Dirty/Harsh top end in high gain amp

[GainFreak]

Hi,

I have a problem in a SLO/Mesa type of high gain amp. Even with the Treble control at 0 you can still hear that harsh top end that sounds like too much odd harmonics /it's not an oscillation/. The amp is too bright and "treble-y".
I tried different tubes in different positions but the problem persists. Tried to cut the high end with a cap parallel to the anode resistor in several locations - at the CF, FX loop return triode section /FX loop is on all the time, switching the jacks off only/ one at a time. A 500pF cap would cut this top end in CF but why then in other builds this problem wasn't there and no cap was necessary?
I would appreciate if you can comment and give me some. Thank you.

[Enzo]

How about what speaker were you using, and did you try different speakers?

[Chuck H]

+1 on the speaker. Also... It may not be an oscillation, but it could still be an instability of sorts. Positive feedback due to proximity could cause this effect even without oscillation. Layout is critical in cascade, high gain builds.

It's also worth noting that when that awful buzzy, fizzy type of distortion presents itself in cascade preamps, it's usually because there is too much bass in the circuit and not too much treble. I've seen this alot with new builds. Guys that want a fatter sound push too much bottom end through the circuit and install a bunch of top end bleeders when the answer is to reduce bottom end. Along with this it helps to have a good power supply that will produce enough clean bass to keep the amp sounding fat. A mediocre power supply and too much bass can never be made to sound "right".

Chuck

[GainFreak]

How about what speaker were you using, and did you try different speakers?

It's a 2x12" closed back cabinet with Vintage 30.

Layout is critical in cascade, high gain builds.

I have another amp with the same topology right next to me and it doesn't sound like that.

It's also worth noting that when that awful buzzy, fizzy type of distortion presents itself in cascade preamps, it's usually because there is too much bass in the circuit and not too much treble. I've seen this alot with new builds. Guys that want a fatter sound push too much bottom end through the circuit and install a bunch of top end bleeders when the answer is to reduce bottom end. Along with this it helps to have a good power supply that will produce enough clean bass to keep the amp sounding fat. A mediocre power supply and too much bass can never be made to sound "right".

The preamp is a SLO/Mesa DR type of high gain preampwithout any modifications. Bass response is balanced - it's not too much. The power supply is beefy enough - with neck humbucker the clean channell sounds like you're playing a bass guitar.

Positive feedback due to proximity could cause this effect even without oscillation.

Now when you mentioned that I have a DPDT relay connected to the CF where one group of contacts switches two tone stacks (channel 2 and 3) and the other group switches Volumes 2 and 3. Could it be that the "raw" signal from CF witch has a very high amplitude bleeds into the post tone stack signal?

[chipaudette]

Hi,

I can't really help you with your problem, but thanks for starting a thread about high gain amps. I'm a fan, and there's not usually much talk about high gain here. I've you've built other high gain machines, I'd love to see some more new threads!

Chip

[tubeswell]

I find vintage 30s are very 'toppy' and not much bass (and to me they have limited usefulness). I suspect they were designed that way to accentuate highs in hi-gain amps which might otherwise tend to be too middy (and even muddy). So if the amp sounds too shrill with them, then maybe go for a darker speaker like an emi blackpowder type of thing.

[GainFreak]

I find vintage 30s are very 'toppy' and not much bass

This is my impresion from them too but after all V30 is a classic.
Elimininating the V30 factor there's something more I can hear on top of that.
By the way I did some experiments and really there's some signal bleeding in the relay I mentioned above.
Maybe I should record couple of samples for you to hear.

[Enzo]

But did we actually listen through other speakers?

[GainFreak]

But did we actually listen through other speakers?

Not yet because curreently only V30s are available but as I mentioned above similar amp doesn't sound so toppy.

[Chuck H]

What are the voltages like compared to an actual SLO?

Chuck

[guitician]

Yeah, I noticed that when I lowered voltages in my pre-amp, it took some of the edge off the highs.

[GainFreak]

What are the voltages like compared to an actual SLO?

Almost identical.

[pdf64]

'I have a DPDT relay connected to the CF where one group of contacts switches two tone stacks (channel 2 and 3) and the other group switches Volumes 2 and 3. Could it be that the "raw" signal from CF witch has a very high amplitude bleeds into the post tone stack signal?'
Yes, capacitive 'bleeding' of the signal could be a cause of this. Possible fix might be to have 2 relays, 1 for high level signal, other for low level.
But you really need to positively identify it as the cause, before you try mitigation.
Peter.

[vanslycken]

I'm working on a high gain channel too (as you may have seen, I haven't been too succesful) but I found out that sometimes you need to reduce the amount of signal being fed to the next gain stage even more than in the original schematic. I've tried with 1M pots between every stage until I found out which stage was distorting in an ugly way.
I also got the hint of using grid leak resistors, which seem to help to take some of the edge off.

[GainFreak]

I think I maybe fixed the problem but here's a sample for you to hear.
First bit mike /Shure SM57/ is at ~45deg, second ~20deg.
Cabinet is 2xVintage30, closed back.