Search:

Type: Posts; User: Helmholtz; Keyword(s):

Page 1 of 18 1 2 3 4

Search: Search took 0.02 seconds.

  1. Replies
    16
    Views
    312

    Maybe this helps: For the tube only the...

    Maybe this helps:

    For the tube only the voltage between grid and cathode matters, consisting of DC bias and AC signal.
    The grid leak resistance always should be smaller than the internal grid...
  2. Replies
    16
    Views
    312

    Sorry, not clear what you mean (double language...

    Sorry, not clear what you mean (double language barrier). Maybe you give an example.
  3. Replies
    6
    Views
    187

    Cable thickness does not matter.

    Cable thickness does not matter.
  4. Replies
    16
    Views
    312

    OK, from tube data the gain of your 12AX7 is...

    OK, from tube data the gain of your 12AX7 is G=72. From this the input capacitance calculates as 120pF. Real value will probably be closer to 150pF, because of wiring influences.
    Worst case treble...
  5. Replies
    16
    Views
    312

    Anyway the second coupling cap and grid leak...

    Anyway the second coupling cap and grid leak resistor don't improve anything. You can't compensate a low pass filter with a high pass filter having a much lower corner frequency. A bright cap of...
  6. Replies
    16
    Views
    312

    I can't think of a reason why the frequency...

    I can't think of a reason why the frequency response should improve.
    Connecting the grid to the pot wiper eliminates the second coupling cap and actually gives slightly better bass response.
    ...
  7. Replies
    16
    Views
    312

    What benefits do expect over connecting the grid...

    What benefits do expect over connecting the grid directly to the vol pot wiper?
  8. Replies
    16
    Views
    312

    If " operational input voltage condition swing "...

    If " operational input voltage condition swing " means an AC signal, average DC cathode voltage (determing the bias point) will not change. To be precise, this only holds for linear operation....
  9. Replies
    16
    Views
    312

    A tube needs a negative bias voltage. Bias is...

    A tube needs a negative bias voltage. Bias is grid voltage minus cathode voltage. So if grid voltage is zero and cathode voltage is +1V, bias is -1V.
    A large grid leak resistor develops a negative...
  10. I just quickly checked but I think you need to...

    I just quickly checked but I think you need to load the sine output with a 15k resistor replacing the square generator input resistance to allow for ample DC current.
  11. Good idea! A cascode is another way of wiring...

    Good idea!

    A cascode is another way of wiring 2 triodes is series. Achievable gain with the 6SN7 is around 100 (a little more than with the ECC82 in Merlin's example), using a plate resistor of...
  12. The principles yes, but not the results. Clipping...

    The principles yes, but not the results. Clipping characteristics and clean headroom strongly depend on input stage gain. And the different triodes vastly differ in gain (G).

    6SN7 : single triode...
  13. In the National 1212 amp the field coil...

    In the National 1212 amp the field coil definitely serves as a PS filter choke.

    In the Supro schematic I don't see a field coil, but if there is one it is most probably also wired as filter choke.
  14. But he is using a very low 6SN7, resulting in...

    But he is using a very low 6SN7, resulting in less than 1/4 the gain of a 12AX7. Gain values with Ra = 100k and bypassed cathode resistor 6SN7: G=14, 12AX7: G=60.
    With the two 6SN7 triodes in...
  15. True and the extra dB gain with the 6SN7 triodes...

    True and the extra dB gain with the 6SN7 triodes in parallel would be even less as this is already a very low impedance (Ra) tube.

    OTOH using a single 12AX7 would increase gain from 14 to 60...
  16. Replies
    15
    Views
    269

    Yes, see post #6.

    Yes, see post #6.
  17. Replies
    15
    Views
    269

    You're right. I only thought of plate current,...

    You're right. I only thought of plate current, which is wrong. Do you have values for screen currents with zero plate voltage?
  18. Replies
    10
    Views
    444

    I wouldn't use the thing without an isolating...

    I wouldn't use the thing without an isolating transformer.
  19. Replies
    15
    Views
    269

    What makes you think the OT is involved? A...

    What makes you think the OT is involved? A defective OT won't cause idle current to rise. If the primary was open there would be no tube current. If there was a short, it would not influence tube...
  20. Replies
    15
    Views
    348

    I would look for a small electric deep fryer....

    I would look for a small electric deep fryer. Might need a little modification. Use it as a "double boiler" and/or connect it to a suitable PCD (phase control dimmer) to regulate temperature.
  21. That would be the only way to achieve a total...

    That would be the only way to achieve a total gain like a pentode (or more as tube gains multiply). Or use a pentode :).
  22. As there still seems to be some misunderstanding,...

    As there still seems to be some misunderstanding, I will try to answer your questions explicitely.




    The screen is not only controlled by average DC but rather by total screen voltage which...
  23. Paralleling 2 triodes does not increase the...

    Paralleling 2 triodes does not increase the (resulting) amplification factor , but decreases the plate/output impedance of the circuit. This allows actual circuit gain to be closer to the...
  24. OK, I will try an explanation using your terms. ...

    OK, I will try an explanation using your terms.

    You are right, bypassing the screen resistor lowers the screen circuit/supply (AC) impedance.
    Lower impedance means less NFB (= more gain) and...
  25. ...and that proves what? BTW, how does this...

    ...and that proves what?

    BTW, how does this Fender history stuff relate to the amp selector topic?
  26. Or maybe an AT&T/Western Electric circuit?...

    Or maybe an AT&T/Western Electric circuit?...
  27. Just like with grid No.1, the control effect of...

    Just like with grid No.1, the control effect of the screen grid depends on momentary/instantaneous voltage. The momentary screen voltage can be seen as composed of a bias DC voltage (being the screen...
  28. Replies
    15
    Views
    270

    That would correspond to 424mV peak. But the...

    That would correspond to 424mV peak. But the meter also somehow averages over time. Not a reliable method.
    To see and measure the high initial peaks you need a (storage) scope.
Results 1 to 28 of 500
Page 1 of 18 1 2 3 4