Why does this amp sound so good?

[nickb]

Thanks NickB

Only slight difference I see is that R22 (bass slope) is 47k and R21 (cathode ) is 56k
OF COURSE, looking at an actual schematic shows what parts DO relative to others and the circuit makes sense, instead of just showing what goes where.
Thanks again.

PS: and might very well miss some other small difference

Thank you 66Kicks & JMF

I was pretty sure it needed a second set of eyes. Fixed one attached and I'll edit the earlier post to avoid contagion.

[teemuk]

Don't ignore the power amp folks!

820R/100R (1/8th) is a whole different ratio of applied negative feedback than "standard" Marshall 100K/5K (1/20th) and that will have significant impact on damping and frequency response. A reactive load such as a loudspeaker will introduce a notable low frequency boost at driver's resonant frequency, which will somewhat explain why the amp isn't all that "bright".

The fact that OP doesn't seem to hear much difference between closed and open loop modes is also somewhat alarming. Maybe the amp isn't working right. In closed loop mode you should expect to hear a "tighter" sound due to flatter response, reduced distortion and more abrupt transition to overdrive.

In addition, a cathode biased amp will also have quite different overdrive characteristics than similar fixed bias amp (cathode biased amps generally clip somewhat "softer" and introduce less crossover / blocking distortion in sustained overdrive), and human ear probably perceives the difference (overdrive creates less high order harmonics) as lesser amount of brightness.

...And like previously noted, tone stack values are also different. It would be interesting to calculate response of each stage, but I'm not -that- interested why this amp sounds "warmer" than expected. Even a quick glance shows that, although similar, it's not exactly the same circuit as a genuine Marshall.

[pdf64]

If the feedback feeds only the tail, then its purpose is to make the cathode supply look more like an ideal current source. So it affects the balance slightly, but not the linearity.

Could you walk me through that?
To me it seems that such feedback, that is only applied to the tail, would be common mode and so wouldn't do anything much, ie not closed loop in the normal sense.

Don't ignore the power amp folks!
820R/100R (1/8th) is a whole different ratio of applied negative feedback than "standard" Marshall 100K/5K (1/20th) and that will have significant impact on damping and frequency response. A reactive load such as a loudspeaker will introduce a notable low frequency boost at driver's resonant frequency, which will somewhat explain why the amp isn't all that "bright"...

Bear in mind that the regular Fender 820/100 is usually taken from a 4 ohm output, whereas the Marshall's 100k/5k from an 8 ohm output, so the actual voltage ratio discrepancy may not be so great.
Also Marshalls may have been 'tuned' used closed back sealed cabs, which may tend to have a bass resonance with higher values of Q, magnitude and frequency than the open back cabs (all else being equal) generally used for Fender guitar amps.
So given the highish output impedance, at bass resonance, the signal output being sampled by the feedback would be higher with a closed back than open back cab, again perhaps reducing the discrepancy between the original intended feedback ratio of the Fender and Marshall systems.

[Mike Sulzer]

But there is no feedback; not in goes to the right hand grid.

Don't ignore the power amp folks!

820R/100R (1/8th) is a whole different ratio of applied negative feedback than "standard" Marshall 100K/5K (1/20th) and that will have significant impact on damping and frequency response. A reactive load such as a loudspeaker will introduce a notable low frequency boost at driver's resonant frequency, which will somewhat explain why the amp isn't all that "bright".

[Mike Sulzer]

Right, it is not normal NFB. Normally when you put a fraction of the output signal (in phase with the signal on the left hand grid of the PI) "below" the cathodes you are making the impedance into the left hand grid very high and better balancing the PI gain. But for that to work right you have to have an effective voltage just a bit smaller than the voltage on the grid. That means that the gain of the output section needs to be stabilized by means of feedback to the right hand grid. But this design uses no feedback to the right hand grid, but rather uses the RH grid as input for the other channel. There is no logic to this design; you just get whatever happens, and if you like it, well, good. This is not something I consider properly designed.

