Tube DIY Asylum

Can someone explain "primary inductance" of an output transformer?

76.174.175.12

Iczerman

Is more better?
Higher primary inductance mean more bass?

Follow Ups:

RE: Can someone explain "primary inductance" of an output transformer? - VoltSecond 16:05:08 08/16/24 (8)
Maybe this will help.
-Volt

Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.

. . .Oh!. . .Remember: Modifying things voids their warranty.
- 211 load line with primary inductance (Open in New Window)
- RE: Can someone explain "primary inductance" of an output transformer? - engineer@baymoon 10:14:51 08/22/24 (4)
  Looking into some Push Pull Output Transformer models,
  for example Dynaco ST70 A470
  
  .SUBCKT A470 P1 Sg1 B Sg2 P2 Sp1 Sp2
  * Dynaco A470. Push Pull transformer, with Ultralinear taps at 33%, 5600 to 16 ohms, 3db 7 to 56000 hz
  *
  * model generated by TransformerModels.xls yyyy Jan 22
  *
  LP1 1 Sg1 7.29664888730406
  LS1 2 B 1.77011598090312
  LS2 3 Sg2 1.77011598090312
  LP2 4 P2 7.29664888730406
  LSA 5 Sp2 0.18300530603261
  
  *Seems* to be violating this criterion:
  
  OPT primary inductance > (load impedance)/120
  
  Well, A470 was designed for EL34.
  
  Looking into a manufacturer's published data for 5K push pull DHT OPT:
  
  https://shop.piemme-elektra.it/Categories/92/Products/8266
  
  Total Primary inductance: > 40H @100Hz 5V
  
  5K/120 ~ 40H
  
  criterion seems to be observed.
  
  Does tube plate impedance matter wrt primary inductance ?
  Any insight ??
  - RE: Can someone explain "primary inductance" of an output transformer? - engineer@baymoon 20:25:06 08/23/24 (0)
    Kind of answer my question regarding Dynaco ST70 A470
    OPT primary inductance ...
    Due to global negative feedback, output impedance is scaled
    by a factor of 1/(1+ beta * mu).
    Plug in some numbers for ST70, mu = 16, beta = 1/5 (not sure ??).
    Impedances are scaled.
    The criterion:
    OPT primary impedance > reflected load impedance
    is valid.
    Make sense ?
    
    Edits: 08/23/24
  - RE: Plate Impedance wrt to Primary Inductance - VoltSecond 21:52:53 08/22/24 (2)
    In an open loop system, the plate resistance working against the primary inductance sets your low frequency roll off frequency. When you add feedback this "small signal" roll off frequency can be pushed to a lower frequency, BUT, but you still have to drive the large signal response to get the power to the speaker.
    
    When the XL (2 pi F inductance) of the transformer is less than the reflected load impedance, the tube will have to push more current into the transformer for the same voltage to the speaker. This extra drive can lead to various forms of distortion. This is part of the large signal response.
    
    Play safe and play longer! Don't be an "OUCH!" casualty.
    Unplug it, discharge it and measure it (twice) before you touch it.
    
    . . .Oh!. . .Remember: Modifying things voids their warranty.
    - RE: Plate Impedance wrt to Primary Inductance - Paul Joppa 08:38:46 08/23/24 (1)
      Only for triodes or pentodes with plate to grid feedback, of course :^)
      - There are tubes that aren't triodes? :-) - Tre' 18:54:05 08/26/24 (0)
        
        In Reply to: RE: Plate Impedance wrt to Primary Inductance posted by Paul Joppa on August 23, 2024 at 08:38:46
        
        .
        Have Fun and Enjoy the Music
        "Still Working the Problem"
- RE: Can someone explain "primary inductance" of an output transformer? - engineer@baymoon 15:21:02 08/20/24 (1)
  Similar to the concept of critical inductance in power supply,
  using this low frequency model, let's try:
  
  At low frequency (f=20Hz)
  
  X_L > R1
  2*pi*f*L > R1
  L > R1/2*3.1416*20 (critical inductance of output transformer)
  
  e.g. R1 = 16K
  L > 127H
  
  Does it make sense ?
  - RE: Can someone explain "primary inductance" of an output transformer? - VoltSecond 16:16:00 08/20/24 (0)
    For steady state, yes. You've got it!
    
    For tone bursts it is more complicated. (See link below.) However, the "attack" time for music from natural instruments may render this issue moot.
    
    Play safe and play longer! Don't be an "OUCH!" casualty.
    Unplug it, discharge it and measure it (twice) before you touch it.
    
    . . .Oh!. . .Remember: Modifying things voids their warranty.
    - 2X margin for tone burst. (Open in New Window)
- Thank you. nt - Tre' 19:05:01 08/17/24 (0)
  .
  Have Fun and Enjoy the Music
  "Still Working the Problem"
RE: Can someone explain "primary inductance" of an output transformer? - Tom Bavis 16:30:26 08/13/24 (0)
More or less true. Low end power is bought by the pound.

Single ended transformers have less inductance in general in order accommodate the DC current. That current flows in both directions in a push-pull transformer, canceling out, so push-pull transformers can have much more inductance. BTW, inductance is not a constant value - it varies with applied voltage and frequency. Typically it increases with signal level, until reaching the saturation point where inductance drops drastically. Saturation will limit low-frequency power, even if there seems to be plenty of inductance.

And there's also leakage inductance, which is the portion that doesn't couple to the secondary - higher leakage inductance will limit high frequency response.

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