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#1
Posted to rec.audio.tubes
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Harman Kardon A500
Hi, I recently bought a HK A500 from ebay and would like to know
something about this amp... 1- I noticed that it´s very weak in power, I use it with a pair of JB L Jubal, I have to listen beyond the midlle (1 or 2 o clock) to hear in a good volume, is it right? 2-One transformer was changed, the channel B, how can I see if the job was well done? 3-I noticed that at this channel B when I turn the balance to only this side, the treble increases, what can be the problem? 4-the ambience function is not working, what it is the funcion of this? How can I repair? 5-there are two OUTPUT BAL controls near the tubes, one for each channel, what are their purpose? Thank you all, sorry for all the questions. Murillo |
#2
Posted to rec.audio.tubes
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Harman Kardon A500
First, what, if anything, have you done to it since it arrived to your
care? Then: Note the interpolations: wrote: Hi, I recently bought a HK A500 from ebay and would like to know something about this amp... OK. A generally solid low-powered, full-featured vintage tube integrated amp. Not terribly common. 1- I noticed that it´s very weak in power, I use it with a pair of JB L Jubal, I have to listen beyond the midlle (1 or 2 o clock) to hear in a good volume, is it right? 25wpc/rms assuming all other things being good, from capacitors to tubes. 2-One transformer was changed, the channel B, how can I see if the job was well done? Does it work correctly? Was it replaced with an OEM type, an after-market purpose-picked substitute or a parts-box leftover? How clean is the work? 3-I noticed that at this channel B when I turn the balance to only this side, the treble increases, what can be the problem? And the same thing _Does Not_ happen when you go the other way? As noted before, could be tubes, capacitors, drifted resistors, or the wrong output transformer, or simply dirt on the controls. 4-the ambience function is not working, what it is the funcion of this? How can I repair? Schematics he http://intra.engr.uark.edu/~lar/fireamps.html as well as many other places. Could be caps, tubes, dirt or a bad Packaged Circuit. 5-there are two OUTPUT BAL controls near the tubes, one for each channel, what are their purpose? The description lists this unit as having continuous balance from Mono to Stereo. I suspect these controls are for that purpose. Some sort of accomodations to exaggerated stereo sources were built into many early integrated and pre-amps well into the 70s. Most did it with a single blend (separation) control, however. Thank you all, sorry for all the questions. Don't be, this NG is pretty much for such questions despite rumors, vendettas and diatribes to the contrary. Now, some CAUTIONS and WARNINGS: CAUTION: Lethal voltages exist inside most tube equipment. Unless you know what you are doing and are experienced working around such voltages, DO NOT mess around inside beyond changing and testing tubes and cleaning the controls and setting the bias. If you choose to clean the controls, do so only with the unit COLD and UNPLUGGED. WARNING: Most vintage equipment such as this (40+ years old) may need its capacitors replaced, it may have drifted resistors and it may be out-of-bias. Your unit has adjustable bias controls, you need to find a manual and/or good source of information as to how it is adjusted and do so, first thing. It may be written somewhere on the unit, there may be some means to do it 'by ear', but whatever it is, do it. With caution, of course. LAST OBVIOUS WARNING: Do not operate the unit (excepting testing, of course) with the cover off. Tubes get very hot apart from the voltages. Small children, careless adults and pets may be injured. Even though this unit is described as being 'easy' on tubes, it remains quite old and tubes do wear out. You should test both the output tubes and the small-signal tubes even if only for emissions. I know of no direct substitutes for the 7355 but they are available... not cheap. The small-signal tubes are generally cheap and common. Good luck with it. Take your time and be careful. Peter Wieck Wyncote, PA |
#3
Posted to rec.audio.tubes
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Harman Kardon A500
Hi Peter, thank you very much for all explanations. I´ll try to look
at all that you mentioned. Murillo |
#4
Posted to rec.audio.tubes
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Harman Kardon A500
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#6
Posted to rec.audio.tubes
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Harman Kardon A500
Brian wrote: Your amp should be quite loud! It's a very good sounding amp. Depends on the first, yes on the second. That amp into an AR3a, for instance will give substantial volume, but will not be "loud" with a very dynamic source without clipping, it is just a little thin for very inefficient speakers. However, I suspect that the OP needs to do some maintenance and perhaps repairs before he will realize the full potential of this amp. Peter Wieck Wyncote, PA |
#7
Posted to rec.audio.tubes
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Harman Kardon A500
wrote in message
oups.com Hi, I recently bought a HK A500 from ebay and would like to know something about this amp... 1- I noticed that it´s very weak in power, I use it with a pair of JB L Jubal, I have to listen beyond the midlle (1 or 2 o clock) to hear in a good volume, is it right? Volume control position versus loudness is a poor test of power output. 2-One transformer was changed, the channel B, how can I see if the job was well done? If the transformers are visibly different, then it wasn't an exact replacement. Note: this was not one of HK's best efforts, even in the days of tubes. The OPTs rolled off the bass. 3-I noticed that at this channel B when I turn the balance to only this side, the treble increases, what can be the problem? Who can tell? There's a reason why I have test equipment on my bench, and looking pretty ain't it. 4-the ambience function is not working, what it is the funcion of this? How can I repair? The schematic posted here http://intra.engr.uark.edu/~lar/fireamps.html Says that there is a separation control, but no ambience control. It's basically a sliding stereo/mono switch. It's effect might be subtle enough that you're not hearing it with speakers. 5-there are two OUTPUT BAL controls near the tubes, one for each channel, what are their purpose? I seem to recollect that you are supposed to adjust them with a test tone and a THD analyzer. Got either? |
#8
Posted to rec.audio.tubes
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Harman Kardon A500
On 29 Oct 2006 16:59:01 -0800, "
wrote: 5-there are two OUTPUT BAL controls near the tubes, one for each channel, what are their purpose? Based on the link that Arny posted, these are DC balance trims. Unfortunately, the circuit provides no convenient place to measure. There are a couple of clumsy ways to do it, if you're bound and determined: 1) You could measure the OPT's primary resistances (cold, of course), measure voltage drops across them, calculate plate currents, and adjust the currents to be equal. 2) You could make a pair of plug/socket combo's with cathode current sensing resistors built in. 3) You could permanently insert a small resistor in each cathode lead, between pin 5 and the trimpot, and measure across them. The purpose of the controls is to balance standing primary currents, which minimizes large-signal distortion in the transformer. So you could also 4) Measure full power distortion at a low frequency, maybe 40 Hz, and adjust for minimum. This often proves to be fairly confusing, though, even with the needed equipment ready to hand. All good fortune, Chris Hornbeck |
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