Selecting a Choke

[cminor9]

I am working on building a small el84 p-p amp. The schem I am working from indicates a choke, but doesn't give any info about it.

I am building this into a Hammond AO-35 chassis (Hammond organ reverb amp) and am using the transformers from there. I know the transformer puts out about 318v on the HV secondaries with no load. Not sure about the current.

Looks like there are three vectors to consider when choosing a choke: inductance, resistance, and current. I understand resistance, but how do I know how much inductance I need? Also, the current rating...is that max current? How does that work?

[black_labb]

think of a choke as a filter cap. the higher the capacitance/inductance the more filtering there is and thus the stiffer the power supply. some modern amps dont use chokes and use larger filter caps instead.

[Wakculloch]

Chapter 31 of the RDH4, gives the relevant formula for calculating the relationship between ripple reduction and choke size.
In the Osram 912 circuit (two EL84s, one 12AT7, and an 8D8), presuming you are talking about a choke before the OT node, it calls for a 10H, 125mA choke, so something of this order should suffice.

[Bruce / Mission Amps]

think of a choke as a filter cap. the higher the capacitance/inductance the more filtering there is and thus the stiffer the power supply. some modern amps dont use chokes and use larger filter caps instead.

Some nit picking...
Except that more inductance or more capacitance can improve filtering and capacitors are cheaper then chokes... I'd be careful in saying to "think of a choke as a filter cap" as they don't operate the same at all.
Also, in my opinion a choke always works much better then a resistor in a filter setup when used in a PI configuration as seen in many amps.
It's just that some amps are built fairly cheaply and smaller low powered amps don't need as good of ripple rejection.

[cminor9]

Thanks for the responses, as well as the reference to the RDH4. I'll read that and see if I can understand the theory.

So one question: chokes are rated for current. Is that a max amount of current they can take? For example, a choke rated for 60mA: how would this behave hooked up to a power supply with a PT that puts out 120mA?

[octal]

In most amps, the choke only filters the supply to the power tube's screens and the preamp. There's no need for it to be rated to carry the full current of the transformer, as the bulk of the current is flowing to the non-choked plate circuit.

Looking at my Mullard book, 2Xel84 = 22mA total screen current @ full honk. Add another 10ma for the preamp (more if you have reverb, many channels, etc.) Your 60mA choke should be fine.


Nathan

[cminor9]

Whether you intended to or not, you just taught me something big that should have been obvious: most of the DC voltage flows through the OT and into the plates. This makes the datasheets make a TON more sense now. In hindsight, it's obvious. I guess I have been so interested in taking voltage measurements I never paid much mind to where the current is going.

Thanks!

[Old Tele man]

...as a general 'rule-of-thumb' I guesstimate:

1) 1mA per active triode (remember, two triodes in each 12A_7 tube)
2) 5mA for the Reverb driver
3) 2 x Is.q for output tubes (typically about 1/10-1/20th of Ip.q)

...then, simply apply the equation: L(min) = RL/1130, where RL = Vdc/Idc.

...so, if screen voltage is Vs.q = 420Vdc and Idc = 22mA (7*1mA + 5mA + 2*5mA), then RL = 420V/0.022A = 19.1K ohms, so L(min) = 19.1K/1130 = 16.9H, or use next closest larger size, 20H.

...here, the low Idc value requires the use of a BIG choke; and increasing Idc actually enables you to use a SMALLER choke value.

[DougH]

In most amps, the choke only filters the supply to the power tube's screens and the preamp. There's no need for it to be rated to carry the full current of the transformer, as the bulk of the current is flowing to the non-choked plate circuit.

True, except in the old Vox ac15 and ac30 circuits there is a pi filter w/choke before the plate connection. So in that configuration, the full current draw of the amp including the plates are across the choke.

I was looking at the Hammond "Vox" chokes and trying to figure out why they were rated for 100ma. When I checked the schematics again it dawned on me...;-)

[tubeswell]

The Valve Wizard

There is a part of this page that covers how-big-a-choke vs how-much-capacitance for optimum filtering

Those 3H-5H 90mA chokes are good for most amps for the screen supply onwards.

[tedmich]

FYI Handmade Electronics Indexpage has some good priced closeouts on Hammond chokes!