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Bassman AB165 midrange hiss

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  • #1

    Bassman AB165 midrange hiss

    Ok, Y'all... next project. My Peavey Special 130 is fires up and SMOKING! in a good way.

    I have a 66 Bassman on my bench now. It has a hybrid circuit, best I can tell... Bias ckt from AB864 and PI ckt from AB165. I have replaced ALL electrolytics and all coupling/tone caps except in the Bass channel, replaced the grid wires with shielded lo-capacitance wire, disconnected the brite sw on the normal channel, new carbon comp 1 watt plate load resistors, re-tensioned sockets, cleaned pots and sockets... inspected, with magnifying glass, the entire board for cold /cracked solder joints and I still can't get rid of an annoying mid-range hiss.

    If I pull the first (bass channel) tube and the third (normal channel) tube it's still there. It goes away if I pull the mixer (second) tube, so I disconnected the grid wire at the socket of the second tube and it's still there.

    The hiss has a few pops here and there and sounds more like holding a conch shell to your ear. The amp otherwise sounds great and in a garage band you probably would disregard it but my friend wants to be able to play this in his church band...

    My un-educated opinion points to something in-between the mixer and the PI. That's where I'm going next...

    Any help/suggestions would be VERY APPRECIATED!
    Tags: None
  • #2
    Here's a few pics...

    Here's a few pics...
    Attached Files
    Last edited by tboy; 10-11-2009, 08:47 PM. Reason: attached images

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    • #3
      Did you try swapping V2 with adifferent tube? try a new cathode cap at that stage? How about grounding the grid instead of opening the circuit? Maybe a bad plate resistor. That's 2 cents worth

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      • #4
        I agree. IMO, the plate resistors should have been changed instead of the signal caps... "pops here and there" being a clue to the resistors, providing tubes have been ruled out. Still have the original caps?

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        • #5
          I did swap about four brand new tubes in that position. That cathode cap is brand new, as is the 1 watt plate load resistor... but I will try swapping those out next.

          I forgot to mention that I swapped out the plate load resistors when I did the caps.

          Also, this amp came to me with a fried 6l6 socket, pwr xfmr, and two heater center tap resistors... if that gives anyone a clue...

          Thanks Dave and Booj...

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          • #6
            Originally posted by Steve Casas View Post
            Also, this amp came to me with a fried 6l6 socket, pwr xfmr, and two heater center tap resistors... if that gives anyone a clue...
            No... Any of those issues that have been corrected are independant of anything else. I assume they have been corrected.

            You might try using a different resistor (other than carbon comp) for your your plate load resistors. Plate loads are the absolute most hissy, crackly place to use carbon comp resistors. At least try metal film for plate loads, slope resistors and cathode follower cathode resistors. Hiss will be improved. I've had old carbon comp resistors that misbehaved very badly in hi V environments. Sometimes it's due to moisture exposure and leaving the amp on for 24 straight hours will clear things up. But overall I suggest avoiding carbon comp resistors for the HV positions in an amp. Tons of thermal noise (hiss, crackle) can be avoided.

            Chuck
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz

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            • #7
              What Chuck said

              Also I have found that Carbon comps are susceptible to overcooking when you solder them in, which can have a number of undesirable side effects. One such bastard hissed and popped like crazy, and when I replaced it it fell to bits as I was taking it off the board! But replacing it did cure the issue. But I'm not saying that's your problem tho (but it might be worth another look).
              Building a better world (one tube amp at a time)

              "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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              • #8
                Chuck: Yes, the other issues have been corrected

                I guess I fell into line regarding "vintage tone" as far as Carbon Comps in the signal path... I have a Vibro Champ that I put all metal Oxide resistors in and it sounds GREAT!
                And I am definitely guilty of less-than-diligent soldering and could very well have cooked a component.

                Anyway, I just replaced the Plate load resistor on the mixer tube with another brand new carbon comp and it still hisses. The hiss goes away when I pull the mixer, so it couldn't be the plate load resistors on the PI, right?

                So I took Booj's advice and grounded the grid on the mixer tube and the hiss goes away. what the hell does that mean??? Remember, this is with the 1st and 3rd tube removed... just the mixer, PI, and power tubes in. So it hisses with the grid wire off at the socket, but not with it grounded. Any Ideas?

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                • #9
                  What circuit reference is the pre-amp a best fit to?
                  http://www.schematicheaven.com/fende...a270_schem.pdf
                  http://www.schematicheaven.com/fende...a371_schem.pdf
                  http://www.schematicheaven.com/fende...b165_schem.pdf

                  The above 3 have mixer stages so maybe the most likely candidates. But we really need to define what the schematic is, before much useful help can be provided. Peter.
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    The first post I made above describes the circuit... It really is a stock hybrid. It was common in the late blackface era.

                    Ohhhhhhhh... I see what you're saying... The AA864 doesn't *really* properly mix in the normal channel. I'll have to analyze that better...

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                    • #11
                      >no hiss with mixer tube removed
                      >no hiss with mixer tube grid grounded
                      >not the mixer tube
                      >not the mixer tubes plate resistor
                      >not the mixers cathode cap
                      >not the .01 coupling cap

                      Next step for me would be to temporarily remove one end of the .01 coupling cap and see if it still hisses. If it does, then check your power supply cap to that stage. If it does not hiss, then check your power supply cap to the previous stage.
                      That's about a nickles worth now.

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                      • #12
                        Originally posted by booj View Post

                        Next step for me would be to temporarily remove one end of the .01 coupling cap and see if it still hisses. If it does, then check your power supply cap to that stage. If it does not hiss, then check your power supply cap to the previous stage.
                        That's about a nickles worth now.
                        Power supply caps? Hiss? Not likely.

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                        • #13
                          You mentioned swapping tubes in V2 and grounding the mixer tube. You did swap the mixer tube, right? You might try re-soldering any old joints remaining in the mixer circuit vicinity.

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                          • #14
                            What I do in this situation is isolate the problem. Take the signal from the 1st stage after the coupling cap and send it to the PI / output. hissing ? Move to the next plate load coupling cap and check again. The place where it starts hissing again is where the problem is. If you can't get the first tube hiss free you won't get anything else down stream hiss free.
                            KB

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                            • #15
                              What did you end up with finding Steve???

                              I have a AA371 on the bench and it has some hiss, I haven't gone into full swing on troubleshooting that issue yet as I'm waiting on a few more parts and then will get on it - but I just wondered what you found on the AA864 hybrid

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