Differences b/n Power Tube and Preamp Tube "drive"?

Discussion in 'Amps and Cabs' started by crazy4blues, Oct 28, 2005.


  1. crazy4blues

    crazy4blues Member

    Messages:
    141
    Joined:
    Oct 19, 2005
    Location:
    SF Bay Area
    Okay, explain this to me like I'm a 5 year old:

    What are the sonic differences between power tube overdrive and preamp tube OD? Is one preferable to the other?

    I know that with a gain/volume control, you can play with OD at lower volumes, but is there something I'm missing here? Why wouldn't all amps be set up this way? It seems to me that there would be more control over your sound this way. Is there some advantage to having just a volume control and relying on power tube OD?
     
  2. drbob1

    drbob1 Silver Supporting Member

    Messages:
    20,783
    Joined:
    Aug 9, 2004
    Woo-hoo you've picked a loaded topic...

    First of all, there's some argument about how much of "power tube drive" is from the power tubes and how much is from the phase invertor (on push-pull amps, anyway), so perhaps the question would be better asked: "What's the advantage of a balanced amp where every stage contributes its portion of the distortion vs an amp that soley relies on stacked preamp gain stages?". In other words, a classic non-master volume amp, vs a master volume amp with a clean power stage.

    To that end, the more balanced (or distributed if you prefer) architecture tends to give a drive that's more touch sensitive, has more harmonic complexity (each type of tube contributes its own signature to the sound) and is less likely to be harsh. As an example of this, run a tube preamp into a clean power amp-it's hard to get the organic sense of an amp working together that you do with a unified amp. I have a Morin high gain preamp that sounds very good into a Mesa 2:90, but it sounds absolutely stellar into the effects return of a modded 73 Marshall. Why? The Marshall is overdriving the PI and the power tubes as well as the preamp gain stages...
     
  3. ZiggY!!

    ZiggY!! Member

    Messages:
    163
    Joined:
    Oct 28, 2005
    relying on preamp overdrive usually just results in a fuzzy mess... But at some point of cranking an amp up the preamp and poweramp tubes must both hit saturation... so I dont think its just the powertubes creating that tonal bliss.

    Simplicity... thats why some amps don't have a gain control and a master volume. There is something about plugging a guitar straight into a non-master volume amp at full throttle that gets the blood pumping...
     
  4. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    They sound different and respond differently :).

    That depends on what type of sound you want.

    That's right.

    That depends what you mean by relying on power tube OD...


    Just because an amp has no MV does not necessarily mean that all (or even any) of the overdrive is coming from the power tubes - it can easily be from a later stage in the preamp, like the phase inverter. The standard Plexi Marshall circuit is a good example. And there are amps where there's not enough gain in the preamp to really make it distort, and all (or almost all) of it does come from the power tubes... 18W Marshalls are a good example. And they both sound good, and both like Marshalls.

    Pushing the power stage hard does sound different from running it quiet though, because it puts more demand on the power supply - whether it's being fed a clean signal and distorting itself, or being fed an already-distorted signal from the preamp. There are many (most) amps which use a combination of the two when they're turned up loud - using a MV just changes the balance of the two, it doesn't necessarily give you one or the other. It's also perfectly possible to design an amp with an MV that is totally out of the circuit when it's turned right up, so is that then a MV amp or not?

    Confused yet? ;)

    :)
     
  5. crazy4blues

    crazy4blues Member

    Messages:
    141
    Joined:
    Oct 19, 2005
    Location:
    SF Bay Area
    Actually, I think that I WAS thinking just that. Now, know that you might not believe me, but I actually "get" what you're saying in spite of my 5 year old's knowledge of basic electronics. But this gets me to thinking a lot about, uh, stuff . . .

     
  6. Unburst

    Unburst Member

    Messages:
    4,221
    Joined:
    Jan 19, 2004
    Location:
    Portland, OR
    Like those fuzzy/messy Dumbles that sell for$30k?
     
  7. ZiggY!!

    ZiggY!! Member

    Messages:
    163
    Joined:
    Oct 28, 2005
    Im glad you picked up on the conjunctive words in that sentence that made it anything but a definative statement...

    :rolleyes:
     
  8. Tag

    Tag Gold Supporting Member

    Messages:
    30,917
    Joined:
    Jan 5, 2002
    Location:
    New Jersey
    IMO, all amps that have a a preamp hot enough to cause some overdrive should be equiped with a MV. You get EVERYTHING a NON MV amp will do, and lots more. For the "purists" just make it defeatable similar to a Matchless...... Many lower gain amps simply gain nothing with a MV however. Why put them on then if they do not achieve anything?
     
