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6SC7 Phono pre amp
6SC7 Phono pre amp
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Old 7th February 2018, 12:02 AM   #1
knockbill is offline knockbill  United States
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Default 6SC7 Phono pre amp

I built this pre amp mule while ago, and recently dug it out, rebuilt the PS and added a second channel,,, Played a couple dozen albums and find it to be a little lacking in bass response with a TechnicsSL-D30/P28,,, it seems to respond better with the Dual 502/Shure M91E...
I want to try a different coupling cap, replacing the .0047 to .0068 or .01 or so to see if there is any boost in the bass response...

Question is,,, Will changing that cap change the EQ of the pre amp? How about if the 22M grid resistor is left in place?

Some searching turned up this post,,,
"I noted that in the Tronola articles, the coupling cap in an Eico phono section was changed from a 0.05 ceramic to a 0.1 f film cap. This made sense and improved the LF response. So sometimes it is beneficial",

Para phrase from...POST #15
http://audiokarma.org/forums/index....or-values-in-tube-amp-preamp-rebuilds.744091/
Any thoughts from the designers out there!!
Thanks for any info,,,
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Old 7th February 2018, 02:07 AM   #2
lexx21 is offline lexx21  United States
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6SC7 Phono pre amp
You have 22mR for your grid stoppers. The data sheet only shows a max of 10mR under certain conditions. Outside of that it isnt critical, so I suppose what you are using is ok, but 22meg...wow.

You say this is a phono preamp, however I am not seeing a RIAA circuit in your drawing. That would leave your bass response totally hosed.
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Old 7th February 2018, 02:12 AM   #3
lexx21 is offline lexx21  United States
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6SC7 Phono pre amp
Ok, I see where your schematic came from. It is a GE design.
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Old 7th February 2018, 02:33 AM   #4
kevinkr is offline kevinkr  United States
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6SC7 Phono pre amp
It's not for modern pick ups playing RIAA equalized recordings. Take a look in Analogue Source for a bunch of appropriately EQ'd designs.

Eli has an improved version of the RCA passively equalized design, you might want to check it out.
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Old 7th February 2018, 04:00 AM   #5
PRR is online now PRR  United States
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Quote:
Originally Posted by knockbill View Post
........ Any thoughts from the designers out there!!...
There's not enough there, in several ways.

6SC7 is "high Mu" but not as high as 12AX7.

All 2-stage 12AX7 phono preamps have shortcomings. Worse with a lower-Mu tool.

The classic connection is P2 to K1 NFB, but the 6SC7's common cathode foils that.

NFB from P2 to G2 will not really cover the whole 40+dB needed to do the RIAA curve well.

Passive EQ may give a nice flat RIAA response but is likely to get hissy at the high end.

This design dates from the late 1950s, when speakers were all like +/-10dB lumps, when GE was trying to promote their low-output cartridges and needed a LOW-cost preamp to make them work "acceptably" (by the standards of the time).

Sow's Ear. Even if not aiming for a silk purse, there's not enough here to make a plain purse you would carry. (If you carry a purse...)

Also the supply voltage is rather low (cost-cut).
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Old 7th February 2018, 04:01 AM   #6
Eli Duttman is offline Eli Duttman  United States
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Thanks for the "plug", Kevin.

It is possible to get some "mileage" out of 6SC7s in the tweaked RCA setup. The common cathode connection is not an impediment to sharing a tube between channels as the grid leak biased 2nd gain block triodes. When sharing tubes between channels, the 6H2Π-EB (6n2p-ev), with its internal shield, is a prime candidate for 1st gain block duty. Those folks with large bank balances can consider an ECC808/6KX8 as the 1st gain position tube.

Don't give even passing thought to AC heating, with the types mentioned.
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Old 7th February 2018, 05:34 AM   #7
PRR is online now PRR  United States
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Quote:
Originally Posted by PRR View Post
There's not enough there.....
I'm going to walk this back.

The old topology will work. Those NFB EQ values are just not right.

At least according to the Idiot Assistant.

Simulation notes:

No 6SC7 model, 12AT7 is loosely similar. Most SPICE models do not do 22Meg gridleak bias with realism (although the tubes do work well this way); I added small batteries to fake what really happens. For real parts, these batts should be shorts.

