lapdawg wrote:
Thanks shaunf for tips. There's a bit there for me to digest & work out. Also, I have given it a bit more of a run through its paces. I am not sure that the sound is 100%. There's definitely no hum. The bass is very pronounced, but I can tweek it out. I am also picking up a little crackle when I crank it. The most disturbing is that when crank the master volume, it fades away from about level 9 - 10. Its perfect up until then, but then the volume dips completely.
I did run the impedance at 16 and not 8 - that's what my cab is.
The KT66's are too hot too touch after a bit of of run time. Is this normal?
Theres still a bit of binding etc. to sort out on the cab, but I do like the way the leather covering has worked out. I took some plain leather, dyed it deep green. It still needs a few more coats of leather dressing. I think the leather will age nicely and get good character. Here's a pic:
Looks cool! Love the green cab!
What you describe when you crank the volume past 9 sounds like you are experiencing blocking distortion. My first thought is that this is caused by not having any output grid resistors installed. I'd suggest installing a 5k6 resistor in line with the shielded leads from the PPIMV to pin 5 of each of the power tubes. It's important when you install them to a.) have little to no lead length between the resistor body and the tube socket pin, and b.) insulate the lead on the other side of the resistor body properly with a piece of heat shrink. It helps to push the heat shrink onto the resistor body a bit before shrinking it, to ensure it stays in place. I'd also remove the orange/green leads also connected to pin 5 of each of the output tubes entirely.
Regarding the Negative Feedback circuit, I'm not sure from your pic, but it looks like there may be a problem there too. If you look towards the right side of your turret board in your gutshot, you will see a carbon comp resistor to the left of the grey metal oxide resistor. This carbon comp resistor is called the Negative Feedback resistor (NFB). The Negative Feedback circuit works by supplying a fraction of the output of the amp, back into the input of the amp, opposing the input signal. The value of the NFB resistor controls the amount of Negative Feedback that is fed back into the circuit. The higher the value of the NFB resistor on the board, the lower the amount of Negative Feedback put back into the circuit. The more Negative Feedback fed back into the circuit, the more stable the circuit, but this comes with a tradeoff in that it can make the amp sound dull and lifeless.
The other thing that controls the amount of Negative Feedback fed back into the circuit, is where the other end of the NFB resistor connects to. One end should connect to the Presence Pot, and the other end should connect to either one of the taps on the impedance selector, or onto the output speaker jack. Note that where this connection terminates is not governed by the impedance of the cab you are using. You can connect it to any tap you want to, regardless of your cab impedance.
The way it works is as follows: Connecting the NFB resistor to the lowest impedance tap, ie the 4 ohm tap, will supply the lowest amount of negative feedback back into the circuit. Connecting the NFB to the 8 ohm tap, doubles the amount of NFB supplied, compared to that supplied by the 4ohm tap. The 16ohm tap again doubles the amount of NFB supplied by the 8ohm tap.
You can connect this side of the NFB circuit in one of 2 ways - either connect the lead to one of the taps directly on the back of the impedance selector, in which case your amp will supply the given rate of negative feedback, regardless of what cab you are using, and how the impedance selector is set. The other way to do it is to connect it to the output jack itself, on the same pin that the centre pin of the impedance selector is connected to. This makes the NFB variable, in that it will change depending on what impedance cab you are using, with the impedance selector set accordingly. That is, if you are using a 16 ohm cab, with the selector set accordingly, you will have the most amount of Negative Feedback being supplied back into the circuit. If you change to an 8 ohm cab, and change the impedance selector accordingly, you will have half the amount of Negative Feedback supplied.
For example, I started off with mine connected to the output jack, but changed it to be connected directly to the 8ohm tap, even though I'm using a 16ohm cab, and have my impedance selector set on 16 ohms. This meant that I halved the amount of Negative Feedback put back into the circuit, and I was very pleasantly surprised with the results.
Here is a picture of the internals of my amp on which I've tried to explain above. Click the link below the image to go to the full size version so you can see the labels I put on it. Please note that I'm also new to all of this, and what I have written above is my understanding and interpretation of these subjects and concepts, upon which I do not profess to be any kind of expert.
http://img.photobucket.com/albums/v206/cliosport/Amp.jpg