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Thread: DIY Bassman 5F6-A treble knob acting very strange...

  1. #1
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    DIY Bassman 5F6-A treble knob acting very strange...

    This is my first post here so hello everyone!

    here is the thing;

    couple of days ago I finish building my first tube amp project, a bassman 5F6-A I built using the ceriatone layout and also the original schematic for reference.

    after some time spent on grounding with minor revisions, only a slight hiss remains and the amp runs and sounds great, except for one serious problem.

    The treble knob acts pretty weird, it controls the treble up to 3/10 normally, if you turn further the cathode currents jump almost to twice their normal currents, producing strange noises (not too loud), i have on-board analogue ammeters telling me that. At that point the output volume decreases dramatically almost to the point where you can barely hear any guitar playing, turn further and the currents changes asymmetrically, one tube decreasing the other one continuing to increase ... towards the end it all turns to some mixed up background noise gradually.

    It all goes back to normal when i turn the knob back to 3/10 or less.

    and also the presence knob seems to have no affect on sound at all but its not the priority now.

    I checked all the pots and connections for any faulty wiring or shorts, it all looks good.

    Can anyone tell what the problem could be ? Please ?

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    My guess is that your amp is oscillating above the audio range; have you fitted grid stoppers, 1k5 on the power tube control grids?
    They weren't specified on the original 5F6A, which is weird as they were on the 5E6
    http://www.webphix.com/schematic%20h...5e6a_schem.pdf
    If not fitted, amps are prone to this issue.
    Pete.

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    What Pete says...

    Plus it might be handy to have some pics of what you have done with resect to routing of wires etc.

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    No Pete, i don't have grid stoppers, it is indeed interesting that its on 5E6 and not included in the 5F6A schematic, ceriatone layout is also missing the grid stoppers .. I will try adding some tomorrow, im not sure if the local electronics shop is working on Saturdays though ..

    i will post pictures, once i take some,

    Cheers!

    Orcun.

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    Ok, before installing the grid stoppers, i separated all the pot grounds together with the input jack ground from others and made a separate bus dedicated to them on another location on the chasis, still the same result but seems like the ,what feels like a threshold point, on the treble pot has moved further up to about 50-60% closer to the open end but still ends up the same... so i installed the grid stoppers and tried that way.

    with the grid stoppers, i realised that if (and only if) the treble pot stays close to its "threshold" point where it starts acting strange and increases cathode currents, the other pots, including the volume pot from the unused channel but except the bass pot, can push through the threshold as well ..

    maybe this was happening before too but i didnt had the chance to find out, maybe it came with the grid stoppers im not sure.

    there was no change in noise, only that slight hiss on the background remained the same and doesn't disturb much.

    the only change was when the guitar is on the normal channel and the other volume is also turned on it produce some serious hum and background noises, some very high pitched towards the end of the knob, which swings much like the ones you can hear on a distorted LW or MW radio signal.

    guitar on the bright channel and normal channel volume is turned on it was just a hum without the high pitch stuff which makes sense i guess as the normal channel is not as bright as the bright one

    here are some pictures i took today:




    here is the inside,







    i know it looks messy but i designed and built the chassis myself with very little knowledge of tube amp design, obviously is not the best configuration but at least now i know what to improve on the next design

    i tried to keep all the wires to the tubes as short as possible and close to the chassis

    the green and yellow wires are the ground wires which i will shorten up when i find the optimum grounding map.

    the empty socket hole is for an M80 magic eye tube as a VU meter, didn't want to install it before i solve all the major problems.

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    That's never going to work, strip it down & start again with another chassis. There is good reason why every classic amp design has the inputs at one end of the chassis and the power transformer at the other. dc voltage should flow from the PT, to power tubes, to preamp tubes, towards the inputs. AC signal should flow from the inputs, towards the power tubes & OT. Neither should be criss-crossing back & forth. Look closely at how the 5F6A is physically laid out, don't try and reinvent the wheel in this respect.

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    Last edited by MWJB; 03-19-2011 at 05:16 PM.

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    Sorry, I agree - your inputs are so close the the output tubes etc. It will always be a problem with a layout like that, you need to start again. Pete.

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    I see, thank you for the comments! My intentions were not to reinvent anything though. I simply did not have the necessary knowledge to properly design a chassis layout. During the assembly I realised that I got the tube layout in reverse .. didn't know it was going to cause so much problems.

    Though all the connections are done per the original physical layout, only with longer cable runs on the heaters because of the reverse tube layout. So the initial heater cable is reaching all the way to the other end of the chassis to start flowing from the power tubes. I tried to take some measures placing the long running heater cables and rectifier supply very snug to the chassis with good turns and as and far from each other and everything as possible.

    I will try and make another one reversing the tube placement with the power tubes on the PT side far from the outputs, is that the only problem you can see with the layout/chassis ? anything else to change ?

    is this the reason why the treble knob goes crazy ?

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    The treble knob is working fine...it's just as you turn it up the amp is suffering with ultrasonic oscillations that are beyond your hearing range, this is causing the syptoms you are experiencing.

    Always try an keep the grid wires (feeding 6L6 pin 5, 12A#7 pins 2 & 7) as short as you can, if they stretch right across a wide chassis, then shield them, grounding the shielding at one end. Search for images of Victoria, Clark & Kendrick bassman amps/circuit boards.

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    I replaced the grid wires, which i thought were over exposed, with shielded cables, now the symptoms are cured, all the knobs work normally and it also got even more quite


    Thanks again for the diagnosis!

    I will soon start designing a wider chassis with a better layout for the next project, hopefully that one will look more like it should.

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