One-Bottle Preamp

[Svart]

Hey CJ, you said no trafo, but you mention using at least a 1:14 elsewhere..

also you mention that amplification changes with cathode R.. could this be a reliable way to apply a trim to the gain or should i stick with some kind of T attenuator on the input before the trafo?

I am thinking a dual primary toroid using one of the primaries as my 150VDC B+, secondaries being either 6V or 12V for heaters. which would you suggest?

 

[CJ]

I was on crack last night. I have a DuKane on the input, but just a cap on the output, so I was half right. I must have stayed under water a bit too long on the river last weekend.

 

[Svart]

hmm ok.. I still would like to know if my ideas might work.. :green:

 

[Svart]

i dug up some beyer input trafos, 1:15 200:45k

beyer 3.51.0.15.006

would these work with the 417based preamp? they are tiny little things though..

I still wonder about the cathode R adjustment for gain adjustment.. why or why not?

I also see that the 5842 valves are close enough to work.. what differences are there?

 

[CJ]

DC on the heaters, working on the tingy micrphonic thing with shock mounted socket/tube cooler, yes the Beyer looks perfect, cathode resistor easy to tweak, so just put something in. Keep plate volatage at around 150 for super quiet performance. Still plenty of gain at this level.
5842 is same tube. Right? I have another secret "Dumpster" tube, similar to the 417, but ten bucks compared to forty. Lower Gm. PM me if interested.

Hopefully a few lab members are going to get together for a mic pre ho-down tomorrow. I am bringing a V72, V76, Valley People, Langevin AM16, and the 417, so you might want to wait for the results on Sunday.


cj

 

[Svart]

Thanks CJ. yeah the 5842 is a cross reference part with close specs for about 11$ a piece. I'm going to try those, but I might still need some of those specials you have.. :green:

I forgot i had those trafos until this morning when digging through some other stuff.. Maybe i can find some more!

I've already ordered the caps/resistors and other stuff for the preamp and plan on just putting a number of 10 or 20db pads in front of the input trafo for gain control.

I found some LDO 6.3v regulators that i am going to try on the heaters..

as for the cathode resistor.. I was actually wondering if putting a pot in there for gain control would work? I don't know what it will do but i might just try it.

I am planning on using a dual primary toroid with the second primary as the B+ and the secondary as the heater.. anyone recommend this or condemn it?

I'm stoked about this project!

thanks!

 

[Svart]

starting to get the parts for some preamps in!

shouldn't be long now.

I'm still kinda wondering about a few things in the above post..

:thumb:

 

[Learner]

[quote author="Svart"]

Also, Learner, CJ and everyone else.. I am interested in this "tingly" sound you are getting.. why does that happen? is it pleasant? something to be used?
[/quote]

Sorry, been totally flat out busy lately didn't catch the post.....

I donno the cause for that tingly noise, it comes with biasing the tube above the optimum current level. To me, it sounds like tiny little metallic pebbles striking a metallic plate.... :shock: like a faint tinging..ing ing ing.... kinda sound to........ ztss..ztsss..ztssz..... when youy increase the coupling resistance. It is still audible if you got it down to 50mV you just need to put your ears close up to the speaker to hear it.

Personally I do not think its pleasant or useful, more like a annoyance that I can't seem to get rid off but have to put up with. :roll:

 

[Learner]

[quote author="Svart"]

I still wonder about the cathode R adjustment for gain adjustment.. why or why not?

[/quote]

Personally for me, I don't think that would be a very good idea because I have found that there is really only 1 optimum baising region(point) for each tube with no FB and minimal noise but not no noise :roll:

If you start messing with the biasing, you'll be adding HUMMMMM and tinginginging..... to you signal while you vary the gain.

Try it for yourself!!!!! ITS A MUST!!!! :green:

YMMV

 

[Svart]

oh i'm sure i'll be trying it if i mean to or not!

 

[PRR]

The little Beyer iron will work. They overload easy, but the tube gain is high enough that the output will be insanely high before the transformer complains.

Variable bias will kill output level much faster than gain. Your main gain control would be bypassing (BIG cap) or not-bypassing the cathode resistor for about 6dB change. With hot mikes, you might want a lower ratio input transformer. Generally the gain structure works better with an output step-down, but that needs BIG iron.

Noise should not be affected much by bias current or voltage. Up to a point, more current gives less noise, but improvement is slow. Excess noise at high voltage suggests a sick part: dirty tube, leaky cap, or even a strained power supply. Let old tubes cook for a week before judging noise. Use GOOD caps of appropriate types.

For the resistance-coupled version, the cathode must be significantly more positive than the grid, and the plate should sit in the range of 1/3rd to 2/3rd of supply voltage. Finding your plate voltage slammed to ground or B+ is sure to be wrong. Letting the grid go positive of the cathode is sure to give trouble.

 

[Svart]

Thanks PRR, that answered my questions. Should get my tubes in a day or so and i can wire something up. Now for another question.. I've never wired tubes before and wonder if it would be more benficial to actually use wire or layout a PCB with the components on it. If i wire it, what is the best way? I have access to 18ga teflon wire and would use that if i can. I'm using polypro caps throughout and metal film power resistors.

Heater 6.3vdc will be supplied via a LDO Regulator, B+ will be supplied by the second primary winding on my trafo and rectified via HEXFRED diodes(nice and quiet). I have thought about it and will supply phantom via a voltage multiplier and adjustable regulator circuit much like what we usually use.

wish me luck! :green:

 

[Svart]

also, NYD has a 20db pad in front of his input trafo.. How did you calculate those values? I need to calculate some other values too..

:thumb:

 

[Svart]

bump.. :thumb:

 

[PRR]

> more benficial to actually use wire or layout a PCB with the components on it.

Tubes do NOT belong on printed circuit boards. Call me old-fashioned, but that's the way I see it.

Anyway: what parts? There are not that many, they are not small, some of them are hot.

A 5-lug terminal strip and a socket should be plenty. What wire? Except for B+ distribution, heaters, and controls, tube gear is wired with the component leads. Keep it tight and neat like the one already built up-thread. Don't over-think. Don't decorate with Teflon: most of a tube amp can be wired bare-lead.

 

[Svart]

Thanks PRR!

 

[Svart]

i've cobbled together a PSU for the CJ version of preamp if anyone would like to see it.

Let me finish the layout and i'll put it up for constructive critique.

 

[Svart]

http://www.groupdiy.twin-x.com/albums/userpics/10015/TUBEpsu.pdf

ok there it is so far. let me know of any problems you see. I'm almost finished with a layout too.

:guinness:

Edit: here's the layout
http://www.groupdiy.twin-x.com/albums/userpics/10015/PSUoverlay.pdf
http://www.groupdiy.twin-x.com/albums/userpics/10015/psutraces.pdf
http://www.groupdiy.twin-x.com/albums/userpics/10015/psutracesmirrored.pdf

let me know what you think.

:thumb:

 

[NewYorkDave]

No interest in my little 12AV7 jobbie?

Beeeaaaah! :sad: :sad: :sad:

Just kidding! :razz:

Actually, I've been messing around with my pre and I'm quite pleased with how it sounds. (I don't claim to be any sort of "critical listener", though). I think I'm going to make a 4-channel rackmount version one of these days.

 

[Svart]

oh no NYD, i'm gathering the parts to do yours too! i just thought i'd get the easier one out of the way first!

I plan on making two channels first and then upping to 4 once i get it how i like it.

what trafos are you using? are they the same that are in the schemo? I'll need to find some.. any suggestions?

EDIT:

I put the layout up. check the post i made earlier with the schemo!


:thumb: