One-Bottle Preamp

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The switch actually gives treble boost, not bass boost, eh? :grin:

The resistance looking into the cathode is about 136.4 ohms and that in parallel with the 62 ohm Rk gives us a high pass corner frequency of about 374 Hz when you switch in the 10uF.

It would take a 330uF bypass cap or greater to get full range.

Nice design! Pics please...

regards, Jack
 
I think your right. The bypass cap might have been a 100 uf. I will see what's laying on the carpet when I get home. It was late when I was playing A/B games with the cap.

Nice design? What design? :grin:
Oh, you mean design-er. I better get that off of there before they sue my nuts off.

Ok, pics tomorrow.
 
Nice work, CJ. Jack has already pointed out the deal with the cathode bypass cap, but I'd like to make a couple of suggestions: use a larger output coupling cap, add a tiedown resistor across the output terminals, and wire a resistor across the "boost" switch to minimize thump when switching. Use a value slightly higher than ten times that of the cathode resistor.
 
Good idea on the resistors Dave! Thanks.

I bridged the output cap with a 10 uF and did not detect any difference. But that was feeding a headphone amp. It might change if feeding another load.

Since that cathode bypass cap makes such a difference, it can be tweaked to give the preamp the tone you are after with a particular mic. Maybe a rotary sw with different values.......

I think EMI and the G9 use something similar, only in the feedback. But this is a feedbac issue also. Since only one stage, feedback is a problem, so an easy way to play with feedback is with that cathode.
 
What's the open-loop gain (of the tube stage alone, not including the stepup from the input transformer?). Since you're using such a high-ratio input transformer, you might be able to afford to sacrifice a little electronic gain if you wanted to add voltage feedback.

The classic way to add voltage feedback around a single stage with a transformer coupled input is by "floating" the low side of the secondary winding on an ac-coupled signal voltage divider coming off the output. The connection would be: plate, coupling cap, resistor to low side of winding, resistor from low side of winding to ground.

Note that as far as the grid of the tube is concerned, the latter resistor appears in series with the signal source impedance (the impedance reflected to the secondary by the mic). For this reason, and to avoid stray capacitance effects from floating the secondary too high above ground, the smallest practical value of resistor that will give the desired feedback ratio should be used. But then this means the series resistor in the feedback network must be lower for a given ratio, which means greater loading on the plate. It's a series of tradeoffs as usual.

This is an interesting "what if" exercise, but I'm certainly not trying to imply that this is worth doing, especially if you're already happy with the sound and the amount of gain.
 
Ah yes, like the input x-former on the V76.
Good idea. Plenty of room for xtra parts with this circuit.

I have a WeCo 407A also, which is a similar tube, only with two triodes. Different plate R and Gm, but still promising. I was thinking of a one stage push-pull mic pre. This would take two chunks of iron, which is not a problem!
:grin:
 
[quote author="CJ"]
There is some high frequency ringing when the tube is tapped, way above what most tubes ring at. So I will shock mount the socket and maybe add a tube cooler.[/quote]

I get light tingly ringing noise too embeded in the noise when the coupling resistor value is too low, while the gain is big.

Got a pic of this tube cooler?

Bias sat at about a buck and a half with 250 B+.
Went to 75 cents with 125 B+.

Just out of curiosity, what is your bias current for the tube??

Let me know if you want some pics. Not much to see with that parts count.

I would love to see a pic of the output signal 1Khz or whatever preferably on a CRO not digital, also closed circuit input and output connected with no signal noise voltage.

Any chance of seeing that?? :green: :thumb:
 
Hi Learner!
Ever get that T4 installed in an LA2?

I have a big pwr supply that I would have to bring to work, but tell ya what, I will take this old scop home tonight (CRT, and see if I can get some shots.

You can do the bias yourself, Ohm's law. No screen current, so real easy. That will be your homework assignment for tonight!

Tube coolers, do a search. I goota go watch a local buddy dj on Hit Me Baby right now. (Greg Kihn)
 
[quote author="CJ"]Hi Learner!
Ever get that T4 installed in an LA2? [/quote]

Haha, still working on it give me some time dude!! :green:

I PROMISE I will keep you posted when I have done so, in fact I was just thinking of playing it with my new tube amp recipe to see how it sounds.
:grin:

Cloning is way down on my yet to do list at the moment......

I have a big pwr supply that I would have to bring to work, but tell ya what, I will take this old scop home tonight (CRT, and see if I can get some shots.

COOL!!! Looking forward to see some pics! :thumb:

You can do the bias yourself, Ohm's law. No screen current, so real easy. That will be your homework assignment for tonight!

Yes, have already done so with my 8 tube pre amp and done about 10 biasing for 10 different types of tubes. The reason that I want to see your biasing figure is because all my biasing of RF pentodes and TT are like less than 1 mA except 6SK7, 6BA6 and the beam tubes which is more than 1 mA for optimum voltage gain and a clean output on the CRO. I just want to see if you get similar results for comparison, othewise I suspect that something is definitely not right with what I am doing. I no longer calculate my biasing but tweaking it on the CRO, which so far has worked very well for me comparing to the load graph calculation business.....

