Rocket choke question

[hans-jörg]

Hello,
I wonder about the choke in the schem at Trainwreck files.
There is a 15H 25mA shown in the schemata.
The usualy used is a Hammond 185L (15H 75mA 400 Ohm) or similar.
Question: is this 15H 25mA not too low from current side? 25mA!
Or only a copy paste mistake?

Best

Hans-Jörg

[tsutt]

I looked at bom i have it says Hammond 158l 15 h 75ma.

Which brings me to my? what is the difference in say A 4h or a 15h what effect is this going to have i have been looking for several days on this have not found any info other than what a Henry is but how does that fit into the equation.

Todd

[M Fowler]

I've come to consider the DCR and mA ratings first then range out the Henrys example anything from 5-15H will work. If Hammond doesn't have exact I go close.

[overtone]

For a Rocket:
At maximum signal the EL84 data sheets show that the 25mA is not adequate, even though it is downstream of B+1.
At idle 25mA would be cutting it fine.

Regarding the choice of Henrys, my understanding is once you start going over 10H on a guitar amp, it may result in a too "stiff" response.
Too low and it is less effective in the reduction of ripple.
So for us it is a balancing act.
I can see the ripple on PSU designer,
but for stiffness you have to suck and see.

Did I just write something naughty?

[tsutt]

So it's more magical mystery vodooas with apparently everything else in these things.

Todd

[overtone]

not to my mind, voodoo is too passive.
It depends really on what you want.
The choice range from 4H to 15H gives you a chance to tailor the response of the amp in line with your concept.
But I stand to be corrected.

Best, tony

[tsutt]

Haha, 4- 15 doenst tell me anything having only done a couple amps was just asking how effects things that apparently cant be qauntified other than preferred tone. which is completely subjective ie the vodoo. so the only way to get an answer is to have several different chokes and change them out. right? just like every other part in an amp.

cheers
Todd

[overtone]

yes, that is what I mean by suck it and see.
That is what I do. But I'm useless on the tone subject, I have no idea what all the discussion is about. I'm looking for response in an amp.

Best, t

[Tillydog]

...so the only way to get an answer is to have several different chokes and change them out. right?

The reason the choke is there is to smooth the B+ voltage (in conjunction with the reservior capacitors and/or additional resistors).

A bit like reservoir capacitors, the actual value isn't massively critical, as long as there's "enough" and not "too much".

If you want to play, you can download Duncan's PSU designer and experiment with the effect of all the components on ripple voltage and source impedance:

http://www.duncanamps.com/psud2/index.html

(A high source impedance will tell you that the b+ voltage will be 'saggy'; a low source impedance that it will be 'stiff')

The trick is relating this to aspects of the way an amp feels or sounds - modelling some 'known' designs can provide an insight.

[tsutt]

Thanks Tillydog i will do that.

Todd

[hans-jörg]

Thanks too.

That means that the 25mA are right on the edge? Better 75mA.
BTW the 25er has aroung 1k resistance and the 75er ca. 400 Ohm. As higher the mA as lower the resistance (20H, ca. 180mA, 100 Ohm) with the Hammonds. Only around the 15H range!

Hans-Jörg

[Roe]

a traditional ac30 choke will measure around 20h, 400ohms. it doesn't sound or feel stiff

[C Moore]

For a Rocket:
At maximum signal the EL84 data sheets show that the 25mA is not adequate, even though it is downstream of B+1.
At idle 25mA would be cutting it fine.

I guess I am a bit confused by this (surprise surprise).
I thought that (and always seem to read the same) a choke that is not part of the Power Tube B+, only has to handle current for the PI and whatever is used by the pre tubes.
Thanks

[hans-jörg]

Hello,
the choke in the Rocket is between the anode and the screens of power tubes. So its a part of the EL84 not of the ecc83 of PI.

Best

Hans-Jörg

[overtone]

It varies from circuit to circuit.
The screens of the power tubes on the Rocket are taken off the power supply after the choke.

So you need to consider four EL84 screens.
At idle that is 4 x 5.5mA = 22mA + the PI and Preamp current draw.
At maximum signal the screen current will shoot up to about 40mA in total. Hit your E7 chord and, on paper at least, there will be smoke and silence.

Going madly over the top with the mA rating is probably not a good idea either.
It is in the nature of chokes being non linear WRT the current pulled through them. Swings and roundabouts.
Hope this helps.

Best, tony