Quicksilver 8417/12FQ7 mods - diyAudio
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Old 19th September 2008, 04:06 AM   #1
llwhtt is offline llwhtt  United States
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Default Quicksilver 8417/12FQ7 mods

After finding four Bogen MO-200s full of the dreaded 8417s (mostly Sylvanias) I "quickly" thought of the Quicksilvers or the Dynas. I've done enough Dynas to last a lifetime so I began thinking of a DIY Quicksilver. After searching the different boards I came across the 12FQ7 circuit debacle and the Burnedfingers (also found you on the Son of Ampzilla threads, more on that later I hope) schematics and pictures. Did anybody figure the circuit out yet? If so I didn't find it, but I have figured it out, I think. The color pictures reveal all. The input tube is connected normally, cathode to ground through 240(?) Ohms and 26.1(?) Ohms with the feedback coming in at the junction. The second half of the input tube is also grounded, the grid straight to ground and the cathode thru 200(?) Ohms and the trimmer (100 Ohms?). Both plates are DC coupled to the phase inverter tube. Both cathodes tied together and grounded thru the 18K(?) resistor. Both plates cap. coupled to the output tubes. The only unusual part of the circuit is what the second half of the input tube is doing. All it is doing is setting the undriven side of the phase inverter to a certain DC level, that is adjustable for lowest distortion I would presume. At the same time it allows the feedback from the output tube plates thru the 47pf(?) cap to its thing. Any opinions? Craig P.S. The smaller of the two photos shows where everything is grounded, just to left of the nut that fastens the terminal strip to the chassis.
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Old 21st September 2008, 08:37 AM   #2
llwhtt is offline llwhtt  United States
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Just a couple of minor corrections to my first post. The second to last sentence should end "do its thing" not to its thing. And lastly the small photo, the close up of the terminal strip shows where all of the grounds are tied together not the actual grounding point which is probably at the RCA input jack. My first post refers to the discussion between several people waaaaaay back in January of 2006, the hand draw schematics and photos from Burnedfingers were my references to figuring all of this out. Hey Burnedfingers where you at?

Craig
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Old 21st September 2008, 02:42 PM   #3
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Old 21st September 2008, 06:10 PM   #4
llwhtt is offline llwhtt  United States
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Burnedfingers,

I know it's been a few years since that Quicksilver discussion but was there ever any conclusion? I couldn't find it if there was. What did you finally end up doing? Does my explanation make sense to you? Also was interested in the GAS discussions but that is a
different forum.

Craig
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Old 21st September 2008, 06:18 PM   #5
kevinkr is offline kevinkr  United States
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I would suggest cloning something other than the Quicksilvers, while not bad the old monos certainly were not the ultimate, and being Williamsons tended to be rather unstable.

How about a circuit somewhat along the lines of a Citation II or similar? OR an MFA design or ?

Eli, SY and others may even have specific recommendations of designs you can build, mine aren't publicly available ..
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Old 21st September 2008, 06:43 PM   #6
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Craig,

There is a thread where SY ended up designing a amplifier off the Quicksilver platform. I haven't built it yet but will hopefully start on it before the year ends. I am trying to find my hand drawn schematic of the KT88 amp that I had but due to a hard drive failure it may be lost.

Kevin,

I guess each person has an idea of what sounds good. Personally I like the sound of the Quicksilver 8417's or modified 8417's. I have found them to be stable with the exception of the bias circuit.

I believe that Triode has a driver board that may be very close to the Quicksilver that they market for the Dynaco stereo 70 amps.

I never cared for the sound of the Citation amps and found them to be more complex than what they needed to be. As you well know sometimes simple designs sound better. Then again its just my personal opinion and choice.
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Old 22nd September 2008, 03:49 AM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by burnedfingers
Craig,

There is a thread where SY ended up designing a amplifier off the Quicksilver platform. I haven't built it yet but will hopefully start on it before the year ends. I am trying to find my hand drawn schematic of the KT88 amp that I had but due to a hard drive failure it may be lost.

Kevin,

I guess each person has an idea of what sounds good. Personally I like the sound of the Quicksilver 8417's or modified 8417's. I have found them to be stable with the exception of the bias circuit.

I believe that Triode has a driver board that may be very close to the Quicksilver that they market for the Dynaco stereo 70 amps.

I never cared for the sound of the Citation amps and found them to be more complex than what they needed to be. As you well know sometimes simple designs sound better. Then again its just my personal opinion and choice.

I'd say the Quicksilver is probably close to as good as it gets with the Williamson topology. I probably wouldn't build anything I or anyone else have recommended, but wanted to throw some other ideas around for consideration. Replicating the QS shouldn't be too hard provided he has access to good enough output iron.

I design and build SE amplifiers, but my 19yr old ST-70 driver design would work well enough with the 8417 - it's still on my site under amplifier projects if you're interested.
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