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5F6A Bassman Build Question

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  • 5F6A Bassman Build Question

    Hello Everyone,
    I recently finished a bassman clone and I am very happy with the sound of it except for one thing. The bass response is kind of farty or mushy/spongy if you will. All component values are stock and I really don't want to raise the filtering. I am using 44uf mains, 22uf screens, 22uf phase inverter, and 16uf for the preamp. It has all NOS RCA Blackplate tubes and I am using a NOS Mullard GZ34 rectifier. Also using Mercury Magnetics transformers. I was thinking of raising the V1 (shared) cathode bypass cap from 250uf up to maybe 10-25uf? Should I still keep the V1 (shared) cathode resistor at 820? I read somewhere that raising the value of the phase inverter input cap (.022 stock) can help with this also? Does anyone have any other tricks to tighten up the bass with this circuit? Any help would be appreciated!! I hope some of you tweed gurus will have some good answers!! Thank You!!

  • #2
    You can try decreasing the PI input cap to .01uF or .005uF. Sometimes you have to go even smaller. Also try decreasing the cap between the PI and the power tubes from 0.1uF to .047uF or .022uF.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !


    • #3
      Thanks for the info loudthud. I meant to type decrease the cap values instead of raising them. My bad!! I will try the PI input cap first and see what that does before changing out the ones to the power tubes. What do you think of my idea to decrease the V1 cathode cap from 250uf down 10-25uf? Will it make much of a difference? Thanks Again for your input!!


      • #4
        By the way, is this the right place in the forum to post my questions? Or would I be better off in the mods and repairs or some other section? Thanks Again Everyone!!


        • #5
          If you decrease the cap on the cathode of the first tube, you get some bass bleed into the other channel. This is not necessarily bad because you have a volume pot to control the amount that gets thru. Many times you'll see independent cathode resistors used with a different cap on each channel ala Marshall. You can also tweek on the .02uF that feeds the volume pots. Plenty of threads on this subject because the 5E3 uses the same type input stage.

          Your question could go either place. Because it's a new build and you are tweeking to what you want, here is fine. You didn't mention what speakers and guitar you are using.
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !


          • #6
            I think we need to try and ascertain whether this spongy/mushiness is typical (I think not), or is a symptom of a problem with the amp that needs fixing before doing any mods.

            What plate voltage/current do you have on the power tubes?

            Have you wired the main filter caps as a totem pole, with 2x 100uf 350/450v caps in series, each cap bypassed with 220K 2W resistors. You can go up to 40/50uf for the screen supply filter cap.

            Pics of the bild might be useful.

            Do you have a 12AY7 in V1, most folks prefer a 12AX7 especially with single coils, but if your pick ups have a large output then 12AY7/12AV7/12AU7 may be more appropriate.

            I like Loudthud's suggestion of independent cathodes for V1, I sometimes do this - Normal channel has 1.5K/220uf, bright has 2.7K/15uf (try with & without bright cap on vol pot). If the bright channel is too "strident" try a little more voltage drop to the preamp by upping the value of the 10K dropping resistor on the board (27K-47K?)


            • #7
              Thanks for the responses guys!!

              I am using Strats for guitars and Scumback speakers in the amp. The speakers sound awesome!! (I have played other amps through them)

              My plate voltage is 462vDC, 36-38mA current (RCA 6L6GC Blackplates) @ 117vAC. (My line voltage is 124-126vAC so I use a variac to step it down to 117vAC). I am using the Mercury Toneclone transformers (380-0-380) which put out higher B+ with todays line voltages. I was actually thinking of getting one of the lower B+ trannies or maybe trying some other brand? What do you guys think/recommend?

              I am using two 22uf 500v caps wired in parallel for a total of 44uf on the mains like original bassman. Other cap values are listed in my fist post. All electrolytics are F & T brand. Should I switch to the totem pole cap wiring?

              I am using all 12ax7 preamp tubes. RCA long blackplates. I am using Jensen copper foil PIO coupling caps and Riken carbon film resitors. I will try to post some pics sometime soon. Thanks again for your comments and ideas!!


