Peak choke current in a 5G9 tremolux

Can anyone help me out with a calculation of the peak current seen by the choke in the 5G9 Tremolux?

http://www.prowessamplifiers.com/schematics/fender/Tremolux_5g9-Schematic.html

> peak current seen by the choke

It is a Cap-Input filter. The Peak/DC current ratio is similar to the amount of ripple on that first cap. We have 40uFd feeding two 6V6, I would expect ripple to be under 10%.

What IS the DC current in that choke? We see about 5mA out the 10K resistor. We also have screen current. At idle, the 6V6es plate current can't be much over 60mA. I'd expect idle screen current in 6V6 to be ~~5% of plate current, so 3mA. I'd expect screen current to rise sharply in OVER-load, say 20mA.

So talking roundly, 12mA-25mA DC current in choke. Peaks can be 10% higher or 13mA-28mA.

And the *right* answer is "small Fender replacement choke", a part stocked by all guitar-amp part suppliers at price as low as any choke around.

PRR said:

And the *right* answer is "small Fender replacement choke", a part stocked by all guitar-amp part suppliers at price as low as any choke around.

Click to expand...

Anyone have specs.  Inductance, resistance, max current, max saturation voltage for this beast?

CJ?

took apart a Twin Reverb choke once, (print below)

here are the specs for a Deluxe Reverb (6V6 pair)

"Replacement for Fender Deluxe and Vibrolux. Equivalent to #125C3A & #022707. Specifications Rating: 50 mA Open Circuit Impedance (Ohms): 1,500 (min. at 10 V, 60 Hz) "

we can do a little math using the inductive reactance formula to try and guess at the Henries,

XL=2 pi f L,  so L=XL/2 pi f, 

L=1500 Ohms / 6.28 * 60, so L = 1500/377= 4 Henries,

but ripple from a Fender pwr supply is at 120 Hz, so reactance will be double the ohms for the same 4 Henry choke , so XL=3K,

so if you have 10 volts ripple, ripple current will be about 10/3K = 3.33 ma

Thanks PRR and CJ, much appreciated. 

CJ implies this choke has an air gap.  If so, it is unlikely to saturate until very high DC currents and its Inductance will be 'constant'.

Will Gurus CJ & PRR confirm this please.

we don't need to worry much about inductance vs frequency in a pwr supply choke  as it sees a constant 120 Hz,  or 60 Hz in a primitive supply which is no longer used much,

so the gap is mostly to keep the core from saturating from the DC current,

here is the inductance vs freq graph for the Fender choke,

to design a choke you need to know AC flux, DC flux, min inductance, and current capacity of the wire used,

you can look up the cross section specs of the lams used in a Mag Met catalog,

then you can calculate flux,

spread sheets are great for this task,

here is an AC flux generator sheet>

DC flux is a bit more involved,

you use the magnetic path length of the core (given in Mag Met catalog pages) and use this to compute effective perm of the core for various gaps,

then you use MPL again  (magnetic path length) combined with the gap thickness and magnetic force to calculate DC flux,

magnetic force is simple amps times turns (amp-turns) times the constant 1.2566,

to get effective perm you use the ungapped perm specs given in transformer books for various metals, as it turns out, the max perm of variolus core materials does not matter that much in a gapped core as the air gap reduces any max perm by a great degree, air has a very low perm compared to any metal, so even the smallest of air gaps spoils all the perm of the best of lams, this is why the size of the air gap is very critical,

magnetic flux flows easily thru a hi perm core, but as soon as it hits that air gap, it has to skip across an area of  very low perm, so this air gap acts as a resistor to the flux, this resistor is in series with the resistance of the core, and is many times the resistance of the core, thus, flux is reduced in the core/gap series circuit which keeps the core from saturating,

so using expensive metal to build a choke makes no sense as it's perm will be dropped down to that of a cheap lam, and besides, expensive metal like 80Ni takes less flux than cold rolled (barn roof) or grain orient steels,

Stored energy>
Neglecting losses, the energy (measured in joules, in SI) stored by an inductor is equal to the amount of work required to establish the current through the inductor, and therefore the magnetic field. This is given by:

    E = 1 \ 2  L I^2 ,    L=inductance, I = current,

notice the similarity to Kinetic Energy,  KE=1/2 mv^2,

as it turns out, most of the energy required to energize the inductor is stored in the air gap, 

a spread sheet for DC flux>

so to build a choke you juggle AC and DC flux along with inductance,

AC voltage feeds the AC flux formula, DC current feeds the DC flux formula,

you add AC and DC flux to get B-max, or total flux, then you make sure the core material can handle it, usually silicon steel is used in chokes which can take a combined AC and DC flux of 18 Kilo Gauss before saturating,

an air gap will reduce DC flux, but has no effect on AC flux, but usually in a choke, AC flux will be pretty low compared to DC flux because the ripple voltage used to calculate the AC flux in the core will be pretty low, maybe 10 volts, which is low compared to say, a tube output transformer which may have to handle 400 volts AC,

so after you add up the flux, you need to know inductance,

wire size is easy,  just use the DC current and compare it with the current specs on a wire chart,

so the last thing we need is inductance, this is calculated by using effective perm from a gapped core along with the core inductance coef.  which is also listed in the Mag Met lam catalog,

so you go back and forth between Henries produced by a gapped core and the AC and DC flux of this core which depends upon the cross section of the core for AC and the MPL (magnetic path length) and air gap for the DC flux,

if your Henries are OK, but flux is too high, you need a bigger core which will have a bigger cross section area and a longer magnetic path length, which will reduce both AC and DC flux,

if your flux calcs come out low and your Henries are low, you can add more turns to the core, which will give you more Henries, but you will have to re-calculate DC flux as your amp-turns will be higher, more turns will decrease AC flux but increase DC flux, but remember that DC flux levels will be much higher than AC flux levels in a choke, so dropping AC flux by increasing turns will not cancel out the increase in DC flux by much,

here is an easy sheet for Inductance>