Quicksilver front end mods - diyAudio
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Old 27th March 2003, 02:03 AM   #1
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Default Quicksilver front end mods

Does anyone have any experience they would like to share in regard to modifying the front end on a 8417 Quicksilver amp? I have seen the 6922 mentioned but so far I haven't seen any schematics to look at changes. Is a 6922 superior to a 12AX7 for a front end?

Joe
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Old 24th December 2003, 02:18 AM   #2
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I'm trying to figure out why folks aren't modding the 8417s to use 12FQ7s in the front end to drive KT88s/6550s?

Isn't that what Mike Sanders did when he came out with the KT88 monos? Same rectifiers, same iron, same power supply, 2 9-pin 12V tubes for the front-end and KT88s. At least that's what I've read.

Seems to me that he wouldn't have changed the front-end tubes if he could have got away with it.

I've got a pair of 8417 monos and my 8417 supply is slowly dying. I figure that it's reasonably cost effective to update the whole amp and make them KT-88 monos. I'm also starting to get some hum, so the rebuild makes even more sense.

My pair is serial numbers 103 and 104. They have the 850uf and 320uf caps on them. I was looking at reducing the 850 down to 500 or below (perhaps even trying the 100uf Solen polypropylene cans). Otherwise, I just wanted to change the outputs to 6550/kt88 (I have a quad of GE 6550As and a quad of GEC KT88s). Obviously, these tubes are far easier to find and cheaper to buy.

I've looked at the changes suggested on the Triode Electronics site, but I also saw that Mike Sanders was offering his own mod, which would use EL34s and 6922. But, after much research, it seemed to me that the 12FQ7 front-end probably works better, since you can use the same heater and just change some resistor values to get things right.

Anyways, I've been searching for the KT-88 mono schematic to find my starting point and I've had no success.

Cheers,
Jeff
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Old 24th December 2003, 11:57 AM   #3
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Hi Jeff

I probably own more Quicksilvers than most people on this site as I have 3) sets, 2) 8417's and 1) KT 88 . The KT88 version uses 2) 12BH7's in the front end. In talking/email with Mr. Sanders the KT 88 version could be purchased in a number of circuit topologies. Mine is a rare one it seems as it mimics a Marantz design instead of being the carbon copy of the Bogan amp like the 8417 was. The only thing that needs to be changed is probably the size of the resistors on pin 5 of the 8417sockets and the value of the bias pot. The front end will drive the 6550, KT88, or KT90 without a problem.

J
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Old 24th December 2003, 12:14 PM   #4
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Joe,

If you look at this page:

http://www.quicksilveraudio.com/products/vac.html

you'll see the KT-88s listed with 2 x 12FQ7s. Also, some recent recent sales on-line have KT-88 amps with the 12FQ7s.

I guess my question is which is better optimized for the KT88/6550? Seems like it would be easy to convert to the 12FQ7, which are supposed to be good sounding tubes, without radical modification.

Also, he reverted to the 12AX7/12BH7 combo for the Silver 90 monos and the latest 100 monos.

Does anyone have a preference for the sound of either combo?

Jeff
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Old 24th December 2003, 12:23 PM   #5
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Quote
I'm trying to figure out why folks aren't modding the 8417s to use 12FQ7s in the front end to drive KT88s/6550s?

Most 8417's came with the 12AX7 as the input tube followed by the 12BH7 or 12AU7 as both will work here. If my memory serves me correctly the 12FQ7 not only costs more money but has a different pin out than the 12BH7 does.

Quote
Isn't that what Mike Sanders did when he came out with the KT88 monos? Same rectifiers, same iron, same power supply, 2 9-pin 12V tubes for the front-end and KT88s. At least that's what I've read.

The KT88 version used the same rectifiers,same iron,same power supply.
I will make the assumption that they came with either the 12BH7 or the 12FQ7 in the front end because the writing on my chassis lists both tubes for the front end.
Quote

've got a pair of 8417 monos and my 8417 supply is slowly dying. I figure that it's reasonably cost effective to update the whole amp and make them KT-88 monos. I'm also starting to get some hum, so the rebuild makes even more sense.

Personally, I would purchase new caps and add some 1N4007 diodes in the power supply. This makes the rectifiers(tubes) more of a soft start function without the worries of arch over that some have experienced.

Option#2

Since I have purchased more Quicksilvers than I have room for you could purchase my KT 88's. I favor this option...LOL
Quote

Anyways, I've been searching for the KT-88 mono schematic to find my starting point and I've had no success.

I actually tried to purchase a schematic from Mike but was told Quote " You have one of the rare ones that I don't have a schematic for"
I could draw this out for you in wonderful "handcad" if you wish.

Regards
J
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Old 24th December 2003, 04:04 PM   #6
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Hi,

Quote:
but has a different pin out than the 12BH7 does.
Yes and on...It's compatible but only has a 12V6 heater.
No problem for use in the Quicksilver.

In theory the 12BH7A should be the better candidate for this application.

Cheers,
__________________
Frank
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Old 24th December 2003, 06:38 PM   #7
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Well, I guess the answer is that there is no basic preference and I should leave well enough alone!

The 12AX7/12BH7 combo stays.

Quote:
Originally posted by burnedfingers

Personally, I would purchase new caps and add some 1N4007 diodes in the power supply. This makes the rectifiers(tubes) more of a soft start function without the worries of arch over that some have experienced.
Is there any sonic penalty? My amp came with a quad of GE 5AR4s and I've never had an issue with them (even with the big dog 850uf cap!).

Thanks,
Jeff
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Old 24th December 2003, 07:33 PM   #8
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Is there any sonic penalty? My amp came with a quad of GE 5AR4s and I've never had an issue with them (even with the big dog 850uf cap!).

Jeff

I would say that using the diodes and the 5AR4's is the safer way to go
as it takes some of the burdon off the 5AR4's. I haven't noticed any sonic penalty from using them. Actually I would say the bass is slightly tighter without any being lost. Its an easy install that can be pulled off in a few minutes. Heck I will even send you the 1N4007's to do it. I have this mod on all sets of the "Quickeys" I own.

I have heard KT88's with 12BH7 and 12FQ7 front ends and I like the 12BH7 front end better. With the 8417's I have I am using the 12AX7/ 12BH7 front end. I have tried the 12AX7/ 12Au7 and I can hear a difference that I don't like.

Feel free to email for pictures or schematics.

J
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