My friend converted his Marshal guitar amp (he had a tube amp repairman do it) to a stereo audio amp. The Marshal amp has 12AX7 driver tubes and he would like to try different pre-amp tubes.
The high frequencies (above ~14Khz) are weak meaning at least 6db too low and the mids are slightly grainy. But the bass and mid bass are great.
1) What kind of tubes will make it sound less like a guitar amp and more like a HiFi audio amp?
2) are there any tubes that are a simple direct replacement so that he doesn't have to take it back to the tube tech for a mod?
Eddie on behalf of Joe M.
The high frequencies (above ~14Khz) are weak meaning at least 6db too low and the mids are slightly grainy. But the bass and mid bass are great.
1) What kind of tubes will make it sound less like a guitar amp and more like a HiFi audio amp?
2) are there any tubes that are a simple direct replacement so that he doesn't have to take it back to the tube tech for a mod?
Eddie on behalf of Joe M.
That was a very, very bad idea.
Just by changing some tubes, you won't get the missing treble back. A competent technician should review the schematic and fix it. The output transformers may need to be changed, too...
Guitar amps often use very high values for grid stopper resistors, as compensation for the appalling wiring layout and resultant instability. These create a low HF rolloff, which can then be adjusted a little by 'tube rolling'.
To correct the problem you need to rebuild the amp to a different circuit, different layout and different components - as others have said.
To correct the problem you need to rebuild the amp to a different circuit, different layout and different components - as others have said.
But... why? Marshalls are great amps, especially the tube ones, and especially those that would be simple enough to convert. They're not at all suited to reproducing high-fidelity audio. These are amps which sound best when cranked up and distorting in a glorious fashion, and what you've done makes the guitarist in me want to cry.
I suspect something like this: Rack Power Amps | Marshall Amps
EL34 50/50 or EL34 100/100 rack mount stereo amps. I had a friend who had a 100/100...
Not sure why you would not just go out and get a pair of VTL 100s or similar..
EL34 50/50 or EL34 100/100 rack mount stereo amps. I had a friend who had a 100/100...
Not sure why you would not just go out and get a pair of VTL 100s or similar..
+1^^
the output transformer of marshall amplifiers has a cutoff at approx 15Khz.
the preamp stages are designed to be overdriven and biased hot.
there is an HPF integrated in preamp signal chain before it reaches the tone stack and PI.
i guess you won't rebuild it...
my best suggestion is to put a 12AU7 in all the preamp stages and in the PI.
they have a lower gain and might give you a smoother not distorted frequency response.
i just hope it wasn't a plexi...
PLEASE tell me it wasn't a plexi!
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