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Overheating resistors in guitar amp

mr fixit 52

Jul 9, 2021
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Help! I recently found an overheating issue with 2 5w 330ohm ceramic resistors in my guitar amp(known issue) which was causing crackling,distortion and lack of volume, even when running the amp on the clean channel.
Problems identified were discoloured resistors,loose connections and lifted tracks on PCB. Tracks I have repaired and 2 ceramic 10w 330 ohm have been fitted with heatsinks and thermal paste and a 5mm gap under each to improve air flow,which has fixed the unwanted noise issue and amp will run well but still a lot of heat generated.(I set up a test rig(see pic) which included a 120mm Arctic pc fan,driven by a 9v battery,which provided a further significant reduction in temperature.)
I now have 2 10w 330 ohm Arcol aluminium housed resistors,which I am intending to mount off PCB on a heat sink with thermal paste then bolt direct to chassis.Will that significantly reduce temperature? Problem is,I only have an area of 70mm x 220mm between PCB and rear of chassis to fit heatsinks.Most heatsinks I have seen are too big to fit.Any suggestions? If I need to install a fan,then I will be needing a 230v to 12vDC(fan current voltage 0.16A or 0.20A),any suggestions on a suitable power supply?
1 resistor is handling +42v input, +16v out and resistor 2 is handling -42v input, -16v out.Any advice on successfully resolving this issue will be greatly appreciated and I wont be binning the Amp!
Many thanks
 

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Harald Kapp

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Do you have a circuit diagram or trimming instructions? The issue could be due to a higher than normal idle current of the output stage.
 

VenomBallistics

Aug 30, 2018
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Yours is an issue I've just come to accept about solid state guitar amps. I've killed and resurrected many before I started designing and building tube heads.
I'm spitballing here as we do not have a schematic to confirm. such resistors are usually employed in the output stage of a class B amp where parallel transistor pairs are used. and they are usually of smaller value, like 33 ohm. The purpose is to limit current so that it better balanced across devices. Otherwise, the most enthusiastic of the transistor set will hog the current, self destruct, then shift load to the next victim.
That said, I'm going to guess that there's some other issues in that amp causing the heat problems you are having.
Further spitballing ... something in the bias generator and or it's thermal compensation is letting your OPS run away.
 

Audioguru

Sep 24, 2016
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42V input and 16V output wastes 26V and produces a current of 79mA in the 330 ohms resistor. It heats with 2.05W. A 2.5W resistor will get very hot.
 

mr fixit 52

Jul 9, 2021
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Do you have a circuit diagram or trimming instructions? The issue could be due to a higher than normal idle current of the output stage.
Thanks Harald,Yes I have a pic (attached) of part of circuit diagram showing the power stage.I also replaced Diode D58 which had blown,I assume through heat,which in turn caused the PCB track to lift.While identifying issues,a capacitor C39(not shown) was also replaced,which is part of an FX loop connection PRE OUT,output to FX device input.20210710_080044 -2.jpg 20210710_080044 -2.jpg 20210710_080044 -2.jpg
 

Harald Kapp

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These resistors together with zener diodes (R144 + D57 and R145 + D58) form very primitive voltage stabilizers. Dropping 26 V they will of course become rather hot. The heatsink is probably the best you can do while preserving the circuit.
Alternatively you could replace this stabilizer circuit by a switch mode power supply, but that is a major modification and may introduce other issues (switching noise).
 

mr fixit 52

Jul 9, 2021
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Thanks Harald. Before I started on this repair,I was advised this was a rather crude form of voltage stabiliser.Amazingly,the manufacturers have continued to use the same ceramic resistors in a number of their products for 20 years+ without change! My model is on a 'Do not repair list' in the USA and the companies I have been in touch with in the UK (with a view to fixing it) didn't want to know. I intend to keep it for personal use,so I think it is worth attempting a fix.Do you think adding a fan is worth the effort? If so,can you suggest a suitable 230v-12vDC power supply? The fan is an Arctic F8 TC 80mm pc unit,which has a built in thermistor,current voltage 0.09A/12vDC. Excellent fans,in my experience and they are quiet and reliable in use. Many thanks for your valued advice.
 

Harald Kapp

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A fan will help, but unfortunately I don't know which one to recommend.
 

bertus

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Hello,

As regulators, you could also use a LM317 tracking regulator for the positive side:
LM317_tracking_regulator.png

The adjust to ground resistor should be 1500 Ohms for 16.8 Volts output.
For the negative side, you could mirror the circuit using LM337 regulators.

You should use this configuration as the input voltage is more than 40 Volts.

Bertus
 

mr fixit 52

Jul 9, 2021
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Hello,

As regulators, you could also use a LM317 tracking regulator for the positive side:
View attachment 52304

The adjust to ground resistor should be 1500 Ohms for 16.8 Volts output.
For the negative side, you could mirror the circuit using LM337 regulators.

You should use this configuration as the input voltage is more than 40 Volts.

Bertus
Thanks Bertus,much appreciated
 
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