  9. Loves_LPs

    Loves_LPs Member

    Messages:
    100
    Joined:
    Sep 24, 2005
    Location:
    Plant City (Tampa area)
    So, if we were going to design our "perfect amp" it would have a preamp capable of enough gain to go into O/D, a power amp w/ enough gain to go into O/D; and have MV, tone controls, and reverb which can be individually switched out of the circuit. Let's also have an "outrigger" board which can take, and switch between different tube configurations: setting #1 = 6L6, #2 = 6V6, #3 = KT88, #4 = El84, #5 = EL34. Oh yeah, and 1/2 power and 1/10 power settings, too.

    Anything else? :D
     
  10. Tag

    Tag Gold Supporting Member

    Messages:
    30,917
    Joined:
    Jan 5, 2002
    Location:
    New Jersey

    No. My perfect amp does not need to go into overdrive. I do not need to hide behind it. :D I simply think it is a waste of many great tones not to incorporate a MV into an amp that utilizes pre amp gain, like most amps that overdrive do. :)
     
  11. riverastoasters

    riverastoasters Member

    Messages:
    2,924
    Joined:
    Mar 1, 2005
    Location:
    Planet X
    Yeah, it's those gunshots coming from the direction of the lead vocalist.
     
  12. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    How about a ZVex Nano? :) That's a non-MV amp that can be cranked to the limit through a big cab and you can still sing over it easily. Many people don't think it sounds very good either (I do, but it's not for everyone).
    Yes, it needs to sound good.

    Forget all these 'rules' about what makes an amp sound good, they're all BS. You can have a great-sounding amp which doesn't fit any of the 'recognised definitions' of what makes an amp sound good (especially as some of them are wrong or misunderstood anyway), or an amp which on paper fulfills all the criteria but doesn't sound any good.

    Remember that the 'holy grail' amp by some people's definition (Dumble) is a preamp-distortion, master volume, solid-state rectified, Class AB, fixed-bias, negative feedback amp. All the things that the purists say are wrong.

    And there are old tube hi-fi amps which are everything the purists say makes for a good tone - PTP-wired, non-MV, tube-rectified, cathode-biased true Class A with no negative feedback and which distort only at the power tubes - and which sound like crap for guitar.
     
  13. Guinness Lad

    Guinness Lad Silver Supporting Member

    Messages:
    15,513
    Joined:
    Nov 11, 2004
    Location:
    On top a mountain of Chocolate Chips
    +1 Dumble, Guytron, Fuchs, Two Rock, Soldano yeah these amps suck and are known for their horrible gain structure.:NUTS

    Give me break all you it's gotta be power tube distortion only guys. I went that route for 15 years and was never happy unless either A, I was so loud no one could hear anyone but me or B, I used a overdrive pedal which is nothing more then clipped diodes or Mosfets and sometimes tubes.

    Good master volume amps are much more user friendly and they can sound the same as power tube only distortion amps if you set the gain control wisely.

    Maybe we should ask people like Robben Ford, Larry Carlton, Warren Haynes, who else? if they want to trade their Dumble for a non-master amp.
     
  14. drbob1

    drbob1 Silver Supporting Member

    Messages:
    20,783
    Joined:
    Aug 9, 2004
    So, why didn't Tony put a good MV on my UG30 combo-it would have been a MUCH more useful amp in the (very) small venues I play:D
     
  15. Loves_LPs

    Loves_LPs Member

    Messages:
    100
    Joined:
    Sep 24, 2005
    Location:
    Plant City (Tampa area)
    Originally posted by Tag
    No. My perfect amp does not need to go into overdrive. I do not need to hide behind it. :D :)
    ---------------------------------------------------------------------------
    LOL This ^ reminds me of an old ad for gibson accoustics!
    Seriously though, I was just trying to understand you, Tag. :)
    ---------------------------------------------------------------------------
    Originally posted by John Philips
    ....Forget all these 'rules' about what makes an amp sound good, they're all BS. You can have a great-sounding amp which doesn't fit any of the 'recognised definitions' of what makes an amp sound good......or an amp which on paper fulfills all the criteria but doesn't sound any good.

    .....the 'holy grail' amp....(Dumble) is a preamp-distortion, master volume, solid-state rectified, Class AB, fixed-bias, negative feedback amp....the things that.....are wrong.

    ....old tube hi-fi amps....everything....for a good tone - PTP-wired, non-MV, tube-rectified, cathode-biased true Class A with no negative feedback and which distort only at the power tubes - and which sound like crap for guitar.
    ------------------------------------------------------------------------
    Don't you just hate it when reality screws up a really goood theory? :D

    Does anyoine else like they idea of a board that holds various different output tubes, and has the means to switch between them, and can interface with a preamp, etc? Kinda like the idea behind the THD amps, but without the need to physically exchange the tubes. :)
     
  16. drbob1

    drbob1 Silver Supporting Member

    Messages:
    20,783
    Joined:
    Aug 9, 2004
    Groove Tubes has you 1/2 the way there. They make an amp called the Dual 75 that lets you use pretty much any octal power tube, and you can run your preamp into one side and footswitch to the other side or both together. Now if you had 3 of those set up with 6 cabs and El84s (with YJs), 6V6, 6L6, EL34, 6550, KT88 you'd have a pretty nice pallette of colors. Failing that, replacing tubes in an amp with external bias points and wide bias adjustment (London Power, Maven Peal, VHT, Komet, Kendrick) doesn't take THAT much longer...
     
