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  • Helmholtz
    replied
    Originally posted by hylaphone View Post

    Here is a shot of both grids with a different pair of KT88. Second trace inverted for comparison.
    Same settings, top trace is the grid of the "top" KT88 on the schem, driven by the non inverting half of the PI. 220K grid leaks are still in place.
    Sorry for the blurriness, it's hard to photograph a trace!
    Not nice, because signals are not balanced and look triangular instead of sinusoidal - but no top side clipping above 20V like in your picture #4 above.
    Did you use same scope settings?
    Can you confirm that different KT88s cause different grid signals?
    Do you see the PI output imbalance and triangular waveshape also with KT88s out?
    Last edited by Helmholtz; 06-12-2021, 09:58 PM.

    Leave a comment:


  • hylaphone
    replied
    Originally posted by Helmholtz View Post

    1. Means 52Vpp or 42W into an 8R load.
    4. Assuming AC coupling, the positive going grid signal gets clipped above 20V. Looks like grid conduction but way too early.

    Do both KT88 grid signals look the same?
    Do you have another set of KT88s to try?
    If not you may temporarily try a set of 6L6s
    Here is a shot of both grids with a different pair of KT88. Second trace inverted for comparison.
    Same settings, top trace is the grid of the "top" KT88 on the schem, driven by the non inverting half of the PI. 220K grid leaks are still in place.
    Sorry for the blurriness, it's hard to photograph a trace!
    Attached Files
    Last edited by hylaphone; 06-12-2021, 08:40 PM.

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by hylaphone View Post

    Here are some.
    1. Output max clean signal. 1V/div, 10:1 probe. 1kHz
    2. Output max signal clipping about 75W, same settings.
    3. Grid max clean signal, same settings.
    4. Grid max signal clipping, 2V/div

    I see no signs of oscillation, my scope goes up to 100mHz.

    What's interesting to me is how the signal compresses, but I'm able to feather it back up... go slow enough and I can almost hit 60W clean, but this isn't the kind of dynamic I want in a bass amp
    1. Means 52Vpp or 42W into an 8R load.
    4. Assuming AC coupling, the positive going grid signal gets clipped above 20V. Looks like grid conduction but way too early.

    Do both KT88 grid signals look the same?
    Do you have another set of KT88s to try?
    If not you may temporarily try a set of 6L6s

    Leave a comment:


  • hylaphone
    replied
    Originally posted by g1 View Post
    I'm not clear on the amplitude of those grid waveforms. 60Vp-p unclipped, 120Vp-p clipped?
    Something odd there.
    Indeed. Here is a shot at full clip. 2V/div, 10:1 probe.
    Top trace is one of the grids, bottom trace is across the dummy load.
    Attached Files

    Leave a comment:


  • hylaphone
    replied
    Originally posted by g1 View Post

    Yes, halving it would be too much I think.
    But just to be sure, the tap arrangement on this model OT is not so simple. Are you using one of the wiring schematics from the attached datasheet?
    Sorry, should have specified it's the "easy wire" version. Just measured the secondaries and it is wired correctly.
    Last edited by hylaphone; 06-12-2021, 06:48 PM.

    Leave a comment:


  • g1
    replied
    I'm not clear on the amplitude of those grid waveforms. 60Vp-p unclipped, 120Vp-p clipped?
    Something odd there.

    Leave a comment:


  • g1
    replied
    Originally posted by hylaphone View Post

    I've not tried changing load resistance. I could halve it using a different tap/same load but 2.5K seems like an awfully high load for this amp?
    Yes, halving it would be too much I think.
    But just to be sure, the tap arrangement on this model OT is not so simple. Are you using one of the wiring schematics from the attached datasheet?
    Attached Files

    Leave a comment:


  • hylaphone
    replied
    Originally posted by g1 View Post
    Have you tried altering your load resistance?
    And that GEC datasheet seems overly optimistic to me. Plugging your numbers into Nick's calculator gives around 60W with your values and 70V p-p signal applied.
    Not 70W, but more than 50 anyway. I had it set for -46V bias and didn't re-try with the lower bias number (-41V).
    I've not tried changing load resistance. I could halve it using a different tap/same load but 2.5K seems like an awfully high load for this amp?

    Leave a comment:


  • hylaphone
    replied
    Originally posted by Helmholtz View Post
    Wow cool! Thanks --

    Leave a comment:


  • hylaphone
    replied
    Originally posted by Helmholtz View Post

    Doesn't seem to make sense.
    Something is missing. Maybe oscillation involved?

    Can you post scope pictures of grid and output signals? And don't forget the scope settings.
    Here are some.
    1. Output max clean signal. 1V/div, 10:1 probe. 1kHz
    2. Output max signal clipping about 75W, same settings.
    3. Grid max clean signal, same settings.
    4. Grid max signal clipping, 2V/div

    I see no signs of oscillation, my scope goes up to 100mHz.

    What's interesting to me is how the signal compresses, but I'm able to feather it back up... go slow enough and I can almost hit 60W clean, but this isn't the kind of dynamic I want in a bass amp
    Attached Files

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by hylaphone View Post

    What is Nick's calculator?!?
    See here:

    http://bmamps.com/ivds.html

    Leave a comment:


  • hylaphone
    replied
    Originally posted by g1 View Post
    Have you tried altering your load resistance?
    And that GEC datasheet seems overly optimistic to me. Plugging your numbers into Nick's calculator gives around 60W with your values and 70V p-p signal applied.
    Not 70W, but more than 50 anyway. I had it set for -46V bias and didn't re-try with the lower bias number (-41V).
    What is Nick's calculator?!?

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by g1 View Post
    Have you tried altering your load resistance?
    And that GEC datasheet seems overly optimistic to me. Plugging your numbers into Nick's calculator gives around 60W with your values and 70V p-p signal applied.
    Not 70W, but more than 50 anyway. I had it set for -46V bias and didn't re-try with the lower bias number (-41V).
    When I feed the numbers into Nick's tool, I get 72W.
    But it's essential that the grid Vpp is at least twice the grid bias.

    In my experience those original tube datasheets were very reliable. Contemporary KT88 clones may be inferior, though.
    Last edited by Helmholtz; 06-12-2021, 04:51 PM.

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  • Helmholtz
    replied
    Originally posted by hylaphone View Post

    Just tried this, no difference.
    I have also tried higher supply voltage to the PI, upwards of 450V and it didn't help.
    Doesn't seem to make sense.
    Something is missing. Maybe oscillation involved?

    Can you post scope pictures of grid and output signals? And don't forget the scope settings.

    Leave a comment:


  • g1
    replied
    Have you tried altering your load resistance?
    And that GEC datasheet seems overly optimistic to me. Plugging your numbers into Nick's calculator gives around 60W with your values and 70V p-p signal applied.
    Not 70W, but more than 50 anyway. I had it set for -46V bias and didn't re-try with the lower bias number (-41V).

    Leave a comment:

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