one 12ax7 complete amp problem

[Carlsoti]

I know this is an old thread, but I'm digging it up, anyways.

Rex, how is that amp holding up? Have you made any changes to it? Did you try the 12DW7 tube?

[rexindigo]

Hi Carlsoti,
i put it in a small case i found and added a 6" Speaker and now ist probably the smallest tube combo known to mankind ;-)
I use it to play guitar in the livingroom.
I never tried the 12DW7 but I tried the 12AU7 and that did not make any sound at all, probably the amplification of the preampstage is not enough to drive the powerstage.

I built a battery powered version using 4 AA 1,5 Volt batteries and a thing called inverter i robbed out of a computer case modding thing called cathode light. (this thing makes up 400 Volt AC out of 6 Volt DC..)
It worked for 3 minutes and than died forever..... (but even this 400Volt can hurt, believe me)

Greetings Rex

[vince76]

it seems more easy to find a transformer with 6.3V and 200V secondaries. I'm wondering if one of these can be used and what to modify...

[rexindigo]

in the "original" design i just used two 6volt transformers back to back:
- one to step down the AC from the wall plug to 6 volt, this Voltage for the heaters
- Then one to step it up again to 230 Volts, this will give You about 300 Volt DC after Rectification

[Boogdish]

rexingo, have you tried this with other tubes of the same pinout? 12AU7, 12AT7? It's an interesting idea.

A friend of mine on another forum has made a similar project:
http://www.diystompboxes.com/smfforu...?topic=73317.0

First post, y'all have a great forum here.

[rexindigo]

I've tried it with 12AU7 and it did not work, probably due to the low amplification of the first triodestage, I did not try 12AT7 but i have a few very good NOS telefunken lying around so I`'ll try this occasionaly, ill keep you informed.

Nice that you mention the Valvecaster Project, I build one some month ago und i like it., good idea to team it up with a OT and use it as an amp.
What does the IC do? its an Voltage doubler but how do the caps affect the doubling factor?

The last single tube amp i built was an amp with the pcl86 tube (triode/pentode in a single glass envelope) but i dont like it as much as the "medianox"

in the moment there is nothing on my bench except my "all-alder les paul project" so I'm more into sanding than into soldering in the moment.....

[darrellcheng]

I just stumbled upon this post and I have to say that your project is really great! I'm definately going to try to build myself one of these real soon, and get as many friends as I can to as well. Great job and thanks for sharing

[bnwitt]

I'm curious....what is your B+ voltage

[Sir Cuitous]

Would the transformers be anything like filament transformers? It looks like I could use
117v || 6.3v - 6.3.v || 230v
for US voltages and still get 300v rectified.

Is that right?

[Sir Cuitous]

These have dual primary and 6.3v secondary @0.6a:

http://www.mouser.com/Search/Product...iqPVPur9FpE%3d

Would two of those work?

[vince76]

OT

another question about transformer:
the OT is a fender reverb driver?
In many european site I can found this OT, but the specs are different.
You wrote 14.5KOhm but I always fin 22.8KOhm. It makes difference I guess.

many people would be happy if you post a layout or a few pics of your interesting amp too.

vincenzo

[rexindigo]

These have dual primary and 6.3v secondary @0.6a:

http://www.mouser.com/Search/Product...iqPVPur9FpE%3d

Would two of those work?

As far as i can see these should work, i built the amp out of my trash bin and I had these transformers :
1.) 230 primary / 12 Volt secondary
2.) 2 x 115 volt primary/2 x 6 volt secondary

with the each of the two windings of n.2.9 connected serial to get 230 volt and 12 volt

Your Mouser Parts should work, the current should be sufficient with 06 ampere ( You need 0.3 for the heaters at 6.3 volt not much needed for the B+)

Id love to hear of your experience with the little amp so keep me informed about your progress.

[rexindigo]

another question about transformer:
the OT is a fender reverb driver?
In many european site I can found this OT, but the specs are different.
You wrote 14.5KOhm but I always fin 22.8KOhm. It makes difference I guess.

many people would be happy if you post a layout or a few pics of your interesting amp too.

vincenzo

hi, this is what was written on my bill from tube town:
Reverb Treiber 3,5 Watt 125A20B
But the thing i got had Printed 6 watts on it and there seemes to be a different secondary printed on it probably its 22,8k.
You will have to try, I m sorry a cant say it for sure

[rexindigo]

Picture of the amp

Now there is a sudden revival for my little design and some of you guys asked me for pictures.

I can give you this one for now:
(It will take to much more time to take it completely apart for the "layout" which is actually what You call rat nest or spider web or so!)
(I will do it but not this week, than i can also measure B+ for bnwitt)

From left to right:
- the two small transformers side by side
- in the background the 6" Speaker that was 2€ at Pollin.de
- then the obvious reverb driver
- then the jj Tube
the speaker plug you see is not in use anymore, this is the back view

[bennieusmaximus]

Saw your thread and had to check it out.that thing sound pretty cool.This may have to be an up and coming project to add to my never ending up and coming project list.