Recommend a good plate choke for 26 tube!!

[andyjevans]

I have tried using two Hammond 156C in series as plate chokes for a 26 tube. They sounded better to me than IXYS active loads, but were rather prone to hum and hard to get quiet.

What would be the ideal plate choke for a 26 with its Rp of around 8.5K?

Looking at Lundahl

LL1667-05mA 810H
LL1667-07mA 580H
LL1667-10mA 405H

Which of those?

Any experiences with these, or something else again?

Andy

[dvb-projekt]

Perhaps the Hashimoto HL-10 K-6 is an alternative:

[djn]

Stupid question, but why are are chokes used on plates? I've heard people talk about it but never asked the question.

[sepolansky]

DJN:

The choke is in lieu of a plate resistor or SS load. The choke offers the advantage of very low resistance, compared to a resistive load, so low B+ can be used. The choke must be closely matched to the tube, with enough inductance to provide good low frequency response.

At any given frequency, the load, as seen by the tube (plate load only, now) is 2*PI*L*F=the inductive reactance, or AC resistance where L is measured in Henries, F is the frequency. Add this number to the DC resistance (DCR) and you get the load on the plate.

To a point, the higher the load resistance, the greater the stage gain. Obviously, the lower the frequency, the lower the "resistance" hence lower gain. To keep from causing a low frequency roll off, enough inductance (L in the equation) must be present for the desired bass response.

Just another way of designing a gain stage. Simple but very expensive.

Stuart

[dsavitsk]

Originally Posted by djn

Stupid question, but why are are chokes used on plates? I've heard people talk about it but never asked the question.

sepolansky's answer is good. For a little more, choke loads compared to other loads here: http://www.ecpaudio.com/pdf/parafeed_basics.pdf

[kevinkr]

In driver applications the choke load also allows you an output swing of almost twice the dc supply rail by virtue of the energy stored in the magnetic field of the choke. This is quite convenient for driving low mu dhts for example on relatively driver stage plate voltages. It also means that for a given swing there is the possibility of slightly better linearity provided that the stage is loaded by a high impedance load.

FWIW I'd use the choke rated for 7 or 10mA as I run 26s in my line stage at 6mA and above. Note also that the plate to line transformer suggestion is a very good one. (This is what I have been running for over 10yrs now in my 26 based line stage.)

Any choke is going to have pick up issues with external AC magnetic fields, this is also true with transformers. Core geometry may help a little, care in physical placement and shielding can help a lot.

[Vinylsavor]

Hi!

Lundahl sepc's the DC currents for operation at full AC signal across the choke. If less swing is required (for example preamp) more DC current can be allowed.

In a line stage, the 5mA type would be ok to be run with 6mA DC

Best regards

Thomas

[andyjevans]

Anybody seen or used this guy's plate chokes?

ANODE GRID CHOKE 40H 200H 300H 500H 12AX7 12AU7 6SN7 83 on eBay (end time 02-Mar-11 05:36:18 GMT)

[mwiebe]

Andy,
I’ve had good luck with Magnequest EXO-010 Plate chokes (10ma/275H, 825 DCR), 04-006 gapped to 15ma/200H, EXO-99 (10ma/750H, 2200 DCR)--prefer the EXO-010s. I’ve also tried Electroprint AC9037 (10ma/100H) but I think the inductance was not sufficient. All are used with parallel feed preamp outputs, QEE-3436 (series primary 20K/600) and Magnequest B7-15K.
Matt

[kevinkr]

Originally Posted by mwiebe

Andy,
I’ve had good luck with Magnequest EXO-010 Plate chokes (10ma/275H, 825 DCR), 04-006 gapped to 15ma/200H, EXO-99 (10ma/750H, 2200 DCR)--prefer the EXO-010s. I’ve also tried Electroprint AC9037 (10ma/100H) but I think the inductance was not sufficient. All are used with parallel feed preamp outputs, QEE-3436 (series primary 20K/600) and Magnequest B7-15K.
Matt

I'd agree, about the minimum you would want here would be 200H..

Andy, I'd be very wary of buying a plate choke from anyone who does not also wind interstage or output transformers as some care in winding and design is required to keep the shunt and stray capacitances manageable, particularly as the inductance goes up. I'd go for Lundahl, Sowter, Electra-Print or Magnequest chokes new or used in preference to the eBay ones you ask about. Even great chokes often roll off surprisingly early on the HF end IMLE..