martin manning wrote:Each pot has one lug connected to a lead from the rectifier board (purple in the layout drawing), and also a 15k resistor from another lug going to ground. The 33k resistor goes across those two lugs, which will put it in parallel with the pot's element. 100k in parallel with 33k is 24.8k. The third lug is the wiper, which is where the bias voltage is taken from.
Excellent, thanks! I just so happen to have 33K/2W resistors handy
martin manning wrote:Determine a target reading for the meter: Estimate the loaded plate voltage at say 95% of the unloaded value, 0.95 * 440 = 420. 6V6GT are rated at 12W max plate dissipation, and we'll choose a bias point at 70% of that, or 0.7*12W/420V = 0.020 A. This is a conservative value since you will be measuring cathode current which includes the screen current. The target 0.020 A will be indicated as 0.020V (20 mV) on the meter since you are measuring the voltage drop across a 1-ohm resistor.
I let the amp stay on in case the bias needed to settle a bit. There was no howl, but a very, very slight hum.
Plate voltage (as measured from node 1 of the output board): 450 V
450 * 0.95 = 427.5;
0.7 x 12W/432.25 = 19.65
So I ensured that none of the voltage at the four test points exceeded 19.65 mV and this was the result:
V8: 18.2 V
V7: 19.6
V6: 19.6
V7: 18.6
Everything seemed to be fine, guitar signal was coming through, until I engaged the OD switch in the back, at which point there was a loud howl. So I turned down the volume low and hit the switch again. At low volumes there was no signal whatsoever from my guitar coming through, just a hum. What could be causing that?
Last edited by Mousey Dung on Mon Oct 06, 2014 10:37 pm, edited 1 time in total.
If it works fine in clean mode, then that indicates that the power amp is working fine.
It seems the issue is in the OD channel side of things. Can you scope it? If not, and time is a factor, find someone whio can put a sinewave on it, and scope it. This will save you a lot of time.
It's the clean that is having problems. When the OD switch is on, I can use the overdrive section - the drive and level knobs work in conjunction with the volume level.
However, when I flip the OD to off position, that's when the hum-and-no-signal problem starts. In that hum mode, when I turn down the master volume, I hear a mechanical rattling inside the amp -- could it be the relays?
Last edited by Mousey Dung on Mon Oct 06, 2014 11:09 pm, edited 1 time in total.
Yes, I did. Although it's the drive channel that seems to be okay and the clean channel that is having problems. I just tried it again, in clean-sound mode, with the standby switch in off position turning down the master volume, at which point the rattling begins. Really sounds like it's the middle relay.
There is a diagram on page 2 of this thread. That's all that I (and everyone else here) have available to work with.
Is it possible that you have the blue wire from the master volume pot that is drawn as passing underneath the middle relay, actually *connected* to the relay, other than at the point where the wire terminates?
