Tone changes at higher volume

[danielalanperkins]

I have built/modded a couple high gain amps that both exhibit an issue where, when turned up via the master volume to the output section, the tone changes in an unpleasing way. At low volumes, these amps sound very much like I want them to (heavily over-driven). When I turn them up, the tone becomes seemingly less over-driven if not somewhat brittle. I don't believe I'm getting loud enough to push the output tubes into compression. Is this just the response of my ear being affected by the higher sound pressure level or is there something to the design of the amp that could cause this?

[martin manning]

Could be a roll-off of high frequencies at lower volume settings is causing this.

[Stevem]

As you turn the amp up your changing the ratio of preamp distortion to output stage distortion even though you may not be pushing the output stage into total compression.
This also changes the ratio of clipping harmonics you hear.

However the wave form offset of the output stage looks at low volume on a scope( top and bottom) is what you want to maintain as you hit the output tubes with increasing signal level.
This will not be easy since what you do at one volume level in terms of the harmonics the output stage produces will effect the other volume levels tonal nature.

[danielalanperkins]

Thanks for the tip. I'll take a look at the waveform offset between the two volume levels and see if that's what's going on.

[pdf64]

…At low volumes, these amps sound very much like I want them to (heavily over-driven). When I turn them up, the tone becomes seemingly less over-driven if not somewhat brittle…

That’s something which has always bugged me. To the point that I shifted to power amp overdrive rigs.
Maybe speakers frequency response changes with signal level, or a combo of the speaker and the amp.
I’ll investigate whether swapping from a G12M25 4x12 cab to my EVM12L reduces the effect.

[Stevem]

Could be part that also since the impeadance of any given driver does change with the frequency(s) being pumped into it, and I guess that one would then also have to expect that this would have a effect on the feedback circuit, no?

Heck, probably having multiple speakers wired in different combinations of parallel or series has a effect since that does change the total inductance of the speaker circuit!

4 drivers wired in ser/ par will without question have softer highs then when wired the other way due to that inductance change.

[martin manning]

I have built/modded a couple high gain amps that both exhibit an issue where, when turned up via the master volume to the output section, the tone changes in an unpleasing way.

Not really much to go on here, so we're just guessing. What do the circuits look like?

[danielalanperkins]

Both amps have basically a Marshall JCM 800 type of circuit with some variations in the preamp. Both have effects loops and master volume. I have tried a number of things with the output stages (EL34s, 6l6GCs, 2 pentode 50W config, 4 pentode 100W config, cathode and fixed bias, inter-stage transformer and long tail pair PI, variations in the negative feedback loop). One amp is a head driving a 4x12 8 ohm closed-back cab and the other is an open-back combo driving a 8 ohm single speaker.

I have also looked at the output across the speaker on a scope at low and high volume. I can't tell a difference aside from the amplitude of the waveform.

[danielalanperkins]

I have attached the schematics if you want a deeper look.

[pdf64]

Both amps have basically a Marshall JCM 800 type of circuit with some variations in the preamp. Both have effects loops and master volume…

Details matter; does JCM800 mean a 2203/4 circuit? As there’s oodles of other models in that range, eg 2205.
FX loop, again, schematics are needed.

[romberg]

Since the starting point is a heavily over-driven signal at low volume (and I don't see a PPIVMV in either schematic) then the signal you like is purely preamp distortion. Perhaps the power section or PI then starts to clip when you turn it up and changes the signal in a way you don't like. I'm not sure how loud things are getting when it starts to sound wrong. At some point even the speakers will start to distort .

Since you have a scope and know how to use it. I'd try and find the approximate voltages where the power section and PI start to distort a clean sine wave. If say your PI starts to distort a 5v peak to peak sine wave then it will also start to distort an over-driven signal at the same voltage. You would then be able to rule in or out the PI and or power section as being a factor by looking at the PI input signal when the sound changes in a way you don't like.

Mike

P.S. That carvin phase inverter is kinda interesting. Never seen a PI setup like that before.

[Reeltarded]

once the output hits capacity.. the NFB collapses and zing goes brrrrr

You don't like output saturation. I only use it about 50% into the curve. Around 7 is the end of the line. I use controls for each driving stage and dual masters then reamp for volume. Every control is a tone knob!

(pre pi and CF master NO PPIMV!!)

All those wicked gripping pinchy sounds we like from recordings are not dimed amps,and they are all driven with pedals... meh.

[danielalanperkins]

When I encounter the unpleasant sound at higher volume, I don't have the master volume set so loud that I am driving either the pi or output tubes into saturation.

[romberg]

When I encounter the unpleasant sound at higher volume, I don't have the master volume set so loud that I am driving either the pi or output tubes into saturation.

Have you confirmed this by looking at the PI input and output signal? Sometimes this is easier to do than one might think (especially with a high gain preamp.

Mike

[Reeltarded]

What is the master at when it starts to suck?