Magnavox Tube Console Preamp Needed

[GordonW]

Actually- that's 300v B+. The plate voltage at the tube plate pin, is only about 60 to 120v or so (depending on tube and cathode resistor).

If you are talking B+... if you can get B+ voltage up to about 175v, it would work well as-is. You might want to increase the cathode resistor to 910 ohms- that should work a little better in terms of the distortion spectrum.

And, the 6CG7 works better than the 12AU7, at that low voltage, too.

Regards,
Gordon.

 

[Tubeglowpio]

Actually- that's 300v B+. The plate voltage at the tube plate pin, is only about 60 to 120v or so (depending on tube and cathode resistor).

If you are talking B+... if you can get B+ voltage up to about 175v, it would work well as-is. You might want to increase the cathode resistor to 910 ohms- that should work a little better in terms of the distortion spectrum.

And, the 6CG7 works better than the 12AU7, at that low voltage, too.

Regards,
Gordon.

Sounds good Gordon, I have not used the 6CG7 tube before but have read good things. Ill have to give it a try. Thank you.

 

[Pio1980]

Single 12AU7 or 6CG7 (prefer 6CG7) for stereo channels, 0.1uf input caps, 470K grid ground resistors and 1K grid stoppers, 820 ohm 1/2w cathode resistors, 22K 2w plate resistors, .22uf 630v coupling caps. Run it on between 275-320v B+. Feed that into a 100K stereo pot, then into the existing first tube in the Magnavox amp, through a 1K grid stopper (change the existing stopper resistor, if there is one, to a 1K- or add a 1K if there isn't an existing grid stopper).

I've lost count of how many times I've done that- and it ALWAYS sounded great.

Regards,
Gordon.

How much gain? Why not 6SN7?

 

[eiraved]

I recently sold a Maggie amp and preamp combo (console pull) to @rossm, and I believe the preamp had a schematic attached. If he still has it, maybe he'll take a picture of it to share.

 

[rossm]

I recently sold a Maggie amp and preamp combo (console pull) to @rossm, and I believe the preamp had a schematic attached. If he still has it, maybe he'll take a picture of it to share.

Hi
Here is the info I found on mine
https://www.radiomuseum.org/r/motorola_console_unknown_hs_960a_hs_961a.html

And

http://audiokarma.org/forums/index.php?threads/motorola-console-vibrasonic-sound.44060/

 

[GordonW]

How much gain? Why not 6SN7?

6SN7 is a drop-in sub for the 6CG7. Exactly the same parameters. The circuit requires no differences. Should sound pretty much exactly the same, with the obvious caveat of typical tube-rolling type differences (different brands and such).

BTW: Total voltage gain for the circuit I spec'd is about 12.

Regards,
Gordon.

 

[Pio1980]

Thanx! Same as my CJ Motif pre, I think.
Iirc, 6CG7 is metal can, 6SN7 glass only.

 

[darkblue94]

Thanx! Same as my CJ Motif pre, I think.
Iirc, 6CG7 is metal can, 6SN7 glass only.

Funny, all of the 6CG7's that I own are glass.

 

[Pio1980]

Funny, all of the 6CG7's that I own are glass.

Technically 6CG7GT.
OK then, only ones I've seen were canned.

 

[kirk57]

I've used my Magnavox with Audible Illusions Modulus 3, Pilot SP216a and 210 and Superphon Revelation dual mono preamps.

I prefer the sound using active preamps.

 

[Pio1980]

Most of mine run passive thru a Noble 10k stepper controller. Gordon's circuit shows a near perfect tube line stage, imo, if I was to use one. Next best is a simple fet setup, either if the gain is needed. Most preamps are redundant signal path obstacle courses and sound like it.
Ymmv.

 

[GordonW]

Thanx! Same as my CJ Motif pre, I think.
Iirc, 6CG7 is metal can, 6SN7 glass only.

Other way around. The 6CG7 is a 9-pin glass tube (Noval, like the 12AX7 and such). The 6SN7 is octal base (both metal and glass bulb versions).

The 6CG7 would be easier to add to a Magnavox amp- most have an extra socket hole that would have been for a 7025 for a phono preamp- that's the same socket type and size as the 6CG7. But, if you're building a stand-alone preamp, then either would be equally easy.

 

[Pio1980]

Other way around. The 6CG7 is a 9-pin glass tube (Noval, like the 12AX7 and such). The 6SN7 is octal base (both metal and glass bulb versions).

The 6CG7 would be easier to add to a Magnavox amp- most have an extra socket hole that would have been for a 7025 for a phono preamp- that's the same socket type and size as the 6CG7. But, if you're building a stand-alone preamp, then either would be equally easy.

I have it confused with an an octal alternate for the 6SL7GT then. I have a couple Calrad phono pre, one that uses a 12AX7 and the other the metal octal tube I apparently misremembered. The 12AU7 would be the alternative then.

A Baldwin carcass now minus opts I have here has that unused socket hole. I considered changing it from paraphase to Mullard, but an additional stage is a possibility, tho it's probable I won't get around to either. It was to be my 6L6 amp.

 

[GordonW]

Maybe you were thinking of the 6SC7? It's similar to the 6SL7, except for having a common cathode for the two sections.

Regards,
Gordon.

 

[Pio1980]

That's probably it, thanx!

 

[punkzter]

@GordonW

Thanks! So, this is hopefully an easy question. But I need to wire a heater to this tube, right? And that's not shown on the schematic?

 

[GordonW]

@GordonW

Thanks! So, this is hopefully an easy question. But I need to wire a heater to this tube, right? And that's not shown on the schematic?

Yes, that's true.

On the 6CG7, the heater (6.3v) is on pins 4 and 5. On the 6SN7 (also 6.3v), the heater is on pins 7 and 8.

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6cg7

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6sn7gt

Regards,
Gordon.

 

[Moonwatcher]

Nice! I play guitar too, been thinking of changing resistors and caps in my fender blues deluxe reissue. For solid state I have a Marantz 2230 that sounds amazing but need to source new transistors before I recap it. I prefer working on tube stuff but this is a ss worth fixing up, warmest ss I've heard. I also have a Pioneer sx 850 that I'm thinking of selling. I don't have a huge budget currently but am trying to get more into the tube stuff. I haven't been able to hear many different tubes yet but am hoping to go to the rocky mountain audio fest next year.

I am using the Marantz 2230's preamp outs with my 9303 Maggie and I gotta say it sounds SO good. Holographic, detailed, dimensional. I love the 2230 power amp, but it seems a little less three-dimensional than the Maggie, which I have outfitted with extremely heavy Edcor transformers.