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Hi Stephen, I thought I would start a different thread for this issue, as it is not part of the other distortion issue I had.

I am still having a problem with the amp breaking up at about 1/4-1/3 volume.

I have narrowed the problem down to the PI. Using a sine wave, the output of the preamp is clean to full volume.

Looking at the output of the PI the amp starts to distort at about 1/4-1/3 volume (21V p-p), clipping the top of the waveform. This is quite audible.

The output of the preamp the signal is about 0.66V at 1/4 volume and is 4.14V at full volume (p-p).

The voltages on the PI are:
p1-163
p2-41
p3-60

p6-166
p7-41
p8-61

So if the PI is operating class A, I have about 163-60 = 103v of plate voltage to play with. If the amplification factor of a 12AX7 is about 100, this tube should go into saturation at about 1V of input.

So if this estimation is correct, either my input signal it too hot, or I need more PI input attenuation or put in a lower gain tube.

What gets me is this sIII worked great before I cleaned up the lead dress. What I just lucky when first building it?

Don

P.S. I have this posted on the 18W site as well. Not sure if you are looking at it these days.

When you do this testing, where do you have the Volume and Master volume Set?

You used the same board and just rewired it, correct?

Is R8 grounded btw?

Voltage chart of a working sIII with AC Mains Voltage 123VAC

V3 (PI) - 12AX7
--------------
Pin Volts
1 222
2 55
3 80
6 217
7 54
8 80

Yours do look a little low.

Here's some other measurements.

viewtopic.php?t=416

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Sorry Stephen, I should mention that this is the normal channel. So maybe I better step back for a sec. On your sIII normal channel at what point on the volume does it start to noticably break up?


As for the other questions:
- yes, R8 is grounded right from the volume to the ground bus.
- yes, I used the same board, just shortened the pot and tube leads, replace a few ceramic caps with silver mica caps.

- I will double check the voltages and the mains.

Is it possible that the normal channel signal is just plain too strong?

My normal channel begins to break up at about 11 O'clock depending on the guitar I am using, I have noticed that humbuckers break up before single coils for some reason. I guess break up is also going to be dependant on the output signal of your guitar, hotness of pickups etc etc. The 18Watt design is not noted for it's cleans anyway, the breakup is the draw of the design.
Nigel

Ok, the Normal Channel - Legin is 100% correct -by design. Yours does seem to be breaking up a bit early tho. Here's some normal channel tweaking taken from our manual:

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps