Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

A good description of Amp bias by Angelfire

Discussion in 'Amp Tech Center' started by peteb, Oct 8, 2017.

  1. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    The AC signal voltages on schematics start with an input that is about fifty times smaller than what your humbucker is going to input. You need to look at the gain along the signal path.

    The good 'ole ab763 is a good example. The gain is about 11 at the grid of V1B, so a 200mV signal from your humbucker will put 2.2V on that grid. The gain at the grid of V4B is around 42, so you will have 8.4V on that grid.

    The Hot Rod DeLuxe has a gain of 145 at the grid of V2B, so a 200mV input will put 29V on that grid.
     

  2. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    to find the descriptions of class AB1 and class AB2 in the RCA receiving tube manual that Ludwig linked,
    go to page 34, left column, 2nd paragraph under the heading: Class AB Power Amplifiers



    Here is the bit from Aiken amps from "the last word on class A":

    What about class A2, AB2, and B2?

    The numerical suffix appended to the class designation indicates whether or not grid current flows in any appreciable portion of the cycle. A "1" suffix indicates no grid current flows, while a "2" suffix indicates grid current flows for some part of the cycle. Class A2/AB2/or B2 requires a very low impedance, transformer-coupled or DC-coupled driver stage. The standard AC-coupled phase inverter or single-ended driver stages used in nearly all guitar amplifiers will not allow grid current flow, so they are class A1/AB1/B1 amplifiers.
    The advantage of class A2, AB2, or B2 is the complete lack of "blocking distortion", or transient intermodulation distortion. The disadvantage is the extra complexity of the output stage required to source current to drive the output tube grids into the positive region.
     

  3. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Notice, however, that that paragraph is specifically referring to OUTPUT POWER tubes and NOT to PREAMP tubes, which (normally) are always operated Class A.
     

  4. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia


    Thanks ten Over, that is what I am looking for and I agree that the signal voltages on the modern schematics are low values, compared to real life playing situation values. Now let me look at your numbers.


    These numbers are great to talk about. I have measured real signals going into the preamp and and real signals going from the preamp to the power amp so I have an idea what to expect, but I am not sure about the signal voltages on the grids in the middle of the preamp. What I measured is a max signal of about 0.1 volt going from a single coil into the amp. The max signal hitting the grids of the power tubes has grown to 10 or 20 or 30 volts depending on the amp, Back to your numbers.


    I gotta admit I am confused right away by your numbers.


    you say the gain is 11 at the grid of V1b. Are you saying the gain at the grid is 11 or the gain of the tube is eleven? If the HB PU is putting out 0.2 volts of signal, then isn't that signal hitting the grid of the first pre amp tube? Somehow, in your explanation the signal from the HB PU was amplified before it hits the grid of the first preamp tube. ? I think I need to understand that before I can understand your whole post.

    then



    you say the HRD has a gain of 145 at the grid. the 12ax7 preamp tube has a max potential gain of 100X but in reality after the plate load reduces the gain of the stage and any feedback reduces it further, the real world gain is some where around 40 max. How can it have a gain of 145?


    and



    why get the signal up to 29 volts at the beginning of the pre amp? after the LPTI bumps it up it will only need to be about 30 max going to the plates of the power tubes, so 29 at the beginning of the pre amp sounds higher than it needs to be, in my opinion.



    hopefully anyone reading this can see that I have put quite a bit of though, research, and measuring and then more thought and then more researching into this subject.
     

  5. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    good input old tele man.



    I have been looking for info on preamp tubes and their is not much else out there.



    my questions: what are the similarities and differences between the operation of pre amp tubes and power tubes.




    thinking about it



    it makes sense



    the pre amp tubes definitely are not operating push pull, so they must be operating class A.




    I will look closer at the RCA manual to see what they say about class A, but






    what difference would it make that they are class A.




    class A output stages have signals less than the bias voltage just like class AB, so whats the deal?
     

