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Old 18th November 2010, 12:14 PM   #1
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Default Back on the forum after a long hiatus 6SN7 Preamp project in the works.

Well it has been some time since I last browsed the forum. I had been out of work for 14 months until March of this year. It has been "feast or famine" since now I have been working almost 7 days a week since March 1st.

Anyway, now that funds have become available I can set to work on putting together my "listening" room system.

My source will mostly be Digital media from a PC via a Behringer FCA202 with ASIO drivers.

My basic plan is to implement the following basic layout.

PC > FCA202 > 6SN7 Preamp > 24dB/Octave LW Xover (solid state sharing preamp chassis) @ ~60-70hz

I have read and read and googled and googled and there are SO many preamp/linestage designs and theories everywhere that I have decided to go as relatively simple as possible.

My thoughts settled on a few basic "rules of thumb" I have come across.

#1. 6SN7's have more than enough gain for the job I am asking of them.

#2. They sound GOOD.

#3. Affordable (since I have a nice supply of various NOS tubes)

#4. They like to be run fairly "hot"

I wanted to go with a basic Grounded Cathode stage directly coupled to a cathode follower. I felt that "balancing" the two stages current draw would be appropriate so that is where the OP's came from. I experimented with battery bias on other projects and was pleased with it so decided to throw that in also.

attached is a draft of what I am thinking.(the 99K output resistor R2 approximates the load at the input of the Xover) Any suggestions as to bias setup? Battery at the grid is how I biased the other projects, using a battery at the cathode is another option how do I implement it? and does that eliminated the input coupling cap?, thus one more component out of the signal path?

I will "float" the heaters at some higher voltage (probably split the difference @ about 60V)

Should I "decouple" the two stages B+ with an RC?
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File Type: jpg 6sn7 pre.JPG (23.3 KB, 453 views)

Last edited by coldcathode; 18th November 2010 at 12:18 PM.
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Old 18th November 2010, 12:17 PM   #2
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I know that a SS crossover is "heresy" but I am using a "proven" PC board and "low noise" OPAMPS. LOL
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Old 18th November 2010, 01:12 PM   #3
SY is offline SY  United States
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Input cap gone, two red leds in the cathode for bias, and you're good to go.
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Old 19th November 2010, 12:33 AM   #4
Bigun is offline Bigun  Canada
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I'm not sure that the biassing on U2 is going to get you to 8mA with -11V grid-Cathode, judging by the tubes I've used.

Edit: cancel that, I was thinking your "Vp" was the voltage on the plate with respect to ground rather than with respect to the Cathode.
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Old 19th November 2010, 01:23 AM   #5
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That general style of circuitry is quite popular. All iterations of the Bottlehead "Foreplay" use it. With a digital signal source, you could easily run into excessive gain and a "hair trigger" volume control. In addition to SY's LED bias suggestion, think about making R1 a voltage divider and connecting the grid to the tap.

Getting rid of the I/P cap. is good, if and only if you can absolutely, positively, guarantee that a DC offset will never be present at the line stage's I/P. A Soviet surplus PIO part is my preference, for tonal and DC block reasons. YMMV.
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Old 19th November 2010, 01:30 AM   #6
john65b is offline john65b  United States
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I can never get the hum down to acceptable levels with a high-ish current 6SN7...

I had a bug up my ^&*) to build a 6SN7 balanced linestage pre (single ended in, balanced out) and is very familiar to what you had posted... thread is here

Dynakit PC-3A as a balance pre?

I built it and had quite a bit of hum. I finally raised the cathode resistor (tripping up the 2/3 - 1/3 follower voltages) to lower gain and current to reduce hum to a level I could deal with. The sound is quite good even with crappy tubes.

I could only muster a B+ of 250V with a Hammond 369EX tranny, a 6X5 rectifier, and a CRCRC filter (28uf/220R/150uf/220R/150uf). Plate resistor is 20KR, 680R on cathode (replaced with two red LEDs at 3.2V - same drop as 680R), Grid resistor of 364K, 25k pot in front (all I have now), 1uf Vitamin Q's at output...

