Gus

6SN7 / 6CG7 preamp
« Reply #20 on: January 09, 2006, 07:55:51 PM »
Paul has a nice circuit.  I saw a friends glass audio? mag with it years ago.  From what I remember he found some interesting things.  It used 6sn7s IIRC.   Good article


rafafredd

6SN7 / 6CG7 preamp
« Reply #21 on: January 09, 2006, 08:57:07 PM »
Wich Paul?

pstamler

6SN7 / 6CG7 preamp
« Reply #22 on: January 10, 2006, 03:05:57 AM »
It was me. The articles were in audioXpress, Feb-Mar-Apr 2002. In brief, I used as a gain block a 6SN7 voltage amplifier direct-coupled to another 6SN7 cathode follower. Electronic gain is about 19dB per block; with a 1:10 transformer that's 39dB. The simplest preamp had just one gain block, with a level control on the output and short cables to the recorder. More complex one had two gain blocks with a level control in between. Nice sound, with the best sound (but least flexibility) coming from the simpler circuit. Regulated power supplies, two filament supplies (one hoisted up to 100V or so for the cathode followers), unbalanced output into 10k. Basically designed for prosumer-type recorders and sound-cards; with a 1:1 output transformer it'll drive balanced +4dBu 10k inputs nicely, but won't drive 600 ohms nicely at all.

Peace,
Paul

skipwave

6SN7 / 6CG7 preamp
« Reply #23 on: January 11, 2006, 05:52:50 PM »
Quote from: "NewYorkDave"
http://www.groupdiy.twin-x.com/albums/userpics/10031/RCABC2BPreamp.jpg


Apologies if this has been covered before, but I was looking at the RCA schemo. They run the B+ through the output TX, hence the need for expensive or custom iron. Big drawback for DIYers.

Is it worth trying to revise the design for a non-gapped output TX? It seems like the result would be close to Dave's One-Bottle pre, but with the AY7's lower transconductance it wouldn't drive the output as well as the AV7 does?

Am I starting to grasp these concepts, or am I delusional? :green:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

NewYorkDave

6SN7 / 6CG7 preamp
« Reply #24 on: January 11, 2006, 08:13:53 PM »
The "One-Bottle" was born when I looked at the BC-2B preamp schematic and thought, "How can I do something like this, but with inexpensive transformers?" I liked the idea of a two-triode preamp but hated the idea of having to fork over the bucks for the special iron.

The AV7 has higher gm/lower Rp than the AY7--and it's a hell of a lot cheaper, too. It's definitely more suited for the sort of "shunt-fed" output arrangement seen in the One-Bottle, where the plate is being loaded pretty heavily. If you plugged an AY7 into the One-Bottle, it'd collapse into a sobbing heap at higher output levels.

But, having said that, Oliver Read published his line amp circuit in 1952, and it uses a grid-leak biased AY7 in a shunt-fed output arrangement. I have no idea of its performance.

(By the way, after I scanned and uploaded the schematic, I noticed that it contains a serious error. Can you spot it? Hint: it's in the power supply).

rafafredd

6SN7 / 6CG7 preamp
« Reply #25 on: January 11, 2006, 09:01:18 PM »
heater wiring...

skipwave

6SN7 / 6CG7 preamp
« Reply #26 on: January 11, 2006, 09:42:03 PM »
The center tap pin 9 is connected to one side of the secondary, instead of grounded for 6.3V or unconnected for 12.6V. It looks like the latter was the intention, since pin 5 is grounded, or that was a mistake too.

I love the caption "using quality components for noise-free amplification."

Thanks for the reply and sorry for the off-topic. Looking at the RCA finally crystalized in my mind the discussion of output iron for Raf's pre.

Please carry on, this thread is full of great information.
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

PRR

6SN7 / 6CG7 preamp
« Reply #27 on: January 12, 2006, 02:44:19 AM »
> The center tap pin 9 is connected to one side of the secondary, instead of grounded for 6.3V or unconnected for 12.6V. It looks like the latter was the intention, since pin 5 is grounded, or that was a mistake too.

