Recent content by JohnW

Never mind, I see them.

Maybe it is hidden in the picture or you have the input RCA jacks in a different place, but I don't see the input going anywhere.

The light blue wires may need swapped, if disconnecting the yellow and green wire removes the noise.

Not the output taps, but, if the primary leads to the transformer are connected to the output tubes backwards you get positive feedback rather than negative and lots of noise. Removing feedback altogether is a quick way to see if this is the problem. If it is, you need to swap which tubes the...

I would try de-soldering the feedback loop ( off the output transformer) as a first step. Its easy to get this switched around by accident.

From what I remember, I based this on measuring the overall resistance through the tone controls. Not wanting to add too much feedback I ended up with 120k and was happy with the way it sounded. Here is a plot taken of the response with network engaged, eq centered, and with it switched off...

On mine, I installed a switch that either engages the compensation network (within the dotted lines on the schematic), or routes feedback through a single 120k resistor. Its best to flip the switch with the power off, but it sounds great and the overall volume of the amp remains the same.

Thanks!

One other question I read that TR114 was originally configured CBE, rather than ECB. Looking at the picture in the service manual it looks like the center leg is the collector, not the base. Does the circuit board have the center leg collector, like the picture?

I too am repairing a P2200, and along with a number of driver transistors, it has one bad 2SA909. I've checked and replaced all the bad transistors and diodes, basically all from TR109 on up Can I replace just the 1 2SA909, or do I need to do all 6? If I do 6 on one side, should I do the other...

http://sound.whsites.net/valves/preamps.html Section 8, Bootstrapped Load Feedback Amplifier. I've built and used this a number of times, mostly without feedback. It sounds great.

A recently finished 175 amp, with added input buffer.

This is the method I used on a pair of 6L6 PPP amps I built. The B+ was a little high at the start, and taking the bias off a floated ground brought things right in line. It has been working great. My observation is that it lowered the noise floor as well, since it helps to isolate the first...

On this 9300, I cut off the side flanges prior to fitting the case.