One-Bottle Preamp

[NewYorkDave]

Here's the best advice I can give you:

1. Don't worry about what anybody else thinks.*

2. Build a breadboard of the preamp circuit. That's easy enough since there's only a few parts.

3. Try out the transformers in different combinations and draw your own conclusions.

(* Everyone's got an opinion. They'll tell you that the UTC tastes like chocolate and the Edcor is peanut butter, and if you put 'em together you'll get Reese's, but that doesn't account for the contribution of the preamp circuit, how certain attributes combine, reinforce and cancel, etc. ).

 

[cannikin]

Much appreciated D

cheers, I let you know how it goes

 

[kiira]

urk... I can't remember if the filiment voltage will go down after I put a load on the HT winding?? I was testing the power supply but I just hooked up the filiments of the 12av7 and the 5879 so my HT is high acourse, 350V. but my filiments are getting 7.0 volts from the 6.3V winding, 119 volt lines. I'm using that an 8416 250V Stancor 25 milliamps . BTW that 8416 that was bin on ebay recently was for sure wider than the one I have - it looked newer too.

Finally getting to the end of building the NYD 2 bottle... after making two enclosures, all new inside brackets (shock mounted tubes woo) and two faceplates. I have enough junk left over to build another pre or DI. Of course I am short "that one little thing"... the three resisters to make the -20 pad but I can't wait again I'll just leave it off until I make another parts order and test the pre with my sm57 and M260.

Kiira

 

[skipwave]

[quote author="kiira"] the three resisters to make the -20 pad[/quote]

We haven't discussed the need for the pad with the 5879 stage. There is a volume control following it, so I'm curious what level it would take to distort the first stage? How hard do you have to punch that grid to make it cry?

 

[NewYorkDave]

The rule of thumb, in a common-cathode stage with a bypassed cathode resistor, is that you're about to overdrive the grid when the peak signal voltage approaches the value of the grid-cathode voltage.

Take the cathode voltage, multiply by .707, then divide that by the input transformer ratio. And that'll give you the RMS input voltage at which the grid of first stage will start to conduct.

 

[skipwave]

Cathode voltage, meaning voltage drop across Cathode resistor?

If I did the math correctly, 1ma across 2.2k cathode r makes a 2.2V drop, so it would take 156mV RMS signal into a 1:10 input transformer to drive the grid into conduction.

That seems low-ish, but that's the point of a high gain pre for low input signals.

 

[kiira]

uh oh... hope I didn't get burned on these I borrowed the money to get them.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=6611789693

I better throw together CJs pre this week so I can test them right away. Those things usually sell for $30-$50 each these days no joke.

Kiira

 

[skipwave]

[quote author="kiira"]I better throw together CJs pre this week so I can test them right away. Those things usually sell for $30-$50 each these days no joke. [/quote]

Look at you rockin all the one bottle designs. Those were a great deal if they play nice, of course. I've been keeping an eye out for those, just to add the CJ pre to my scroll of projects (the list got so long I had to roll it up).

 

[kiira]

[quote author="skipwave"]Look at you rockin all the one bottle designs. Those were a great deal if they play nice, of course. I've been keeping an eye out for those, just to add the CJ pre to my scroll of projects (the list got so long I had to roll it up).[/quote]

Nah the one I'm finishing up is NYDave's 2bottle. I had given up on getting a reasonably priced 417a and decided to build a redd47 instead because I have good EF86s and 6922s. A 5 component circuit tis just the nazz[1][2]though. :razz:

Kiira
------
1. where did that word come from? I keep thinking Mose Allison or Lord Buckley.
2. yes... footnotes on forum posts.

 

[kiira]

quick question about the 2bottle... does the cathode resistor on the 5879 need a bypass cap? or rather, should it have one?

thanks,

Kiira

 

[skipwave]

The schemo shows a cathode bypass cap on the 5 8 7 9 stage. Use 100uF or larger.

Disclaimer: I'm ever the newbie, so you should wait for the voice of authority to confirm or refute anything I say.

 

[NewYorkDave]

it would take 156mV RMS signal into a 1:10 input transformer to drive the grid into conduction.

Correct. That's -14dBU.

that's the point of a high gain pre for low input signals.

Yep. And that's why building-in the switchable pad is a good idea unless you use nothing but ribbon mics. If you never need that much gain, better to go with the One-Bottle. It's also not a bad idea to arrange the Two-Bottle so that you can bypass the first tube stage when it isn't needed... in other words, a way to bypass the 5 8 7 9 and the pot and effectively revert to the One-Bottle circuit.

need a bypass cap?

The bypass cap is optional if you're using a DC heater supply, but should definitely be used with AC heaters. The gain of the stage will be about 6dB less without the bypass cap. Use 47uF or greater.

 

[khstudio]

I'm getting ready to go to my friends shop & he's got TONs of old shit laying around & wondered if someone could post a list of tubes & output transformer ratio's that would work in here. I'm gonna start building this tonite.

