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Old 2nd October 2009, 02:05 PM   #1
surfstu is offline surfstu  United Kingdom
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Default Easy question for experienced tubies

I have a heathkit s99 integrated amp, I have posted the schematic for s88 but I feel its very similar. I think I'd like to scrap the preamp section for a cleaner sound. In the middle of the schematic is a clear marking for volume pot, left and right. Can I just feed my input signal direct from source, using original pot to control volume and therefore making this unit into a power amp only? If so can i Just run the amp without the preamp valves or will this put unusual stress on th HT supply?
I have heard that the s99 used relativley good quality output transformers in this model,
any help would be much appreciated, also if someone could point out essential caps to replace with a view to priority as I will not be replacing them all initially,
thanks in advance to those more in the know than I
stuart
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Old 2nd October 2009, 02:54 PM   #2
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On this one, it looks like you could use the tape outs as power amp input. Number one priority for caps would be output coupling caps. Power supply electrolytics are likely bad too...
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Old 2nd October 2009, 03:01 PM   #3
surfstu is offline surfstu  United Kingdom
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Thanks, which ones are the output coupling caps?
and can i go straight into the volume control instead of the tape ins, this way i can keep the volume
cheers stuart
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Old 2nd October 2009, 05:51 PM   #4
surfstu is offline surfstu  United Kingdom
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I tried going straight into tape record, very low volume and not to clear either, is this the same as wiring it straight into the volume pot, i must admit that it looks like it on the schematic, also could someone point out which are the coupling caps? I presume they are the ones taken from the anode into the next stage to block dc, is this correct? If so i can see 4 of them in the power output section
cheers stuart
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Old 2nd October 2009, 06:42 PM   #5
tomchr is offline tomchr  United States
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The coupling caps would be the two 1 uF caps from V4 to the output and the 50 nF (0.05 uF) caps from V3 anode to V4 grid.

~Tom
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Old 2nd October 2009, 06:51 PM   #6
surfstu is offline surfstu  United Kingdom
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cheers Tom, any thoughts on just using this amp as a power amp, i.e. no preamp section?
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Old 2nd October 2009, 06:56 PM   #7
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Yes, they're the caps you're looking for... perhaps the S-99 has a different tape output section? A bad balance control will drop gain considerably... does that seem to work?
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Old 2nd October 2009, 08:28 PM   #8
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Quote:
Originally Posted by surfstu View Post
cheers Tom, any thoughts on just using this amp as a power amp,
Don't expect more than 7W per channel. The pentode section is a good sounding tube and the global nfb loop isn't frequency selective..(on cheap stuff the tone controls were often used in the nfb circuit often upsetting stability). That's a good sign. Instinctively at some time replace every electrolytic and wax oozing coupling cap in sight.
Any views on this next one...The selective treble cut circuit V2B (pent) ECF80 ? I'm tempted to not use this by coupling the vol outer via cap to the anode of the ECF80 triode anode sect. If you want the tone control then this tube has to stay as it is part of the Baxandal tone circuit. I presume the selective treble/ filter cut was historical from Vinyl days. Opinions up for grabs.

richy
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