How to 'fine tune' your amps?

[Family]

People always say they will 'fine tune' their amps after building it. Actually how to do so? Any steps or procedure?

[Sch3mat1c]

I fine tune stuff like hum, parasitics and NFB. Hum is often caused by PSU ripple, heater-cathode induction and the ever-mystical, ever-simple, ground loop. Parasitics such as HF (squeal if audible) or LF (motorboating) resonances must be tackled before anything and may be caused by PSU wiring, wiring layout in general, or an unstable (such as a Williamson) or undamped circuit (grid stoppers, zobel networks). NFB in particular magnifies any phase shifts and really gives your troubleshooting a run for your money. I enjoy a challenge.

Tim

[gingertube]

This is a sadly neglected topic in most tube amp books and probably the thing which causes most grief to newbies.

Rule 1: Get the amp as linear and as hum free etc. before trying to add the feedback loop (assuming it has one).

For amps using global feedback its best to try and suppress any high frequency resonances in the output transformer at this stage - before closing the feedback loop. This is done by adding zobel networks to output transformer connections - usually on the primary side.
The best explanation/method fro doing that can be found here:
http://www.siteswithstyle.com/VoltSe...in_xfmrs.html.

When it comes time to close the feedback loop it may be necessary to adjust the frequency and/or phase response in order for the amp to not oscillate.
The best explanation/methods for this is probably:
TUBE AUDIO DESIGN - The Beginners Guide by Bruce Rozenblit

I hope this is of some help.
Cheers,
Ian

[ray_moth]

As Gingertube says, get it right before adding feedback. Adding NFB to a poorly performing amplifier will be disappointing.

[Sch3mat1c]

Well, except for like I said. Example: you can have an amplifier perfectly flat from 20Hz to 20kHz, but if there's any phase error at the ends, you won't see it until NFB kicks it in the middle anatomy and it gets red and swollen. Williamson topology, and OPTs being used well beyond their useful range, are wonderful things to watch LF rise at 3Hz on, or resonance at 50kHz. Seriously, I had to drop the coupling caps to like 0.022uF, and there was still a 3dB rise at 13Hz! On that one I eventually rewired it to something better...

Tim

[ray_moth]

Example: you can have an amplifier perfectly flat from 20Hz to 20kHz, but if there's any phase error at the ends, you won't see it until NFB kicks it in the middle anatomy and it gets red and swollen.

As you rightly point out, misapplied feedback can actually make things worse in an amp that was behaving well prior to its introduction. This is certainly true of the Williamson and any other design that has too many coupling caps in the loop and/or has an OP tranny with deficiencies such as excessive leakage inductance.

Incidentally, I wonder why Mr. Williamson designed a global FB loop in his amp, when he was using triode OP tubes? It seems to be inviting problems unnecessarily, considering the already low distortion and low OP impedance of his triode-strapped KT66s. A bit of local FB would have done the job with no risk of instability. He designed his amp to be built by hobbyists and must have known that many people wouldn't have access to a sufficiently good OPT to make it work properly. However, one can easily provide a simple and effective fix to make global feedback stable in the Williamson, by converting the second coupling cap (before the OP tube) in each half into a step network and rearranging the biasing accordingly.

[Tubes4e4]

Hi Ray_moth,

Incidentally, I wonder why Mr. Williamson designed a global FB loop in his amp, when he was using triode OP tubes?[/B]

This is a simple one: Because he wouldn´t have got the distortion figures he was going for w/o tons of gNFB even with a PP triode output stage.

Tom

[Miles Prower]

People always say they will 'fine tune' their amps after building it. Actually how to do so? Any steps or procedure?

Don't know; don't care.

Solid state or hollow state; RF or audio: makes no difference since I always test as I build. It's SSSSSOOOOO much easier to get one stage working right, to apply optimizations, and do all the tweaking before moving onto the next one. At least, you will know where the durn thing is misbehaving! Saves much aggravation as opposed to wiring everything up, then having to go back to track down a misbehaving stage. Also makes it much easier to do whatever final tweaking may be necessary to get full functionality.

Even if it's a proven design, I still test while building. I've caught lots of potential problems that way. While repricating a solid state design of mine, I caught a dead transistor right away when the problem was easy to find, and, more importantly, before it could potentially cause other transistors to fail in later stages.

[Family]

Thanks for the input from you guys.

Agree with Miles that always test when building the amp. What I can do is to make sure all the calcualtion and values are correct when building (supply voltage, value of the cathode resistor.... etc), and that is what I can only learn from the tube amp books or websites. You guys mentioned the problem of phase shift. Can the problem be eliminate when you're in the designing stage? Or is it an inevitable problem?

[Miles Prower]

You guys mentioned the problem of phase shift. Can the problem be eliminate when you're in the designing stage? Or is it an inevitable problem?

Problems such as these can only be treated empirically. You won't actually know if it will be a problem until you have the amp built and running, as it depends on so many factors that you can't know ahead of time. A phase shift problem may or may not show up, as it depends on the characteristics of the audio iron you use, the circuit stray capacitance, the amount of global feedback you apply to solve linearity problems. Yet another reason to test while still constructing the whole amp. That way, you'll know what characteristics your subsystems have, and can best determine where something like grid stoppers, or phase compensation can best be applied to nail any instabilities that do show up.