(And you want your 47K resistor before my C1.)

I used Koren's reverse-RIAA because it was handy. I did not check it.

The EQ accuracy WILL depend on the tube. Popping a 12AX7 in for TU1 gives a *lower* overall gain(!) and some rise below 50Hz, though otherwise flat. Smaller change for the +/-20% spread of 6SC7 production.

In Real Life: Gridleak bias works, and tends to give about maximum gain; but the exact operating point is uncertain. This means a bit more spread of working gain. Also for a given B+ you won't get maximum output before distortion. Some of these variations will reduce with lots of B+ (overwhelm contact potentials which add/subtract from gridleak potential).

When doing gridleak you want *big* coupling caps, far more than you expect looking at 22Meg.

The THD spectrum seems to be high because of poor operating points and heavy loading, particularly above 1KHz. In that most music falls above 1K, this may not be a real problem.

The EQ lacks the 20Hz corner defined by IEC in 1976. I believe any modern phono should do this. Apparently other folks have other opinions. There is no simple way to add a 20Hz corner (grid impedances are not defined, filtering at input raises random 1/f, at output is load-sensitive).

This is essentially a re-power and re-cap of what you have on the board, with a few added parts for proper EQ, so I don't propose to beat my simulator further.
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Last edited by PRR; 7th February 2018 at 05:39 AM.
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Old 7th February 2018, 11:57 AM   #8
knockbill is offline knockbill  United States
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Quote:
Originally Posted by lexx21 View Post
Ok, I see where your schematic came from. It is a GE design.
First of all thanks for replying!!! I had this over at AK for a week and got nothing!!! and I questioned it not showing RIAA EQ...

It works fine as its built, very quiet at any volume,, good mids and highs, just lacking in bass as mentioned, which is teh point of the thread... so, it seems raising coupling cap value won't change the EQ, as it really isn't there!!!!
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Old 7th February 2018, 12:08 PM   #9
knockbill is offline knockbill  United States
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Quote:
Originally Posted by kevinkr View Post
It's not for modern pick ups playing RIAA equalized recordings. Take a look in Analogue Source for a bunch of appropriately EQ'd designs.

Eli has an improved version of the RCA passively equalized design, you might want to check it out.
Thanks for replying,,, I have built two RCA phono preamps, they may actually be on this forum... First one is a copy of the original,,, 2nd uses 6SL7s,, which to me sounds better... I've seen the improved version schem also...

This project was to see how this simple pre would perform, and seems it is similar to an old Muntz TV,,, gets the job done with less parts!!!!

I appreciate the help, I will try replacing the coupling caps, as I'd like to tweak it as much as possible, without going to a "better" or different design... There's a little learnin for me here, and I appreciate your input and help in optimizing this old design...
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Old 7th February 2018, 12:26 PM   #10
knockbill is offline knockbill  United States
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Quote:
Originally Posted by PRR View Post
There's not enough there, in several ways.

6SC7 is "high Mu" but not as high as 12AX7.

All 2-stage 12AX7 phono preamps have shortcomings. Worse with a lower-Mu tool.

The classic connection is P2 to K1 NFB, but the 6SC7's common cathode foils that.

NFB from P2 to G2 will not really cover the whole 40+dB needed to do the RIAA curve well.

Passive EQ may give a nice flat RIAA response but is likely to get hissy at the high end.

This design dates from the late 1950s, when speakers were all like +/-10dB lumps, when GE was trying to promote their low-output cartridges and needed a LOW-cost preamp to make them work "acceptably" (by the standards of the time).

Sow's Ear. Even if not aiming for a silk purse, there's not enough here to make a plain purse you would carry. (If you carry a purse...)

Also the supply voltage is rather low (cost-cut).
I agree with the gain assessment,,, 6SC7 is closer to 6SL7 in gain than 12AX7,,, however this build is an exercise in using it... Its a nice sounding tube, that is available for for next to nothing... and has a built in shield...

I did build a cathode follower for it, but found it had too much gain, even tho CF isn't supposed to add any...

I don't want to mess with the NFB circuit, however the B+ thru the amp does seem low, I haven't tried to raise it as I can't see where it would help bass response and may add some noise...

Again, I truly appreciate the time you gave this question, I realize its not optimum, but its a fun winter project!!
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