Results 1 to 8 of 8

Thread: What are the major tonal parts of an amp and how strong an effect do they have?

  1. #1
    Ultimate Tone Member
    Join Date
    Aug 2017
    Posts
    380

    Default What are the major tonal parts of an amp and how strong an effect do they have?

    I'm talking about the major stuff like output transformers and rectifiers, but I'd also like to know about anything worth mentioning. Or point me in the direction of somewhere I can learn about the minutia of amplifiers, because I find it very interesting.

  2. #2
    of the Forum PFDarkside's Avatar
    Join Date
    Jul 2005
    Location
    Somewhere around... Chicago!
    Age
    39
    Posts
    17,108

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    Search for “Uncle Doug” on Youtube.
    Oh no.....


    Oh Yeah!

  3. #3
    Mojo's Minions Lake Placid Blues's Avatar
    Join Date
    Nov 2005
    Posts
    3,635

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    http://ampwares.com/schematics/bassman_5f6a.pdf

    The 59 tweed bassman is the most influential tube amp design. The circuit is called the 5F6A. It is the basis for the Marshall tube amps from 60s plexis right through the JCM800s. Even a Soldano SLO and its progeny such the 5150s, the Mesa Rectifier series, the DSL ect... owes it basic DNA to this design.

    On the schematic the signal flows from left to right starting at the four input jacks. Then to the first tube designated as V1. These preamp tubes are actually two mini tubes in one bottle. One tube is designated V1A and the other V1B. Each mini tube or triode handles either the treble or the bass inputs. V1 is the most important tube in any all tube amp. V1 sets the tone that is amplified from then on. In the Tweeds this tube was a 12AY7. A Marshall uses a 12AX7 here. A 12AX7 has a gain of 100%, but an AY has gain or amplification factor of 45%. The 12AY7 was used to decrease distortion for bass guitar. A 12AX7 is usually used now to increase distortion.

    Preamp tubes operate in class A, which means that they amplify the whole wave form. They are usually cathode biased which means that they have a resistor between the cathode and ground to control the flow of electrons through the tube. These resistors and bypass caps between cathode and ground of preamp tubes have a profound effect on the voice the amp. What kind of caps are used as well as their values effect the tone. On the bassman seen here, v1A and V1B share a cathode resistor and cap. On later Marshall plexis the cathodes were split. This means that V1A and V1B each have their own separate cathode resistors and caps. A split cathode plexi has a greater contrast between the bright and normal channels. The bright inputs are brighter and normal inputs are darker than on a shared cathode amp. The split cathode design offers more tonal range by "jumping the channels" using a small patch cord between the second bright input and one of the normal inputs. The volumes for the bright and normal can then be used against each other to get a wide range of basic voicing from bright to dark.

    Going on to V2 you will notice two 270k resistors both feeding the input grid of V2A. These are called the mix resistors, since the signals from the treble inputs and the bass inputs are mixed here.

    Also in between V1 and V2 are the volume controls. There are two, one each for the treble and bass inputs.
    On a high gain amp, or a master volume amp, one or both of the volume pots become the gain controls.

    The treble volume pot usually has a "bright cap" across its wipers. On a bassmann seen here it is 100 pf. On a plexi it is 500 pf. On an 800 it is 1000 pf. The bright cap not only effects how bright the amp is, but also how quickly it gets loud as you turn up the volume. With a larger bright cap, the amp will get loud fast. As the amp is turned up the cap has less and less effect. At full volume the cap itself is being bypassed. However, at low volume the cap allows highs to bypass the pot so the amp doesn't sound dull during low volume operation. The larger the bright caps the more highs and possibly upper mids bypass the volume control.

    On a master volume amp, or a high gain amp, the bright cap effects the distortion tone. As the gain control is turned up, less highs bypass the pot. A high gain amp at lower gain settings can sound bright and thin because a lot of treble is bypassing the pot. The solution is to use more gain. This is why Warren Haynes had his Soldano SLO modded so the bright cap could be removed from the circuit. Warren normally uses low gain settings being a blues type player, so the bright cap was making his tone sound thin. Without the bright cap (or a lower value bright cap) you can get a thicker/deeper tone at lower gain settings.

    V2A in the original circuit only increases the voltage of the signal.

    Notice that V2B's cathode does not connect to ground. This is because the cathode is used to drive the tone stack or the passive EQ section located between V2 and V3. This is called a cathode follower tone stack. On some amps a plate follower tone stack is used. These include Hiwatts and the Marshall Jubilee. Plate followers get a darker, more saturated, tone, and cathode followers get a lighter, more airy, tone. Plate follower eq's are more effective than cathode followers for adjusting the EQ of the amp.

    One of the important bits of the tone stack is the slope resistor. On the 5F6A schem it is the 56k bypassing the treble pot. Plexis eventually went to a 33k slope resistor which fattens the mids. A 100K slope resistor will more scoop the mids.

