6SC7 Phono pre amp

knockbill · 2018-02-07 10:53 pm

PRR said:
> I'd like to try something that could use the 6SC7s...

Use the 6SC7!! I only simmed with 12AT7 because I do not have a SC7 model; and to some degree tubes is tubes.

(However I am uncomfortable with the results I got. I suspect SC7 is not as hot-rod as AT7; also my AT7 model may be hotter than reality.)

You need that many R and C in the NFB EQ to define RIAA well. Anything with fewer parts 'cheats'. One cheat is that cartridge loading, no longer fashionable (data not available). Another is like PR6, let the "50Hz" corner happen where the tubes run out of gain (so low NFB in an important part of the spectrum, and wide variation with tubes).

Also you do not use my 1V grid batts with real vacuum tubes; the model does not have real electron clouds so does not simulate what really happens for 22Meg Rg.

I can build a new mule from your sim, if I can assemble the caps... I'll need a new PS, as the PT I used for this PS won't have 250V B+...
A couple questions,,, First, eliminate the batteries and run 22M grid resistors to grd? 2nd,, whats the 1VAC in the bottom Left corner of the schem?

I think the 6SC7 may be a little hotter than 12AT7,,, its basically replacing a 12AX7 in a phono amp, correct? I'll try 10k loading the Shure first...
Thanks for the continued support, it may just work out!

knockbill · 2018-02-07 11:58 pm

Tried the 10K on one channel with the Shure,,, no joy,,, I'll get the parts together and try to build a new mule...

PRR · 2018-02-08 1:06 am

I've confused you.

The preamp you have now.

There is a 120K+0.0027u network from pin 2 to pin 5.

Disconnect one end of that.

Build the 4-part network shown. Put it from pin 2 to pin 5.

In simulation this gives accurate RIAA.

knockbill · 2018-02-08 1:12 am

Will do,,, and thanks again for your support!! Its just a matter of having the caps to match...

cogsncogs · 2018-02-09 1:07 am

PRR said:
> I'd like to try something that could use the 6SC7s...

Use the 6SC7!! I only simmed with 12AT7 because I do not have a SC7 model; and to some degree tubes is tubes. ...

Code:

* ==============================================================
* 6SC7 Hi-mu twin triode
* Koren model (5 parameters): mean fit error 0.0418049mA
* Traced by Wayne Clay using Curve Captor v0.9.1 on 5/21/09
* from RCA data sheet (1940)
* 6SC7-RCA LTSpice model
* ==============================================================
.subckt 6SC7  P G K
    Bp  P K  I=
+ (0.01302262338m)*uramp(V(P,K)*ln(1.0+exp((7.848813146)+(7.848813146)*
+ (74.21318947)*V(G,K)/sqrt((7.786192876k)+(V(P,K))**2)))/(7.848813146))**(1.07495152)
    Cgk G K 2.4p ; 0.2p added (2.2p)
    Cpk P K 3.7p ; 0.7p added (3.0p)
    Cgp G P 2.7p ; 0.7p added (2.0p)
    d3  G K dx1
.model dx1 d(is=1n rs=2k cjo=1pf N=1.5 tt=1n)
.ends 6SC7

knockbill · 2018-02-09 11:50 am

PRR said:
I've confused you.

The preamp you have now.

There is a 120K+0.0027u network from pin 2 to pin 5.

Disconnect one end of that.

Build the 4-part network shown. Put it from pin 2 to pin 5.

In simulation this gives accurate RIAA.

I have the resistors to build this network,,, the caps will need to be paralleled,,,,, what is max tolerance, 10-15% ?

knockbill · 2018-02-09 2:03 pm

Close as I could get,,, well under 10%,,, I'll tack em in later, and we'll see...
Thanks again for the sim work....

knockbill · 2018-02-09 3:56 pm

Getting closer,,, this network sounds right... Bass is back, not booming but full... highs are just right, not exaggerated as before...

Volume is not loud as originally, but its probably due to the added network and/or big coupling caps (10x original) that are still installed... I'll keep tweaking, but we seem to be on the right page now....

PRR · 2018-02-10 6:39 am

knockbill said:
... Bass is back, not booming but full... highs are just right, not exaggerated as before... Volume is not loud....

Good. And there's only so much gain you can get from one 6SC7.

(Higher supply voltage *would* help some, because the NFB loop does not cover the whole amp. Gm of V1 is uncontrolled and will rise with I which will rise with B+. I don't know what "noise" you fear unless you are running out of B+ filtering, or have damp sockets that crackle with >100V across them.)

(An output FET would help slightly, by making V2's life easier.)

What you really want is a Test Rig. Stable audio sine source, reverse RIAA network, and decent AC voltmeter. Taking a frequency response is a BASIC audio skill. If you are sure the output cap does not leak DC, you can do it on a PC soundcard.

knockbill · 2018-02-10 1:16 pm

Filters are adequate, its very quiet,,, caps should be big enough to easily raise B+...

I'll study on the Test Rig,,, seems a reverse RIAA network can be built,,, I have a Ballentine ACV meter,,, should be just the thing for low ACV readings, as it works well for reading input signals...

Thanks for all your help/info,, guess I need to catch up on some BASICS...

hpeter · 2018-02-10 3:42 pm

..problem with old era preamps, : input impedance&gain on power amps was much higher than today.
you may load the preamp too much and shift his parameters

knockbill · 2018-02-10 4:39 pm

Got the limited gain somewhat sorted... I have a Maggy 185, which I rebuilt, but retained the 7025 input stage, which was for the passive tone stack in the phono only console, and added VC pots to it... With this pre amp plugged in thru this extra stage, we get all the gain/volume you could want, and its still quiet... I didn't raise B+ in the pre yet...

I'll continue reading and try to understand the frequency tests you mentioned along with building the reverse RIAA network...

With your RIAA network added to this old phono amp design, I don't think anyone who built it would be disappointed... Can't imagine a simpler vintage circuit that gets the job done...

Got Traffic spinning right now,,, "Good old fashioned medicated goo"....

Kudos, PRR!

knockbill · 2021-02-04 9:55 pm

Bored and back to this 3 yo project,,, I had a couple of these cedar cigar boxes, so I bent a chassis out of an old metal shelf, and figured its about time to finish it!!!
Thanks again for the help i got here...

bigbargain · 2021-02-05 1:29 am

Ha! Ha! I just read it for the first time!

Wanna provide an update. I’m shopping diy phono amp ideas.

knockbill · 2021-02-06 12:20 am

Chassis update,,,

knockbill · 2021-02-09 4:19 pm

Got one channel wired...

knockbill · 2021-02-10 7:52 pm

Wiring is done, its very quiet, and sounds good, with small amp and crap speakers!!! Fired up on first try,,, needs some permanent ground wires, and terminal for the TT... The cigar box case needs to be finished,,,

Thanks again to PRR for the RIAA sim and everyone else that offered help...

knockbill · 2021-02-11 9:42 pm

A couple pics of the finished (for now!) preamp,,, I reread the thread, and decided to try raising B+ as suggested... Seems 23-25VDC higher got a bit more volume and depth, and its still dead quiet,, so I'll leave it there until I try it with different amps, TTs and speakers...
Thanks again for the support with this project...

Linearojg1976 · 2021-02-11 9:52 pm

It looks really fantastic Nice job. It gives me hope that my collection of 'could come in handy' boxes will be proved to have some point.

knockbill · 2021-02-11 10:01 pm

Thanks, I finally got back to it,,, first schem I printed out is dated 2016!!!

6SC7 Phono pre amp

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