What makes the power section sound like it sounds?

[Alan0354]

Look at the 5F6?

[Steve Conner]

Yes, the story goes that Jim Marshall's first Plexi was a straight copy of the 5F6A. Even down to the screen resistor values that would cause carnage later with EL34s. He owned a music shop in London, and rather than importing Fenders he thought he would cut out the middleman and make his own.

At that point, the difference between "Fender sound" and "Marshall sound" was just the difference between the open-backed cabinets of the Fender amps and the closed-back Marshall 4x12. The two companies diverged later, with Fender getting a reputation for a big, twangy clean sound and Marshall for thunderous riffage.

[lowell]

I have to chime in here. Here are details that I think dictate a LOT of how a tube PA sounds.

Phase inverter: The 5E3 Deluxe concertina has a specific "grainy" sound when overdriven. Some dudes I think call this "swirl." I LOVE this sound... and i'm not alone. Listen to Neil Young Hey Hey, My My with Crazy Horse. Sounds like the amp about to explode. It is farty and overdriven to all hell. You cannot get this sound from a, say, silverface Fender.

Negative feedback. I've read on this forum a few times that once the PA begins to distort the feedback falls apart and is basically not there. Not sure I "hear" that when I crank a silverface Fender. No feedback allows for much greater harmonic content/bandwidth. This is one of the first things I suggest to people that don't want their ears to bleed just to get that sound. Plus, I'm not an electronics engineer by any means, far from it, but I've tinkered a lot, and I believe that there is less damping factor too, which allows more speaker interaction and I hear this as more dynamics and, well, better tone, for guitar that is.

Sag, compression, are huge factors in the feel and tone. A fixed bias tube with your typical low value (470ohm) screen resistor will have a "harder" feel, less compression. A self biased tube, and/or a larger screen resistance allow for more compression. Again, I can't stand playing a "hard" amp. I really like some compression. And I think a lot of people do... this is why we like tubes.

Unfortunately alot of amps out there from the 70s lets-make-loud-clean-amps-days do not sound very tubey (mostly Fenders)... compared to amps of the 50s/60s at least. Now the craze is back to smaller, cathode biased amps, cause guitarists WANT that sound.

If it were me I'd have everyone who wants classic blues/rock tone bring their ridiculously stale and loud Twin Reverb into my shop so I can disconnect the negative feedback, and maybe adjust the screen resistance.

Yes what is fed into the PA is naturally what the PA amplifies, however the FEEL of a PA is very important.
My 2 cents

[soundmasterg]

If you are playing cleaner sounds, then the Fender blackface/silverface designs work better because they have more bass and trebles and less mids, and more tonal variation due to the location of their tone stack after only one stage of gain. The midrange scoop gives room for all the notes to seperate from each other and gives clarity. The Tweed Fender designs and the Marshalls that copied them have more midrange and a more aggressive sound, and they work better for lead and hard rock type sounds. Try to play the same thing on the blackface designs and they blur and muddle up the notes on the bass when you have high gain, but the Marshalls do well. Try to play the cleaner stuff on the Marshall and they don't do that as well as the Fenders beecause they sound a bit thinner. Marshalls typically have higher voltages in the power amp and lower voltages in the preamp, and higher value filtering than their Fender blackface counterparts. The higher voltages in the pwoer amp and the higher value filtering tighten up the sound, and the lower voltages in the preamp allow the preamp to distort quicker. The basic deisgn of both styles of amps are very similar and some of the details in the circuits are different but overall they are all very similar like what Enzo was saying.

Greg

[jpfamps]

A few points I think are relevant to the Bassman/JTM45 discussion.

Firstly Marshall decided to "copy" an amp that in 1962 was not a current production Fender model.

Secondly, the JTM45 used an ECC83 is the first valve position which gave more gain than the 12AY7 used in the Bassman.

Finally, the original JTM45 output transformer has 16/8 and 4 ohm taps (and 100V line), whereas the Bassman has a single 2 ohm winding. Marshall took the negative feedback from the 16 ohm tap, but used the same value voltage divider as the Bassman, so the JTM45 has more negative feedback than the Bassman.

So there are subtle differences between the JTM45 and Bassman, even before the changes Marshall started to implement to the basic circuit.

[woodyc]

...Firstly Marshall decided to "copy" an amp that in 1962 was not a current production Fender model...

I agree, and would add that it is a design that Fender abandoned for obvious reasons. He was trying to make clean amps. For him, it was a swing and and a miss.

Also I think the changes in the Marshall were a bit more than subtle. And by the time the shelving filter (.68u/2.7K) and the presence boost (the 470p bypass on the dividers) were introduced, it was a whole new world.

Its easy to say that Marshall was a copyist or trivilaize the achievement of Fender with smug statements like:

It is easy, Fender "stole" many of his circuits right out of the RCA tube guide. The RCA circuits were provided free from restriction for use by all.