Could you walk me through that?
To me it seems that such feedback, that is only applied to the tail, would be common mode and so wouldn't do anything much, ie not closed loop in the normal sense.



Bear in mind that the regular Fender 820/100 is usually taken from a 4 ohm output, whereas the Marshall's 100k/5k from an 8 ohm output, so the actual voltage ratio discrepancy may not be so great.
Also Marshalls may have been 'tuned' used closed back sealed cabs, which may tend to have a bass resonance with higher values of Q, magnitude and frequency than the open back cabs (all else being equal) generally used for Fender guitar amps.
So given the highish output impedance, at bass resonance, the signal output being sampled by the feedback would be higher with a closed back than open back cab, again perhaps reducing the discrepancy between the original intended feedback ratio of the Fender and Marshall systems.

[darkbluemurder]

I see what you're saying, but note, also from that article, this point:

" As long as the Volume control is set to ‘0’, the grid is at AC ground. "

And so it is on this amp as well. The volume control on the EF86 channel has no audible effect on the 12AX7 channel, and is generally kept off when running the other channel as the way they have the inputs set up necessitates this for noise purposes.

Thanks, understood, that makes sense. So we have to look for other possible reasons.

Cheers Stephan

[wizard333]

Gain:

The 1M/500pF doesn't have the usual 470K to ground so you pickup 6dB of gain that is normally thrown away on treble peaking.

Good point! I missed that detail.

That and the EL84s have significantly more gain (6 or 8dB) than what you see in a Plexi. The 56K on the cathode follower lowers the headroom by rounding off the top of the waveform and that will give more distortion as the Master is turned down. The feedback switch only feeds the tail resistor, I'm surprised it does anything at all.

Sounds significantly different that std. Plexi even when not clipping the 84s though. As noted above I did notice the impact of the 56k Rk on the follower, and while that surely is part of it, not enough to suffice for the difference I'm hearing.

[wizard333]

Don't ignore the power amp folks!

820R/100R (1/8th) is a whole different ratio of applied negative feedback than "standard" Marshall 100K/5K (1/20th) and that will have significant impact on damping and frequency response. A reactive load such as a loudspeaker will introduce a notable low frequency boost at driver's resonant frequency, which will somewhat explain why the amp isn't all that "bright".

The fact that OP doesn't seem to hear much difference between closed and open loop modes is also somewhat alarming. Maybe the amp isn't working right. In closed loop mode you should expect to hear a "tighter" sound due to flatter response, reduced distortion and more abrupt transition to overdrive.

In addition, a cathode biased amp will also have quite different overdrive characteristics than similar fixed bias amp (cathode biased amps generally clip somewhat "softer" and introduce less crossover / blocking distortion in sustained overdrive), and human ear probably perceives the difference (overdrive creates less high order harmonics) as lesser amount of brightness.

...And like previously noted, tone stack values are also different. It would be interesting to calculate response of each stage, but I'm not -that- interested why this amp sounds "warmer" than expected. Even a quick glance shows that, although similar, it's not exactly the same circuit as a genuine Marshall.

Well, no it's not exactly the same, but again the puzzling bits are almost all the differences would point to a brighter/tighter amp, when it's significantly not brighter. It is of course brighter with the 84s pushed to clip than when they aren't, but it's still nowhere near as bright as a kt66 or el34 plexi pushed into clipping.

I went over the amp; I found incorrect power dropping resistors but the rest appears to be functioning properly, and I replaced those. The feedback works, it just doesn't do a whole lot. I agree its sort of an afterthought design wise.

Previously when I've gone between fixed and cathode bias on existing amps, I heard not much difference, most of it when the power tubes were really pushed. It's true this is cathode biased, but I don't *think* that would make the significant difference I'm hearing especially before PT clip. If you take say, a Friedman DSM vs the 40W DS, essentially the same circuit except the big bottle version has .1uf PI couplers and fixed bias; the DSM is NNFB cathode biased, and some PI differnces. They sound less different from each other (a lot less) than this does from say a lead spec plexi.

