all choked up

[andrew]

Hello, I have a project that calls for a 470 ohm resistor after the first power cap. I would like to use one of my chokes but I don't know which one to use. One is a 5H - 135 ohm and the other is a 30H - 595 ohm. The PT is a Toneslut using the 300v taps. Any advise? Thanks!

[David Root]

Depends on the DC volts rating of the choke. If you're using a SS rectifier you're looking at about 425VDC max into the choke, no load.

You're probably OK with either one, given the choke resistances you have.

[David Root]

sorry, double post.

[roberto]

What kind of amp are you building? What is the supply configuration and voltages?

[tubeswell]

The choke's current rating is the most critical. It has to be able to handle the combined peak current draw of the amplifying tubes. So a rating of 2 x the sum-total idle current is usually a safe bet.

In most power supplies where the choke is part of a CLC filter between the OT supply and the rest of the amp, then 90mA (or even 50mA is fine), because it is only supplying the screens of the output tubes and the plates of the pre-amp tubes. (You still should work out what the total current draw will be at this point, because not all pre-amps are equal)

But if the choke is to be used in a CLC filter between the rectifier and the rest of the amp (including the OT supply node) i.e.; where is is supplying the plates of the power tubes as well, or if it is used in a choke input filter, a considerably higher current rating is necessary.

[andrew]

It is an Express style A7 as found in the Trainwreck Files section. The 30H is 40mA and the 5H is a 75mA. The schematic calls for a 470ohm in the place of the usual 1K 25 watt resistor. Another factor is that I am using KT-66s.

[roberto]

5H is the right one.

[andrew]

The 5H only has a resistance of 135 ohms and the schematic calls for 470 ohms resistance. Is that close enough or should I add a resistor to make up the difference? Thanks!

[David Root]

KT66s should each have their own 5K6 screen resistor. At least 2W. Regardless of the choke's 135 ohm resistance. Some folks think that is too much, there can be an audible difference, but KT66s aren't $5 each any more!

In the old days when tubes were cheap 470/1W was better than nothing, but these days folks generally use at least 1K, usually a 5W sandbox, on 6L6 and bigger tubes.

[tubeswell]

The 5H only has a resistance of 135 ohms and the schematic calls for 470 ohms resistance. Is that close enough or should I add a resistor to make up the difference? Thanks!

You can try a single 270-300R resistor from the output filter cap to both screens and see how you like it. (This 'compresses' the grid curves/lines a wee bit, but keeps them 'up' so that the load line passes through or above the knee). But it shouldn't matter if the screens are the same voltage as the plates or a few V higher (within reason), because the screens are so small relative to the plates that nearly all of the electrons just whizz past the screen anyway.

Or you can wire in a switchable CLC/CRC filter. (I tried this on a 5E3 last year, and I used a DPDT switch, so that when running on the CLC filter, I wired a shared 4k7 5W resistor from the output cap to the screens - this dropped the screens by about 20V when in 'choke mode')

[roberto]

Well, if you change the 470R with a choke, you want to change the feel of the amp, so reduce the compression at high volumes. So keep the choke as is. Then if you want you can put a 270R in series with the choke.

[andrew]

Thanks all, I'll use the choke and a 5k screen resistor.

[roberto]

well, try it also without the 5k screen res. The 5k screen res makes the amp sort like "dull". Novosibir used this screen trick as a sort of power attenuation, but now he doesn't.

[img:800:533]http://www.larry-amplification.de/briti ... pur091.jpg[/img]

[andrew]

I just now finished wiring it up and on the KT66 grids I get 383v and on the screens I get 383v. I used a 135r (choke) and a 250r in series in the place of the usual 1K. I put a 500r on each grid of the KT66. Do the voltages look safe?

[andrew]

Sorry- 386 on the grid and 383 on the screens