18 Watt Layout Choke Placement

[Ed_Chambley]

Hi Guys, I am roughly using this layout for a build.  I plan to add a choke.  Where is the best place to add it.   I am using a Dual cap can as in this layout.  Could I add it from c1 to the center tap of the ot and if so, should I change the position of the 1k5/5w resistor between the c1 and c2?

[sluckey]

Just remove the 1K5/5W and connect the choke where the resistor was connected.

[dude]

In this amp, what would the differences in tone be from the 1K5 and the choke...?

Assuming voltages where the same...

Thanks,
al

[stratele52]

Slukey , I'm not sure of that . I will chekc later i should go.  Ed do you have schematic  ? easier to work with that . layout is only usefull to build .

[sluckey]

Slukey , I'm not sure of that

Why not? Doing it the way I suggested would put the choke between the plate node and the screen node, just like 95% of the amps with a choke are done.

[stratele52]

Sluckey , you could be right there are so many desing, but if I look at the Marshall 50 watts Master Volume Model 2204 , Power supply is the same desing.
They put a wire in place of ( the 18 watts)  1K5 ohms and put the choke just after those two capacitors.

The 1K5 and the 8K5 ohms 3 watts and the 2K2, 2 watts ( on B+ line of the 18 watts amp )  are replace by three 10 K ohms on 2204 amp. All after the choke.

Choke have a lower resistance than the 1K5 ohms.
If you simply replace de 1K5 by the choke, the  voltage will be higher on all preamp tubes. This does'nt mean the amp won't work fine . Voltage desing can vary.

As a say slukey ,the amp could work with your suggestion , Ed should take some mesurements before and after he put the choke and do what he want .

[sluckey]

but if I look at the Marshall 50 watts Master Volume Model 2204 , Power supply is the same desing.
They put a wire in place of ( the 18 watts)  1K5 ohms and put the choke just after those two capacitors.

A wire between two caps? I'd just say parallel caps.

Read what I said again and look again at the 2204... Those first two caps are in parallel. Just think one big cap. Now look at the choke and you'll see it's exactly where I suggested Ed put his... between the plate node and the screen node. Sure B+ will increase depending on the choke's DCR. But it will have less ripple.

The high-end 2204 is a big fixed bias amplifier and it uses a stiff B+ rail with lots of filtering. If you look closely you'll see the 2204 uses double 50s at all 3 B+ nodes. OTOH, the puny cathode biased 18 Watt uses 32, 32, and 16µF for it's 3 B+ nodes. A much looser sound. It's also a cheap bottom-of-the-line amp. So, even though the B+ rail looks similar (don't they all?") they are two totally different animals.

I don't think an 18 Watt Marshall amp needs a choke, but if I was gonna put a choke in my 18 Watt based amps, I'd put it exactly where I suggested above. And I wouldn't be a bit concerned about the small increase in screen voltage.

[stratele52]

I agree with you Steve,

I don't use the word "parallel capacitor" to be shure Ed understand I 'll replace the 1K5 resistor by a wire

[Ed_Chambley]

If I had a schematic, I would surely post it.  I am not building the amp as an 18 watter.  I am using a 50/50 cap.  The amp is going to be a EL-34 Cathode power section using the Matchless Chifton Power Section using separate caps and resistors for the screens.  All of the filter caps on the board are 22uf/475. I have yet to power it up, but am getting close.

This parallel comment has confused me.  

I was assuming I would just place the choke between the dual cap and test the voltages in the preamp and adjust the dropping resistors accordingly.

EDIT... fixed that transparent gif... sluckey

[sluckey]

I was assuming I would just place the choke between the dual cap

What I say?

[Ed_Chambley]

I was assuming I would just place the choke between the dual cap

What I say?

That IS what you said. 

[sluckey]

Yes. But then I got sidelined by strato and lost focus. Sorry for the confusion.

[stratele52]

Yes. But then I got sidelined by strato and lost focus. Sorry for the confusion.

strato ?

[sluckey]

slukey? 

I think you and I are just not communicating clearly. I do that a lot.

[Ed_Chambley]

The only difference I can see is the choke has a resistance of 420ohms so I will be losing about 1ohm resistance.  Since the PT is rated slightly more than a stock 18 watter it will probably be necessary to increase the 8k3.

It was asked what adding a choke would do to an 18 watter.  I have no idea as I have never built an amp without one.  I d know they decrease HT ripple which yields a quiet or more quiet amp.

[Fresh_Start]

The only difference I can see is the choke has a resistance of 420ohms so I will be losing about 1ohm resistance.  Since the PT is rated slightly more than a stock 18 watter it will probably be necessary to increase the 8k3.

It was asked what adding a choke would do to an 18 watter.  I have no idea as I have never built an amp without one.  I d know they decrease HT ripple which yields a quiet or more quiet amp.

Won't a choke make the power supply for the screen grids a bit stiffer?  Perhaps make the amp a tiny bit less touch responsive?

If you're worried about voltage drop, you can always put a resistor in series with the choke.  But I probably wouldn't bother.

HTH

Chip