choke

[Lindz]

is a 150ma choke sufficient for 5 12ax7's and 2 el34's (or perhaps 2 kt66's)?

[Colossal]

Lindz,

Are you choking the entire power supply or just the screens and power amp? If you a talking about a choke for the screens and preamp, only about 40mA is necessary but to do the whole amp, I'd give yourself a good margin over the total current the amp will be pulling at peak.

[Lindz]

Thanks for the reply Colossal

I am still kicking around my design.. I am not sure of the advantage or disadvantage of choking the whole supply or just preamp/screens..? I am looking to build a Plexi/JCM800ish build with 2 independent preamp channels with individual tone stacks and a few other tweaks

using the dreamtone PS calculator (http://www.dreamtone.org/Calculate_Current_Form.htm) and a 350/350 PT voltage I get 146ma with el34's and 172ma with kt66's

So assuming I run el34's would 150ma be sufficient for choke (and PT)?

Seems pretty tight though I do not understand the theory well enough to make an educated call - i.e. is there is much headroom in typical PT/choke specs (Classictone trannies are the supplied specs) and how much current really gets pulled during typical operation.

If this seems a bit tight do people have suggestions for other JCM800ish 50w parts with a little more current headroom? Edcor has several that fit the bill but has longer lead time than I would like

[martin manning]

if you are doing a 50W Marshall and you want that sound, then I'd keep the same power supply topology and put the choke after the plate supply node.

A couple of extra 12AX7's means little to the HT supply- another 4-6 mA if they are 12AX7's, but you'll need to have another 0.6A for the filaments. Adding up all the filament current you need 2x1.5 + 5x0.3 = 4.5A. Seems like a Magnetic Components PT and choke meant for a 50W Marshall will cover it.

[Cantplay]

What would be the effect of too much choke?

John

[roberto]

A stiffer sound. You can solve in part adding a 50-150 ohm resistor in series with the choke, but it's better a 60mA 3-5H choke for a 50W. Also take a look at NFB.

[Lindz]

thanks folks..

I always appreciate the input

My concern with the MC PT is that it specs 150ma and I come up with 147ma for 2 el34's and 5 12ax7's. Is that enough headroom for reliability long term?

[Super_Reverb]

When you look at commercial designs, it seems cost is the biggest reason manufacturers didn't LC filter the plate supply on power stages. So, when you connect B+ to CT on OT, 120 Hz ripple is a common mode voltage and effectively gets removed by differential effect of primary with power applied to CT.


IMO, 200mA PS current rating is preferable for 2xEL-34s or KT-66s.

rob

[roberto]

I personally don't care about the reason why they decided to use CLC filters on early guitar amps, but that is the tone we consider THE tone, also if it's not perfect technically.

[Colossal]

IMO, 200mA PS current rating is preferable for 2xEL-34s or KT-66s.

Good call. Good design should provide a margin of safety above the max current for the application and especially if the whole amp is to be choke filtered. You have to take voltage spikes on startup and any other possible stresses into account as the amp is used and parts age.