Dave, I'd like to try these input tranny's on your pre. I'm also trying to identify them... could you or anyone help.

http://www.groupdiy.com/index.php?topic=14290

I was going to build 1 NYDpre & 1 CJpre but I can't find CJ's tube & want to use scrap parts only so yours is 1st.

Thanks for the project :thumb:

Kevin - KHStudio

 

[Kingston]

[quote author="khstudio"]wondered if someone could post a list of tubes & output transformer ratio's that would work in here. [/quote]

All this is covered in the thread in great detail, but I'll try a quick recap.



I seem to remember NYD was pretty strict on the 12AV7 and 5879 (for the two bottle) requirement. It seems he engineered and tuned the parts to work extremely well with these particular tubes. At some point it was concluded that 5965A *might* work as a replacement for the 12AV7 (something that I will test soon), but that it isn't exactly equal.


Input transformer IIRC can range from about 1:6 to 1:15. The exact gain figures and math are somewhere in this thread.

Output transformer was recommended to be 4:1 or 5:1. It needs to be able to handle the possibly very high output levels. The exact figures, again, are somewhere here. 2:1 was said to be less than optimal.

 

[khstudio]

Hey thanks. I just got off the phone with him & he's got some Stancors, 15k:600 rated for 30+ DB of gain. :shock: think they'll work?

This post is so long I got lost... did anyone ask about a 12aY7. I have a few of those.

Will they work for the 1 bottle. I know they sound great & are one of my favorite tubes.

 

[NewYorkDave]

I assume you mean you got off the phone with your friend, because I haven't heard from you today :wink:. Anyway, your friend's 15K:600 +30dBM Stancors should work great for the outputs, assuming they're full-bandwidth. For the inputs, those 150:60K transformers you posted earlier should also be fine--again, assuming they're full bandwidth.

12AV7 is preferred; 5965A would probably work OK but nobody's tried it yet. A 12AY7 wouldn't work well without a redesign of the circuit. Luckily, 12AV7 are much cheaper than 12AY7 these days. Hold on to those 12AY7s because they are used in a number of other mic preamps.

 

[khstudio]

[quote author="NewYorkDave"]I assume you mean you got off the phone with your friend, because I haven't heard from you today :wink:. Anyway, your friend's 15K:600 +30dBM Stancors should work great for the outputs, assuming they're full-bandwidth. For the inputs, those 150:60K transformers you posted earlier should also be fine--again, assuming they're full bandwidth.

12AV7 is preferred; 5965A would probably work OK but nobody's tried it yet. A 12AY7 wouldn't work well without a redesign of the circuit. Luckily, 12AV7 are much cheaper than 12AY7 these days. Hold on to those 12AY7s because they are used in a number of other mic preamps.[/quote]

Dave thanks, I'm leaving now to get my stuff.

Yes the Stancors are 20 to 20k.

Man I hope he's got some 12aV's... I can't wait to hear this thing. I'm getting tired of my MP20 & M80... although there cool I like tubes.
I'm not sure if it's the same thing but the gain I get from my LA-2A is so fat & open it really made me want a tube mic pre.

 

[khstudio]

OK, I'm back with 10 more transformers :green:

I've now got 8 of those Triads but still no model #'s... all scratched off http://www.groupdiy.com/index.php?topic=14290

I dug thru about 300 dirty as hell tubes & found 3 GE-5965's & two measured good on my tester so I've almost got all my parts.

I thought he said Stancor but they are SESCOM:
MODEL- MI-69
PRIMARY IMP- 150/600
SECONDAR-IMP- 15k
USE*- LI, LO
PRI POWERLEVEL 600- +18
MAGNETICSHIELDING- ?10 dB
CASES- M

He also gave me an old RCA Gray can 9 pin plug-in, type SXMG-09 - can't find info for it.

And an AMPEX 8 pin light gray can, it says: Input Transformer CAT. No. B-17331-1D - it tells input impedance but no out, just were the pins go.
Input can be 150-250 ohm OR 30-50 ohm.

And an ACDC Electronics Inc. , Large can with 5 taps, part# 7859


I don't want to beat your post up but I need help figuring out what these are & if they'd be good for this or any of my mic pre builds. If pics would help I'll post them.

Thanks,
Kevin

 

[khstudio]

Dave, I started another post about these tranformers to get it out of here.

http://www.groupdiy.com/index.php?topic=166153#166153

Please check it out & let me know what you think.

Thanks Dave,
Kevin

I Think I'll be the first to try the GE - 5965 (no "A" after it though)

 

[Kingston]

a quick info since I had it tested now,

5965A works extremely well in this design. Intially, compared to the RCA 12AV7 it has a slightly more detailed "air" and the shape of mid transients seems different, too. On the other hand the 12AV7 is rougher overall with more bite. The biggest difference between the two is probably in the hi mids, where the 12AV7 is more agressive and 5965A a bit "loudness curve hifi" ie. slightly scooped.

This is good news since 5965A is a bit cheaper around here.

[EDIT]

see next post.