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Thread: 6bm8 Clipping question

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    6bm8 Clipping question

    In looking at a friends very low end looking SE 6BM8 amp that sounds bad, the sine wave coming out of pin 9 looks good. Looking at pin 3, the top of the wave is clipped way off. The resistors in the circuit measure ok and bypassing the coupling cap with a good one didn't help.
    The attached schematic isn't the actual circuit but similar. There is no gird stopper on pin 3 in the circuit that I'm working on. Any ideas on what's likely to cause this? Thanks
    Click image for larger version. 

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    Supporting Member Jazz P Bass's Avatar
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    Sounds to me as if the coupling cap is leaking.

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    Supporting Member mozz's Avatar
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    Bypassing the coupling cap is not gonna prove anything, unless the original was open, which isn't the usual failure mode. You have to take the old one out of the circuit, because it's probably leaky.

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    Thanks. I replaced the cap and it made no change. Any other ideas? Thanks again!

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    don't forget the joker g1's Avatar
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    Can you post the DC voltages for the second 6bm8 stage for the pins shown on your pic.

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    Here are some voltages- thanks:
    edit: the pot isn't 22k. Also, the 22k Rk seems big so I tried a range of smaller values & it didn't help.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Supporting Member Jazz P Bass's Avatar
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    I take it that you do not have another 6BM8 to try.

    If the signal looks good coming out of pin 9 & it looks bad at the 6BM8 grid, then something internal to the output tube may be bad & it is affecting the signal.

    You could try removing the cathode bypass capacitor.

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    Last edited by Jazz P Bass; 06-15-2018 at 03:28 PM.

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    Supporting Member eschertron's Avatar
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    Is that "4v with the arrow" the bias voltage? Measure across the 180R cathode resistor. Also, what's the AC signal that you are applying?

    If the signal coming off the triode's plate is OK, no need to look at that stage any further.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

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    Does your output tube grid have a path to ground via the trem circuit?

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    Supporting Member Jazz P Bass's Avatar
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    See if this Gibson schematic is close: GA5 T_ 6BM8.pdf

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    Supporting Member eschertron's Avatar
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    The hand-drawn schematic shows 180R for the pentode's cathode resistor, but the two printed drawings show bigger values (680R and 750R). The valve data sheet I looked at suggested -16v bias for 200v plate-to-cathode. How close to these design values does this amp operate?

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

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    Jazz P Bass- I have tried 3 known good tubes with exactly the same results. I'll remove that bypass cap & give it a try.
    eschertron - The 4 v is the grid to cathode voltage. The voltage across 180R is also 4 v. The AC input signal to the amp is 110Hz at about 1/2 volt. I vary the input to the 6bm8 via the volume control and it clips at almost any audible volume. I'll check exactly when it clips. I'll also check at higher Hz.
    Mick Bailey: - I couldn't figure out the term ckt last night but I'm guessing it supplies a grid leak path to ground. I did clip a 1M to ground on the grid as a check and it made no change at all.
    Jazz P Bass - thanks for the schem, that will help me figure out the tremolo ckt.
    eschertron- The printed schematic isn't the actual circuit but similar. I used it as a pin reference for discussion. Sorry if it caused confusion. The hand drawn is the actual circuit.
    -4v bias is quite small compared to -16. I will muck with the pentode bias & see what happens but I wasn't thinking that would clip the input to the pentode.

    I don't understand blocking distortion & need to read up on it. Could that be involved?

    Thanks everybody. After trying your ideas, I think my next move will be to disconnect the trem and supply a grid leak at pin 3.

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    Supporting Member Jazz P Bass's Avatar
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    That 110Hz is a tad lowish.
    Try 1K as an input frequency.

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    Supporting Member eschertron's Avatar
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    +1 ^^^
    the lower the frequency - and 110Hz is more than a tad lowish - then the more easily blocking happens. So the test freq could be a factor.

    edit: I just saw "input 1/2v". What's the signal AC voltage on the plate of the driving triode? If it's more than about 2.5vac, you will be driving the pentode into grid limiting.

    I looked again at the data sheet for the 6BM8. By your numbers it's dissipating about 3.3W, close to the design of 3.5W. So my suspicions of a 'wrong value' cathode resistor are probably unfounded.

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    Last edited by eschertron; 06-15-2018 at 09:03 PM.
    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

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