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Dowdy choke for an headphone/preamplifier.

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Posted on July 6, 2016 at 02:01:07
FrancescoM
Audiophile

Posts: 4
Joined: July 6, 2016
Hi Michael and all,
i'm ready to use a tl404 that i bought one year ago. I found some VT52 WE....

I need a wonderful plate choke, the best for a VT52, with the highest inductance value.

Your best proposal is the Dowdy (125H and 40mA)?
Could you design other chokes with higher inductance?

The vt52 work point could be between 35 and 40 mA.

Let me know.

Regards
Francesco

 

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RE: Dowdy choke for an headphone/preamplifier., posted on July 7, 2016 at 13:50:36
mqracing
Manufacturer

Posts: 4307
Joined: June 29, 2000
What is the I sub r of the plate at your operating point? What is the nominal primary impedance of your output transformer?

125 Henries is a (as the Donald would say "a huge load". At twenty hertz it's impedance is 15.7 kilo ohms. At 30 hertz it is 23.5 Kilo ohms. At one thousand hertz it is above 750,000 ohms.




Builder of MagneQuest & Peerless transformers since 1989

 

RE: Dowdy choke for an headphone/preamplifier., posted on July 7, 2016 at 13:59:09
FrancescoM
Audiophile

Posts: 4
Joined: July 6, 2016
Hi Michael,
The output transformer is your TL404, standard version.
The I on the plate of the tube could be between 35-40mA.

Thank you for your reply

 

RE: Dowdy choke for an headphone/preamplifier., posted on July 7, 2016 at 14:13:09
mqracing
Manufacturer

Posts: 4307
Joined: June 29, 2000
The Dowdy choke is more than adequate for use with a TL-404.

I could make you a plate choke with double, triple or quadruple the L but at great expense monetarily and an even greater cost in terms of performance.

The Achilles heel becomes at least twofold. As the size becomes greater and greater so doesn't the winding capacitances increase to the point where your tube would have difficulty driving the capacitance.

As the core becomes larger and larger the energy required to kickstart the core and supply the magnetizing current becomes a limiting factor.




Builder of MagneQuest & Peerless transformers since 1989

 

RE: Dowdy choke for an headphone/preamplifier., posted on July 7, 2016 at 14:24:55
FrancescoM
Audiophile

Posts: 4
Joined: July 6, 2016
Some times ago my friend measured the primary inductance of your tl404. He found about 27H.
So the anode choke and primary of the tl404 are in series in a parafeed design.
The total inductance isn't a great value.
But I trust you. If in your opinion the dowdy is great for my application (vt52 parafeed) i'm ready to buy a pair.
Have you got in stock a pair of dowdy chokes?

Regards

 

RE: Dowdy choke for an headphone/preamplifier., posted on July 8, 2016 at 10:19:50
mqracing
Manufacturer

Posts: 4307
Joined: June 29, 2000
The two inductors (plate choke & output trans) are in parallel.

What was the signal level and frequency that you used to measure the L? What test instrument did you use?

To get true inductance readings you really need to use a professional inductance bridge and drive the unit under test properly.






Builder of MagneQuest & Peerless transformers since 1989

 

RE: Dowdy choke for an headphone/preamplifier., posted on July 8, 2016 at 10:40:03
FrancescoM
Audiophile

Posts: 4
Joined: July 6, 2016
Sorry for the mistake….
I thought to parallel connection but i wrote series…..
We are speaking about parafeed design…
:-)

About the measurement, ASAP i'll send you more information about instrument that my friend used.
Thank you very much Michael,

Regards

 

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