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Hi all. I am in the final stages of finishing a custom build amp. The chassis is all wired up, and almost everything works perfectly now, except for one one strange thing.
My trem oscillator is the same basic design as this one on the Valve Wizard's site -
The differences are that the cathode is biased with a red LED instead of resistor/capacitor, and I have changed some R and C values to alter the speed range of the oscillator.
The trem works perfectly as is, but when i try to engage the switch to turn it off, instead of stopping oscillations, the trem just oscillates at a fixed frequency. Altering the 'speed' pot does not change the frequency of this 'trem turned off' oscillation. The freq is ~ 50Hz. If I try grounding the first or third feedback network junctions, i also get fixed freq oscillations, but just at different frequency at each junction.
The wire that connects the switch/ground to the feedback signal point runs ~ 15 cm to the edge of the board, where the footswitch wiring attaches. I have not connected the footswitch wiring yet, I have just been using wire jumpers to test first. The wire was initially unshielded, but running beside the chassis for most of it's length.
So far I have tried -
Changing the connecting wire to a shield one, with the shield grounded at one end - no change
Jumpering a 10uF cap across the red LED, in case for some reason it wasn't effective at AC bypassing - no change
Trying grounding the long 'off' jumper wire at various points in the grounding scheme, including at the oscillator PS node - no change
Using a very short jumper wire and grounding right at the trem circuit position - problem solved
Now this short jumper works, and is how I had the circuit set-up when i breadboarded it. But it doesn't help me much now, as i need the wire to go all the way to the footswitch! What it may do is give a clue as to what the problem is. I suspect that it is related to stray capacitance, but any help would be greatly appreciated
My trem oscillator is the same basic design as this one on the Valve Wizard's site -
The differences are that the cathode is biased with a red LED instead of resistor/capacitor, and I have changed some R and C values to alter the speed range of the oscillator.
The trem works perfectly as is, but when i try to engage the switch to turn it off, instead of stopping oscillations, the trem just oscillates at a fixed frequency. Altering the 'speed' pot does not change the frequency of this 'trem turned off' oscillation. The freq is ~ 50Hz. If I try grounding the first or third feedback network junctions, i also get fixed freq oscillations, but just at different frequency at each junction.
The wire that connects the switch/ground to the feedback signal point runs ~ 15 cm to the edge of the board, where the footswitch wiring attaches. I have not connected the footswitch wiring yet, I have just been using wire jumpers to test first. The wire was initially unshielded, but running beside the chassis for most of it's length.
So far I have tried -
Changing the connecting wire to a shield one, with the shield grounded at one end - no change
Jumpering a 10uF cap across the red LED, in case for some reason it wasn't effective at AC bypassing - no change
Trying grounding the long 'off' jumper wire at various points in the grounding scheme, including at the oscillator PS node - no change
Using a very short jumper wire and grounding right at the trem circuit position - problem solved
Now this short jumper works, and is how I had the circuit set-up when i breadboarded it. But it doesn't help me much now, as i need the wire to go all the way to the footswitch! What it may do is give a clue as to what the problem is. I suspect that it is related to stray capacitance, but any help would be greatly appreciated
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