Still want to try DC on heaters, but I don't expect a whole lot of benefit...
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Last edited by john65b; 19th November 2010 at 01:45 AM.
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Old 19th November 2010, 12:00 PM   #7
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Quote:
Originally Posted by john65b View Post
I can never get the hum down to acceptable levels with a high-ish current 6SN7...

I had a bug up my ^&*) to build a 6SN7 balanced linestage pre (single ended in, balanced out) and is very familiar to what you had posted... thread is here

Dynakit PC-3A as a balance pre?

I built it and had quite a bit of hum. I finally raised the cathode resistor (tripping up the 2/3 - 1/3 follower voltages) to lower gain and current to reduce hum to a level I could deal with. The sound is quite good even with crappy tubes.

I could only muster a B+ of 250V with a Hammond 369EX tranny, a 6X5 rectifier, and a CRCRC filter (28uf/220R/150uf/220R/150uf). Plate resistor is 20KR, 680R on cathode (replaced with two red LEDs at 3.2V - same drop as 680R), Grid resistor of 364K, 25k pot in front (all I have now), 1uf Vitamin Q's at output...

Still want to try DC on heaters, but I don't expect a whole lot of benefit...

DC heaters might help but they need to be well filtered and you will need another transformer since your best bet for DC Heaters on this would be to use a bridge rectifier on something like a 15V transformer and a 7812 regulator with decent caps on it.


I suspect that the "HUM" is coming in from component placement and or inadequate ground scheme. Your B+ looks to be pretty well filtered.

Not sure what your doing with the balanced out? I assume there is a transformer at the input of the next component?
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Old 19th November 2010, 12:01 PM   #8
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John also, Pin 9 of the 6CG7 is an internal shield, run that to ground, it might clean up some of the hum.
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Old 19th November 2010, 12:10 PM   #9
john65b is offline john65b  United States
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Coldcathode - sorry, I seem to be taking over your post - I just saw how similar our schematics were and felt a need to post.

I am using a 6SN7 not the 6CG7 - so no internal shield. My balanced out is going into an amp with balanced input - I have a Pass Aleph 5 Clone and an Icepower ASC200 - both with balanced in.

Input source (CD player) is a clean - no DC offset...
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Old 19th November 2010, 12:19 PM   #10
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Quote:
Originally Posted by Eli Duttman View Post
That general style of circuitry is quite popular. All iterations of the Bottlehead "Foreplay" use it. With a digital signal source, you could easily run into excessive gain and a "hair trigger" volume control. In addition to SY's LED bias suggestion, think about making R1 a voltage divider and connecting the grid to the tap.

Getting rid of the I/P cap. is good, if and only if you can absolutely, positively, guarantee that a DC offset will never be present at the line stage's I/P. A Soviet surplus PIO part is my preference, for tonal and DC block reasons. YMMV.
As far as I can tell the FCA-202 has an output impedance of ~1K and has DC blocking caps, if I decide to add any other inputs or a phono preamp onto this chassis I would be sure that there would be NO DC present (add coupling caps at those inputs.

LED's are one possibility but I am leaning towards battery biasing at the cathode with a NiMH cell.

As far as the source is concerned the FCA202 has maximum of 2dBV (big V) so I see no issues with "overdriving" either stage since I rarely run the "volume knob" up past 10. I plan to use a linear rather than audio taper pot and might increase the value a bit.

As I said before a unity gain solid state Xover will actually sit at the outputs. This will have individual attentuators for the outputs.

This layout IMHO seems the simplest way to get some gain and present a low impedance source for the crossover. Besides the foreplay, VTV's preamp and many others used this configuration. I am simply combining all the things I have gathered over a couple of years and "cobbling" together a preamp. I am somewhat of an "Octal Maniac" hence the 6SN7's
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