is 32 volts. Using basic Ohm’s Law, voltage drop squared divided by the resistance will give us the power being dissipated across said resistor.
In this case, it’s 2.17 watts. If we do the same calculation with the 330 Ω resistors, we come up with 3.10 watts. Almost a full watt more! So yes, you are correct, they will run hotter. They are, of course, still within their power rating of 5 watts, but the downside is that the hotter they run, the more potential damage they will do to the circuit board ... especially because they are not in an open-air environment.
I’d guess this forum repair suggestion references the fact that the other amps in this family (the Blues DeVille, Blues Deluxe, and Hot Rod DeVille) all use 330 Ω resistors here. However, the reason is that they’re all fed with an initial DC voltage of approximately 33 volts, not 48, which allows them to run with a cooler dissipation of less than 1 watt.
My suggestion would be to install the correct 470 Ω resistors and keep them as far away from the circuit board as possible. You also need to check the integrity of the traces on the board as well. These boards are constructed with very thin, single-sided traces, with no plated-through holes, and this makes them very susceptible to damage, both from overheated components, as well as simple servicing. In instances like this, I’ve carefully scraped the solder mask from the traces for a good distance and added an extra-thick layer of solder to reinforce the trace. It not only helps to stabilize the connection, but also helps to dissipate the heat at the leads.
Here’s hoping the only extra heat coming off your Deluxe is from your playing!
Warning: All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.