Mods to the 3 channel amp

[Bob-I]

After talking to dogears, Scott, I made some additional changes to the 3 channel amp. He suggested that the 150K load to simulate the FET has more tone than adjusting the PS rail for correct voltage. I used a combination of the 2.

Before my voltages were rignt in spec, or at least very close to the 200V suggested. It does make a big difference.

So I added the 150K load to the end of the PS rail and adjusted the resistor right after the PI to give me the same voltages. All it took was lowering the resistor to V2 from 15K to 8.2K and it's back to the same numbers.

The result is a smoother, softer OD with more bloom. The best think about it is that this happens at very low volumes. So low that I swapped the Linear 1M master to an audio for finer control at lower volumes. It's a smooth increase from 0- about 2:00 then it starts to jump, but since I never play that loud this is good.

I made a few other minor changes but basically the ckt was about what Scott suggested. One of the keys to the Hybrid ckt is proper loading of the OD and PI. Without the HRM OD2 likes to see a 100K load going out or the coupling cap, but the PI likes to see a 1M load.

I had a 47k/47k voltage divider going out of OD2 coupling cap after the 150K series resistor, then a 1M linear pot going into the PI. I made some changes to clean up the layout. Instead of the 150K -> 47K I swapped to a 200K and left the 47K to ground so the values are the same, but fewer components.

I've also been fighting an intermittant bass control. From time to time it simply stopped working. I've swapped all the wiring, re-flowed all the solder connections and no change. Yesterday I swapped the pot out and it's been working ever since. Keep your fingers crossed.

[tonelab2]

The result is a smoother, softer OD with more bloom.

To quote the late physicist Dr. Julius Somdemiller
"Why is this so?".

[llemtt]

The result is a smoother, softer OD with more bloom.

To quote the late physicist Dr. Julius Somdemiller
"Why is this so?".

Good question!

Does anyone have ideas?

First thing that comes to my mind:

class A triode stages don't draw a costant current under signal because of even order distortion (non-linear transfer function), overdriven stages experience a huge increase in current under signal

lowering the voltages with the additional "constant" load of a resistance instead of bigger ps resistors could help to stabilize B+ voltages and maybe this has a good effect on OD?

teo

[Bob-I]

Why? I have no clue......

I'm not an engineer even though I have a sheepskin that says otherwise. It's real sheep skin because it was earned before paper was invented. They had to hire monks to write them because there were no printing presses yet. During electronics labs we just guess that things would work, that guy Franklin hadn't elecrocuted himself with a kite yet.....

I don't really know the answer, and I don't believe HAD did either. He just experimented until things sounded right. That's basically what I'm doing. Teo may be right about the way a tube acts in distortion, bottom line, it sounds good.

[BobW]

Bob,
I don't doubt you but it's interesting the B+5 only resistively loaded at 150K can make a sonic difference. Probably due to a transient supress at turnoff? Hmm...I'll do the same mod and make scope measurements before and after, will let you know what I see vs. hear. Am hoping for an additional tone improvement too. Thanks for the info. BobW

[llemtt]

..and I don't believe HAD did either..

In this case probably he didn't even experimented, he just connected his fet preamp... something I never did because I don't find it useful

As soon as I have two identical amps again I am going to modify one with the fet-load-simulation-resistor and A/B test for differences.

cheers

[Bob-I]

As soon as I have two identical amps again I am going to modify one with the fet-load-simulation-resistor and A/B test for differences.

cheers

That will be interesting. Sometimes your ears deceive you when there's 10 minutes between listening.