  6. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia

  7. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Not trying to confuse you, but you are aware that a Long-Tail Phase-Inverter (LTPI), which commonly uses preamp tubes, operates "quasi Class-B" with the two triodes alternating between a conduction pulse and deep cutoff. Thus, to say ALL preamp (small-signal input/large-signal output) tubes operate class-A is not ALWAYS true; rather, it's their intended mode of operation that defines their actual operating class.

    Thus, Class-A, AB, B, etc. operation can be applied to ANY tube, *IF* that's how they've been configured to work. For example, there's PUSH-PULL Class-A, where both tubes operate Class-A ALL the time (hence, the "A" designation)...which is fundamentally different from Class-AB1 operation, where the tubes alternate operation.
     
    Last edited: Oct 12, 2017 at 3:46 PM

  8. Ten Over

    Ten Over Tele-Meister

    175
    May 13, 2015
    Central California
    We put 200mV AC onto the grid of V1A. V1A has a gain of 44, so there is 8.8V AC on the plate of V1A. The plate of V1A feeds the tone stack which knocks the signal down 75% to give us 2.2V AC at the grid of V1B. We started with 0.2V in and got 2.2V at the grid of V1B which is a gain of 11.
     

  9. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    OTM, I gotta check that out!

    Tenover, thanks, that makes sense. The gain of eleven is the combined gain for gain stage one and the loss of the tone stack combined, 44X(1/4)=11, got it. So we still have relatively low signal volts on the grids of the first two preamp stages, .2VAC and 2.2VAC. If it were single coils it would be .1VAC and 1.1 VAC. This is in line with my expectations




    But your comment of the HRD signal volts being 50 times smaller than HB and my put that they are 25 times smaller than single coils GIVES MY THEORY A HUGE PROBLEM, thanks for pointing that out.



    the HRD schematic shows 580 mV on the grid of 4th preamp tube, which is biased like a typical preamp tube, .58 X 25 = 14.5VAC. By theory does not allow a 15 volt signal to be used with a 2 volt bias. THIS COULD DEFINITELY BE SHOWING MY THEORY WRONG.




    So I had to do a test
     

  10. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    First test is a champ. (AA764)



    I almost stopped before I started, first thing is I didn't want to poke around the preamp tubes because they pop so much, but more importantly, just by looking at the schematic I knew what I would find before I even measured the signal voltage on the grids of the preamp tubes, low voltages.



    How did I know?




    I know the power tube takes max about 10 volts of signal on its grid. The second preamp tubes outputs straight to the grid of the power tube. If the second preamp tube puts out 10 volts of signal max, how much input signal does the second preamp tube stage need? Not much. The most I could measure, and there were no pops, was about .4 volts of signal on the grid of the second stage preamp tube.


    10/.4=gain of around 25


    More testing will follow on bigger amps.





    But just looking at schematics in the same way I looked at the champ schematic, and knowing the max signal at the power tubes, and figuring in some gain for the PI, in most amps there is no need for elevated voltages on the grids of the preamp tubes.




    Think about it.





    5VAC times 44 is 220 volts, why would an amp need 220 volts of signal in the preamp?



    220VAC times one quarter (tone stack loss) = 55 VAC for the next stage. No stage can handle that.







    The mystery continues for me
     
    Last edited: Oct 12, 2017 at 9:30 PM

  11. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Riddle me this, Batman.

    References: The 65 Princeton Reverb Reissue schematic for AC voltages, Fender AA964 schematic for DC.

    In the cathodyne, the cathode voltage is 65VDC at idle. The grid is 63.8VDC. Measure at the indicated point on the schematic for the grid voltage. Now, directly measure voltage drop across the 1M grid leak for the cathodyne and see what voltage drop exists, if any. If there is none, or even a reading in the low mV range, will you agree that the grid voltage effectively equals the reading at the indicated point? After all, a number + zero is equal to the original number, yes?

