Tube Amps / Music Electronics
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|1/1/2006 6:51 PM|
||Traynor YGM-3 – Ultrasonic Oscillation|
Ok, I’m officially stumped, and would appreciate any assistance.
My favorite amp, a Traynor YGM-3 (1x12 combo with reverb and trem, schematic here: http://www.lynx.bc.ca/~jc/730911_YGM3_4.gif ) has problems. It was fine up until a couple of weeks ago when I took it for a ride to check out a guitar. When I got there it sounded distorted, like the speaker or cabinet was buzzing. When I got it home I tried it with another cabinet, same thing. I stuck the scope on the speaker output (with no input, and output connected to a load box), and saw a 6-7v peak to peak sine wave, at about 52 KHz.
Swapped power tubes, then preamp tubes with no change. The oscillations appear to start at the input to the phase inverter, right around C10, and go right through to the output from there. For the hell of it, pulled all tubes except for the power tubes and the phase inverter, still there. Disconnected the ‘output’ side of C10 (closest to R14 and V2A pin7), no change. Checked the value of all phase inverter and power amp stage resistors, as well as the caps with what I had (which only measures capacitance not ESR etc). All were well within tolerance including C13 which is supposed to kill ultrasonics, right? Tried tapping all of those components in case something was microphonic, nothing found. Since it seems to originate in the phase inverter area (V2A, pins 7 and 3/8), I tried shielding these grid and cathode connections between the tube socket and eyelet board, with no success.
What am I missing? As I said, any assistance would be greatly appreciated!
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|1/2/2006 1:10 PM|
Check R18, lift one end to open the NFB loop and see if the Oscillation changes.
Also check all ground connections and solder joints, sounds like something changed during the "ride"
|1/2/2006 10:21 PM|
Hi j, and thanks for the reply!!
I tried lifting one end of R18, no change.
I also posted this to alt.guitar.amps, and Walter Campbell suggested looking at the trem (bias modulation) section. This is promising! With only the phase inverter and power tubes in (*not* V6B, which is the trem oscillator), power up with wiper of trem intensity pot disconnected, no oscillation. Reconnect wiper, oscillation is back. Checked all caps in trem circuit, as well as all grounds and touched-up solder connections in that area. All grounds and cap values ok.
So, getting closer, but not there yet...
|1/3/2006 8:45 PM|
With *only* the phase inverter and power tubes in (*not* V6B, which is the trem oscillator), power up with wiper of trem intensity pot disconnected, no oscillation. Reconnect wiper, oscillation is back. Consistently. Checked all caps in trem circuit, as well as all grounds and touched-up solder connections in that area. All grounds and cap values ok.
Then, with the wiper connected, noticed that I could turn down the oscillations' amplitude by turning the trem intensity pot up. Sometimes with the pot up the oscillations would stop completely, then try to start up again, etc.
So, since it was *most* stable with the pot wiper towards the trem output, I started looking at the bias circuit. I had installed a bias pot a long time ago, so I tried turning the bias pot to see if running colder affected the oscillations. It did so I checked my bias supply, which was fine. I figured I'd check to see just how cold they were running. With my meter on the 300mV range, checked one of the 1 ohm sensing resistors to ground, ~5mV. Checked the other, instant over-range! WTF? Found it was 105vDC, so I knew that 1 ohm resistor was 'exhibiting anomalous behavior'. The damn thing was wide open! Replaced it and biased the amp with no problems. Life is good again!
Thanks again, j!
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