Tube swap in a Marshall Valvestate failed...
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the Marshall is long gone. But my 5751 theory was correct..it was incompatible with that amp. In fact 12AX7/ECC83 is standard on most amps out there ..all tube or hybrid. With a 5751 in the pre amp , an overdrive or distortion pedal will not work effectively...
If it's any indication, I picked a Crate Blue Doodoo over the Marshall Valvestate in 2001. I wanted a head and even as bad as the BV-120H was, the Marshall was worse.
The 5751 tube has different requirements to run it at its optimal spec as opposed to a 12AX7. Just as a 12AU7 can be swapped and will function, without actual component changes it will not operate 100% as intended. The Marshall Valvestate uses only 1 X 12ax7 tube in its circuit! Which without actually looking at a schematic, my best guess is that it is run as more of a buffer stage that exists just before the tone stack than an actual gain stage. Because the amp is designed more around IC and transistor based gain stages with diodes being the most likely form of distortion, I wouldn't be surprised if the tube is being run in starved plate mode even. The point being that because the amps has all its value placed in silicon, the tube portion of the circuit may be very finicky and marginal at best. A typical tube amp has an HT supply rail voltage of around 200 - 300 volts for the preamp. This amp very likely wouldn't need an HT rail that runs that hot and is probably rated tight on its current draw capability. A 12AU7 draws almost 10X as much current as a standard 12AX7, and a 5751 also draws more current than a 12AX7 ( I don't recall exactly how much more though ) and this extra draw on the power supply could be what caused your issue? Tube amps are more forgiving because they usually have a stouter or more robust power supply design which can handle a little more draw. And since in a tube amp that extra draw is stolen from the other tubes ( which can tolerate the subsequent voltage drop without issue ) the amp simply sounds or responds differently. In a SS based amp, the other components require a certain current and voltage supply to work, if the voltage is too low ( from excessive current draw ) because the power supply is unable to provide it, the circuit can do all kinds of strange things. In your case buzzing.
While it sounds like this is a necro thread, at least future readers will have some reference. In hybrid amps, run only what the manufacturer suggests. In 99.99% of cases, it will be the same as what was already in there.
The 5751 tube has different requirements to run it at its optimal spec as opposed to a 12AX7. Just as a 12AU7 can be swapped and will function, without actual component changes it will not operate 100% as intended. The Marshall Valvestate uses only 1 X 12ax7 tube in its circuit! Which without actually looking at a schematic, my best guess is that it is run as more of a buffer stage that exists just before the tone stack than an actual gain stage. Because the amp is designed more around IC and transistor based gain stages with diodes being the most likely form of distortion, I wouldn't be surprised if the tube is being run in starved plate mode even. The point being that because the amps has all its value placed in silicon, the tube portion of the circuit may be very finicky and marginal at best. A typical tube amp has an HT supply rail voltage of around 200 - 300 volts for the preamp. This amp very likely wouldn't need an HT rail that runs that hot and is probably rated tight on its current draw capability. A 12AU7 draws almost 10X as much current as a standard 12AX7, and a 5751 also draws more current than a 12AX7 ( I don't recall exactly how much more though ) and this extra draw on the power supply could be what caused your issue? Tube amps are more forgiving because they usually have a stouter or more robust power supply design which can handle a little more draw. And since in a tube amp that extra draw is stolen from the other tubes ( which can tolerate the subsequent voltage drop without issue ) the amp simply sounds or responds differently. In a SS based amp, the other components require a certain current and voltage supply to work, if the voltage is too low ( from excessive current draw ) because the power supply is unable to provide it, the circuit can do all kinds of strange things. In your case buzzing.
While it sounds like this is a necro thread, at least future readers will have some reference. In hybrid amps, run only what the manufacturer suggests. In 99.99% of cases, it will be the same as what was already in there.
..OK
On the subject of the 5751, I wanted to mention I put one of those in place of an ECC83 in my AC10..the one closest to the input. A common swap. the idea being it 'cleans up the sound'. the interesting thing is this. A) the sound I got was more sterile or bland, and B )if you then apply an overdrive, treble boost or distortion pedal -as required, it wont work! or wont work as well. In effect ,it seems you need a certain amount of distortion in the circuit for those things to work. a low gain tube prevents this -IMO . if you look at the spec of nearly all tube amps, the 12AX7 / ECC83 seems standard fit. With basic master volume/gain so long as the volume is opened up more than the gain, the sound wont distort much....5751 now sits in a box...
I had the original 100w head in early 90's. It was OK at best on the dirt channel, but it transformed into a beast once I started running a Metal Zone into the clean channel and using a new 1960a cab in combination with a peavey 212 open-back scorpion-loaded cab. It was an almost dead-ringer for the Carcass Necro album tone. "tools of the trade".
A 12AX7 is a 12AX7 for a reason. Just as a 12AU7 is what it is. While preamp tubes don't need biasing in the sense that we the user needs to worry about, they are still biased to work within a window of operation. I.E. the component values to bias a 12AX7 to its desired operating range will be different than a 12AT7, 12AU7, or 12AY7. That's not to say that the tube won't operate with either ones specific parts values, but where it operates may not be ideal.
For instance, a 12AU7 is probably the best tube to use for driving a tone stack because it moves more current than a 12AX7 and isn't loaded down by the tone stack. A 12AT7 is probably the best fit for phase inverter operation. Its characteristics lend it to be able to maintain stability when pushing the power tubes deep into distortion while maintaining linearity with the input signal. From a technical standpoint, most of the other A" "7 tubes are designed to do something the other isn't. There is a relatively unknown tube called a 12BZ7 which kind of has the characteristics of both a 12AX7 in terms of signal output ( MU ) and a 12AT7 in terms of current driving capability.
Most amps use a 12AX7 because it is A) Very common and B) has the highest amount of gain ( MU ). For what we are trying to do in a guitar amp, it is the best all-around, easily attainable, and functional tube there is. You can design an amp around it entirely and quite literally using circuits from the RCA Tube Handbook, build an amazing sounding amp with very few components.
For instance, a 12AU7 is probably the best tube to use for driving a tone stack because it moves more current than a 12AX7 and isn't loaded down by the tone stack. A 12AT7 is probably the best fit for phase inverter operation. Its characteristics lend it to be able to maintain stability when pushing the power tubes deep into distortion while maintaining linearity with the input signal. From a technical standpoint, most of the other A" "7 tubes are designed to do something the other isn't. There is a relatively unknown tube called a 12BZ7 which kind of has the characteristics of both a 12AX7 in terms of signal output ( MU ) and a 12AT7 in terms of current driving capability.
Most amps use a 12AX7 because it is A) Very common and B) has the highest amount of gain ( MU ). For what we are trying to do in a guitar amp, it is the best all-around, easily attainable, and functional tube there is. You can design an amp around it entirely and quite literally using circuits from the RCA Tube Handbook, build an amazing sounding amp with very few components.
Aceman
I am your doctor of love!
See - Carcus. Right there is a perfect use of a Metal Zone.
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