First amp build questions

[martin manning]

The Hammond flat-pack transformers mentioned above are great for that purpose.

[pdf64]

Being as there’s people here with sim skills -
Doesn’t C13 0.1uF DC blocking cap feeding the tonestack render the 0.1uF C15 bass cap redundant?
ie change C13 to 0.05uF and link across C15; does that give the same response?

[GAStan]

Hopefully not too late, Mojotone carries a PT that may work nicely. 250v 120mA and 6.3v 1.5A

https://www.mojotone.com/GA-5-Reissue-R ... ransformer

[szabi43]

Hopefully not too late, Mojotone carries a PT that may work nicely. 250v 120mA and 6.3v 1.5A

https://www.mojotone.com/GA-5-Reissue-R ... ransformer

I live in Hungary so ordering from Mojotone would be quite expensive, one of the Hammonds mentioned earlier will do the trick. Thanks for the recommendation though!

[szabi43]

I've managed to build the (pre)amp but there is a problem. Besides the nice sounding distorted guitar signal there is a high pitched squeal as well. Here are some things I managed to discover so far:

• Knocking on the tubes with a wood stick has no effect

• Noise completely stops when the input is shorted

• The noise has harmonics (probably because of the high gain nature of the amp)

• Gain and mid pots modify the guitar signal as they should but they have some weird effects on the noise

• The noise goes from 2482 hz to 4205 hz as I turn the gain pot from min to max

• With the gain at max the guitar sound is no longer heard, just the squeal

• The mid pot at lower settings reduces the volume of the noise and at higher settings it increases the pitch a bit (but not the volume)

• Volume, high and bass pots work as they should

• Removing first tube has no effect

• Removing second tube reduces the noise slightly

• Connceting the grid of the last gain stage to ground slightly increases the pitch of the noise, connecting the other 3 grids to ground has no effect

• I can hear the squeal from the inside of the chassis very very very faintly, but I can't locate the exact source

Everything is inside a metal chassis and the components are on a PCB designed by me except the power transformer and the inductance which are fixed inside the metal chassis with screws. Because of this I'm suspecting a ground loop somewhere originating from poor ground layout on the board.

Gain at minimum:

Screenshot 2023-12-27 011952.png

Gain at maximum:

Screenshot 2023-12-27 012132.png

Does anybody know what might cause this?

[szabi43]

Update: moving the input jack around with its wires changes the pitch of the squeal

[TUBEDUDE]

If that's the case, i would improve all the grounds, resolder ground wires, sand connections at jacks and pots, clean with solvent and reassemble. That might kill your oscillation.