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Birder 30th September 2012 03:46 PM

Trouble with Aragon 8008bb
Hi, I have a Aragon 8008bb that I noticed the other day was very hot compared to normal. It is just the right channel. It runs too hot to touch after 20-30 minutes. There is no other signs of trouble, still sounds great etc.
I checked voltages on the power supply and they are 72.4 volts left ch, and 71.6 right ch.
I checked the output transistors on the right ch(at the .33ohm resistor) and it was 40mv-47mv. The left channel is much harder to get to, but I read one at 16mv. I think 16mv is close to spec for this amp, and 40mv is way too high.
I am more familiar with tubes than solid state, but can usually figure stuff out....but I am over my head on this one. The bias pots still have factory paint on them, and no mods/repairs have been done.

One possible idea: I was testing a prototype tube preamp, and one time the amp shut down(opened the relays), and then came back on? So this could be self inflicted....
Any ideas??
Thank you!!!

Chamberman 18th November 2012 02:09 PM

A little late to respond, but did you correct this issue?
The bias is too high on the right channel, this is evident in both the heat of the right channel heat sink and the lower right side rail voltage (higher loading). I would not run the amplifier with the right channel heat sink too hot to touch or you may over heat and ultimately destroy the output devices in that channel.

The factory bias spec for the 8008 ST/BB & MkII's was 8mV outer channel & 12mV inner channel. I'm guessing they had a heat problem with the outer [right] channel due to the fact they spec'd a lower bias for it. The fact they made no mV distinction between the ST and BB is baffling as the BB version has 50% more output devices yet they both use the same heatsinking. So one would think that you should be able to run slightly more bias on the ST than the BB. Anyway I would set both channels to spec and then run for an hour or more and check the temp again, then move the bias up a couple of mV. I would be surprised if you could not run at least 15 - 20% more idle bias as long as you operate the amplifier in a well ventilated location.

Birder 18th November 2012 02:39 PM

Thanks Chamberman, I did "fix it" sort of. While I started to turn the bias pot, the mv jumped back down from 40mv to about 12 mv. It was kinda weird, but I guess the pot was "stuck" at one spot until I moved it?
Anyways, the amp runs warm now and still sounds great! I did not want to lose my amp!

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