one 12ax7 complete amp problem

[rexindigo]

hi experts,
i built a complete working amp out of one 12ax7 and the thing is loud but squeals when i open the volumen pot full.
the preamp section is a classic fender first stage connected to a simple outputstage on triode two driving a fender reverb transformer.
where can i tweak to reduce the squeal?
the tube is not microphonic!
thanks for any comment

[cbarrow7625]

1.) Reduce the lengths of any lead wires inside the amp.

2.) Change / Improve your grounding scheme. Use a star ground instead of randomly placed grounds.

3.) Add a low value film cap to the power supply (maybe even in parallel with a mica cap) at the plate resistor node. This can help overcome the high ESR found in typical electrolytic power supply caps.

4.) Shield the wires going to and coming from the Volume / Tone stack (is there a tone stack?).

5.) Add pF bypass caps at strategic points in the circuit (around plate resistors, from plate to grid, plate to cathode, grid to ground, etc.)

I would do these things in this order if it was my amp with this problem. I like to attack build problems & power supply problems first before I go shielding wires & adding caps around a tube. The latter just seem to muddy up the sound too much and remove the "sparkle" from the tone.

Good Luck.

Chris

[stokes]

I would try tweaking the cathode resistor on the output section to tame this.Are you using a bypass cap on the output cathode resistor?The fact that you get the problem only when you turn the volume full out tells me you just have too much gain.

[rexindigo]

Cable Moved: Problem gone

Hi Friends, thank you for Your help!
I#ve moved one lead, the one going from pin 6 to the output transformer, and the squeal is gone. this lead was very close to the 68k resistor from the input.
now the sound is perfect: overdrive at bedroom level, I will call it "media nox" because this you can play at midnight without disturbing the neighbours.
Thank You again, this forum is very helpful!

[Rocks]

Do you have a schematic for this amp? I'd love to see it.

Rocks

[rexindigo]

Hi Rocks,
I will have to draw the Schematic first, this may take some days!
please have a little patience.

[Rocks]

Thanks! I'd love to see this. I've been dying to build a small bedroom practice amp that I can crank late at night and not wake up the whole house.

Rocks

[rexindigo]

I hope this upload works, i did a drawing with SWCAD
Please excuse the simple form, but thats the way i did it, i didnt care about the math and just tried it and it worked.
Yes Rocks You are right, all the usual design with 12ax7 PP or Parallel are way to loud and would wake my kids, thats why i build this thing.

Now for the Power section I just used two 220VAC-12VAC Transformers (Europe) Back to Back and the usual filter Caps...

Always build Your amps with Fuses, Ground everything Correctly and take care, Im not responsible for any harm caused from that thing ;-)

Hope You like it!

[hasserl]

Your cap values, shouldn't that be C1 = .047uf & C2 = .01uf?

But very cool, I'd love to hear any clips of it you could make.

[Rocks]

I know very well about the volume problem. I have two small amps that sound great but are way too loud. One is an old Vibro Champ and the other is an even older Kalamazoo model 2, Both are about 5-6 watts and can still wake the whole house when they get turned up to get the nice tone.

Rocks

[Rocks]

Yes, some clips would be great!

[rexindigo]

here comes the clip, sorry for not playing to tight but i did it right away when You asked, the guitar is sort of PRS Clone:two Humbuckers, maple neck, mahagony body with maple top

[dave////irongater]

Nice tone esp the rhythm!

[hasserl]

That's sweeeet!

[Rocks]

rexindigo,
You are a great guy, you were very fast to respond with the schematic and also the clip. I love the tone of this amp, was it used for both the rhythm and lead? Also what kind of speaker/cab were you using?

Rocks