Choke question

[tubeswell]

Motorboating. (sounds like an outboard motor on a boat)

Chokes in a Pii filter offer very good filtering. A choke steadys out changes in the current, helping to keep the voltage constant, whilst at the same time offering a low resistance to voltage. In the simplest terms I can come up with, this is to do with the interaction of the coil and the iron producing a magnetic field when a current goes through it, which helps buffer changes in current that result from rectification and capacitive filtering. It does this by offering a 'resevoir' of current (if you like) in the EM field surrounding that part of the power supply wire.

Chapter 3 on this page is a good tutorial site for valve amp power supplies.

http://www.tpub.com/content/neets/14178/index.htm

[mooreamps]

SIngle ended amps do hum more than p-p if using simple filtration. putting the choke before the power tube plates does smooth it out and reduces hum considerably.

COnsider the 5E1 CHamp. By the 5F1 they cheaped out and drew the plates off the first node.

Look at Princetons as late as 5F2 while they were still single ended.

5E7 Bandmaster draws plates after the choke even though it is push pull.

Later versions of these amps reverted to plate off the first node. ANother example of Leo Fender going for "good enough" rather than good as can be.

I have a Fender service bulletin somewhere here where they tell all us techs not to "repair" the hum in the Champs, it is normal. This was a later version with less filtration and no choke.

But since daz opened this thread by stating his amp was a 50 watty Marshall style amp with EL34s, I had to assume his was push pull.

Well, in that case, this is something I'm going to have to look at.

-g

[Steve Conner]

Yes, you can quieten a Champ-style amp considerably using a choke.

The inductance of the OPT doesn't count for hum filtration, because it has the reflected speaker impedance shunted across it, and any 120Hz ripple flowing through this impedance, oddly enough, comes out of the speaker as hum.

The high plate impedance of a pentode tube does count somewhat, though, and you can help the hum by extra filtering of the screen supply.

But the choke really does work great. On a SE amp I was messing around with, a choke of around 8H took hum from annoyingly loud down to below the noise floor.

[CitizenCain]

Thanks for the 411 on the choke's function, gents