It's a PopElectronics plan. Don't trust it too far.

4 and 5 are tied together. 6V is fed to pin 9 and pins 4+5. Heat is correct.

Heater should be AC grounded and at some rational DC voltage. In high-level work you can ground one side, in low level work you can ground the center-tap, in mike work you may want an adjustable "center"-tap such as the pot (which was such a standard part that no value is shown: try 100 ohms to 1K). Therefore the connection from 4+5 to ground is either an alternate or more likely just a drafting error.

Other than that, Read's plan is not the dumbest thing I ever saw. The output biasing is odd but seems to work. He can't be getting much over 12V peak at the 600Ω load, but that is +20dBm. The gain-pot input reduces output S/N.... but actually it is constant at about 95dB output S/N at all settings. In days of yore, this was ample for the recorders. It is not real efficient, but making it more efficient would not reduce the cost... for a one-mike system, this is fine. (Consoles had to worry about heat buildup with 4 to 40 2-tube preamps next to the operator's knee.)

> the discussion of output iron for Raf's pre.

Think about this.

For a "perfect" device, used single-ended (not in pairs):

With transformer, the maximum sine audio output is 50% of the power dissipation.

With resistor coupling (no tranny or tranny just for matching), the maximum sine audio output is 8.3% of the power dissipation.

With a non-perfect device, the performance sags very badly. Almost as the square of the device resistance.

The small twin-triodes don't give Rp much lower than 5K. It is hard to find a transformer over 5K. To avoid DC in the iron, we have to add about a 5K resistor. Doing the resistor means we can NOT swing above the supply voltage, a large chunk of supply is wasted in the resistor, and for most practical proportions this also sucks most of your audio power. Read's plan takes 3,000mW of plate power to give about 110mW of sine audio output power, 3.5% efficiency. Transformer-coupled "pro" preamps might do the same with only 1,500mW of plate power.

A lower Rp output device would also be more efficient, approaching the 8% maximum for resistance-coupled amps. But Rp is proportional to Mu, inversely proportional to cathode size and heater power. A low-Rp tube has low Mu and low voltage gain, unless we get one with a bigger cathode and feed more heat to it. If heater power costs as much as plate power, this is a no-win game. Plate needs to be clean DC and most heaters can eat raw AC, so there is some headway here. Unless of course it is the total dissipation that is the problem (the operator's knee-burns).

Read's output plate sits about 90V-100V (very variable with tube leakage). This is opposite of an NYD plan where the plate resistor was small and the plate voltage was large. Might be interesting to try both ways. Biasing that grid as close to zero as possible is key; yet without objectionable grid current on musical peaks. (It may clip test-tones yet not stink-up the music.)

PRR

6SN7 / 6CG7 preamp
« Reply #28 on: January 12, 2006, 03:08:51 AM »
AH!

The rule of thumb for voltage amps is to make the plate DC resistor 2* the Rp, make the next stage input at least 5* the Rp. (Or given a next-stage resistance Rg: find the highest-Mu tube that has Rp less than Rg/5.)

BUT for current amps, reverse this. Make the plate DC resistor 5* the Rp, make the next stage input about 2* the Rp. It isn't great efficiency, but when low Rp is hard to get, it may be your best deal.

Since we are stuck with 10K-15K transformers, we want Rp about 5K-7.5K and the DC plate resistor about 25K-30K. This gives Read's plan.

To get the lowest Rp for a given tube, use low voltages (but above the bend at about 50V in many tubes). Plate voltage maybe 100V, grid voltage as low as you can go.