    Next, do we agree the bias voltage, and therefore the idle point, of a preamp type triode like a 12AX7 is established by the difference between grid and cathode voltages? And can we also agree that 65 - 63.8 = 1.2?

    All good so far?

    Now look at the Reissue schematic.

    The cathodyne grid AC signal is 14.7VAC, supplied by V4a. The plate AC and cathode AC output signals are both 13.9VAC.
     

  12. LudwigvonBirk

    LudwigvonBirk TDPRI Member

    Age:
    115
    96
    Aug 26, 2017
    Madison

  13. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    59 Bassman Reissue schematic, V2b (the DC coupled cathode follower)

    V2b grid (also the V2a plate voltage) = 213VDC
    V2b cathode = 215VDC.

    Vk - Vg = (215 - 213) = 2.

    AC signal on the V2b grid under test = 4.81VAC.
     

  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
     

  15. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Clint,

    Thanks for the replies.



    yes, the cathodyne phase invertor (princetons and tweed deluxe)and the cathodyne follower (Bman preamp) serve as interesting examples, possibly the most interesting examples for all preamp type tubes and bias.



    I agree that the circuit analysis points to a small bias and a large signal. I was putting that one off until later while focusing on the true preamp tubes first.



    they have my attention.
     

  16. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    thanks rob,

    about reading and posting, I feel like I have been running at 99 mph lately, sorry if I missed something, but I did read it all, looking for the most pertinent stuff.



    about running AB2. that's good info, thanks.




    I am going to test my next amp, a Princeton, and I am nearly certain I will find small signal voltages on all true preamp tube stages, all two of them.





    what is this telling me?





    I still think that the tubes exhibit their best behavior when the grid remains negative. This is why the true preamp tubes have a signal voltage less than the bias. this would make for a clean preamp. Fender is known for its clean sound and amplifiers only get credit for generating a clean signal at a specified power level.






    Is this what it is all about? For clean signal amplification, the grid should be negative. For distorted sounds, anything goes. But if the signal is distorted, the amplifier does not get credit for cleanly amplifying the signal at that power level.







    Is this why tweed deluxes and brown princetons sound so crappy when cranked? Because the cathodyne PI grid is positive?





    It makes total sense to me that if the positive grid is attracting electrons then the signal gets distorted.







    In my simplified ideal tube. the plate is highly positive with respect to the cathode and draws current from the cathode. the grid is negative and placed in between the plate and the cathode and its role is to repel electrons, which will limit or block some of the current flow. It is NOT the role of the grid to attract electrons, but it will happen if the grid goes positive, but the signal will get distorted, possibly in undesirable ways.





    Old Tele Man,,


    You said the preamp tubes are class A. Class A has class A1 and class A2 as well and the same applies (I think). The best performance will be class A1, with class A2 being a less desirable alternative.






    yeah, I admit that I am expostulating.
     

  17. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Is anyone reading between the lines?????
     
    Bendyha likes this.

  18. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Thanks Wally.



    Approximate gain structure for an AA764 champ, voltages are maximum.


    [input voltage]>(12AX7)(tone stack)(12AX7)(6V6)

    [0.1V](X16)(1/4)=[0.4V]>(X25)=[10V]>(6V6)


    0.1 volt max input into the first 12AX7 with gain of 16 and tone stack with gain of 1/4. This puts 0.4 volts onto the grid of the second 12AX7 with gain of 25 putting 10 V max on the grid of the 6V6.



    The bias of the two preamp stages is around -2V and the max signal on the grids of the two preamp stages is less than half of that. The bias on the 6V6 is around -23V and the and the max signal is around half of that.



    All three grids are negative 100% of the time.
     

  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX

  20. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Ok Wally,

    See the positive 1.5vdc and 1.6 VDC on the cathodes of the preamp tube?


    That makes the grids 1.5 and 1.6 volts negative with respect to the cathode, that's cathode biasing.



    Good question, thanks.



    I wanted to add this comment about the champ. Even though the grids are negative, there is plenty of overdrive available st higher volumes.
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.