18 watt choke

[Lindz]

I'm messing around with my 18 watt build and thought I would try installing a choke to see if I like how it sounds (and to perhaps help quiet it down a bit hum wise) - but alas even something simple like this gives me pause given my limited electronics knowledge

I tried looking for a layout with a choke in a TMB style 18 watter but did not find anything obvious.

I did find Mojotone's kit instructions and they suggest installing a choke right across the terminals 32/32 cap can in place of the resistor.

Amps like the JTM45 show chokes installed between the cap can and the 8.2k resistor on the board

1. Where in the circuit would folks suggest? How critical is the physical orientation and position in the amp?

The math is a bit over my head though I did try reading up on it. Power tranny is 315v dropped with some zeners and b+ runs about 355v. Solid state rectified, 2x el84, 3x 12ax7

2. Is a 75ma choke suitable? 4.8HY

http://www.mojotone.com/amp-parts/trans ... mp#details

or this? 100ma 3.5HY
http://www.mojotone.com/amp-parts/trans ... r-6L6-Amps

MM has a 1974x choke they sell that is 100ma 10hy though if the above would work for only $11 I thought they might be the ticket

[iknowjohnny]

I wouldn't waste your time. I tried this too only to find it doesn't change a thing. This was with a 18 watt i had modded to accept 6V6. I was told it has something to do with the tubes, and i believe i was told it would be the same with EL84's. In any case, choke vs resistor makes a huge difference in a el34 amp, but i don't think you will find it does anything noticable with an 18. I was actually quite surprised that it did nothing at all. Maybe someone will chime in with the technical reason for this, as i don't recall what i was told.

[Mr. Lime]

Long time ago this thread was opened but I consider adding a Choke to my 18W amp as well and wonder if some other guys experienced no changes in 6V6/EL84 power amps?

[pompeiisneaks]

Chokes do make a tonal difference. They're a combined resistive and inductive load. Is it a massive difference? That's opinion really. It's easy to temporarily wire one in and see how it sounds. You'd need to figure out how to adjust the dropper resistor in the circuit. The typical chokes are say 200-700 ohms resistance, and often replace the first dropper resistor in the chain, so either you'd want to drop more voltage if the existing one was higher, or ensure the circuit can handle the higher B+ after the swap out. Or you could just put it before the first dropper and lower B+ a bit.

~Phil

[Stevem]

Hum?
Are your power tubes heaters wired the correct way, like pin 4 to 4 and pin 5 to 5?

With 355 volts on the plates what are your outputs idling at?

[Mr. Lime]

Chokes do make a tonal difference. They're a combined resistive and inductive load. Is it a massive difference? That's opinion really. It's easy to temporarily wire one in and see how it sounds. You'd need to figure out how to adjust the dropper resistor in the circuit. The typical chokes are say 200-700 ohms resistance, and often replace the first dropper resistor in the chain, so either you'd want to drop more voltage if the existing one was higher, or ensure the circuit can handle the higher B+ after the swap out. Or you could just put it before the first dropper and lower B+ a bit.

~Phil

In my current project the goal is to archive a litte bit of higher voltages on the plates and to make the low end tighter. It's an EL84 PP kind of Trainwreck amp and has cathode bias. Got ~300V after the rectifier so the first gain stage gets around 125V with Rp 120k.
I wonder if a choke could help the bass response. Coupling caps are already small and there's still some muddiness when the bass control is turned up.

Which Hammond Choke would be recommendable in this situation?

Thanks a lot!

[pompeiisneaks]

You'd need to know your amps requirements, since you just called it an '18 watt amp' we have no visibility into the number of tubes or expected current draw etc.

Here's a good article to help understand choke sizing:

http://www.aikenamps.com/index.php/chokes-explained

~Phil

[sluckey]

Hoffman uses the Fender 125C3A choke on his stout. Choke is located between the plate and screen nodes. Here's the link...

https://el34world.com/Hoffman/files/Hoffman_Stout.pdf

[tubeswell]

Its an old thread for sure.

Chokes make more of a noticeable difference if used as a whole-of-supply filter in SE amps. In PP amps, hum noise tends to get cancelled out in the opposite sides of the OT primary. (But you can use chokes in PP amps to eliminate ripple hum with virtually no loss in supply voltage - and that's where they are an advantage over resistors, which always will drop more supply voltage than a choke).

[Mr. Lime]

You'd need to know your amps requirements, since you just called it an '18 watt amp' we have no visibility into the number of tubes or expected current draw etc.

Here's a good article to help understand choke sizing:

http://www.aikenamps.com/index.php/chokes-explained

~Phil

Thanks, I've already read Aiken's article and that's why I would choose a Hammond 158M for example.
Inductance: 10 H
DC Current: 100 mA
Resistance: 195 Ohm
Max. DC: 400 V

Hoffman uses the Fender 125C3A choke on his stout. Choke is located between the plate and screen nodes. Here's the link...

https://el34world.com/Hoffman/files/Hoffman_Stout.pdf

Thanks for the schematic. This one looks good!

Its an old thread for sure.

Chokes make more of a noticeable difference if used as a whole-of-supply filter in SE amps. In PP amps, hum noise tends to get cancelled out in the opposite sides of the OT primary. (But you can use chokes in PP amps to eliminate ripple hum with virtually no loss in supply voltage - and that's where they are an advantage over resistors, which always will drop more supply voltage than a choke).

A stiffer power supply and less voltage drop would be fine for my amp!
I have to draw a whole schematic of my amp the next days..

B+.jpg

That's my power supply.

[M Fowler]

Look at the Matchless Lightning Reverb choke placement.

Mark

Matchless Lightening 15 Reverb.gif

[pdf64]

As the amp currently has a 100 ohm HT dropper to the screen grid node, won't replacing it with a ~200 ohm choke result in the amp being saggier rather than stiffer?

[M Fowler]

My experience with Marshall 18w amps is they sound better with slightly higher voltage on the plates and no sag, to my ears.