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Thread: Carvin Bass Amp tube question

  1. #21
    GAStronomist DrNomis_44's Avatar
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    It's also possible that the 12AX7 Valve is contributing a subtle effect on the overall sound of the amp.

  2. #22
    Overlord of Music dave.king1's Avatar
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    Quote Originally Posted by DrNomis_44 View Post
    It's also possible that the 12AX7 Valve is contributing a subtle effect on the overall sound of the amp.
    I'm often wrong ( just ask my wife ), but if pulling the 12AX7 made no difference to the sound or output I doubt there's any signal getting to it, hence my switch diagnosis.

    If the sound stopped when the tube was pulled there would be a signal path, if the switch worked and there were other elements in that signal path then under the current circumstances it would go silent when the switch was thrown.

    If it's a plastic switch it is probably something as simple as the toggle inside the switch not throwing the contacts, still a switch replacement job.

    JM2CW

  3. #23
    GAStronomist Simon Barden's Avatar
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    The valve and the switch are working, so any lack of added valve harmonics us down to the low signal level passing through it. It is mimicking the effect of a valve power amp stage, so it needs lots of volume to become noticeable.

    No point in changing the valve. The 12AX7 spec is fairly tight so another manufacturerís valve wonít perform significantly differently in terms of the breakup point. Youíd need to change the surrounding circuit to do that. Another valve may have lower or higher noise, or sound slightly different once it starts to distort, but that point will always require about the same input signal level as the gain of the circuit is fixed. The other valve types that are often changed with/are pin compatible with the 12AX7, the 12AT7, the 12AU7 and the 12AY7 have less gain so will need to be driven even harder to distort.

    Your ampís power output is above the range of the attenuators Iím aware if, which normally go up to 100W, so if you want some light drive to your bass sound at low volumes, itís easiest to get a cheap pedal that does subtle overdrive sounds.

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  5. #24
    Mentor fender3x's Avatar
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    Quote Originally Posted by Simon Barden View Post
    Your ampís power output is above the range of the attenuators Iím aware if, which normally go up to 100W, so if you want some light drive to your bass sound at low volumes, itís easiest to get a cheap pedal that does subtle overdrive sounds.
    I use the Sansamp BDDI to add a little grit.Normally the only pedal I add.

    I had read enough reviews when I bought the amp to know there might not be much effect from the tube. I'm not unhappy with the sound of the amp at all. I just wanted to see if I could coax another useful tone out of it. If not, no harm no foul ;-)

    Thanks!

    Sent from my ONEPLUS A3000 using Tapatalk

  6. #25
    Mentor Marcel's Avatar
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    Had a look at the circuit last night and how the tube is used, and it certainly doesn't look like a gain stage to me.

    Seems more like a limiter or compressor type thing. And if it is then there shouldn't be much difference in tone when switched in or out. As a limiter or mild compressor you would/should notice a variation or constraint in volume dynamics with the tube switched in, which I expect would be more noticeable at higher volume levels when driven hard.

    Couldn't find a user manual or handbook so cannot confirm Carvins intentions in regards to the inclusion of the tube

  7. #26
    GAStronomist Simon Barden's Avatar
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    Here you go, it's on this page.

    https://carvinaudio.com/pages/archiv...-bass-amp-head

    They don't really state anything about usage except about 'turning the tube off if you prefer a pure solid state output stage'.

  8. #27
    GAStronomist Simon Barden's Avatar
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    Just a thought, if the tube circuit isn't configured for unity gain, (Marcel or someone else might be able to tell by looking at the schematic), a lower gain tube may reduce the overall level going to the power amp, which mean you can turn up the master volume and drive the tube harder and you may then notice some difference in the sound (solid state level will remain the same and be louder). Your maximum volume will also be reduced by 6dB or so, but that doesn't seem to be an issue at the moment.

    Or you may be able to change a resistor value to get the same result in the tube circuit, so you may only get 50W output, but drive the valve hard to get there and add some warmth. This could even be made into an added control pot or selector switch to give another master volume control.

  9. #28
    Mentor Marcel's Avatar
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    Hmmm... Specs and reviews ... great advertising pamphlet ... lol

    Zilch from the designer builder ....

  10. #29
    Mentor fender3x's Avatar
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    I appreciate the hard looks at this. It may be because I have a very efficient set of cabs, but I rarely need to turn the amp up very high. Playing outside I have never gotten much past the "6" and in my garage, no matter how loud my son plays I don't need much above 2 on the master. I don't usually use the drive knob on the amp because I prefer the drive from the SansAmp BDDI.

    I don't really hear compression with the tube in. I am not great at reading schematics, but it looks to me like it comes in after the solid state compressor. But I think I at least have a better understanding as to why the consensus seems to be that tube in/out makes little to no difference in sound. I actually stopped using my compressor in a stomp box because the compressor on the amp seems to sound pretty good.

  11. #30
    GAStronomist Simon Barden's Avatar
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    A tube/valve will only start to Ďcompressí the sound once it reaches its clean gain limit and starts to distort slightly. From that point on, youíll be getting an increase in compression at the same rate as harmonic distortion increases. Unlike a compressor which kicks in on the whole signal once it crosses the threshold and only returns to non-compression mode after the signal level drops below the threshold for a period related to the release time of the compressor (almost always a preset value on non-studio compressors and any compressor with three knobs or less), a tube will only reduce the peak signal value above the clean limit. Doing this will alter the signal wave shape, so introducing distortion and adding harmonics. So itís a very different type of compression to a normal compressor, and hand-in-hand with extra harmonics and an initial Ďwarmerí sound, before audible distortion kicks in and you know the tube is definitely overdriving and adding grit.

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