7 posts / 0 new
Last post

410-65 Oscillation - Transistor Driver

Hey everyone, I've got a 410-65, 2275-65 chassis on the bench that I can't get stable. Before I saw the amp someone tried to run it at "half-power" by pulling one of the power tubes. Apparently the whole output section blew up including the output transformer. The transistor driver board was rebuilt by another tech and the output transformer was replaced (incorrectly)with a P-TM050 Marshall-style 50 Watt transformer.

The rebuilt amp came in with a blown output tube and one fried driver transistor. I replaced both driver transistors with 2N4488's and installed new power tubes. I rearranged the secondaries on the output transformer so rather than the 3.2K load of the Marshall-style OT the tubes are seeing a reflected impedance of 6.4K (closer to the original 6K impedance). The negative feedback is coming off the 16 ohm tap.

Now I'm getting 117KhZ oscillation on the cathodes of the power tubes. It's enough to make the tubes over-dissipate. The source of the oscillation is somewhere in the driver stage after the master volume and before the cathodes. I've been able to get it a little more stable by adding small amounts of resistance and capacitance in parallel with the negative feedback resistor but it seem pretty finicky. I've replaced the flyback diodes and 3KV caps on the tube sockets. Voltages at the grid and screens are stable. The oscillation is visible on the scope at the cathodes.

The obvious thing here is the OT. I'm trying to save the owner the cost of a proper replacement from Mercury. Does anyone else have any idea how I can get the amp stable otherwise?


Forgot to mention that all

Forgot to mention that all electrolytics have been replaced as well as the opamp in the driver stage.


Positive Feedback?

Looking at the 2275-65 schematic shows the feedback coming off the 8ohm tap. This is only guessing but perhaps the phase angle might be different from the 16 ohm tap and cause a positive feedback loop. Try using the 8ohm tap. As mentioned, just a guess. http://www.pacair.com/mmamps3/sites/default/files/docs/2475-65_and_2275-... Good luck. -mgriffin


Thanks so much for the reply,

Thanks so much for the reply, I'm using the 16 ohm tap because the primary is 3.2K on the replacement transformer (as opposed to 6K for the original), so I was thinking that would be a better match for what the amp expects to see. I'll try the different taps when I get it back on the bench tomorrow. It occurred to me to try adding a little capacitance between the base and collector of the driver transistors. I do think it's some kind of phase/feedback issue.


Tried taking the negative

Tried taking the negative feedback from different taps. The bias is stable off of the 8 ohm tap but I get a gated fuzz on the low notes. Looking at it on the scope there's a parasitic-looking transient that appears on a 500hZ sine wave in the opamp that feeds the driver transistors (pin 1 of IC8). When I switch the function to a square wave the transient appears on the leading edge of the square. I made a little Zobel network with a .001 uF cap and a 25K pot and placed it across the primary but even when I increased the cap value to .01 I couldn't dial out the ringing. Hmmm..... Everything looks good up to the master volume.


Is it stable with the NFB loop disconnected?

Hi there,

is the output stage stable with the NFB loop disconnected? It should be.

Lars Verholt

Bill Roberts

410-65 Oscillation - Transistor Driver

Had the same problem with repairing a HD-130. A previous tech had installed some caps to try and stabilize the board. I ended up rebuilding the entire power supply and driver boards. The 16v B+ B- was a little out of balance so replaced the 620 ohm resistors with 1%. Solved the problem amp is incredibly quiet with tones of output. Checked all resistors to make sure they were within tolerance. Replaced all the Electrolytic Caps, The D17 Diode was replaced as again a previous tech had replaced it with a close 15v diode ad not the 16v specified. Also replaced the output Transistors with the recommended Service Bulletin #7 Driver Transistors. The amp sounds great but still has issues Bright, Deep Switches don'rt work, Tremolo circuit also not working quite right. I am suspecting a number of the Film Caps will also need replacing.

Log in or register to post comments