Quick Quicksilver preamplifier question

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Posted on November 10, 2013 at 07:43:40

Lew
Audiophile

Posts: 11012
Location: Bethesda, Maryland
Joined: December 11, 2000

I have a vintage Quicksilver full function preamplifier.
The linestage uses a 12AU7 for input gain, followed by a 12AX7 for yet more gain, because that is followed by a 12FQ7 cathode follower, the cathode of which also gives NFB to the input tube, thereby using up a lot of the gain from the 12AX7. There are many obvious ways in which one might tinker with this circuit, but it seems to me that the very simplest thing is to swap the 12AU7 and 12FQ7. Use the 12FQ7 (a better sounding tube) at the input for gain and the 12AU7 as the cathode follower.

As far as the power supply is concerned, this minor mod should be invisible.
Has anyone tried it?

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I am very familiar with the preamp. .

John PA

23:09:53 11/27/13

RE: Quick Quicksilver preamplifier question

Caucasian Blackplate

10:21:44 11/10/13

RE: Quick Quicksilver preamplifier question

Steve O

08:40:39 11/10/13

7

...

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I am very familiar with the preamp. .

John PA

23:09:53 11/27/13

RE: Quick Quicksilver preamplifier question

Caucasian Blackplate

10:21:44 11/10/13

RE: Quick Quicksilver preamplifier question

Steve O

08:40:39 11/10/13

7

What Steve said~nT

Cleantimestream

23:08:18 11/10/13

What I said,...

Lew

07:28:39 11/11/13

pin 9

Lew

13:43:35 11/10/13

RE: pin 9

unclestu

14:25:31 11/10/13

OK

Lew

14:58:34 11/10/13

RE: Quick Quicksilver preamplifier question

Lew

10:51:04 11/10/13

RE: Quick Quicksilver preamplifier question

Lew

10:42:58 11/10/13

RE: Quick Quicksilver preamplifier question, posted on November 10, 2013 at 08:40:39

Steve O
Audiophile

Posts: 13324
Location: SE MI
Joined: September 6, 2001

I'd try getting rid of the 12AU7 altogether and repl w/12FQ7. Watch out for different pinouts of FQ7 vs AU7 and make sure filament voltage is correct (not familiar with preamp specifically). Another pin compatible alternative for 12AU7 is 12BH7...if the filament supply can handle 2x current, it just plugs in. The AU7 and FQ7 are similar enough that no circuit changes other than pinout should be reqd. The BH7 will play OK in an AU7 position but optimum results might require changes to anode and cathode resistors.

Personally, I've used 6CG7/FQ7s in AU7 positions with good results in power amps.

RE: Quick Quicksilver preamplifier question, posted on November 10, 2013 at 10:21:44
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	The bigger question is which stage(s) are directly coupled, though the 12AU7->12FQ7 should be about the same.

RE: Quick Quicksilver preamplifier question, posted on November 10, 2013 at 10:42:58

Lew
Audiophile

Posts: 11012
Location: Bethesda, Maryland
Joined: December 11, 2000

Steve, thanks. I know about all the other options, and I have more ideas as well, but I am just asking about this most simple of changes that really does not involve a modification per se of the circuit or alter the demands made on the PS. Next step would be to use a 12FQ7 at both the input AND for the CF, thereby getting rid of the 12AU7 entirely. But I don't (yet) own any extra 12FQ7s, and I don't know whether the filament supply of the Q can handle the extra current demand of a second 12FQ7 over that of a 12AU7.

Next step after that might be to get rid of the 12AX7 and the NFB that makes the 12AX7 necessary. Then one could add a CCS for the CF, etc, etc. One could use a higher Gm/lower Rp tube for the CF, too. But I would rather not alter the fundamental "flavor" of this unit, which despite its rather "old fashioned" topology (for want of a better word) does sound great.

RE: Quick Quicksilver preamplifier question, posted on November 10, 2013 at 10:51:04
Lew Audiophile Posts: 11012 Location: Bethesda, Maryland Joined: December 11, 2000	As to your comments on filament V, so far as I can tell, the Q has a 12V supply. Thus there is no need to worry about the fact that the 12FQ7 needs to see 12V between pins 4 and 5. The 12AU7 is almost certainly also running on 12V.

pin 9, posted on November 10, 2013 at 13:43:35
Lew Audiophile Posts: 11012 Location: Bethesda, Maryland Joined: December 11, 2000	Apparently, pin 9 in a 12FQ7 goes to a screen and is supposed to be connected to ground. If it is so connected in the Q preamp, it would not be a good idea to just swap the 12AU7 from front to back, since pin 9 in the latter tube is the center tap in the filament connections. I will have to check the wiring first. It's an easy fix, in any case, once you know enough to look for it.

RE: pin 9, posted on November 10, 2013 at 14:25:31
unclestu Dealer Posts: 5851 Joined: April 13, 2010	Not according to Duncan amps page. Pin 9 is a NC no connection although Sanders ma have used the postion to anchor another component. 12FQ7=6FQ7=6CG7, with different heater voltages. On a 6CG7, pin 9 is tied to the internal shield between triodes. FQ's eliminate the shield and pin 9 is left unconnected.

OK, posted on November 10, 2013 at 14:58:34

Lew
Audiophile

Posts: 11012
Location: Bethesda, Maryland
Joined: December 11, 2000

Good to know. I have yet to look at an actual data sheet. I read about the shield by doing a search on "12FQ7" here on Tubes Asylum. Someone else apparently thinks that the 12FQ7 has a shield, like the 6CG7. If that is not the case, then it is even simpler to swap the 12FQ and the 12AU around.

PS. You are correct. I just looked up a pinout diagram; pin 9 is "NC".

What Steve said~nT, posted on November 10, 2013 at 23:08:18
Cleantimestream Audiophile Posts: 7623 Location: Kentucky Joined: June 30, 2005	~! The Mind has No Firewall~ U.S. Army War College.

What I said,..., posted on November 11, 2013 at 07:28:39

Lew
Audiophile

Posts: 11012
Location: Bethesda, Maryland
Joined: December 11, 2000

but my response to Steve has been moved down below.
I recognize all the possibilities and more. I was specifically asking about this one simplest change which requires no new tubes and no re-wiring.

I am going to give it a try today. I was thinking this morning however, that because there is NFB from the CF back to the input tube, the CF may have a more than usual influence on sonics. Don't know if that's an over-riding fact.

I am very familiar with the preamp. . , posted on November 27, 2013 at 23:09:53

John PA
Audiophile

Posts: 4038
Location: The lower 48.
Joined: August 27, 2000

as it was my first really good one. I have 4 power supplies and converted mine to run all 8 pin tubes as in 12sn7 and 12SL7. I have done all kinds of things to mine. Nice unit. Mike never really made any money on them as they were too expensive to build.
iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.