All the component values are mere details. Most amps are extremely similar to the others...

Its true that designers "arrange" more than they "invent", but smart and purposeful design is very difficult and worthy of admiration. I am in awe of what Leo Fender accomplished. But I appreciate the fact that a lot of people can't tell the difference between a strat and any other guitar -- it is very much in the details.

[Alan0354]

Woodyc

I agree, yes, the stages are limited to common cathode, common plate or common grid. But I feel putting a cathode follower that don't provide much gain ( any gain is due to buffering of the previous stage) but lower the impedance of the output to drive the tone stack thereby lessen the interaction between the different tone controls a little bit. I can even see Fender could have file a patent application on just this if nobody in the hifi do the same thing( big if as I don't know the hifi circuit in those days). This goes way beyond just changing the value of some components.

[Alan0354]

Woodyc

I agree, yes, the stages are limited to common cathode, common plate or common grid. But I feel putting a cathode follower that don't provide much gain ( any gain is due to buffering of the previous stage) but lower the impedance of the output to drive the tone stack thereby lessen the interaction between the different tone controls a little bit. I can even see Fender could have file a patent application on just this if nobody in the hifi do the same thing( big if as I don't know the hifi circuit in those days). This goes way beyond just changing the value of some components.

[shocki]

Negative feedback. I've read on this forum a few times that once the PA begins to distort the feedback falls apart and is basically not there. Not sure I "hear" that when I crank a silverface Fender. No feedback allows for much greater harmonic content/bandwidth. This is one of the first things I suggest to people that don't want their ears to bleed just to get that sound. Plus, I'm not an electronics engineer by any means, far from it, but I've tinkered a lot, and I believe that there is less damping factor too, which allows more speaker interaction and I hear this as more dynamics and, well, better tone, for guitar that is.

Ok I tried this. Disconnected the vibrato circuit (don't need it anyway) and added the 50K Depth pot to the existing 800 Ohm feedback resistor. So I have variable feedback now. Could not crank the amp yet. Sounds prett interesting though. When dialing in more resistance (less feedback) I get less bass and more treble. Also the volume goes up. I would describe it as more "raw" as supposed to smooth. I think I will notice more of a difference in feel when I crank the amp.

Next mod I am going to try is bypassing the tone stack of one channel. I think I will use the existing bright switch. Using a Footswitch would also be an cool option. I think this might get even more in the Bassmann direction.

[lowell]

How is the 50k depth pot wired?

[shocki]

In series with the existing 880 Ohm resistor. Which gives a range from 880 to 50,8K.

I figured out I will bypass the connection to ground at the tone section with a foot pedal. This will give me more gain and will bypass the tone controll more or less.
So I have footswitchable tweed sound. Well at least kind of

I think it does not make much sense to go any further than that.
I might try some of those changes though.

http://music-electronics-forum.com/a...98125-mod1.gif

[walkman]

How about a amp with two separate switchable phase inverters one, a cathodyne and the other a long tail ? Would it really make very little difference to be worth the effort and the extra Tube.
(Could you even feed both and mix the return signals in parallel with each other? and again how much difference would it make)

I believe the difference is not in the power section, but what it gets fed by the preamp. Hard rocking amps like the JTM45 filter out some bass by using undersized cathode bypass and coupling caps, and have different tone stack values that emphasise the midrange. When this signal is distorted by an overdriven power amp, the result is an aggressive crunch. The big 4x12 cabinet then puts the bass back in.

Fender preamps boost bass and treble, and when this is distorted by the power amp, the result is a more farty distortion, unless you turn the bass right down.

The cathode follower in the Marshall preamp also clips more smoothly and symmetrically, which could explain why the distortion doesn't sound "broken".

[walkman]

How about an amp with two separate switchable phase inverters? One a cathodyne and the other a long tail. You could even feed both and mix the return signals in parallel with each other, though I don't know what problems could occur.

Would it really make very little difference compared to the differences that can be made by changing preamp components, NFB , OT Speakers etc, to be worth the effort and an extra Tube?

I believe the difference is not in the power section, but what it gets fed by the preamp. Hard rocking amps like the JTM45 filter out some bass by using undersized cathode bypass and coupling caps, and have different tone stack values that emphasise the midrange. When this signal is distorted by an overdriven power amp, the result is an aggressive crunch. The big 4x12 cabinet then puts the bass back in.

Fender preamps boost bass and treble, and when this is distorted by the power amp, the result is a more farty distortion, unless you turn the bass right down.

The cathode follower in the Marshall preamp also clips more smoothly and symmetrically, which could explain why the distortion doesn't sound "broken".

[Enzo]

This is only my opinion, others may disagree...

Imagine a car with two steering wheels, you could hit a lever and the other one would swing into place. One would be more padded and family friendly, the other might be more industrial/NASCAR. Would that make more difference than different shocks, carburation, headers.....

[walkman]

that little difference...