What is causing buzzy distortion in the preamp?

[Mark]

Hi, thanks for looking at my post. The amp in question is a AX84 modified P1 amp using a EL84. The power transformer is a Hammond 369EX which is the export version of the 269EX. The output transformer is a Heyboer HTS-7687-2, this output transformer is actually larger than the power transformer.

IMG_0715.jpeg

The clean sounds of the amp are acceptable but the distortion is pretty hideous.
https://youtu.be/lWQ2WvXRIgg?si=230XsrW8DUBvp-Sv

I’m somewhat perplexed as to the reason for getting this obnoxious distortion. I will continue in another post with waveforms….…

[Mark]

The distortion, I find it I put it into You Tube the clip gets compressed and sounds much better. I guess I found out Oete Thorn’s secret.

[Mark]

Here is the layout of the amp. The amp was bought to me by a friend of a friend not working, this turned out to be bad solder joints, but it had other issues regarding the primary tap chosen on the output transformer and the rail voltage was low.

IMG_0714.jpeg

[Mark]

Here are waveforms from the grid of the second stage, the plate of the second stage and the top of the master volume. I find the amp sounds good if it’s pretty much going flat out, if I turn the master volume down it sounds buzzy.

The first lot of photos are at low “E” which is 82hz, the waveform if simply rolled off and no signs of harsh treble.

IMG_0719-compressed.jpeg

IMG_0720-compressed.jpeg

IMG_0721-compressed.jpeg

I’m surprised that the tone controls distorted the signal more than the clipping.

Next post is clipping at 1Khz. …

[Mark]

Here are the waveforms taken at 1Khz.

IMG_0722-compressed.jpeg

IMG_0723-compressed.jpeg

IMG_0724-compressed.jpeg

What I should point out is the treble peaking 470pF across a 470K resistor was lifted so it didn’t contribute to the waveform.

So that’s the information, what do you think is causing the buzziness?
Could it be the low voltage on the first two stages 137VDC and 143VDC, typically a Marshall would have a much higher DC voltage.

[LOUDthud]

Have you tried removing the diode on the cathode follower ?

[Mark]

Have you tried removing the diode on the cathode follower ?

No I haven’t, I can try it but the amp sounded buzzy before I put it in. In its sitting state it is biased off, the link discusses the theory behind this mod.

https://www.valvewizard.co.uk/dccf.html

[angelodp]

This is gonna get good....

[Roe]

the power supply voltages seem quite low (and the 22nf on the first stage is too big for high-gain, try 1nf)

[LOUDthud]

No I haven’t, I can try it but the amp sounded buzzy before I put it in. In its sitting state it is biased off, the link discusses the theory behind this mod.

https://www.valvewizard.co.uk/dccf.html

I suggest you read it again. It says without the diode you get more even order distortion that warms up the sound and reduces fizz. The plate Voltage of the gain stage is a little on the high side, try lowering the 820 Ohm in the cathode, or raising the B+ for that stage. Another thing to try is the bootstrap idea just below the part that talks about the diode.

[Mark]

This is gonna get good....

I certainly hope so.

[Mark]

No I haven’t, I can try it but the amp sounded buzzy before I put it in. In its sitting state it is biased off, the link discusses the theory behind this mod.

https://www.valvewizard.co.uk/dccf.html

I suggest you read it again. It says without the diode you get more even order distortion that warms up the sound and reduces fizz. The plate Voltage of the gain stage is a little on the high side, try lowering the 820 Ohm in the cathode, or raising the B+ for that stage. Another thing to try is the bootstrap idea just below the part that talks about the diode.

I did try removing the diode and I bypassed the tone controls, the sound was even more buzzy. I did reduce the 10K dropping resistor in the power supply and the “character “ the amp improved but the buzziness was still there.

[Roe]

does it sound scratchy when you lower the volume pot of the guitar? have you tried adding an input cap before v1

[Mark]

does it sound scratchy when you lower the volume pot of the guitar? have you tried adding an input cap before v1

Okay, there isn’t any DC on the grids of the 12AX7, the sound that I’m referring to is the nature of the distortion.

There is an attached MOV file that reasonably captures the sound of the distortion, though it’s even worse in person. I would have put it on You Tube, but the sound gets compressed and it sounds okay.

[Roe]

try to remove metal oxide resistors entirely from the signal path!