Fender Champ style pre amp/amp

Hi,

I am starting a build and thought I'd document this amp/pre amp build as I'm going to use it as an amp as well as a preamp. Pretty simple build,  have only attempted LA-2A's and Gyraf G9 before in the valve pre/ compressor world so this is a fair bit simpler but I have heard its a great sounding preamp for summing etc.
Having dummy load for direct out, will document all but just didn't know if guitar amps are part of this forum really? Even though this kind of qualifies as it will be used as just a preamp.
Going to make a nice metal/wooden enclosure for it that's rack able so will have vintage feel about it.

Regards

Spence.

Hi,
The Rude Tube Ivory in fact is designed after the Fender Champ preamp. Add some power tubes as last tube stage and you're done.
http://groupdiy.com/index.php?topic=53011.0
Bernd

Hi,

Gonna build just the guitar amp first then put in txf And mods afterwards.

Inside.

Ok, you've already built it. Nice work!
Bernd

Hi,



Yes bernbrue, this is the first go at it before I attempt a stereo unit.
Will probably try and section off a 2U unit for the  stereo unit,  I have some transformers which I'm hoping I can use. Sowter 4062 which is a 3.23:1 and some OEP A262 6.45:1 and some other ones.
Is there an equation  to work out what ratio  I need.

Regards

Spence.

right,

got this little sucker going and powered up today, will test more tomorrow.
Voltages look good, i haven't stuck exactly to the precise values but after a few bangs and i know what 300+ volts feels like now!!!
Ive gone with silicon diodes rather than the tube rectifier, i prefer silicon on my mesa boogie to give more punch, i find the tube recitifers a bit flimsy and flappy on the low end.
ill take some more photos when i get it all tied down and in some sort of box which i can pickup safely!!!

regards

Spence.

On further inspection there seems to be some  problems with the sound,  plugging in a sine wave seems fine until I turn it up. When turned up volume seems good until nearly at full volume where it over saturates and goes very quiet.
Going to check all voltages again.

Regards

Spence.

Hi,

this is what voltages i'm getting now and my conclusions.

360v at B+

6V6 voltages

pin3/4 = 350v
pin8 = 60v?

12AX7

pin1 = 178v
pin3 = 1.653v
pin6 =213v
pin8 = 1.691v

just seems im going wrong on pin 8 of 6V6,  going to change the 22uf/25v cap there as well as check the resistor.

regards

Spence.

Hi,

Changed 470R and the 22uf cap and now getting 20v at 0in 8 on 6v6, all other voltages look pretty close.
Getting massive amounts of hum!!! Anyone got any suggestions?
I have star Gnd which everything is back to, I have power txf one side and output txf on other.
All leads to valves are all pretty short and have twisted the heater AC wires.
Also when turning up the volume the amp just makes an awful noise, turning the volume the last quarter turn it does it.
The treble control just adds more buzz!!
And the bottom end is a very subtle, would like to make both the treble and bass controls do a bit more than they are doing at the moment.
Suggestions welcome please.

Regards

Spence.

Hi,
It's quite difficult to follow as long as we do not know to which schematic you refer.  Voltages seem quite high.
Bernd

Hi,

The schematic I'm going by is:

http://jalbum.com/files/attach/images/3153/237/007/df8dd855eadea74cd5ae3cb25b81009e.gif

But i have changed the following:

1st cap at B+ is 47uf/400v, the next two are 22uf/400v
The 250pf is a 220pf/630v polypropylene cap
Pin 8 12ax7 bias cap I've put in 10uf/100v as that's the smallest I Have lying around
Apart from having a bridge rectifier rather than the tube that it.
I have just connected the heater wires to the tubes and haven't connected one down to Gnd?
I can see the schematic asks for it, could this be where I'm picking up all this noise and hum from?
I'm pretty sure it about 100hz hum?

Any ideas?

Regards

Spence.

One  heater side (-) needs a reference to ground.
Bernd

Even if it's AC from the txf?

Regards

Spence.

http://www.valvewizard.co.uk/heater.html
Bernd

That's great stuff,

I have tried the two 100r from each heater pin 9 and 4/5 down the Gnd, I just get big sizzle noise and resistors start burning!!! They are 2w I think or 1 w?
I must have something wrong with the way I've wired the main txf so I'm going to double check.
Something is a miss!!!

Regards

Spence.

Still haven't got much further, amp is oscillating at full volume sounds ok up to about 7 volume then goes into oscillation and saturation?
Hum is present so this need a serious look.!!!
Anyone got any ideas about the oscillation?
Regards

Spence.

looking into a new layout and using screened wire!!!

On closer inspection where the output txf comes through the chassis I didn't use a rubber grommet, the wires had been cut by the chassis a little so this could have been my main problem!
Have repaired leads with heat shrink and used grommet to feed cables through chassis now!
Top tip, always use rubber grommet when putting leads through chassis!

I have moved the 12ax7 further away from main txf and included 2 x 100R 2w resistors for Gnd connection on heaters.

Regards

Spence.

After moving the 12ax7 away from the main txf and repairing the output txf wires I've fired up the amp today and sounds great. I have put in a negative feedback switch so I can go between having it or not.
Not much volume but great sounding amp.
Will try and building a stereo version now and put in the  mic txf.

Regards

Spence.