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Back on the forum after a long hiatus 6SN7 Preamp project in the works.

coldcathode

Member
2009-01-11 4:31 pm
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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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.
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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

http://www.diyaudio.com/forums/tubes-valves/166176-dynakit-pc-3a-balance-pre.html#post2175525

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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coldcathode

Member
2009-01-11 4:31 pm
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

http://www.diyaudio.com/forums/tubes-valves/166176-dynakit-pc-3a-balance-pre.html#post2175525

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?
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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...
 

coldcathode

Member
2009-01-11 4:31 pm
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
 

coldcathode

Member
2009-01-11 4:31 pm
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...

Try floating some DC at about 60V on the heaters.
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
Try floating some DC at about 60V on the heaters.

Yes, this is Plan B. I have the pre out to a friend who wanted to hear it, so I should get back on it later this week. So far its pretty good...need to get hum down and better tubes -

Since your knowledgeable on 6SN7, have any advice on brand? I have four nice Tung Sol 6SN7s, but its currently in use in my Aikido. The ones I have left over for the Balanced are a few old odd RCAs and a Magnavox - The pair I have now in the balanced pre, a Magnavox and an RCA, look identical - dual round side getters, same bottle and base size, and test close on my Knight 600...

I have a single New 5692 red base, but don't wanna spring huge cash for a another matching one, I hear the only thing great about the RCA 5692 Red Base is the 10,000 hour life - soundwise its just OK. Hard to make my own evaluation with just one. But I have been impressed with the older Tung Sols...
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Red base 5692s are extremely linear, some of the best I've ever seen. Lowest microphonics I've ever... uhh.. not heard. :D

The 10,000 hour life is only valid if you run them within the VERY conservative ratings (much lower than conventional 6SN7). Linearity champ is still the CV1988. The ElectroHarmonix seem to be quite good, though I've only tested two of them.
 

coldcathode

Member
2009-01-11 4:31 pm
I have found that in some circuits just about ANY 6SN7 sounds good. My favorites are some of the Tung Sol's

and my "babies"

RCA 6SN7 WGTA NOS Military tubes. Really nothing special about them other than they are "ruggedized" for aircraft and military use. I have about 7 or 8 of them from the same lot.

IMHO the circuit design, chassis layout, quality of passives and the wiring job are more important than the tube itself.

I have yet to acquire the skills and money needed to eliminate all of the various "flaws" in the work I have done to get to a point where I can clearly hear sonic differances from tube to tube.

Most of the work I have done so far with tubes (actually any audio) has been experimentation and learning. You know like how to "keep the magic smoke" IN the components!! LOL

I now feel that I have learned enough to make a go of building a system from scratch, (Albeit a few Solid State PCB projects) (stuffing them myself though)

I think I can now look at circuits and simulate them in LTSpice well enough to make an educated guess as to their merits.

I wish I knew more about "balanced" inputs to be of help to you. What little I have gleened about them has pointed me towards understanding one thing. GOOD transformers COST A LOT!! LOL, Most of the funds I can scrape up for this overall listening room project have to go towards the "rehab" of the basement into a room and on the drivers themselves. Relatively high quality,highly efficient, well designed and built speakers will cost more than most of the other signal and amplifying gear.

Because I am not really "in to" speaker design and building I plan to find a well reveiwed set of plans and follow the directions to build them. Once I know I have a good "reference" set of drivers then I can take my time with things like "tube rolling" and LED Bias etc.
 

coldcathode

Member
2009-01-11 4:31 pm
One thing I have learned is that there are MANY MANY opinions and "stereotypes" (excuse the pun) about topology's, tube types, bias methods, etc. Some people will choose to "ignore" or "take issue" with someone's "critique" or "comment" about X or Y. I on the other hand feel that most "stereotypes", "rules of thumb" or "common opinions" are based in some FACT and TRUTH. I would rather hear someones comment and look to see it from that point of view or do some research or "simulation" to see why it came up rather than "blindly" go on into the night.

Someone once "set me straight" on this forum (could have been SY or maybe KevinKR? not sure) "that I needed to sit down and READ a couple of the sections of tube manuals to get a "foundation" before trying out ideas"
. This was initially taken by me as "look kid your still wet behind the ears" and I chose to ignore it for about a week. I downloaded a copy of RC-30 and read it like a book. Within about another week things started to get clearer and make sense.

So ANY input should ALWAYS be considered and then you decide if you want to apply it or not.
 

coldcathode

Member
2009-01-11 4:31 pm
That being said, you have peaked my interest in Balanced inputs/outputs and IF I get some free time I will play with your schematic in LTSpice and get an idea of maybe some "tweaks" to apply.

This is not a "noble effort" I have an alterior motive (the FCA-202) has balanced outputs as an option. LOL
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
Have a look at the anode follower. I'm using one in my 300B DeathTrap 500 amplifier. You could build one anode follower with one 6SN7... I use it as a driver for a 300B and it's the best sounding 6SN7 driver I've been able to come up with. It has reasonably low output impedance - you could make it even lower by using unity feedback around the second tube. That would give you a preamp with a gain of about 20 V/V...

Just a thought.

~Tom
 

coldcathode

Member
2009-01-11 4:31 pm
Input cap gone, two red leds in the cathode for bias, and you're good to go.

SY,

Any suggestion as to what LED's to use? I know I should be looking at fairly "run of the mill" RED LED's but finding good Spice models and accurate data sheets for forward voltage at specific currents is almost impossible.

I really want to stick to about -1.5 to -1.7V about 8 mA.

I chose the operating points around a couple of thoughts/restrictions.

1: I have a huge potted transformer (Gilfillan Brothers?) that will get me about the 400 - 450 VDC @ at least 50 mA if need be for the whole preamp project (possible RIAA preamp ala RCA 12AX7) added

2: Wanted the follower stage to draw almost identical current so the current swings are out of phase effectively balancing the draw on the supply.

3: Want to run the tubes a bit "hot" (.9-1 Watt Plate dissipation)

So basically what I'm looking for is an "off the shelf" red LED with about 1.5 forward voltage @ 8mA
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
You'll be closer to 1.7V with a red LED. Which is fine, considering the tube tolerances involved. Best "new" LED I've found is the HLMP6000. And they're really cheap. Otherwise, old surplus units seem to work nearly as well- I bought a bag of 2000 red LEDs of unknown origin for $20 and they have a dynamic impedance under 5 ohms, compared to about 2-3 ohms for the HLMP6000.