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72 twin reverb need help , have pictures of inside

Discussion in 'Amp Tech Center' started by JohnFender, Jan 16, 2021.

  1. JohnFender

    JohnFender TDPRI Member

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    I just bought this 72 twin reverb for a good price, its the non push pull master volume but the master volume was pulled out of circuit, but the issue is that it starts to make minor some pops here and there after it gets warm for awhile and when i move or hit the first power tube it also make that noise could it just be inserts for the tube or what do you guys think it might be? Alot of work has been done already to this amp not much is still original
     

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  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    My initial thought is tube pin contact. Clean the tube pins/sockets and check the sockets. Poor/cracked solder joints can act the same. Plate resistors would be next on the list.

    Does the amp hiss? That could indicate resistors.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Swap the power tubes. Does the crackle follow the tube or stay with the socket?
     
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  4. Wally

    Wally Telefied Ad Free Member

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    JohnFender, welcome. Imho, you would do well to find a good tech and have the amp evaluated and put into proper working order. Then, you can play the amp with assurance that it is working properly, This work should provide you a minimum of power tube plate voltages and Biasing numbers...if not a full voltage chart. I am guessing that filter caps have been replaced, but we don’t see them...they are under the doghouse o; the underside of the chassis. Wan pr ing...there can be dangerous voltages. Yes, there should be draining; resistors, but until you know that the power rail circuit is proper one never knows. And one should always work as if there are voltages present. We also do not Se the bias cap, which is an electrolytic. I am going to think that it has been replaced since that bias section has been modified to a voltage adjustment circuit rather than a balancing circuit. From what I see, the amp is not far from original but with some minor mods.
    I agree with lower left coast that the movement of that power tube indicates a connection problem in that socket and or a problem with one tube. His advice is good in doing an analysis of that. However, imho at this point you would do well to have an experienced tech helping you with ascertaining what it is that you have bought.
     
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  5. JohnFender

    JohnFender TDPRI Member

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    i know about the voltage and how to drain them.. filter caps have been changed but the resistors in the dog house are still original, i will try cleaning the tube socket and switching tube placement to check if its the tube or not and if that doesn't work then i will probably take it to an amp tech, i also noticed that the eyelet board is warped , isn't there a resistor that sometimes crack when that happens?
     
  6. JohnFender

    JohnFender TDPRI Member

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    The previous owner told me that is was modded but he didnt know any details so i just figured that it was just the master volume that was disconnected, are you saying that the bias section has been changed?
     
  7. JohnFender

    JohnFender TDPRI Member

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    Would you leave the bias circuit like it is or would you return it to its original specs?
     

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    Last edited: Jan 17, 2021
  8. Wally

    Wally Telefied Ad Free Member

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    editted due to hi jacked mental functions.
     
    Last edited: Jan 18, 2021
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  9. Ten Over

    Ten Over Tele-Holic

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    I think your account has been hacked.
     
  10. Wally

    Wally Telefied Ad Free Member

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    How’s that ????? Or...why?
    Okay.....I see. It is not my account that has been hacked but the function between my ears, right? Lol....
    JohnFender, forgive me. I see better now that the bias has been reverted to a bias voltage adjustment circuit.
     
  11. JohnFender

    JohnFender TDPRI Member

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    I dont know alot about mods i like to keep things original, why would you remove the capacitor thats on the bias pot?
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The blackface and silverface twins are *fixed bias*. It is the terminology used to describe how the power tubes handle the voltage and current to *bias* the tubes. The silverface twin differed by using a pot to balance the fixed bias. Essentially there are two tubes that handle the crests of the waveform and two tubes that handle the troughs of the waveform. The pot used to balance the bias can balance the waveform. In a guitar amp an imbalance may be more desirable. The balance pot can do that as well. This balance pot can also be helpful if the tubes are not a matched set.

    The ability to balance the tubes is not as important to some users. They prefer to be able to adjust the fixed bias point. (How hot or cold the bias is.) The blackface amps used a pot to perform that function. Many silverface amps, like yours, were modded to do just that. As you can imagine both methods can be made to bias and balance the amp to the same place. The amp could sound the same with either method.

    As Wally suggested you can mod the amp to do both. I would leave it as it is, but if you have an oscilloscope put in both and have fun pushing the waveform around.

    I don't know why a capacitor would be removed. ???
     
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  13. Wally

    Wally Telefied Ad Free Member

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    I will observe that the early SFTRs as well as all of the other AA/AB763 circuits that existed in most BF and early SF amps...4/‘67 until about 5/‘68....had true bias voltage adjustable circuits. The AA165 and AB165 Bassman amps were the first Fenders to utilize the bias balancing circuit. The AA165 schematic was drawn in January, 1965. The AB165 was a revision of that circuit that replaced the AA165 very soon after that. This balancing circuit was introduced in the other fixed biased amps....except for the Princeton’s of course....in the circuits that were introduced in the summer of 1968.
     
  14. JohnFender

    JohnFender TDPRI Member

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    Thanks for the tips guys , i decided to leave the bias as it is, since now we can easily get a matched set, which is what I already have... i cleaned the output tube sockets with some contact cleaner and got the pin holes a little smaller using a very small flat screwdriver and it seems to have worked great i dont hear any pops anymore even of i hit or move the tubes, but there is a little bit of hum ( more on vibrato channel) , not too bad but i was woundering if that would be preamp tubes , can i put an 12at7 ?
     
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  15. Wally

    Wally Telefied Ad Free Member

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    You can put a 12AT7 anywhere it will fit, but whether or not it is needed or proper or corrective is another question. In the way of Leo Fender, with whom I agree, there are only two positions in which a 12AT7 is ‘proper’....phase inverter and the reverb driver position—V2. 12AT/AU7 tubes were designed for driver positions. They are different from the 12AX7/5751/12AY7 family. Yes, all of these tubes will ‘function’ where the others are, but those are two different ‘families’ of tubes regarding their design purposes.
    A lower gain Noval tube will perhaps reduce hum IF the hum is in the preamp or those driver positions, but unless the tube is the problem it is only masking the actual problem by reducing the level of the hum.
     
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