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Motorboating


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5/30/2000 2:16 AM
Winnie Thomas
Motorboating
I re built my November amp using a tag board. It worked okay except for a hum which I thought might be lead dress and figured building another board might solve the problem. It did in my first November.  
 
Anyway, the hum seems to come from the P/T, but I now have a motorboating and squealing problem when I turn up the treble or Mid pot. The bass pot works fine. I noticed this same problem once before on a BF preamp, but this is a cathode follower. What happens is that when the treble or mid pots are turned up, the Voltage on the pin 7's (the ones connected to the O/T) of the EL84's goes from 340 V to 800V.  
 
I plan on going through the amp to make absolutely sure i wired it properly.  
 
Any thoughts?  
 
Winnie
 
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5/30/2000 2:20 AM
Matt

I had that problem with my Marshall 2204 and *properly* shielded inputs was the trick. Prior owner had shielded the inputs on mine, but butchered the sheild in process making for a terrible job and squeel like a pig on helium.  
 
Cheers,  
Matt
 
5/30/2000 2:24 AM
Peter S

Winnie,  
I would check all the grounds very thoroughly. Make sure you have all your filter and decoupling caps grounded in the right places. That's where I would start.  
Peter S
 
5/30/2000 3:00 AM
Graywater

Winnie,  
 
Haven't paid much attention to the amp in question but your mention of voltages jumping to 800v caught my eye. Where would you get 800V (DC I assume) in this circuit? If you're using a DMM I suspect you've got an artifact of a complex waveform thats confusing your meter - first, replace your battery - makes a world of difference. If that doesn't help, borrow an analog meter - a VTVM, or Simpsoon 260, or use a 'scope - then you can get a reading that will allow you to realistically troubleshoot your problem.  
 
GW
 
6/2/2000 12:03 PM
Ray Ivers

Winnie,  
 
I believe what you're seeing is ultrasonic oscillation. The voltage jump on the output tube plates is probably due to the 'flyback effect' from the OT at full power; I bet that if you hooked the amp up to a 'scope, you would see the amp operating full-tilt at some ultrasonic frequency (which the meter may integrate into a DC reading). Turning up the treble and mid controls would make this problem worse, of course. Small animals can be great troubleshooting tools for these type of problems, especially if you can read their facial expressions; got any?  
 
Is it possible you reversed the OT transformer's plate leads? Try disconnecting R35 (feedback resistor) and see what happens. It may also be a lead dress problem, or anything that affects amp stability, really. I would first try disconnecting the feedback loop, and see what happens.  
 
Ray
 
6/2/2000 12:50 PM
Winnie Thomas

Ray,  
 
Thanks, I'll try that tonight and get back to you.  
 
Winnie
 
6/3/2000 3:54 AM
Winnie Thomas
It was lead dress!
I tried detatching the feedback loop, but no difference. I started poking around and pressed on of the O/T wires. The noise stopped. I re-wired the circuit board in with shorter lead runs and the pronlem is gone. The hum, unfortunately remained, but I'm relatively certain that it's my new Hammond 270FX. I'l replace it tomorrow.  
 
Winnie
 

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