At this point I'm highly tempted to impose the pre-amp sans PI and bias differences on a fixed bias, standard plexi PI circuit with big bottle tubes and see what happens. Nothing like empirical knowledge.

[Mike Sulzer]

The feedback works, it just doesn't do a whole lot. I agree its sort of an afterthought design wise.

What feedback? Why are you still talking about feedback? Putting a signal onto the cathodes of a differential pair of triodes is not feedback. OK maybe some output signal can get to the right hand grid depending on how the pot is set, providing a small amount of negative feedback, but it is just as true that some could to the left hand grid causing positive feedback.

I am not at all surprised that an amp with no significant feedback is not so bright when pushed into clipping. An amp with feedback is brighter when pushed into clipping because of the sudden onset of distortion, causing higher harmonics.

[wizard333]

What feedback? Why are you still talking about feedback? Putting a signal onto the cathodes of a differential pair of triodes is not feedback. OK maybe some output signal can get to the right hand grid depending on how the pot is set, providing a small amount of negative feedback, but it is just as true that some could to the left hand grid causing positive feedback.

I am not at all surprised that an amp with no significant feedback is not so bright when pushed into clipping. An amp with feedback is brighter when pushed into clipping because of the sudden onset of distortion, causing higher harmonics.

Better question is why are you and everyone else still talking about it? I thought I put that to bed Friday when I noted that it is kept off since it has little effect anyway, but people keep bringing it up as if it were doing something.

It exhibits the characteristics I noted before the power tubes are pushed into clipping, as noted multiple times above.

[Mike Sulzer]

Better question is why are you and everyone else still talking about it? I thought I put that to bed Friday when I noted that it is kept off since it has little effect anyway, but people keep bringing it up as if it were doing something.

It exhibits the characteristics I noted before the power tubes are pushed into clipping, as noted multiple times above.

You mentioned feedback, as did others, and I responded.

But in any case, I was thinking you might be clipping the output tubes a bit at a somewhat lower level than you are aware of (just on some peaks of the waveform). At this level feedback/no feedback can make a difference.

[nickb]

And not to be overlooked, what speaker & cabinet are you using?

[J M Fahey]

Why does this amp sound so good?

WHO says it sounds good?

Got a Ceriatone 36RP and it sounds frickin great, but I don't know why looking at the schematic.

The schematic is only half of the picture ... or even less in a Guitar amp.
The speaker is both the strongest equalizer *and* the definitive one, since it sits at the end of the chain.
It will affect everything before it and is affected only by cabinet and room placement ... which also affect all earlier stages.

Forget the EF86 channel a moment (which also sounds great)

Oh, please don´t, specially because it partly answers your question.
We are supposed to look at the Plexi-ish channel, analyze it gain and EQ and pick " the magic element" ...yet a lower gain, *flat* EQ (at best you cut some bass) stage still sounds killer ... logic says maybe the secret sauce is somewhere else.

Why do I call a raw guitar speaker "an equalizer"?

Ok, imagine a 31 band EQ, an equalizer if I ever saw one.

Now imagine at what extreme positions you would have to set sliders to get *this* frequency response:
Here´s a sampling of Eminence, Jensen and Celestion speajkers, all 12"; if we added other sizes from 6" to 15", PA types like EV, JBL or PA Eminence we´d have hundreds of different curves, all sounding different:

[J M Fahey]

(mumbling) "ok, ok, nice drawings, but I only trust my ears"

Ok, plese do.
Here´s 15 speakers, all tested with same cabinet, microphone, position, etc. but mainly exact same amp, settings, player an guitar.
Differences are mind crushing:

EDIT: as a side note, maybe now you´ll understand why I got into the *headache* (and that´s an understatement) of making my own speakers, from scratch.
Started 40 years ago, and would do it again, even knowing all the trouble involved (plus heavy investment).

[wizard333]

And not to be overlooked, what speaker & cabinet are you using?

Tested it through a few, the same few I run about 20 other amps through (or have run about 100 or so other brands/models of amps through) so the speaker is not the cause of any difference.