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Re: Hammond 193B and 193C. (plus 157J) - What is the significance of the self-resonance in both power supply and parafeed applications? Have you built amps using Hammond output transformers?

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In Reply to: Hammond 193B and 193C. (plus 157J) posted by Tom �. on November 27, 2000 at 11:34:31:

Hi Tom
does the 'in-band' resonance reder these chokes useless for parafeed?
also - I assume there's not much problem in pi-filters (?)
in another forum I asked you abut 'maximizing the Hammond output transformers in two cases and wonder if you might agree on these methods and results?:
(1)Case of a zero feedback amp (or feedback amp with the 1st stage running 'free') = I would assume either partial cap or RC bypassing of the 1st stage cathode resistor will provide the 'lift' need to compensate for the Hammond transfomer's HF rolloff - can this bit of eq make the Hammond 'competitive' with the pricier transformers and squeak better sonics out of the unit?
(2)Case of feedback amp using loop nfb from the output transformer's secondary - will a RC 'snubber across the whole transformer primary allow deletion of the RC 'lead-lag; network in circuits where the driver/inverter gain is low sucha s a gain/split-load inverter or a cathode-coupled inverter with the nfb injected to one side ala Fender?
many thanks - very cool tests on the Hammond chokes - have you also tested Hammond and Lundahl output iron under dc current?
Freddy

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Re: Hammond 193B and 193C. (plus 157J) - What is the significance of the self-resonance in both power supply and parafeed applications? Have you built amps using Hammond output transformers? Paul Joppa 21:18:45 11/28/00 (2)
The "resonance" is not significant in a plate choke. The function of a plate choke is to present a high impedance to the plate; this resonance is merely the peak of the impedance curve, which is hopefully high everywhere in the audio band. Effectively, the resonance is highly damped and dominated by the tube's plate resistance.
In theory, a resonance in the middle, say 800Hz, gives a well balanced response, with the impedance falling to the same value in the bass (say, 30Hz) and the treble (say, 20kHz). In practice, it seems that stray capacitance sounds worse than stray inductance. Good audio plate chokes should be made with (among other things) good materials that have good dielectric properties. Even in that case, I would like a plate choke to resonate an octave or two higher, at least 1500 and preferably 3kHz.
If you are using chokes not designed for audio, I prefer to err on the side of low capacitance, so I like to see a resonance of at least 4kHz in that case. The one in the Paramour resonates around 5.5kHz (I just checked a sample).
- Paul's got it right. Voltsecond 21:48:32 11/29/00 (0)
  Tech talk:
  For an LC tank the resonant frequency is given by
  F = 1/( 2 * PI * SQRT(L*C) )
  For an output transformer with identical windings and capacitance: if we use a core with 10X higher inductance the resonant frequency is SQRT(10) = 3.16 times lower. The lower resonant frequency transformer is probably a better one in an application with out DC current. This one of the reasons why Paul is right when he says to watch the capacitance.
  Because the inductance on most of these magnetics changes with frequency, this topic becomes even more convoluted.
- -Significance of self-resonance in plate chokes - Thanks Paul! freddyi 15:33:03 11/29/00 (0)
  Thank you Paul
  best wishes,
  Freddy
short title Tom �. 20:47:56 11/27/00 (0)
When used as a filter choke, the main frequency to be concerned about is 120hz (or 60 if you're a half wave freak). The "resonance" won't be much of a factor. Personally speaking, I wouldn't use any of these chokes as a plate load unless the plate resistance of the tube used was 500 ohms or less. I just thought I'd throw out that info if it would be of any use to someone.
Also keep in mind these values were measured with 10Vac. The measured values can change significantly with level of signal input.

Tom �.

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