one 12ax7 complete amp problem

[rexindigo]

Please wait for Rev.2

yeah, its all the little amp, rhythm is the bridge pickup, lead is the neck pickup
i recorder it with logic from emagic using an shure sm58 and the speaker/cabinet is an old guyatone transitor amps 12" speaker ( the guyatone is gonna be a nice tube amp soon...)
after re-checking the values from my schem with the real thing, i think i ll have to correct it. so please wait for revison 2 (thanks hasserl)

[Rocks]

I'm looking forward to revision 2. I'm going to build this amp and I'm going to order the parts on Monday. It will be my first amp build

Can you post some pictures of the amp so I can see the layout?

Rocks

[Paul P]

yeah, its all the little amp, rhythm is the bridge pickup, lead is the neck pickup

Nice little amp. I'm going to have to make myself one of these as well.
Rexindigo, could you give us some idea of the volume level that the
amp was putting out when you made your recordings ? This may not
be easy to do but something like "you can carry on a conversation
with someone at the same time" or "you'll wake up the downstairs
neighbours" would be appreciated.

Paul P

[rexindigo]

Hello Paul P,
Lets put it This way:
- when I recorded The Clip it was 9 pm, my wife watched TV in the next room
(i only closed the door of my room AND She didn't complain!) and the children where asleep.

Hello Rocks,
Please give me some more time, I Have to make some major Changes to the Schem, I realized that what I Painted out of my mind seemes not to fit to the real thing, For Example some Values are wrong and there are more Components in it.
I'll Try to post a Picture of the build but this is gonna be hard for the pros, (Rat Nest) because i soldered everything directly to the Tube socket and this is not very helpful for a layout.
But as i have to do some more cad painting i will also paint the Powersection.

[Rocks]

Thanks rexindigo,

I'm going to order the parts as soon as I know what parts to order. It will be so good to be able to play at night again without using headphones and solid state.

Rocks

[rexindigo]

Hi Rocks,
here comes REV.2. Please be careful. if this is Your first Tube Project I think it might be good to read the safety tips at:www.aikenamps.com in the Techinfo section and When probing a live amp, keep one hand in your pocket - it's the current across the heart by way of the arms that kills you ;-)

Also, read everything in this forum and at AX84 and 18Watt

Have fun!

[Rocks]

Thanks very much. Two questions, what would I need to change in the power section other than the two transformers to use this on 120V? And what amperage are the two transformers? I might have one that is powerful enough by itself.

Rocks

[rexindigo]

If You have a transformer with 120V Primary and 6,3 volt / 230v or so secondary You can Use it of Course.
I Actually built this amp from the crap i had lying around, so that were the transformers i had.
The heater current of a 12ax7 is 150 mA at 12,6 Volt and 300 mA at 6,3 Volt
Plate Current is about 1,2 mA at 250 v.

I understand what You think about playing without disturbing:
I just finished an other little amp with a self split push pull 12au7 and a12ax7 preamp but this amp is way to loud,
The medianox is better for "Quite Playing" altough im not sure if its a good first project.
Because input, preamp and output section are very close, ist not easy to get it going without sqealing, (thats what actually started this thread...;-)
i would recomend the ax84 Pi project for a start, beacuse everything is described very accurate there

[Rocks]

What fun would it be if there wasn't a little challenge involved? And this may be my first build, but I have done some minor work in tube amps before. I am currently working on an old silvertone 5 watt amp in another thread on this site. I'll most likely sell that one as I've already got 2 other 5 watt amps.

Rocks

[rexindigo]

Thats great, please keep me informed if You get the little thing to work!
I have forgotten to point out (in the Power section) that all the (electrolytic) filter caps go to ground with the minus pole ( just in case You need the information)
I Think i will put up rexindigo.com soon and maybe show this little project there as well as some others i am working on.

[Paul P]

Thanks for the schematics. What purpose does the 220K R6 resistor serve ?
I know they are used as mixer resistors when there is more than one channel
but I don't see why one would be necessary with only one channel.

Paul P

[rexindigo]

hello paul,
to be honest, i tried this design first in my sandbox amp were i had the second half of a 12ax7 free and just connected the first half with the "output section". As it worked and sounded good, i copied the design and tweaked just a little.
For example i added a cathode bypass cap on the power section, but this increased sqealing so i removed it again.(Sqealing of the circuit started this thread...) in the process of reducing the sqealin i thought it might be a good idea to add a voltage devider (R6) to attemuate the signal and after soldering it in (and before adding the one going to ground) i accedently moved the cable coming from the OT to pin6 , which terminated the sqealing.
so i forgot about R6 there and did not think about it in my drawing, because this the way the amp is right no and was when i recorded the clip. But you are right, it is probably not of any use there (and wasnt there in my revison 1 wich i drew out of my mind.)
feel free to change it. Its easy because its few parts. keep me infomed if You find ways to improve it.
Thanks for Your interest in the "medinox" i would be very pleased to here Your what You Experience with the circuit!

[DirtyGeorge]

A 12dw7 (JJ ecc832) would be a good choice for this application. It has an AX section and and AU section. Using the AU section for the output would be a better match for the OT. The AX section keeps the high gain for the preamp stage.

DG

[rexindigo]

Thanks George, thats a very good idea, i did not know that there are such tubes out there. a qick check in the internet says that its pretty expensive.
the idea of the medianox first build was to keep it cheap in case something failed (or gets burned) i would not ruin anything valuable. But as there is some interest in this little amp and i love to play at night i realy right now think about rebuilding it with a real PT and then maybe the 12DW7 could be a very nice try.

[DirtyGeorge]

JJ has new production 12dw7 listed as ecc832. Bob at Eurotubes sells them for $11.00.

DG