I believe that if a triode with significant Mu is worked with a DC plate resistor much larger than Rp, and the grid left nearly open, grid leakage tends to force the grid voltage to about Vp/Mu, where Vp is the self-adjusting plate voltage for that DC plate resistor and that B+ voltage. If the grid resistor is less than infinity but much higher than the value specified for predictable biasing, the grid approaches Vp/(2*Mu), which is the maximum symmetrical grid-swing point, where we want to be for maximum output. Assuming the plate resistor is much higher than Rp, this is actually quite stable. You have to watch that the plate dissipation can NOT get too high: here the worst-case would be (360V/2)^2*30K or 1.08 Watts, safe for a 2W tube. Note that the resistor dissipation can be higher: 4.3 Watts if the tube shorts, 3.2 Watts if the tube goes to 5K, 2.5 Watts if this is a 5K-7K tube that self-biases to about 10K DC plate resistance. I'd say that 3 Watt is the absolute minimum rating for that 30K resistor, and if you hate smoke then three 10K 2W resistors would be wise.

Another point about working low plate voltage: the output plate voltage is small. If feedback factor is also small, you can return feedback to the first cathode without a blocking cap, as Read has done. This saves a few bucks. It avoids a bass-pole in the feedback, which may be good or bad (the cap forces a rising bass response which can be exploited to flatten sagging transformers). You will have to re-figure your input cathode resistor to account for all the DC in it, and note that the first tube is not cathode-biased but essentially "fixed" bias (the cathode voltage is mostly due to output tube DC levels, not first-tube cathode current).

skipwave

6SN7 / 6CG7 preamp
« Reply #29 on: January 12, 2006, 11:49:57 AM »
Quote from: "PRR"
> The center tap pin 9 is connected to one side of the secondary, instead of grounded for 6.3V or unconnected for 12.6V. It looks like the latter was the intention, since pin 5 is grounded, or that was a mistake too.
4 and 5 are tied together. 6V is fed to pin 9 and pins 4+5. Heat is correct.


I missed that connection and thought that 5 was jumping right over to ground. Thanks for pointing it out. Now I will re-read your post for the third time today and learn some more. :thumb:

Edit: More questions about application specific heater connections; In the Dyna preamps I see the second stage grounding one side of the heaters and the first stage grounding the center tap. As seen here. Is this because, as PRR stated, the grounded center tap is better for low level work? If so, I need to change my One-Bottle pre, since I grounded one side.
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.


PRR

6SN7 / 6CG7 preamp
« Reply #30 on: January 12, 2006, 04:49:33 PM »
> In the Dyna preamps

I was speaking of AC heat, the only kind practical for many decades.

The Dyna uses DC heat. Huge selenium rectifier to handle the current, and a couple fat caps (we do it in 1/10th the size today). This normally makes heater ground-balancing unimportant.

> I need to change my One-Bottle pre

Does it hum? If not, don't mess with success.

skipwave

6SN7 / 6CG7 preamp
« Reply #31 on: January 12, 2006, 06:48:47 PM »
Quote from: "PRR"
> I need to change my One-Bottle pre

Does it hum? If not, don't mess with success.


No hum. It runs very quiet, so I'm :cool:. If I didn't mention, I copied the Dyna PS because I already had the parts around. I read that replacing the selenium rectifiers with silicon (which I've done) increases noise and shortens tube life. Is that a real concern?

Coming back to the Rafa pre, I think I have a line on some 6SN7s, so I might be able to give it a try. There is some appeal to a zero feedback two triode design.

It seems like this would be possible with the 6EM7 that you have mentioned a few times. However, the typical plate current of the second triode scares me, 50mA. :shock: Across a 4k7 plate resistor that's a 235V drop, so B+ of 385V. With the low Rp, 750ohms, I could use my Triad A69J TXs strapped for 6250:500 and still be over 5 times the Rp for clean amplification and not too much power lost. First triode gets a plate resistor of 100k, and we run from there through a 400k pot to grid 2.

Sorry about the rambling, I just wanted to throw out some ideas and see if I'm starting to understand. :green: The 6EM7 does seem like a really cool candidate for a simple two triode design.
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

NewYorkDave

6SN7 / 6CG7 preamp
« Reply #32 on: January 13, 2006, 05:47:03 PM »
Quote from: "PRR"
I spent an hour looking for a classic two-triode broadcast mike amp schematic. Where are they hiding?


