Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

6SC7 Phono pre amp
6SC7 Phono pre amp
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th February 2018, 01:35 PM   #11
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
Quote:
Originally Posted by Eli Duttman View Post
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.
Eli,,
Nice to hear from you again,,, you were one of the first to assist me when I retired and started this hobby!!! I do appreciate your improved design, and am familiar with it,,, however this project is more for me to see what simpler, if any improvements can be made to this vintage circuit... Its been on the bread board for at least a year, as it had an annoying hum at first... I dug it out a month or so ago, and rebuilt the PS with bridges and oil bypass caps which made it silent, hence the effort to proceed...
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 02:08 PM   #12
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
Quote:
Originally Posted by PRR View Post
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.
Thanks for the simulator work, I will study the schematic... I want to try raising coupling cap values first, to see how it improves bass response....
I did find quite a discrepancy in the noise level of the tubes... RCAs seem to be the quietest and best sounding so far... I can also adjust the B+ but if that is necessary, it would be better to just follow a different design... I'm sure GE designed for a specific cartridge back in the day, but its fun to experiment with old designs... and it may lead to following your new mods....

Thanks again to all that contributed,,, I don't expect miracles!!!
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 06:12 PM   #13
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
I just did a comparison to the 6SL7 RCA phono pre I built a while ago,,, the GE/6SC7 is no comparison.... I think I'll retain the nice PS and come up with another try!!
I could try the schem PRR posted but I have a few questions about it... I'd like to try something that could use the 6SC7s...

Also, I boosted the coupling caps incrementally up to 10X and it still was lacking bass,,, so the missing RIAA network is very necessary, as was posted here... maybe this GE offering was for cheap portable record players, as 78s also have a bunch of different EQs depending on who recorded them...
We learn from our mistakes! thanks again for all the input...
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 07:36 PM   #14
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
The GE setup relies on the inductance of the cartridge to "match" the RIAA curve. Give GE their due, as they were 1st in making mag. carts. available.

Quote:
I'd like to try something that could use the 6SC7s.
I've already provided a phono preamp option. You could use a 6SC7 as the LTP in a Mullard circuit power amp, instead of a 12AX7/ECC83, provided you use full pentode mode "finals". High RP/low gm types, like the 6SC7, get into trouble attempting to drive anything resembling a substantial load.
__________________
Eli D.
  Reply With Quote
Old 7th February 2018, 07:56 PM   #15
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
I have two RCA pre amps... I'll keep searching...
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 08:57 PM   #16
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
Here's this pre amp with a Fisher badge... I remember seeing this a while ago,,, it says it has RIAA for any magnetic cartridge,,, but I guess that was true with 1954 cartridges...

http://www.preservationsound.com/wp-...PR6_Manual.pdf
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 09:44 PM   #17
PRR is online now PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
Quote:
Originally Posted by Eli Duttman View Post
The GE setup relies on the inductance of the cartridge to "match" the RIAA curve.....
Yes. Confused here because post #1 shows 47K input. Given that, there are not enough corners in the EQ to do RIAA. The old-school trick of R-loading the needle's inductance does work, if the needle maker commits to a specific value. GE did, today nobody does.

Now that we see the Fisher PR6, we see the same thing with R1 noted as 6.8K or per needle maker's recommendation. L(cart)+6.8K makes the 2KC corner. 120K makes the 1KC gain. 2700p+120K makes a 491CPS corner. 50Hz happens just because it runs out of gain.

Mis-done with 47K input, I would expect screaming highs, which is not what knockbill reports. The shy bass may be weak 6SC7 or poor biasing (DC checks would be first- are the tubes even running happy?).

PR6 booklet with notes:
Attached Images
File Type: gif PR6-booklet.gif (104.0 KB, 86 views)
  Reply With Quote
Old 7th February 2018, 10:07 PM   #18
knockbill is offline knockbill  United States
diyAudio Member
 
knockbill's Avatar
 
Join Date: Feb 2013
Location: SE PA
The highs are a little pronounced, I wouldn't say screaming,,, As far as cartridge loading, seems the specs were accurate at the time it was designed,,, Pickering cart at 2500R or a GE at 6800R ( factory wired), were they the only Mag cartriges available at the time?... I really didn't give a thought to the timing until I saw this manual,,, As I have pair of Fisher 50C clones from the same era, but it has a lot more EQ controls...

FWIW,, 6SC7 voltages are well within specs... Would changing the input load resistor from the common 47K put this back to the RIAA specs, the amp is designed for? or maybe it will only be accurate with one those two vintage cartridges?
__________________
Regards,
John
  Reply With Quote
Old 7th February 2018, 10:15 PM   #19
PRR is online now PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
> 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.
  Reply With Quote
Old 7th February 2018, 10:21 PM   #20
PRR is online now PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
For Shure M91E, 720mH, try 10K input resistor.

BTW, with this trick your turntable-preamp leads may be 30 feet, whereas for 47K we can't go over 3 feet. Also the extrapolated response runs far past the audio band. OTOH you have to change the resistor for every cartridge you use, and there is some rise in effective hiss.
  Reply With Quote

Reply


6SC7 Phono pre ampHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
6SJ7 and 6SC7 not on roll chart famousmockingbird Tubes / Valves 5 21st November 2015 05:01 AM
How Can I connect A Balanced RIAA Phono Pre-Amp to a Phono Plug amp input jmartinbaugh Analogue Source 3 25th April 2011 02:24 PM
6sc7 vs. 6su7 in LTPI duty aletheian Tubes / Valves 3 25th November 2006 02:17 AM


New To Site? Need Help?

All times are GMT. The time now is 10:04 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki