Choke/R switch

[iknowjohnny]

can anyone find any issues with putting the choke and a resistor on a switch in order to be able to choose between a choke filter and a resistor? i'm finding i like the tone with a resistor a lot, tho the choke too has it's assets. so i wanna make it switchable. any reason not to? seems it would be just like switching the standby out for a split second as the switch changes, so i don't see why not. But because i never see anyone doing this i just wanted to be sure.

[Wayne]

Never seen that before, either. The two things that come immediately to mind are:

- use a good quality switch - you're switching B+, after all.

- might make a helluva POP when you switch it.

W

[benoit]

Would you want make before break for this application?

[markr14850]

You could try doing it on the standby switch as is shown here:
https://tubeamparchive.com/files/bm_dcl ... 12_171.pdf

[iknowjohnny]

I'm not worried about saving space or adding a switch, so the standby idea isn't something i'd do. But it does show that i CAN do this w/o issues since that amp has it. Thanks.

[Luthierwnc]

Works great. Look under the Robben "Ford mod" on the Dumble Discussion. You'll find several opinions on the resistor value. I use a 330R -- usually an aluminum housed 10 watter. That isn't so much for the wattage capacity as that you can mount it almost anywhere. Depending on the layout, you can also have a separate switch a bit more out-of-the-way than the standby. Either way, use a stout switch.

Skip

[Structo]

I don't think I would flip that switch while the amp is on.

Probably be a hell of a arc on the switch.

Putting it into standby before switching would probably wise.

[iknowjohnny]

Thanks. A few more Q's..... which one do you guys with switchable choke/R prefer? And which would feel less like a choke, a 1k or a 330 ohm? How low a resistance can you use?
(amp is 2xEL34's with a 300-300 PT and about 430 on the plates and 1k screen R's)

[Jana]

If it was me I would stick a 2k/ 25watt resistor in there. If you're going to go for a change, go for it big! The thing is, the bias requirement will probably change between the choke, which is probably about 100 to 150 ohms and the resistor. So, I would experiment, see where the amp biases at with the choke and see where it biases at with the 2k resistor. Maybe it is such that you can set the bias just on the edge for one setting and then when you go to the other you are still within the parameters you want.

to answer your questions, a 1K is going to feel less like a choke.

How low can you go? As low as you want. The lower you go, the less filtering effect for the stages downstream however.

[iknowjohnny]

I see. Well, i wanted to use a smaller one to make the bias stay close enough which is why i mentioned the 330. I now have a 1k there and the bias changes enough to be safe, but enough to where i'd want to use a DPDT and have one side add a R in the bias circuit to keep it even for tone reasons. But rather than do that i figured a 330 would leave the bias close enough not to worry about doing it that way. But if the 330 is going to bring me closer to the sound/feel of the choke then i guess i'll stick with the 1k and do the bias switch thing, because otherwise why bother if the difference isn't enough to matter.

[Luthierwnc]

Structo;

One side of the switch is the standby. The other side routs the B+ to either the choke or the resistor. The middle position is the standby so you do disconnect the HV when you go to the outer throws. It pops a little but I've only done this on amps with bleed resistors on the first node. sh

[Structo]

Thanks for that.

[roberto]

In some builds I've used a similar arrangment:
a switch that inserts a resistor in series with the choke. This let the amp sag more if you need it, but let the filtering effect of choke as is.

[Zippy]

A resistor in the range of 250-500 ohms will do the deed. More resistance will increase sag as current increases due to dynamic loading. The inductance - opposition to a change in current - will be eliminated and you may "feel" a change in the amp.

[iknowjohnny]

So far i've tried a 1k, 270R and 100R, and i couldn't really say for sure whether there was any difference between them. But if it wasn't placebo effect, it seemed like the 270R was best.

I tried a sag R at the beginning of the B+ in the past and didn't like it. But i like it like this, presumably because it's AFTER the power tubes and therefore affects only the PI and pre. So i guess i like the sag in the pre but not the PA. as to using a R AND the choke, what would the purpose be? So far the amp is dead quiet no matter what i've tried, so if noise would be the only reason then i don't need a choke at all. I have yet to try all the different R's and the choke by itself in a listening test, but i plan to when i have the amp where i can crank it up a bit. So far at lower volumes it's looking like i have no reason to use a choke at all.