Saw the data sheet where it suggests 25mA for normal operating condition but I have doubts that you will get any voltage gain biasing it at more than 2mA, before the voltage gain get all swallowed up by current with a flatliner on the CRO. I suspect you might end it up at about 0.8-0.9 mA mark for optimum voltage gain and as it past 1.2mA mark the gain will start to diminish...

I am looking forward to see your results!!!

Tube coolers, do a search.

I have seen those 2 ring tube dampers around, was just wondering if that's what you are refering to.

I goota go watch a local buddy dj on Hit Me Baby right now. (Greg Kihn)

Have fun!!! :grin:
 
The 417A is an entirely different animal than most audio preamp tubes. It was designed as an IF tube in a microwave reciever. (70 mHZ) It uses a frame grid instaad of the usual posts, which means you can get the grid a lot closer to the cathode, which is why the Gm is so high. I did some more tests last night, and even with the B+ dropped to 150 volts, the preamp is way louder than API, Langevin, and V72.

The crackling noise has disappeared. I guess after 40 years, the tube just needed a little burn in. Running at 150 will help keep noise down over the long hall.

Current is high, so you have to be careful on your plate resistor wattage. At 300 vdc, it' cooks pretty good since Ip is about 27 ma. At 150 B+, current drops to about 11 ma, so a 2 watt resistor is just fine.

This is a great vocal pre, I like it just as much as the Tele V-72.
I forgot the signal generator last night, so all I had was the scope and a 57 in front of a guitar amp, but everything looked real smooth.

Unfortunately, people seem to have caught on to this "dumpster" tube and prices have gone ballistic. I just checked my new Antique catalog and they have "call" written in instead of the price, which is always bad news.

cj :sad:

I just called Antique and their price is $38.20!!!
The darn janitor at the surplus store was kicking a bunch around because they kept getting in the way. Oh well. Some unwary fool probably has a bag fulf of them for a buck a piece, which is what I paid.

The bypass cap I used was 10 uF. I tried it last night and it makes quite a difference, even with the low cathode resistor.
I am sure because I used no polarized, and 100 uf is hard to come by in non polarized.
 
Oh boy are you in for a treat.
Wire up something like this.
I know you have iron by the bucket loads.
Skip the cathode cap. It sounds just fine without it.
I am actually using 65 ohms cathode resistor, but whatever you have that is close will work.
Keep your B+ at 200 or lower so you can use a stock 2 watt plate resistor.

Parts List for you:

1 ea. 4.3 K or close resistor. 2 watt
1 ea. 62 ohm resistor or close 1/2 watt
1 ea. cap 1 uF at 250 vdc
1 9 pin tube socket

1 input transformer- ratio of youer choice-enough gain in tube for even a 1:4.


cj

one_tube_micpre.jpg


Learner, I lowered the plate resistor, not much difference, just reduced gain.
NYD-I tried a 20 uF Poly on the output, no difference.
 
No difference because you must be running it into a fairly high-Z load. If the load was lower-Z, then you'd notice a difference. Crunch the numbers into the capacitive reactance formula and see for yourself. :wink:

Of course, you never want to go very low with the load impedance if you're coming right off a tube plate, which is always going to be a pretty high-Z source in the grand scheme of things.

I specified a 2.2uF "or greater" in my one-bottle pre because unless an output transformer is used it should never be run into less than 10K. So with the minimum rated load impedance, the -3dB point is around 7Hz.
 
I will try a load resistor on the output and a resistor across the input iron tonight. I do not know if I can improve this thing. There is not much to work with. But I will try.
 
Post your pictures. I think people here need to see some tight-right tube wiring. I know you did tight-right, because a 417 is a hot tube to tame.

For general use, I really think you want the output transformer. Even though it will be hard to find. With R-C coupling, you have way-high gain in hi-Z loads, low gain in low-Z loads, and unbalanced output.

5K:600 30mA is an odd transformer; might have to DIY. For hasty tests, a 5K:8 power-amp transformer will give the tube the same loading, though about 18dB less voltage gain than with 5K:600. BTW, with 5K:8 and efficient speakers, this should be a sweet little half-watt loudspeaker amp.

Also, the gain should not shift much with supply voltage. Certainly not enough to be a useful gain control (may be a great favor control). The gain will be even more solid with transformer coupling.

The cathode impedance is VERY low, lower even than the obvious 60 ohm resistor. Proper full-bass bypassing will need to be at least 500uFd. This can be a Polarized electrolytic: polarity is ensured, and maximum voltage is only a few volts. Try 2,000uFd 6V.

You may not "need" bypassing with R-C output, but with iron output you probably will.

Noise should not be a strong function of supply voltage. I suspect that indeed your dumpstered long-sleep tubes needed a wake-up burn-in. Bias up to maybe 80% of plate dissipation rating, on a flame-proof bench, and let them cook a week.