  17. waxnsteel

    waxnsteel Member

    Messages:
    3,131
    Joined:
    May 14, 2005
    [QUOTE
    Does anyoine else like they idea of a board that holds various different output tubes, and has the means to switch between them, and can interface with a preamp, etc? Kinda like the idea behind the THD amps, but without the need to physically exchange the tubes. :) [/B][/QUOTE]

    They're out there. The Budda Dual stage 30 has EL 84's(4) and 2 6L6's, footswitchable, and you can switch cabs. The 6l6 section has a master, and the EL84 section has an attenuator, and there's one preamp channel. The tonal change is not bite your face drastic, but it is noticeable. The EL84 section is a bit looser, and of course the 6L6's tight. Amazing and very dynamic amp. Tough to find. There were very few made, and they sound killer. If you see one get it.

    Mesa makes the Road King (and stopped Budda from making the DS 30) which has more ridiculous options, like 2X, or 4X 6l6, or 2 EL 34's or combos of both, and cab switching, AND 4 pre channels, 2 clean, 2 dirty, channel masters, overall master, and solo feature. Way more sounds than the DS 30, and unlike the Budda, it DOESN't sound great at any setting. It's a Mesa. None of them do. The Mesa doesn't respond like the Budda. Don't get me wrong, I totally dig the Mesa, but the Budda has this amazing liveness and bounce, that the Mesa barely approaches clean or dirty. But ALL the sounds are there. Just use the footswitch. Don't bother with your guitars volume knob. In this amp, the differences in output tubes is less subtle to me.

    I'm sure there are more amps like them, but I haven't really seen them. The Blue Angel does 6V6 and/or EL84's, but not footswitchable. I'm sure if you got the rocks and a soldering iron, you could make it happen, but one section's 18 watts, and the other is like 33, combined, 38. That amp bounces like the Budda. It has tons of great tones in it, but my favorite is the barely not clean.

    Any others out there?

    By my experience with these amps, I have to say responsiveness is inversely proportional to the number of knobs. And those touch sensitive amps feel/sound louder to me, too. I opened up the King with all the channels on 50W(MC90's in the combo, MC90's and V50's in a 4x12), and the Budda (phat 12's) and the Angel (2x10 alnico's) seem at least as loud, if not louder.

    As far as the original post, in the simplest terms, to my ears, pre OD is fuzzy and buzzy, power stage is crunchier. My favorite is being right on the edge of both.

    John P. mentioned phase inverter distortion. How do you get there, and what does it change about your tone? Can you give some tonal signature examples?
     
  18. Tag

    Tag Gold Supporting Member

    Messages:
    30,917
    Joined:
    Jan 5, 2002
    Location:
    New Jersey
    Hey Dave, I have dug the amps of yours I have played.
    However, you must have it backwards. (At least the way the ear perceives it) The least fuzzy amp I have ever played is my Fuchs OD SLX. That is pure preamp drive at the lower levels I usually practice and play at. I have owned and or played nearly every quality vintage amp known over the past 25 years. Not one of them can produce the smooth, non fuzzy tones of the Fuchs or a good Dumble\Dumble style amp. I can post as many clips of it for comparison as you want.
     
  19. gulliver

    gulliver Member

    Messages:
    5,924
    Joined:
    Jun 25, 2005
    Location:
    Buffalo, NY
    When I was shopping for a new amp, part of my test was turning up the amps to 8-10 with the clean channel and/or distortion channel set to low gain. This was to test for power tube distortion. It seemed like a lot of amps didn’t even clip the power tubes, most sounded terrible. I ended up buying a Voodoo Modded ’86 JCM800 (2205). Playing that in the above manner sure seems to offer gobs of power tube distortion (or at least “post preamp distortion”). To me, it sounds much like Pete Townsend’s ‘70s tone … kind of thin and buzzy with explosive dynamics. Actually, I prefer the amp’s preamp distortion, if it was a choice of one versus the other. So then, the tone quest turns into mixing the preamp and power tube distortion to find the best tone. Getting good preamp distortion and turning up the amp to bleed in power tube distortion is the experiment. At first, the power tubes add compression without clipping, which shapes the tone differently than at low volumes, where the dynamics are huge. Beyond that, I haven’t found the perfect balance as I feel I need to test with an attenuator given my room acoustics, speaker setup, and concern regarding damage to my hearing. Right now, I’m thinking an attenuator may not be for me as I’m liking the preamp distortion and wide dynamics, which makes playing more difficult but better sounding to my ears.
     

Share This Page