One-Bottle Preamp

[NewYorkDave]

Please tell us more about "filament sag."

So, far, I've taken the manufacturers at their word when they say "mounting position: any."

 

[Gus]

What is the heater supply voltage?

 

[burdij]

Sagging of the filament in miniature tubes is unlikely. The heater is wound and then coated with the alumina insulator. This structure is sintered in a furnace until the alumina forms its ceramic phase. The structure is then inserted into the nickel tube that forms the cathode.

 

[Kingston]

[quote author="Gus"]What is the heater supply voltage?[/quote]

it *should* be 6.3VAC, right from the high quality Welter PSU 6.3VAC output (floating virtual earth). There's enough current reserve for a third channel if I want one.

checked just now, 5.7VAC!! what the heck? that shouldn't be right. :shock: If I had to guess it's my bad quality volt meter. I remember measuring it a month back and it was 6.1VAC. This far I've understood that a bit under or over the target voltage on the heaters might even help fight ground hum issues. That shouldn't kill tubes.

Can it be that when I turn on the unit it initially fires major over voltage on the heaters and this might slowly kill the 12AV7 over time? I mean, even lightbulbs always die on the initial fire up.

as for the bias supply voltages, those are within 1-5% of the target and AFAIK that's well within safe limits, and when it gets distributed to the all the stages it stays that way (just double checked).

I oughta drag this to someone with a decent oscilloscope.

 

[pstamler]

Filament sag: something I've heard about but never actually witnessed. The filament sags and gets closer to the cathode on one side of the latter. Burdij, I agree that it sounds unlikely on miniature tubes, so maybe I'm barking up the wrong tree here.

So let's try another tree: what frequency is the hum? 50 Hz or 100Hz?

Peace,
Paul

 

[Kingston]

50hz is the highest peak. then 100hz, and then the usual steadily declining assortment of harmonics all the way to 1-2khz. I've had 8 12Av7's to select from and some of them were duds and looked exactly like that to begin with.

Normally there's a negligible 50hz peak at around -104dB and even more negligible harmonics from 100hz barely showing on my spectrum analyser. Within two months it'll look like I described above with the 50hz hum starting at even -40dB (very much audible already).

 

[pstamler]

Have you tried the old trick of hoisting the filament supply up about 50V? Make a voltage divider from the B+ supply with resistors designed to draw about 1mA, which would make your shunt resistor 51k and your series resistor (B+) - 51) times 1k. Hook the junction to the center tap of the filament winding (or a couple of 10k resistors to make a center tap if the transformer doesn't have one). Cuts hum radiation from the filament, in some cases dramatically.

Peace,
Paul

 

[Kingston]

I remember reading about that in the MILA-1 thread. I've been meaning to give it a go so thanks for reminding me. At one point I already had the heaters at 6.3vDC, but I thought it was a waste of current (for the smoothing caps) for a measly 1dB of hum reduction it gave me. (I'll use it up for the extra 3rd channel in the future)

That still leaves me the mystery of the dying 12AV7's to solve.

 

[APMA]

First, I would like to say thanks to all, this forum is great. Second, I'm looking for the PSU and input and output options for NYD's 1 or 2 bottle. Does Anyone have the original one Dave posted? Does the PSU differ if I build a 2 channel 2 bottle? This will be my first main project, I've only done some simple things such as Alice mics, and a pair of omni's. I'm a little shaky without having a specific map right there infront of me. Thanks again for the wealth of info you guys have in these pages. Looking forward to a great summer.

Aaron,
[email protected]

 

[blandman74]

Haven't seen that schemo, and no one else has posted, but I thought I'd post something for you to try. I haven't used this specifically, but I've made enough of these that I'm pretty confident it will work.

You can use many different methods. (I am re-using another power supply from a different tube circuit, so that's why I'm not using this one.)

[/url]

You could also use the old-style tube-circuit transformers from Hammond like the 269AX but those are overkill.

Hope this helps.

 

[martthie_08]

I'd also be interested in PSU options for the one bottle, as I'd love to build a 2-channel version of NewYorkDave's design! I have seen the PSU from the MILA, which looks as if it would be suitable for the NYD one bottle, but I am looking for an alternative using one of these power transformers:

http://www.musikding.de/product_info.php?cPath=84&products_id=808
http://www.musikding.de/product_info.php?cPath=84&products_id=944

As stated before, BTB electronic have the 12AV7 tube available for people in or around Germany.

Is there a decent layout for a turret board floating around? I have been through almost the whole thread and found very little (which was partly incorrect).

btw, this preamp will be built into a guitar amp head style enclosure, I am starting to get bored with 19" standard housings, time for something different.

cheers, Marten

 

[martthie_08]

slowly more progress, I thought I'd post some pics (click for larger image). The tolex is on:


A rough sketch of the frontpanel:


I need some advice concerning the mounting of the power transformer: Is it ok to make a largish cutout and fix the tranny to the back of the chassis with four bolts?


I need to do this to save some space and by doing this all the connection leads will be inside the chassis. The PSU section will then be separated from the actual preamp circuit by a sheet of metal with a through hole for the B+, 48V, heaters and ground.

 

[keefaz]

[quote author="martthie_08"]...
I need some advice concerning the mounting of the power transformer: Is it ok to make a largish cutout and fix the tranny to the back of the chassis with four bolts...[/quote]
I mounted a very similar transformer at the back of a 19" chassis, making a square hole and fixing it with four bolts. On the back of the chassis, I added a metal cover like this: transformer cover, available at: this site

 

[martthie_08]

keefaz, yes I ordered the trafo from Musikding but it is manufactured by the site you referenced. I'll look into getting one of those covers, they look neat!

 

[3nity]

Alright i've completed my PSU it gives a voltage of 322V. now the question is what's best to drop it a little to 300 voltage. if i'm not wrong R: E/I gives me a drop resistor of about 6k???

Right now i'm working on the preamp layout using turrets...
Thank you for the patience.

 

[3nity]

I tried my mathematics.....on OHMs law....and i found that on the One Bottle pre..Dave gave 300V and then drop it trough an 10k/1w resistor to 235V Not really shure if this is a RC parallel circuit with C6 and R13. But OHM law taught me that a 3.06mA is needed.

So i have 322 V X .0306 = 10733.46464 or something similar....

Now is this the resistor value (10K7) to drop the voltage or whatever the voltage is 235 or more what i need is the Current ???

Any help please??
what i'm seeking is to have the exactly 235 Volts dave has on the schem...as i have 322 before R13....

 

[keefaz]

I found 6.5 mA through the resistor with my maths...

How much voltage drop
300 - 235 = 65V

How much current
I = V/R
I = 65 / 10000
I = 0.0065 A = 6.5mA

So with 322V
Voltage drop
322 - 235 = 87

Resistance
R = V/I
R = 87/0.0065 = 13384R = 13K384

 

[3nity]

Voila....Merci beaucoup Keefaz.
Sincerement j'etais perdu...je vais pouvoir commencer mon "Une boutteille preamp" ou "un tube preamp??

Merci.

 

[3nity]

Has anyone used Edcor transformers on the input?? a 1:10 ratio would do the job??
thanks

 

[hodad]

1:10 should be fine I think. The potential issue with edcors is that their shielding is not that great, so you might get hum. There was a thread here about the edcors not so long ago.