Howdy!
I just built a clone of a Bassman inside an old solid state Marshall 2x12 combo. My only deviations from the original design are a SS rectifier and moving the filter cap board inside. It sounds killer... just not through the two speakers built in. When I connect it to a 16-ohm 4x12, located about 6 feet away, it sounds normal, but when I connect it to it 16-ohm internal load it makes a very high pitched squeal before switching to a deep WOOOOOOOOOOF! Terrifyingly loud, even with the volume all the way down. THe only thing that seems to affect it is fiddling with the presence knob. The negative feedback is connected to the 4-ohm jack, though I have tried connecting it to both the 8 and 16 to no avail.
I am utterly stumped and would appreciate any help.
I just built a clone of a Bassman inside an old solid state Marshall 2x12 combo. My only deviations from the original design are a SS rectifier and moving the filter cap board inside. It sounds killer... just not through the two speakers built in. When I connect it to a 16-ohm 4x12, located about 6 feet away, it sounds normal, but when I connect it to it 16-ohm internal load it makes a very high pitched squeal before switching to a deep WOOOOOOOOOOF! Terrifyingly loud, even with the volume all the way down. THe only thing that seems to affect it is fiddling with the presence knob. The negative feedback is connected to the 4-ohm jack, though I have tried connecting it to both the 8 and 16 to no avail.
I am utterly stumped and would appreciate any help.
First off it is a basic law of physics that oscillation requires positive feedback. The trick is to find the path
The simplest mistake is to connect the NFB to the wrong side of the secondary so it becomes a positive feedback. Let's hope it's that simple.
But before you do anything, What if you pull out all the preamp tubes (leave in the phase splitter, it's part of the power section.) If you still have the problem with the preamp tubes out then you've proved the problem is down stream of the master volume pot. Maybe a backwards NFB?
But if moving the speaker fixes this then I'm thinking it might be in the preamp and maybe one of the tubes is microphonic and the feedback path is acoustic. The speakers are simply shaking the bad tube. Microphonic tubes are not uncommon
I realized another change I made was putting a 12BH7 in as the PI. I always thought it sounded killer there in Fender amps.
The power tubes are EH 6L6s.
Forgot to mention, the OT is a Hammond 1650NA, which has their easy wire secondary, so I don't think thats hooked up wrong.
I've currently got the thing opened up so I can redo some grounds, so I'll have to wait till tomorrow to see if any of the tubes are microphonic... which would be sad. Brand spankin new EH 12AX7 Golds. I will readily admit that I'm new at most of this, and I'm not sure how I could have wired up the negative feedback backwards, as I thought it was just a wire going from the tip connector of the speaker jack to a 27k resistor on the board to the presence knob. I also realized that some of the wires going to the PI are about 3 inches long; could that be picking up anything?
Thanks for all your help guys!
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Yes from the tip connector but which wire on the transformer is "tip"? You have a 50/50 chance of getting it right. Actually I suspect it is right or you'd be having a much worse problem.
Allright. I checked all the preamp tubes, and none seem to be microphonic.
I also started messing with the phase inverter, which I usually put a 12BH7 in. I connected the amp up to the external cab, with nothing other than the 12BH7 and the power tubes plugged in. Once I turned the presence knob below a point, it squealed. So I popped out the 12BH7 and put in a 12AX7 and got the same WOOOOOOOF I got when it was plugged into its internal speakers.
So I can safely say the problem is with the PI, the lower gain of the 12BH7 just kept me from hearing it. I'm going to start by moving the tube socket closer to the board in order to keep the wires as short as possible.
I also started messing with the phase inverter, which I usually put a 12BH7 in. I connected the amp up to the external cab, with nothing other than the 12BH7 and the power tubes plugged in. Once I turned the presence knob below a point, it squealed. So I popped out the 12BH7 and put in a 12AX7 and got the same WOOOOOOOF I got when it was plugged into its internal speakers.
So I can safely say the problem is with the PI, the lower gain of the 12BH7 just kept me from hearing it. I'm going to start by moving the tube socket closer to the board in order to keep the wires as short as possible.
Not necessarily the PI, but the PI/output feedback loop. The presence control changes the feedback. Swapping valves changes the loop gain, which could make things better or worse. Using a different OPT from the original could do it too.
Does the PSU follow the design, apart from SS rectifier? Did you use bigger caps? You will have changed the supply impedance as SS has lower 'on' resistance than a thermionic rectifier. If the original design has marginal LF stability then you might have pushed it over the edge.
Try increasing the 27K feedback resistor. The bigger it has to be before stability sets in, the worse is your problem.
Does the PSU follow the design, apart from SS rectifier? Did you use bigger caps? You will have changed the supply impedance as SS has lower 'on' resistance than a thermionic rectifier. If the original design has marginal LF stability then you might have pushed it over the edge.
Try increasing the 27K feedback resistor. The bigger it has to be before stability sets in, the worse is your problem.
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