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A few Adcom GFA 555 questions

mixsit

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#1
I've been running this original version of the amp since new -(1985 to 90 perhaps?) and wanted to ask a few questions.
First to it's continued safe use given it's age and concern for some very nice speakers I would hate to see harmed.
Second is simply might there be possible sonic improvements to be had from other amplifiers? I don't hold any particular faults with the amp, but nor have I used anything else in years, plus I wouldn't mind having a bit more headroom occasionally available for the larger of the speaker sets.
Thanks
 

SIY

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#2
I have one from the same era ('85). All it's ever done is work. You can find modern amps that have better measurements, but they won't sound any different.
 

Sal1950

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I'm running 3 GFA545 II's and 2 535 II's in a multich system, none have failed over that last 2 years of heavy use. Internal inspection and cleaning of all revealed no repairs ever done, very minor drift of bias easily tweaked to spec.
Do to age you can be sure there's a chance of failure but I've seen lots of much, much newer amps let the smoke out.
You might look into having it recaped if you want to spend a few bucks.
I doubt you'll ever find anything audibly better in a DBT.
 
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mixsit

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I did the 'bias drift check a few years ago on the output lugs to be ok -per some instructions I found at the time. (I don't recall the number, nor find the source I used at the time.) I assume some components degrade in time correct?
Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Thanks
 

Sal1950

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Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Very, there's little protection outside the rail fuses.
 

SIY

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I did the 'bias drift check a few years ago on the output lugs to be ok -per some instructions I found at the time. (I don't recall the number, nor find the source I used at the time.) I assume some components degrade in time correct?
Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Thanks
Did you measure the bias by measuring the voltage across the emitter resistors?
 
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mixsit

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Did you measure the bias by measuring the voltage across the emitter resistors?
No, it was mVolts at the speaker terminals with the speakers disconnected.
add.. My recent unsuccessful search to find the source I used I came across some things that were similar and perhaps incorrectly picked up the term 'bias check.
 
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Doodski

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#9
No, it was mVolts at the speaker terminals with the speakers disconnected.
Then you measured the, "Idle/offset voltage." That's very different than the, "Bias Adjustment." You need a service manual so you can see where and how to set the bias and the idle voltages.

WARNING: The power supply has some rather large capacitors that can cause damage to yourself in the form of electrocution or even broken bones from the shock. So you need to be veryyy careful you don't touch those caps terminals while you are working on the amp.

Service manual>
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Adcom-GFA-555-Service-Manual.pdf
 
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mixsit

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I wouldn't feel competent enough to do internal tests. Perhaps I should skip to the question; Should the amp, this age, be shipped to someone for refurbishing?
There is one other consideration or option. I have two sets of SP Tech speakers. What I do now for one or the other is swap the speaker line pairs at the amp. I mentioned upping the amp size. I could do that and reserve the Adcom for the smaller set once it's been serviced.
 

Doodski

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I wouldn't feel competent enough to do internal tests. Perhaps I should skip to the question; Should the amp, this age, be shipped to someone for refurbishing?
There is one other consideration or option. I have two sets of SP Tech speakers. What I do now for one or the other is swap the speaker line pairs at the amp. I mentioned upping the amp size. I could do that and reserve the Adcom for the smaller set once it's been serviced.
It is a nice amp. Worthy of a proper recalibration and torture test. I'm not sure what refurbishing would entail but I see big bucks for handfuls of capacitors and labour. It might not even need parts replacement. If you are feeling like a DIY project do it yourself. It's not rocket science. Having a proper sized soldering iron, solder, solder wick and a solder sucker tool from Kester. Just detail all the electrolytic caps that are used, order them in a high quality spec/brand and replace them one at a time. Take a couple days/afternoon and voila you'll have refurb'd it. Otherwise find a decent tech and pay.
 
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mixsit

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Thread Starter #12
There is a fellow named Jim Williams of Audio Upgrades that has been doing services for various pro audio gear including some Adcom amp upgrades. He may be closer to retirement these days(?) but I believe I'll give him a call and ask.
I appreciate all the folks here helping me out by the way. :>)
 

Sal1950

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