Santiago Alvarez (electronics engineer, JVM, YJM, AFD...)

[0XI0]

the curves may look like a pentode, although the input load is completely different to a tube grid input

do designers ever use input eq in front of solid state clipping to mimic the tube input load tonal characteristic? I feel like the primary tonal difference of tube stage vs solid state distortion is a prominent pre gain mid scoop that's always missing from solid state breakup, like the impedance or input load of tubes creates a natural V curve at the input

 

[santiall]

do designers ever use input eq in front of solid state clipping to mimic the tube input load tonal characteristic? I feel like the primary tonal difference of tube stage vs solid state distortion is a prominent pre gain mid scoop that's always missing from solid state breakup, like the impedance or input load of tubes creates a natural V curve at the input

I'd say that more than a pure EQ is some sort of distortion that depends on the frequency and the signal level which also affects the input impedance. The tube input is highly non-linear when the signals are relatively high but it is easy for a guitar to hit that point. I think a way to hear this is to use a buffer in front of a tube amp against connecting the guitar directly. In my opinion, the buffered guitar feels more immediate, perhaps a bit crisper too.

I'm not sure if anyone has done anything, people out there just claim "FET input" as if that would be same as a tube input but other than that I am not aware. I have done some experiments on my own but I don't have anything "polished enough" to go out there and claim that this circuit fully replicates a tube input...

 

[HotRats]

Hi, if that noise is there with the master down I'd suspect of the phase splitter tube. It should be the closest to the power tubes/choke.
Do you have a spare to try?, otherwise take that one out and replace it with the next preamp tube in the amp.

Let's try that before suspecting of something else

I'll try the swap and report back, thank you.

 

[HotRats]

@santiall I've swapped the tube but the noise while knoking is still there, still the same.

 

[James Freeman]

@santiall I've swapped the tube but the noise while knoking is still there, still the same.

Try tapping each tube with a wooden stick.
Clean the tube sockets with a contact cleaner.

 

[HotRats]

Try tapping each tube with a wooden stick.
Clean the tube sockets with a contact cleaner.

Thank you James.

I've tapped them all already but none, but the EL34s, seems to produce more noise than the others. Looks like tapping on the valves is simply transmitting the vibration elsewhere.

I've also taken out and put back all the tubes and pins are very clean, like brand new.

The amp is only 2 years old and I keep it at home in my studio, I move it back and forth for rehearsals or gigs so basically has spent 99.9% of it's time in a warm and dry environment.

 

[Byrdman]

Hi,
those values pretty much convert the preamp into 1959/1987 values. To me still you will be missing the cathode follower, which is a big part of the plexi sound. You may try a MOSFET as "cathode follwer" between V1B and the tone control, there are a few examples over the internet if you search a bit :)

Thank you for taking a look and your suggestions. Have a great weekend!

 

[santiall]

@santiall I've swapped the tube but the noise while knoking is still there, still the same.

try to be methodical when testing the amp. Those noises when hitting the amp are, most likely, a microphonic tube. The thing is that sometimes it can be hard to know which one is the culprit. Try this:
1- remove all the preamp tubes, tap the power tubes. Is it noise?
2- add the phase splitter (the one closer to the power tubes) and repeat. Is it noisy now?
3- add the next tube. Tap the amp with the master both full and zero. Is it noisy?
4- add the next and repeat
5- do the tone controls affect the tone of the noise?
6- does the gain control affect it

I am quite sure it would be relatively easy to diagnose in person but it is hard to do remotely.
Also,that the EL34 tubes are new doesn't mean they work. Precisely right now I was working in an amp here with two brand new EL34s and one doesn't even heat up... dead, zero, nada.... nice...

 

[santiall]

Thank you for taking a look and your suggestions. Have a great weekend!

you are more than welcome!