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Hi All, I am having a bit of a distortion problem with my (homemade) TMB sIII 18W when I turn up the volume on either channel past 6 or so. Here is a sound clip:

http://www.cameronsoftware.com/misc/farting.mp3

To try and correct, I have tried:
- a different speaker
- chopsticked the whole amp for loose connections
- measured voltages
- swapped all the tubes

Here are a sine an triangle wave from the output of the amp:





Line in 116.4VAC
B+ 336V
Heaters 6.22VAC

V1 (1st stage TMB channel, only stage normal channel 12AX7)
1 - 147
3 - 0.76
6 - 144
8 - 0.76

V2 (second stage TMB, cathode follower TMB 12AX7)
1 - 160
2 - 0
3 - 1.42
6 - 244
7 - 159
8 - 159

V3 (PI 12AX7)
1 - 199
3 - 64
6 - 197
7 - 45
8 - 64

V4 (Power EL84)
3 - 10.4
7 - 327
9 - 306

V5 (Power EL84)
3 - 10.4
7 - 325
9 - 306


And Stephen's schematic: http://www.cameronsoftware.com/misc/Tri ... c_v1_5.jpg

I have posted on the 18W forum, but not much luck. I also asked on Hoffman, and they suggested changing the EL84 grid resistors, etc. - But since I am quite true to the schematic and layout Stephen has devised, I thought it best to ask other sIII builders.




thanks
d1

Never heard that sound.

Looks like it's on the bottom end of the ampllified signal on the leading / trailing edge . Almost like clipping but distorting as it starts to clip and as it comes out of it.

I'll give it some thought. I'd start by looking at the [negative half of] the PI though. Maybe measure the value of the resistors around it.

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

Hi Stephen, thanks for the suggestion. I am sure I did something wrong, as this design worked well for me before and I am sure it has worked well for you and others. I am going to trace through the whole thing (again) and measure everything.

thanks
Don

Do the same test at the grids to the EL84; then before the PI to power coupling caps. Work your way back until you get a clean signal and then isolate the bad component / connection.

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

Think it is wiring or component related.

Check that grid resistors are on correct pin; [last resort could move them right to the tube socket]
Recheck wiring around phase invertor to power tubes;
Check components values around PI (Resistors / Capacitors)
Both power tubes are OK?

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

Hey Don, have you got any pictures of the guts of the amp? someone might spot something.
Nigel

Man, I should have known better - every amp I have built I have done the same mistake: my output uses floating jacks and I did not ground it.

But, this is a bit odd, as this amp was working just great before I decided to clean up the lead dress.

However, it is working much better, but the signal after the PI is clipped on the upper half after about 1/2 volume. I did not think this was happening before.

Here is what I have tried:

- retraced the whole circuit (yellow hiligher method) - all OK
- measured all resistor values - all OK
- swapped out different tubes - same result

Is the PI running too hot?? Here are the voltages:

p1 - 169
p2 - 43
p3 - 63

p6 - 170
p7 - 44
p8 - 65



And for those who are interested, here is a pic of the guts:



(ain't this fun? - I love it!)

d1

Heres a PI set @ 123 VAC mains so its not too hot anyway. Bit cool actually. You might want to check why.

BTW, I always float my output jacks. Dont ground them either.


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

Check this posting and there are others just search for "Voltage" in the "Trinity Talk - 18 Watt and 15 Watt" forum

viewtopic.php?t=689

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

Thanks Stephen. wow this is interesting!

On your sIII builds, when do you experience noticable distortion? Have you ever known it to come from the PI? The preamps are driving quite clean - lots of headroom.

One person at hoffman suggested that my OT leads were to close to my preamp and PI, and can induce oscillation - this may be one reason why you don't see it in your builds (your OT is well away from these areas).

Don

Well, the OT is definitely affecting the signal. When I unhook the ground, moveing the OT output leads causes the oscillations to fluctuate.

I will rebuild the chassis to see if I can get this to sound clean using a flotaing output.

thanks Stephen,

Don

Very interesting.

Sure, you get distortion from the PI. You should be able to drive the pre hard as well with the Volume and get preamp distortion. Master Volume will drive the PI.

I haven't had this problem. Our OT is 6" from the PT, 90 degree rotated and pretty well in line with the Power tubes. Try twisting the OT leads to the tubes.

Maybe just unbolt the OT and move it closer to the output tubes and try again.

Can you post a pic of the top of the amp?

I also noticed you had a different layout for the front controls. What have you done there?

Oh, and I'd move the shielded cable from the Vol pot to V2 to be above the turret board, not below it. It is shielded?

Very interesting indeed.

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

The physical location on my OT sounds about the same as your builds:



Looking at your layout though, I think my board is a bit more spaced out so the PI gets overtop of the OT. Also with my selector far over to PI/PA area, the OT lines run under that part. I will move the selector to the otherside and re-route the OT leads.

The is my third 18W build with the same chassis (18W, MarWatt and now the sIII). I did not have any space for the TMB MV so I had to put it on the other side of the jacks.

The Vol pot line is shielded.

Thanks for the help, this is a great learning experience.

Don

Your OT is very close to the PT. I would recommend that you move it over to center it on the output tubes.
I dont think that moving the selector switch will make much difference.

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

Based on your scope pictures, you have some bottom half clipping in your amp so you may want to check the Power Tube Cathode Resistor.

It controls headroom and linearity of the stage. As the bias point is shifted (i.e. resistor value changed from stock), the amplifier will clip more on the top or bottom portion of the waveform.

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

Thanks for the help Stephen, moving the selector switch did remove the oscillation from the signal (the wires are awat from the PI/preamp circuit now).

I put the amp back to stock, but still have the premature clipping.

I will take a close look at it again - do some scope work as well.

Don

How far apart are the output jacks & selector switch now? And where do the wires run between now? I'm suprised that made a difference. But it's all good!

Check to see where the premature clipping shows up from output back. What value is the cathode resistor on the EL84s? Since it's on both channels, check the cathode values on the PI & V1 but I'm guessing there's something about the PI circuitry that's not quite right.

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