Here's another one for the collection: the RCA 41-B from the 1930s.

628kB PDF

Alas, the schematic includes no part values.

NewYorkDave

6SN7 / 6CG7 preamp
« Reply #33 on: January 13, 2006, 09:06:30 PM »
Quote
I could use my Triad A69J TXs


You're thinking of gutting your "One-Bottle" already?

Booo!

 :?

PRR

6SN7 / 6CG7 preamp
« Reply #34 on: January 13, 2006, 10:33:08 PM »
> I copied the Dyna PS because I already had the parts around. I read that replacing the selenium rectifiers with silicon (which I've done) increases noise and shortens tube life. Is that a real concern?

Sounds like BS to me. Silycon rectifiers may have a sharper turn-on than the old stacks: it would be good to add an R-C stage, say 1 ohm and 2,200uFd per side. If you use Dyna-exact parts, you should check the heater voltage: the original rectifiers had more drop than modern ones. Dyna did design for slightly low heater voltage: when you work the tube FAR below maximum cathode current, no harm results, and you may (and may not) get a little less hiss.

> 6EM7 ..., the typical plate current of the second triode scares me, 50mA.

Well, it was made, rated, and marketed to do the most work for the least bucks.

Say 20mA. It is a waste of a fat 100mA-peak tube, but it isn't like the tube cost will hurt you. (The 1.05A heater may be painful.)

> Across a 4k7 plate resistor that's a 235V drop, so B+ of 385V.

Nah, let's see.... 300V supply, 10K DC resistor, 20mA, puts 100V on the plate. We can cleanly swing a low-Z load to 10mA and 30mA, 10mA peak. With a 2:1 transformer, 2K4 primary, we can swing 24V peak primary. We have 12Vpk 20mA peak at the secondary, +21dBm in 600 ohms. The incremental plate resistance here is about 1K, so damping factor is 2.4, so output impedance is 250 ohms, resonably low. It looks like about 14Vp-p on the grid, and 7Vpk in for 24Vpk out is a gain of 3.4, which agrees with Mu about 5 and this ratio of Rl/Rp. Gain to the load is 2:1 less or 1.7. The small-side of 6EM7 can easily give voltage gain of 40 (use 12AT7 tables). Total gain from first grid to 600 ohm load is 70, or 37dB. 1:10 input iron makes it 57dB. That may be too much for some uses, put a pot between stages. How far down can we turn the pot before the input tube clips first? Taking voltage-amp peak output as 20% of supply, the small-side can make almost 60V peak, we can turn-down 18dB. After that, the input stage clips before the output stage reaches +21dBm.

Working the tube richer, with the plate around 60V, buys a little lower Rp and a little more gain. But we may have trouble holding it there. 80V-100V on the plate may be more stable.

You could also work things at lower B+. Some TV sets worked as low as 110V B+. But this works much better with transformer-coupled (DC in the iron) than RC-coupled (iron just for matching).

Working 300V B+, 4K resistor, 50mA current, 2k4 load, we could pull 50V 20mA peak, 25V 40mA peak in the load, +27dBm. Gain rises to 4, Rp drops to 800 ohms. If we don't need +28dBm, the small improvement is probably not worth the added supply power. Note also how little the performance changes when plate resistor and current changes by a factor of 2: we do NOT need precision parts to get consistent results. 4/3.4=1.4dB. Your idiot parts-assistant got 7K4 instead of 4K7 resistors? Slap 'em and ship 'em... customers will never know.

skipwave

6SN7 / 6CG7 preamp
« Reply #35 on: January 14, 2006, 11:13:03 AM »
Quote from: "NewYorkDave"
Quote
I could use my Triad A69J TXs


You're thinking of gutting your "One-Bottle" already?

Booo!

 :?


Not without suitable replacements. I have Altec/Peerless 15095A that I can swap in. I wouldn't maim the One-Bottle. :wink:  I'm doing overdubs with a band called "The Dawn of Things to Come" today, and I'll bet we use the One-Bottle pres on damn near everything.