              • #8
                OK, what do you get without the variac? Your variac may also be dropping heater voltage (bad). What is the A rating on the variac? Your plate voltage is lower side of ball park, a few more volts won't hurt, but don't go higher on the plate current. 380-0-380VAC *loaded* is a peculiar choice for a 5F6A (you'll end up with over 510vdc on the plates), 350-0-350VAC is more like it. A plate voltage of 470vdc at 30-35mA per tube, with a GZ34 is ideal, but I wouldn't panic if you ended up +/-10vdc at the same currents.

                Yes, I would switch to the totem pole wiring & the 40uf at the screen node. This will tighten up the amp a little.


                • #9
                  With the variac set at 117vAC output, I get 6.4 vAC at the pilot light for heater voltage and 5.0 vAC at the rectifier. The variac is rated for 15 amps. The 380-0-380 is unloaded B+ (off spec sheet from Mercury, sorry for the confusion). If I set the amp to run at 30mA plate current, I am getting over 470vDC plate voltage, I just liked the way the amp sounded with the tubes biased a little hotter and I figured they could take it since they are blackplates.

                  So you are saying totem pole two 100uf main caps and go with 40uf on the screens? This won't make the amp to tight will it? What kind of caps are you guys using? The way I built my amp I really don't have the room for Spragues. Thanks for the help everyone!!


                  • #10

                    My 5F6A clone uses 40uf on the plates and screens. I dont have the totem pole arrangement, but I prefer the higher filtering on the screens. F&T e-caps should fit.

                    the schematic is here:


                    (5F7A refers to my particular build, it's a 5F6A, but using a Weber 5E7M chassis and 5E7 cab)
                    "In theory, there is no difference between theory and practice. In practice there is."
                    - Yogi Berra


                    • #11
                      Yes, use 2x100uf totem poled for the main, a single 40-50uf/500v will suffice at the screens. F&T are great. "Too tight"? You're complaining it's not tight enough. Lots of folks use 2x220uf totem pole at the mains, still sounds fine.

                      Still it would be interesting to take the variac out of the equation & see what voltages & change in tone you get.


                      • #12
                        I'm going to order the 100uf caps and try the totem pole config. Also, will up the screens to 47uf. (F&T caps jump from 30uf to 47uf. No 40uf option and I don't have room for Spragues) That is more than doubling what I have on the screens now (22uf). Hopefully this wont make the amp to stiff. I am going to try the split cathodes on V1 next week while I am waiting for the parts to come in. I will report back once I have some news. After I try a few of these ideas, if its still not the way I like it, I may try some other transformer options. I've been reading about the Hammonds, Magnetic Components, and the Heyboers. Any opinions on these? I have always used Mercury Magnetics transformers on all my builds. It may be time to try something else for comparison. I don't want to get to far ahead of myself though. lol. Anyway, thanks for all the help and ideas!!


                        • #13
                          "I have always used Mercury Magnetics transformers on all my builds. It may be time to try something else for comparison. I don't want to get to far ahead of myself though" Nothing against MM, just a bit pricey, wound on plastic bobbins...but I think that you are significantly overemphasising the importance of brand of tranny. As long as the PT supplies correct voltage (470vdc +/-10v, w/5AR4, WITHOUT a variac), with adequate current, it'll do the job. One that's overengineeered will last longer, keep a more constant voltage & have better transient response. OTs will largely be based on similar specs...yes there are differences and people rightly have their preferences, but they are relatively subtle given the big picture. If your amp does not function "properly" now, changing the OT is not going to make it work better. By all means, try different OTs but wait until you have the amp functioning at a reasonable datum line first.

                          RI Bassmans are made with PCBs and low cost parts (compared to what you have built, in reality they are still great value) but they still perform properly.

                          Some pics of the build might be useful. It's common for novice builders to assume that they need to change a major component for the same thing, just of a different brand, when often revising layout & wiring will do the job.


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