Robben Ford Switch?

[tictac]

I've been thinking of a modified standby switch that would allow you to have either the "Robben Ford Mod" in the power supply or the normal choke pi filter.

What do you think?

[Bob-I]

Looks good to me. I've been thinking of the same idea.

[Luthierwnc]

Looks fine. Shrink-tube those throw positions twice if they are near anything conductive.

Skip

[Normster]

I'm hoping to get enough sag out of my 5AR4.

BTW, my build is postponed this weekend. My daughter went into labor this morning. I'm headed back to the hospital to wait it out. Later!

[d95err]

This looks very strange. In the Sag mode, the HT supply goes through the sag resistor AND the choke. Can your choke really handle the current? Also, the screen supply will not pass through the choke, so it will be quite badly filtered. Is this really your intention?

[bluefireamps]

That's not going to do what you want, I don't think. You would have to attach +HT to the first filter cap section, change S1 to a regular SPST switch and add another SPST between the rectifiers and the first filter cap section for your standby. You may also want to upgrade the resistor from 10 watts to 25 watts. It'll get pretty warm.
Dave

[Luthierwnc]

The way I have done mine, the screen supply goes through either the choke or the resistor, not both. Right now I am liking the resistor side better. I haven't noticed any filtration problems on either side. If I were to use them in series, I'd probably go through the choke first. sh

[tictac]

My Bad, try this...

The 10W value is what I've heard Dumble was using, youre not drawing current from the plates and the screens have their own resistance which will reduce the current draw on the 10W as well. 10W seems plenty big to me.

[d95err]

What is it you are trying to achieve? The only difference I can see between the resistor and choke modes is that the resistor one will have more hum (worse filtering).

If you want to simulate a sagging tube rectifier, you need the HT to drop, so the resistor needs to be before the HT supply.

[dogears]

Try it and listen.... There is quite a tangible tonal difference, with the resistor mod having a chewier midrange, slightly subdued highs, and more textured low end. Most I know prefer no choke. I'd say try between 330ohm and 500ohm, no lower. Fwiw, check out a Trainwreck sometime....

What is it you are trying to achieve? The only difference I can see between the resistor and choke modes is that the resistor one will have more hum (worse filtering).

If you want to simulate a sagging tube rectifier, you need the HT to drop, so the resistor needs to be before the HT supply.

[d95err]

Try it and listen.... There is quite a tangible tonal difference, with the resistor mod having a chewier midrange, slightly subdued highs, and more textured low end. Most I know prefer no choke. I'd say try between 330ohm and 500ohm, no lower. Fwiw, check out a Trainwreck sometime....

Interesting. I'll try it out someday. I thought the idea was to emulate a tube rectifier. Thanks for explaining.

[bluefireamps]

tictac,
10 watts will work just fine. I suggested 25 watts to help reduce the heat level. Anything helps. You might still want to add another switch for your standby rather than leaving the power tubes sort of "on" all the time, but your second schematic should work fine!
Dave

[tictac]

The standby switch arrangement Jim Kelly amps use is a switch that disconnects the screens from the B+ like I'm doing here. The power tubes won't be drawing any current without the screens connected so it should work fine as a standby.

I prefer this type of standby over the cathode lift as the latter causes the B+ to rise and could place some voltage stress on caps. I'm not aware of any downside to keeping voltage on the plates and 1st stage filter caps; to my knowledge it's good to keep the filter caps powered up as much as possible because it keeps them formed. If you know something I'm missing here I'm all ears.

[bluefireamps]

tictac,
There's really no benefit to keeping the caps powered up. Why would you want just the first set to be formed? Really it doesn't matter. Regular amp use will keep the caps formed no problem. Even using it a couple of times a year will acomplish that. Keeping the +HT caps connected will still cause the voltage to rise on them since you've disconnected the rest of the power supply and load. That's not a problem if your caps have enough voltage rating, but I'm sure you know that. There's nothing "wrong" or "bad" about your setup. Please don't get the impression I'm ragging on you. I'm not. Let us know how it works out and how it sounds!

[Bob-I]

Looks good to me. I've been thinking of the same idea.

Wait... I see a problem. The B+ only comes from the choke side. Follow the current with the switch set to the resistor, the current will go through the resistor, then through the choke and finally to the B+, not good as you'll overload the choke.

You'll need a DPDT switch so that the B+ can be switched to either the choke side or the resistor side.