Ashdown 60 watt bass head with overheating resistor

[smackoj]

Hello again amigos; I have been finding and fixing SS and a few tube amps lately. Having good luck with most but have a bit of a resistor overheat problem with this Ashdown "Perfect 10 60watt" Yeah kind of a weird name. They have quite a few named Perfect 10 with different features and different output power. I had trouble finding the schemo for it but I will post what I believe to be a close cousin schemo. The output is from two Darlington transistors; TIP142 npn and the TIP147 pnp. When I got the amp all I found was the sister board that sits behind the VU meter had a 5w 330ohm R that had scorched the board, popped the two fuses that handle the B+ and B- pwr rails, and both output Darlingtons were toast. I replaced the Darlings, popped in some new fuses and ran it without the VU meter hooked up. It ran fine, quiet and smooth. I put a ceramic Brown Devil 300 ohm resistor in place of the Xicon one that it came with. The Xicon resistor still tested good but it was positioned with no air between the R and the sister brd and it had gotten really hot. So, tested the amp with the VU meter back up and running and all is well until I check the temps. Both of the Darlings are running cool but the Brown Devil R is too hot to touch after about 5 mins. run time. Anyone have a suggestion on what I should do other than scraping the VU meter which is nothing more than a "woo hoo" gimmick to sell amps???

[sluckey]

This could be normal. It's not uncommon for power resistors to get too hot to touch but still be within their rating. Measure the voltage across the resistor and calculate the actual power. Ideally for safety, the resistor should have a power rating that is at least twice the measured/calculated power dissipation.

If the calculated power exceeds the ration of the original resistor, suspect a problem.

[PRR]

I do not see a "VU" meter on the plan you show.

Does the 300r really power the meter? Or just a lamp for the meter? (Then get a White LED for efficient illumination.)

If the meter "needs" the 300r, post what Sluckey asked.

I am guessing you got 30V (raw supply minus some low load voltage) across 300r. 3.2 Watts. A 5 Watt resistor WILL stand that, for long enough to run-out the Warranty. As Sluckey says, YOU should double, and double 3.2 makes a 10 Watt resistor. And YOU don't want HOT resistors right on plastic or phenolic. So mount a terminal strip out in the open and put the 10W there.

[PRR]

BTW, the plan you show, there is NO protection on the output devices. (Oddly, TR12 does; so they knew better.) This amp WILL fry again, when worked in too-low load impedance or a shorted cable with decent signal trying to pass. Dunno if there is any profit in fixing what the factory considered "acceptable".

[smackoj]

Hi SL and PRR; thanks for helping out. The dc volts across the Brown Devil is 34vdc. The power diss worked out to be 3.85 watts. So, I will step up to the 10 watt resistor. I am adding a diff schemo that does show the meter and the R  values and diodes are the same, so if I could find the exact schemo for this amp, I'm pretty sure that the meter would look the same on the schemo.

Re: the output Ts not being protected. that seems pretty short sighted or maybe it's fry one, go buy another one? I had a high opinion of this company to begin with but perhaps the almighty British pound got the best of the guys over there? I thought it was curious that they printed "minimum load 8 ohms" next to the spkr jack. I was under the belief that bass amps tried to run close to 4 ohms to make more power? Anyway, here is the other schemo that shows the meter. It's not my amp but the meter looks the same although it doesn't show the odd looking small tubular red light that does the backlight for the meter.

[smackoj]

Well gents I think this one is as good as it gets for now. I went to a 10 watt resistor and it heated up quickly but not as hot as the 5 watt R. I found  an old SS main brd laying in the junk bin. It had a nice 2.5" x 4" piece of aluminum that I re-shaped a bit and then mounted it to the Ashdown chassis with a good smear of heat grease in between. I used long enough leads so I could set the big R onto my makeshift heat sink and there she sets running a lot cooler and, hopefully the amp will keep working in appreciation  I would add that this amp runs quieter than any other amp I've owned so it does have it's good points. Thanks for all your help amigos. Until we meet again, via con Dios.