Too much feedback?

[GloBug]

Hey guys, just a general question for you.

I have a 6v6 PPP amp in my avatar that I built. I was fortunate enough to have Lance Cochrane himself supervise on this build

I noticed that on some bass heavy tracks, the upper and mids get quieter with big bass hits. Should I cut back the feedback a little bit?

I can supply technical information about my amp if needed. Just a general question wondering if that is a symptom of too much feedback.

I have over 1000uf as second PS cap, so it's not for lack of power reserve, I mean the amp will play for over 10 seconds after I switch the power off. lol

I suppose I could ask Lance, just figured this would be a little quicker. Fall is here so I am getting the soldering iron out to tinker.

[Frank Berry]

The bass frequencies require the most output power.
I'm guessing that the amplifier is clipping. As the clipping occurs, the mids and highs (which essentually are riding atop the heavy bass) are the first to disappear.
Reducing the playback volume might be the solution. I doubt that feedback is the issue.

[GloBug]

This happens at low volume, it's less noticeable when I turn it up.

I am on "1", I literally can't turn it down anymore.

I'm pretty sure it's not clipping.

[DF96]

Is it the automatic gain control in your ears? As well as adjusting volume, this could also modify the frequency balance.

[Rundmaus]

... remembering some stuff from the MJ books, if the final stage is cathode-biased with bypass caps, large caps can lead to a bias shift in large signal peaks, which can take up to several 100ms for the final stage bias to recover to normal levels.

It's only a theoretical thought, but if this happens with a bass peak, following mids and highs might be affected by that.

Greetings,
Andreas

[GloBug]

I will have a look and see what I put in there and report back. (cathode BP caps)

[richwalters]

Originally Posted by GloBug

I noticed that on some bass heavy tracks, the upper and mids get quieter with big bass hits. Should I cut back the feedback a little bit?

I can supply technical information about my amp if needed. Just a general question wondering if that is a symptom of too much feedback.

Trying to keep this short is difficult as there is so much bearing on sound quality.
One can cut the global feedback down but the fault remains in a lesser form of undesirable intermodulation at the expense of higher thd and noise depending on the leeway of the design, it will always be there unless a drastic circuit review is done: The original 3 stage Mullard 20W amp; the Williamsons suffered from oversized coupling caps; and my Linear Concord 50Watt which uses a sim LTP 3 stage design as per Mullard 20W, all suffered badly from bass transient overhang which modulated the midrange, esp on classics and worse when the bass control was wound up to +18dB or more. Once noticed, one becomes very distracted by it. It can also be a symptom of gross bass overload on a poorly sized output transformer.
The problem could also be a slack power supply not able to supply the current or most likely, too high interstage cap values. Need a schematic on this.

The technical side is complex, nasty and to many boring. There is circuit damping theory associated with a phase locked loop response, which is essentially what global nfb is doing; closing the loop on the stable side solong the values keep within the Bode plot regards octave decay and cutoff response as tampering with the global nfb loop also changes the circuit Q factor.
In my 150-200W amps I address this problem by selecting careful values of the concertina p/s caps by using a very low frequency pulse 1Hz or lower squarewave at a very low level and observing transient circuit overhang...if the scope has an Y output used as a wobbulator then even better.
Once a compromise value is selected, a full-pull test accompanied by a hefty beatnote reveals perfect reproduction of two tones.
In each and every case in push pull circuits I find that the optimum bass overhang/transient response occurs when the f2 distortion is around 1.4% at the LF design cutoff frequency of the output transformer. With other circuits, not the case. The bigger the amp design, the more important the issues become as overhang sucks unnecessary power rides and increased tube dissipation. Big issues.

There are more snags; another subject.....Thought about the loudspeaker ? Got an undersized cheapo box, versus the amp power ? On the instance of a hefty beat note with maximum cone excursion forced against the stops, it's obvious the cone is furthest away from the magnet. That would immediately give the same modulation result one is hearing in the midrange. This is where large cabinets with large speaker frames rule the room. What's next ? the influence with Crossover networks.

I find the selecting the correct circuit capacitors is far more responsive to reducing the problem you outlined.

Sorry for being so long, but having experienced the these problems on a commercial side some 45 yrs ago, the issues deserve an airing.
I hope MJ's new book 4th ed which is due to come out, has some relevance on this.

richy

[Rundmaus]

I guess richwalters is a better help here - my knowledge of valve technology is limited to the MJ books, one half-finished test setup of an all-valve PP1C and the basics that I have from studying physics and working on UHV experiments...

Greetings,
Andreas

PS. Will of course continue to read here - maybe this will also become important for my project - hope I'll ever find the time to continue with that...

[richwalters]

MJ books are one of many references...we all refresh from. The Radiotron handbook 4th ed which is one of my old trade favourites covers some of the issues in chapter 7.
Sometime ago I did some graphs of coupling cap values versus closed loop transient responses in Williamson 4 stage amps which show the circuit problems. I will find them.

richy

[GloBug]

No need to be sorry, thanks for that.

I will draw up some schematics and post it on here, I should have a set anyways, they are Magnovox 142's.

I am going to play around with the speakers because I have a very oddball setup.

12" Jensens in open baffle, 2 3" GE tweeters/mids (One receives fulls range sound while the other is crossed -overed.
Below that I am using 8" Sansui woofers in their SPX-4000 boxes.
I have it wired in a way to increase the bass at the 8"woofers. When I first hooked it up the 12"ers were too loud (sensitive). Part of the reason for this is my amp has 3-4ohm taps.

The Sansui's really move, full excursion for sure. Maybe that "mid" that's taking the full range has something to do with it too, it's identical to the tweeter, just no cap.

So I will play with some speakers and report back with any results.

Here are some pictures of my amp if you want to have a look in the meantime.
The inverters were replaced with 6CG7s also an extra 6CG7 stage was added as a bit of a pre, I was told would make it sound more musical. Truthfully it's too loud. Before I added the extra stage it was dead-nuts silent, now it pics up a bit of computer noise, nothing major, but it was silent before. I can't turn things up very much, my computer volume is at 1, sometimes I "crank" it to 15, this is out of 100!