Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: idontknowyou on June 08, 2021, 12:15:24 pm
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Hello EL34 world! This is my first post here, so please excuse any mistakes.
I have an AIMS Eclipsor head that I bought last year in non-working condition as a project. An old thread on here helped me a bunch (https://el34world.com/Forum/index.php?topic=14405.msg136371#msg136371) and is where I found the schematics. I eventually got it working and mostly really love the amp, especially for bass. One weird problem persists: it creates ghost notes of a really dissonant and unpleasant interval, especially at high gain / low negative feedback settings. I will attach a picture here to help illustrate. This is my scope probing the output of the phase inverter. i have zoomed in on the top of the sine wave.
If I crank the negative feedback up (I replaced the 820 ohm resistor with a potentiometer) the ghost notes go away and the signal cleans up nicely but output volume obviously drops significantly. Also I love the distorted sound of this thing when the feedback is low so I would love to someday fix this problem. I have considered adding grid stopper or a small value cap to ground as in some fender designs in order to stop oscillation but am unsure of what values to try. I have tried chopsticking and moving wires around and it doesn't seem to affect anything. If its caused by lead dress I'm worried that it's somehow in the printed circuitboard itself, which leads me to my next point of business:
I have been thinking of trying to convert this entire amp to a eyelet / turret board but I am going to need some help understanding the circuit and modifying a Fender layout. The circuit is very similar to 60s Fender but with some distinct differences, other than just the two 6550s for the output stage. (I will attach the schematic below) The preamp design is almost identical but with different component values, the power supply has different filtering etc, all of that seems within my capability to change. The things I'm struggling with are these. 1- both channels are fed through the 3rd preamp stage right before the phase inverter instead of just the 2nd channel, and both channels have access to the reverb via switches. 2- The tremolo circuit itself seems to be slightly different from the standard Fender design and I don't know if it's significant. The biggest differences I notice in the tremolo circuit are that the 2nd half of the triode (pin 2 on the schematic) is connected to the wiper of the intensity pot instead of the photoresistor. Also, on Fender amps, the tremolo circuit is connected to the 50/50 cap in the bias circuit but that connection is absent on the AIMS. Could anyone explain the purpose of that connection to me?
I think the circuit is similar enough that I could use a premade board made for most blackface era amps. I've printed out copies of layouts and going through with pencil making the changes to component values and trying to trace the proper signal paths. I know many people here use DIYLC and I would love to use that but I am a linux user and not particularly software savvy so I can't manage to get that program to run on my computer. If anyone out there is at all interested / willing to help me with converting the layout I will GLADLY find a way to repay you!
I know this amp isn't particularly valuable and maybe the idea of converting this to point to point seems kind of crazy, but I like the idea of making it into something reliable and long lasting, and if it corrects the ghost notes that'd be great too.
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The vibrato circuit itself seems to be slightly different from the standard Fender design and I don't know if it's significant. The biggest differences I notice in the vibrato circuit are that the 2nd half of the triode (pin 2 on the schematic) is connected to the wiper of the intensity pot instead of the photoresistor. Also, on Fender amps, the vibrato circuit is connected to the 50/50 cap in the bias circuit but that connection is absent on the AIMS. Could anyone explain the purpose of that connection to me?
There are lots of different tremolo circuits. Yes, these two are different. Both work well. The Fender circuit uses the negative bias voltage to turn off the tremolo oscillator by biasing the oscillator into hard cutoff. When you turn the trem on with the footswitch that negative voltage is quickly removed and the oscillator is quickly shocked into operation, kinda like slapping it hard to wake it up.
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Interesting! So the footswitch connects/disconnects the negative bias voltage to the trem circuit. Does it effectively bias the tube cold to the point of not passing signal? Or does the negative voltage somehow stop the oscillation itself? Sorry for the rudimentary questions, just excited to learn.
As for the AIMS tremolo, since it doesn't have a connection to the negative bias voltage, does it not have the same kind of shutoff method? Does it just rely on the intensity control to pass all the trem signal to ground?
P.S. thanks for reminding me that its tremolo and not vibrato