BTW - I don't know how I didn't see it before, but on the Triads it clear says "2.5ma D.C." next to the primary. I take that to mean I could run the B+ through it as long as the current draw was that low, so I could use them for a BC2B clone. Hmmmmmmmm....... :idea:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

NewYorkDave

6SN7 / 6CG7 preamp
« Reply #36 on: January 14, 2006, 09:29:14 PM »
The 15095A should work great (15K:600).

I have a bunch of them, pulled off of various Altec amps before they headed to the scrap heap. If I had any sense, I would sell them--they've become sought-after for some reason--but I'm such a packrat, I can't ever get rid of anything.

The Triad wouldn't be great for a BC-2B preamp. It would need to be rated for at least 3.5mA.

skipwave

6SN7 / 6CG7 preamp
« Reply #37 on: January 25, 2006, 01:22:53 PM »
Quote from: "NewYorkDave"
Before you invest in custom iron, try this. It might meet your needs well enough, and uses a regular (non gapped) 10K:600 output.



B+ is 300V, well-filtered but not necessarily regulated. R1 represents the secondary impedance of the input xfmr, R11 and R12 represent the voltage stepdown of the output xfmr. I don't use the transformer models in Circuitmaker because they don't seem to work very well.

Electronic gain is about 42dB, or 30dB to the secondary of the output xfmr. With a 1:10 input xfmr, you'd have 50dB total, or 53dB with a 1:15 input, etc.

THD is below 1% at normal operating levels--IF[/i] you believe the simulator.


Got some Sylvania 6SN7s. Did anybody have time to try this plan yet?

I got a 6EM7, too, but that deserves it's own thread. :wink:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

jsn

6SN7 / 6CG7 preamp
« Reply #38 on: January 25, 2006, 11:46:13 PM »
Hey doodz,

I haven't read this thread in detail yet, but I have a couple things to throw into the mix.  I'm a hi fi and guitar amp guy who is slowly getting into studio gear like this.  I have built a couple mic preamps so far, and it is really a lot of fun.  Low level signals like this require a whole other way of looking at things.

Here are my mic preamps.  I have a schematic drawn for the second one, but haven't scanned it yet.

http://boozhoundlabs.com/microphone-preamp/
http://boozhoundlabs.com/microphone-preamp-2/

I think the reason the Altec 15095 and 4722 mic transformers are so popular these days is because people (myself included) have discovered them for use as stepup transformers for moving coil phono cartridges.  Especially the Denon DL-103, which has attained cult status along with a couple old idler drive turntables.

I wanted to suggest another alternative to the whole plate resistor size thing.  It is nice to be able to make this resistor large because you get more gain, and a more linear load line, but as you increase the current in the tube, this resistor drops more and more voltage and ultimately makes you wish you had a higher voltage supply.

The hi fi tube amp guys like to solve this in 2 ways, either by loading the tube with a choke, which has the amazing property of low DC resistance, but high AC impedance, and so is a good linear load, but without dropping the voltage.

The second way is to use a current source, of which there are many designs.  Basically this is little circuit that acts as if it has a very high impedance, but only drops whatever voltage is necessary to pass the current at which it is set.  Using this as a plate load gives you the advantage of a very linear load line, and lots of power supply noise rejection.  k and k audio makes a simple little $10 current source kit you can buy.

Good stuff.  Thanks for the discussion.  This is neat.

jsn

skipwave

6SN7 / 6CG7 preamp
« Reply #39 on: January 26, 2006, 02:36:13 PM »
Quote from: "jsn"
http://boozhoundlabs.com/microphone-preamp-2/


Hi Jsn,

Welcome to the board. You're 6sl7 pre looks just like my 6sn7 will, I was even going to use the 15095s. :thumb:

Dave was being facetious in his remark about the Altecs' popularity. He was just pointing out how they probably aren't worth the money they are selling for now, but so much hype has been attached to them. That's the power of the 'net for ya. :wink:
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.


 

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