    V3 is the phase inverter. Its job is to split the top half from the bottom halve of the sound wave, so each 5881 power tube receives about one 1/2 of the wave each. This is called a "push pul0" power ampl when each power tube is responsible for amplifying about 1/2 of the sound wave . It also called class B amplification. A guitar or audio amp does not use strict Class B because that would sound too harsh, so each power tube actually amplifies part of the other have of the sound, overlapping part of the other power tube's domain. This is called Class A/B.

    Between the power tubes and the speakers is the output transformer. The speakers connect to the secondary windings of the transformer, and the power tubes are connected to the primary windings.

    There's a ton more, such as negative feedback, filtering, damping factor, cascaded gain stages ..and so forth but, I have rambled on enough for tonight.

  4. #4
    Mojo's Minions LLL's Avatar
    Join Date
    Oct 2014
    Location
    Evil Volcano Lair
    Age
    50
    Posts
    3,455

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    The more NFB (negative feedback) in the signal, the cleaner (less distortion)...
    ...and vise-versa; the less NFB in the signal, the dirtier (more distortion).

    V3 (or the typical phase inverter; "PI") is extremely important in that it needs to be cookin' (cranked amp) to get all that juicy, lively, compressed tone... what many refer to as "power tube distortion"... actually is a properly pushed PI.
    Lefty Lounge Lizard's Guitars & Amps Extravaganza



    Fastest ears in the West

    Quote Originally Posted by Edgecrusher View Post
    I thought it sounded great, until I heard something that sounded good.
    Guitaramped.com

  5. #5
    Our Neighbor Totoro FuseG4's Avatar
    Join Date
    Jun 2004
    Location
    SLC, Utah
    Age
    32
    Posts
    6,818

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    A good site, pretty easy to read and lots of useful stuff
    https://robrobinette.com/Amp_Stuff.htm

  6. #6
    Ultimate Tone Slacker Jacew's Avatar
    Join Date
    Nov 2016
    Location
    Finland
    Posts
    2,465

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    Quote Originally Posted by LLL View Post
    The more NFB (negative feedback) in the signal, the cleaner (less distortion)...
    ...and vise-versa; the less NFB in the signal, the dirtier (more distortion).

    V3 (or the typical phase inverter; "PI") is extremely important in that it needs to be cookin' (cranked amp) to get all that juicy, lively, compressed tone... what many refer to as "power tube distortion"... actually is a properly pushed PI.
    Exactly. Learned that hard way trying to add FX loop to Orange TT... Joey Voltage had this schematic how to add loop after the PI, but I never had time to try it out. And never really figured out exactly how it's supposed to work (I have always too many projects going on...)

    What makes the overdriven PI different from regular gain stage?

    I was just thinking that TT uses two ECC83's, where overdriven preamp stage pushes the PI to distortion, creating it's tone before MV.

    If you had similar setup with 3 ECC83's: Separate second gain stage, and PI after MV, would the result be that different? (That's how my Egnater is, but they're so different amps I can't draw any conclusions about it whatsoever).
    "So understand/Don't waste your time always searching for those wasted years/Face up, make your stand/And realize you're living in the golden years"
    Iron Maiden - Wasted Years

  7. #7
    Our Neighbor Totoro FuseG4's Avatar
    Join Date
    Jun 2004
    Location
    SLC, Utah
    Age
    32
    Posts
    6,818

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    What makes the PI different from an overdriven gainstage?

    The Phase inverter, in a long tailed pair at least, has one triode (half the tube) amplifying the positive half of the signal and the other triode doing the negative. This tube has a tendency to get hit with a big signal and overload quickly cuz it's right before the power tubes. And also the triodes and arrangement of the phase inverter doesn't perfectly balance the + and - signals, so there's going to be a slightly different, distorted signal when they combine at the speaker. Many people suggest that a perfectly balanced phase inverter would "kill tone".

    If it's a cathodyne phase inverter there is interaction with the power tubes when they clip, the impedance difference causes asymmetric distortion on the cathode when it can't output anymore and the plate amplifies the signal more, and when the PI tube itself overdrives it can output signal from the cathode side but the plate experiences voltage increases equal to voltage on the cathode that is excess grid current, giving a nasty frequency-doubling distortion.

    So Phase inverters distort differently cuz they are getting at times a lot of signal, and they are configured and biased differently from preamp gain stages.

    Also, say you have the master volume before the PI, in many cases this can sound darker and smoother cuz master volumes are gonna take off some high end and the earlier in the amp that you shave off high end, the more dramatic it seems, cuz you are taking treble off the top before it's even had a chance to get smoothed out or enhanced by the following tubes/circuits. And, you don't get the PI distortion in the low master volume sound.

    For high gain amps, where most of the gain is preamp, having the master earlier can tame fizz, noise, and ice pick.

  8. #8
    Ultimate Tone Slacker Jacew's Avatar
    Join Date
    Nov 2016
    Location
    Finland
    Posts
    2,465

    Default Re: What are the major tonal parts of an amp and how strong an effect do they have?

    Thanks! Great info there^
    "So understand/Don't waste your time always searching for those wasted years/Face up, make your stand/And realize you're living in the golden years"
    Iron Maiden - Wasted Years

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •