check my new 12au7 F-2B schematic

[fatfoohy]

Hello , I wanted to make a good tube preamp for my bass, and the F-2B looked great, however, i wanted to put it in a stompbox, and have some lower voltages.  So this gave birth to my 12au7 F-2B. I wanted to check and make sure i hadn't royally screwed something up.  Any tips or hints would be appreciated, i could upload my picture, so here is the link http://viewmorepics.myspace.com/index.cfm?fuseaction=user.editAlbumPhoto&albumID=1723865&imageID=29146739 thanks in advance!

[fatfoohy]

anybody?

[frequencycentral]

anybody?

The link takes me to Myspace login page. Try using Photobucket for hosting your images. 

[MikeH]

Or imageshack.us - no registration/account/login required

[CynicalMan]

Or the gallery

[fatfoohy]

i couldnt get it on the gallery, i'll try photobucket right now, gimme a second

[fatfoohy]

http://s420.photobucket.com/albums/pp282/fatfoohy/?action=view&current=12au7F-2B.jpg
There we go, sorry about the inconvenience!

[frequencycentral]

There we go, sorry about the inconvenience!

[Brymus]

Dont the cathode resistors and caps need to be tweaked when scaling the voltage down ?
IDK (not sure) it seems I recall seeing this before in other designs, and it was said to improve the sound at lower voltages.
Anyone know for sure?

[PRR]

I like http://tinypic.com/

Free, no registration, usually responsive, and they even give you a URL in BBS code.

And none of that crapola that PhotoBucket forces on the viewer (if you just take the BBS code URL).

Just don't lose the URL they give you. As an unregistered user, there is no way to find your image again.

Registration is free and lets you maintain your images. Organize into albums, delete, tag, etc. Haven't gotten any spam from registering.

[PRR]

> Dont the cathode resistors and caps need to be tweaked when scaling the voltage down ?

Somewhat. Especially below 100V. And we sure are in that zone.

What "concerns" me is using the 1/10th-scaled tone-stack values. A Fender tube-amp will use 100K for the first tone-network resistor, not 10K, and likewise throughout. This is done in FET designs to reduce noise voltage. However it may be an awful heavy load even for 12AU7. And keeping the 1Meg volume pot, the noise voltage to the 2nd stage won't be lower. My gut says to take the whole tonestack with values from one of the mature Fender Twins or similar.

[fatfoohy]

oh, alright i'll put the tone stack back to normal, plus i think im gonna set this thing up to run off of 12 volts since i read the max 1044 can put out enough to power the heater, so maybe that would help somewhat with having to change the cathode resistor and capacitor, does anybody have any clue what i'd have to do to calculate the change on those? oh and also, the original circuit used a 1 meg pot for volume as well

[PRR]

Find your cathode resistor experimentally. Try different resistors until the Plate sits about half the B+. That will work. After everything else works, you may want to repeat the tweak, aiming to set Plate at various points from 1/4 to 3/4 of B+. You will get somewhat different gain, color, and particularly overload sound.

[fatfoohy]

hmmm...i was thinking, if i bumped B+ up to 100volts, then i really wouldn't have to mess with the cathode resistor and anything else, what would be the benefit of running the tubes off of higher voltages? and also what other issues would that bring up?

[Brymus]

It would likely have more headroom and much improved tube tone above 100V,the trade off would be getting a transformer to do the high B+ voltage and the dangers associated with high voltages. (also larger caps more costly,ect)
If you are going to go high voltage you might as well use 12AX7s and go to 250-300V and make an actual F2B clone.(F2Bs ran at 300v plate voltage)

It wouldnt hurt to finish your design, just measure your voltages and ajust like Paul recomended,he really knows this stuff...
Then make a high voltage one if you arent happy with the low voltage one.
I think alot of people would like to see your design finished and working it IS a good idea IMO.
The truth will be in the tone once its done.

If you search PRRs older posts there was a thread where he explained why the design at hand needed the odd cathode cap and resistor values and it was in reference to someone elses low voltage tube design ,it might be worth your while to find that schematic and the thread about it.There are others around here as well.

[PRR]

> benefit of running the tubes off of higher voltages?

Nevermind the theoretical analysis, the volts and currents and loads.

If all your friends jumped off the roof, should you? No, but if "all your friends" going back 40 years of the music tribe ran 12AX7 at 300V and it worked good for them, maybe you should.

The Alembic F-2B is "inspired by" (copied from) the early 1960s Fender guitar-amp preamp and tone controls. The Fender (run at 280V-390V) is THE World's Most-Copied music amplifier. There's a jillion low-volt amps, with some general common themes, but none so widely copied as the old Fender plans.

Drawbacks: Size/heat: it won't fit in a toe-box. Safety: 300 Volts is like a basket of snakes or a running lawnmower: you don't stick your finger inside.

[Caferacernoc]

At low voltage, I've had good luck scaling down the 100k anode resistor to bias the tube. Cut it half to 50k or even lower.

[fatfoohy]

alrighty, here's my second try, http://s420.photobucket.com/albums/pp282/fatfoohy/?action=view&current=scan0001.jpg this time i put the original tonestack back in it, plus it's running off 12 volts, with B+ being 45 volts, i also changed the plate resistor to around 47k as suggested, oh, and the diodes should be 1n4148 not 1n914, sorry

[petemoore]

  Here's an approach:
  Trim the treble early, some of this can be regrown with the gain stages, for bass that may be fully extended lows [which = skip this if you like 'fat' sound through the set]. A lot of the uumph required of the power supply is in the LF region, not drawing power in the lower range allows this to be applied to higher frequencies.
  Along with this, a consideration or experimenting with presenting less impedance to LF's at the output/tone control.
 

[fatfoohy]

hmm, it might be good to make treble cut and put it in as a switch, what would be the simplest way to do this? I was looking for it to just be on and off, maybe control the level via an internal trimpot