What is causing buzzy distortion in the preamp?

[Mark]

I have taken voltages of every point on the amp. Is there any information that you would like me to provide?

The amp uses an EL-84 amp in cathode bias, the rail voltage is 250VDC, it initially used the 6K tapping but I changed that to the 4K tapping as the output was low. I had thought of asking about the correct primary impedance but thought I would try and work it out myself.

Are your scope pictures taken with AC coupling or DC coupling?

The CRO was on AC and I needed to use a 10X setting on the probe at times. I’m having issues with my CRO reading low and I have been using my Fluke 77 meter for voltage measurements. The waveforms are accurate, just not the displayed voltage levels. I do intend to replace the CRO in the future though it’s not my greatest priority at the moment. Thanks for your help.

[R.G.]

What I'm looking for is where the signal is driving the grid in relation to the cathode. The sharp-horned scope pictures will sound buzzy like crazy, all right. I was hoping to get some insight into where the grids were going.
The earlier comments about grid blocking and overdrive were good starts.

[Mark]

What I'm looking for is where the signal is driving the grid in relation to the cathode. The sharp-horned scope pictures will sound buzzy like crazy, all right. I was hoping to get some insight into where the grids were going.
The earlier comments about grid blocking and overdrive were good starts.

The cathodes are bypassed so I won’t see signal there. Do you want to know the DC voltage on the cathodes and the amplitude of the grids?

Thanks for your assistance.

[Mark]

I mentioned that the input signal to the output stage (EL-84) is relatively clean and the output is distorted.

The rail is 260VDC
The plate has 249VDC present
The screen has 256VDC
The cathode has 10.3VDC

The primary impedance of the transformer is 8K and the second is 8 ohms.

This transformer is a Mercury Magnetics transformer that is a known good transformer. The transformer in the amp is a Heyboer transformer with a 6K, 4K, and 2.5K primary, the secondary has 4, 8, and 16 ohm tappings.

IMG_0791.jpeg

The signal measured 3.5vac on my Fluke 77 meter.

IMG_0792.jpeg

IMG_0793.jpeg

It’s a little better but not great.

I will try the same thing with the Heyboer transformer.

[pdf64]

It looks like the EL84 handling the upper lobe of the waveform isn't pulling its weight.

[LOUDthud]

The CRO was on AC and I needed to use a 10X setting on the probe at times. I’m having issues with my CRO reading low and I have been using my Fluke 77 meter for voltage measurements. The waveforms are accurate, just not the displayed voltage levels. I do intend to replace the CRO in the future though it’s not my greatest priority at the moment. Thanks for your help.

Have your probes been adjusted in 10X mode to make square square waves ? This is called probe compensation and it adjusts the probe for the input capacitance of your scope. If this has not been done, Voltage measurements on sine-waves will not be accurate.

[R.G.]

LOUDthud and pdf64 are right. Your probes need to be "calibrated" using the o'scope's square wave calibrator to get as square a trace as possible. This removes frequency loss/peaking from the probe capacitance itself. And the traces do seem to show the upper part of the trace isn't as big as the lower part.

The reason I was asking for both grid and cathode traces is that for triodes and pentodes, the current through the tube is controlled by the voltage between the grid and cathode. The cathode has to be more positive than the grid in normal operation. When the grid goes more positive than the cathode, the impedance of the grid changes from really high to as little as a few k ohms. This causes a sudden discontinuity in the current flow and distortion in the plate current. The cathode being bypassed should make it have little change with signal, so seeing both cathode and grid traces on DC lets you see both any funny movement on the cathodes and also whether and where the grids go positive. It also shows grid blocking movement on the grids as grid voltage changes with signal. That's what I was after.

[GlideOn]

The schematic you drew out for the preamp on post #1 looks fine, nothing really buzzy comes to note except maybe the V1 1.5k/1uf not being quite Marshall...

Preamp voltages seem a bit low, used to seeing high 100s/low 200s for a more traditional Marshall territory.




But the biggest question I have is this:

What is going on in the Power Amp?

[pullshocks]

Wild ass guess: on the 2nd stage, try a lower value cathode bypass cap. Your schematic shows a 22uF in that position.

Different preamp arrangement than the one you are working on, but the Fender Champ AA764 only uses 2 uF in that position

I have the 25uF in that position on SE 6V6 amp I'm working on. Obviously it is a different configuration than you're working on but bear with me.....The OD sounds kind pf ratty with the bass knob on 5, much better with the bass knob on 1 or 2. I . Reducing the Ck would be another way of reducing bass conent.

[GlideOn]

OP, your voltage is on the lower side with that B+ rail, try lowering the value of the droppers and get some voltage back into there. A "brown sound" is like high 170s/180s in V1 which is already low, yours is low 130s. That's absolutely gonna be farty and loose

[Mark]

Please accept my apologies for not responding sooner.

When I initially started working on the amp, I found the amp to sound buzzy and I tried swapping speakers. It made no difference, all the speakers sounded buzzy. I tried things to get the amp to sound better and it did sound better but still buzzy.

I don’t know why, but something made me think that I should try different speakers again. This time the amp sounded good through any other speaker than its own speaker.

Crisis averted. I thought it was unusual that the amp was buzzy sounding and the speaker also sounded buzzy. I’m in the home stretch now which is relieving.

Thanks for interest and assistance everyone.