IIRC, the 3A55A is 1:15, not 1:30. 1:30 from 150Ω source is 150K secondary impedance: winding and grid capacitance would eat all your highs off. I would not even try to Zobel the funky 3A55A winding: let it do its ring-thing, then Zobel the plate circuit to "tolerably-flat". 10,000pFd+1K in the plate side should be a too-much droop: reduce the C to shift the droop higher frequency, until it negates the rising side of the 3A55A ring, then increase the R to get back to ~12dB/oct slope on the high side of the ring.
 
> The 417A is an entirely different animal than most audio preamp tubes.

Yes, but compared to learner's 6SK7, it does the "same job" only shifted 1.5 decades in both history and bandwidth. Both were the hot high-Gm tubes of their day. 6SK7 is Pentode because at its frequencies you can get gain with high-Z (~100K) loading, and the ionosphere makes tube noise a minor issue. At 417 frequencies, high impedances are impossible (stray capacitance dominates everything) so it neglects high-impedance operation in favor of very high current and Gm to make some gain into the few-K load of 400MHz tanks.

Also: you can map a line from 12AT7 or 6BQ7 to 6DJ8 straight toward 417. It would not be far wrong to liken 417 to four or eight paralleled sections of 6BQ7. (And a case of 6BQ7 could be found a lot cheaper than a single 417.) The 417 gives better performance per power-watt, but in home/studio use the pile of 6BQ7 might have lower lifetime costs even with the electric bills.

I'm sure learner's troubles are due to not keeping plate voltage around half of supply voltage, or not watching the grid bias. Many-mA operation of that tube has little advantage for audio voltage gain, but it sure is possible (see the H-P 200AB which has a similar pentode working at fairly high current, or many oscilloscopes).
 
pictures....

OK, alright. Hey, it sounds good, so.....:oops:

The slanted resistor is in series with the 6 volt heater regulator that gave up and put out 7.2 volts.

we_micpre_1.jpg


Image

Looks like I have a 1.5 uf instead of 1 uf. I need glasses.
This tube has 4 connections for the same grid, which is kind of a mystery to me.

Image

Plug and play, trying different components:

we_tuneup.jpg


"5K:600 30mA is an odd transformer....

Will this work?
It's a bit large, but for a test....

It's made by guess who?

we_output_1.jpg


Your right about the supply voltage and the gain. As I dropped the B+, I heard the noise go down and automatically figured that the gain was going down. But the thing kept the same gain! So iif it like 150 volts B+, and the noise is way down, that's where it will sit.
 
"Proper full-bass bypassing will need to be at least 500uFd........

It's a mystery to me too why a 10 uF bypass cap has such a profound effect on the sound, but it does. Maybe it's a NFB issue. This amp works best as a vocal mic, so I am not too concerned with 20 hz response for micing amps, etc. But if I can get it to sound as good as a Langevin when micing a Fender, I might put in that cap with a switch.

" indeed your dumpstered long-sleep tubes needed a wake-up burn-in....

Just in the last 24 hous, the rice krispies have disappeared. With the lid on this thing, it's extremely quiet. :thumb:

I will try a network on the output tonight.

Isn't this a fairly low output impedance circuit, even without a transformer?
I mean 20 ma thru a 4.7k plate resistor seems like it would pull a freight up the Tehachapi Loop?

http://www.tehachapi.com/loop/
 
hey CJ,
What exactly is the supply voltage?? 100V or 200V? and your bias current is 11mA!?? :shock: hmmm...... I would love to see the output on the CRO with a signal running through it, also the noise figure with closed circuit.

What is the gain of this tube?? I would not think it will be too high if the current is at 11mA.

I don't have transformer in my circuit input but I have a coupling resistor connected to the headphone output of my CD player, this resistor reduce the noise but also the input signal. I wondering if that the input transformer works the same way, otherwise by connecting the input directly to the grid makes horrendous noise but may be because I have a grid shunting resistor.... I will go and try that without the resistor.....

Same case at the output, I have a resistor connected in series with the output coupling cap. This resistor kills both noise and gain, so its a compromise. Without it and with no FB for the tube, I get a sine wave like sort of noise at the output and its loud. I forgot to measure its frequency..... :mad:

How did you work out the bias, using load curve? Interested in replacing the plate resistor with a 500K ohm pot then run a 1khz signal through the tube, probe the plate coupling cap and tweak the plate resistor pot. Watch it on the CRO and see what value of the pot you arrive at to get optimum gain and minimum noise, you might want to add another 500K pot for FB as well, since it will help to flatten the noise you'll see it on the CRO!

I don't have cathode decoupling cap on any of my tubes because I found that it added noise, may be I didn't wait long enough until the noise settle down a bit after turning on the circuit. I might try that again.....

I also tried all different coupling caps and could hear NO difference, the only thing I find is that the biasing point is absolute critical to gain, noise and distortion which to me pretty much defines the sound. There is a optimum region for the value of plate R and FB R and screen R(in case of the Pentode), where by combining these 3 variables you can achieve maximum gain with minimal noise in comparison to every other combination of the 3 variables. I only tweak for max gain and min noise because there isn't much I can do to alter the distortion plus its really minimal, can not really see it on the CRO also to me it defines the personality of the tube as well as the characteristic of the sound.
 
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