Choke/R switch

[tictac]

I use a 3way switch for standby [on/off/on] wired as follows:
choke and 330 ohm resistor to B+ source; other end of choke to one "on" terminal of the switch; other end of the resistor to the other "on" terminal.
Center Terminal to B+ load side.

The middle position of the switch is Standby; toggle one way will be choke power supply; other direction = Robbin Ford Mod.

If you wire this way you can switch with power on and it will not pop.

Another way to look at it: In a Dumble or Fender Twin Reverb PS, The switch goes AFTER the choke and BEFORE the 2nd filter/screen B+ node.

This is the best standby setup I've ever used; it really does change the feel of the amp. I'm liking the RF mod more than I thought I would.

TT

[dehughes]

OH NOW THAT'S COOOOOOL! Man, what a great idea. Perhaps I'm the last person to find out about this, but heck yeah I'm doing that on my next build.

What amp is this installed in, tictac? Do you think it would be a worthwhile mod in other amps?

[tictac]

I've got it installed in my ODS 100W clone. The nice thing about this mod is you dont need to drill an extra hole if you already have a standby switch; I haven't tried this in other amps but there's no reason you couldnt use it on any type of amplifier.

The resistor smooths out the way the amp reacts to your pick attack; the choke gives you a tighter, more focused reaction; both are useful.

The key is wiring it up correctly; if you put the switch on the B+ source side of the choke you will get a big surge and a thump when the switch is used.

[iknowjohnny]

I can't do this after all because the bias changes too much between the R and the choke. I could use a DPDT and add in a R in the bias circuit in one side of the switch. But then i'm beginning to think i like the choke much better anyways.

[dehughes]

I've got it installed in my ODS 100W clone. The nice thing about this mod is you dont need to drill an extra hole if you already have a standby switch; I haven't tried this in other amps but there's no reason you couldnt use it on any type of amplifier.

The resistor smooths out the way the amp reacts to your pick attack; the choke gives you a tighter, more focused reaction; both are useful.

The key is wiring it up correctly; if you put the switch on the B+ source side of the choke you will get a big surge and a thump when the switch is used.

Very cool. Might this work for a 6G6-B Bassman-styled amp?

Also, do you have a diagram that shows how you have it wired up (just so I can make sure I'm getting it correctly...)?

Thanks!

[tictac]

This is the general idea: I think I updated the drawing to reflect the correct switch position; the first time I installed the switch I was getting a thump; I reversed the position of the switch & choke/resistor junction & it worked fine.

[Andy Le Blanc]

the point is to switch the B+ off until the device is sufficiently heated to
prevent cathod stripping during the warm up period...... you must do both
switch plate volts and select choke/resistor......

[dehughes]

Cool. That makes sense. So does it matter that, according to this schematic, the plates of the power tubes would still have B+ on them? That is, would this still be a proper standby solution?

[roberto]

Andy, I've read about many hifi gurus prefer a stand-by bypassed with a eg 100K resistor, so caps can start slowly charging while heaters warms up. I think this was mostly to reduce PT stress on charging big capacitors.

[Andy Le Blanc]

earlier in the thread theres a scheme that does both...... but the later is ment
to have a reduced POP...... or transient..... which would be fine for a "sag"
on the fly...... but it wouldnt function as a "stand by"..... theres a lot of options
for a stand by function....... the AC legs or the ground of the power supply
the cathode return of the power tubes ..... ive seen some projects with timed
relays...... the pop can be reduced with a resistor..... not good to hammer
parts with current...... I do like the idea of a standby with an added function
and in guitar amp the standby pop almost seems traditional....
with a tube rectifier and cathode bias the start up is softer and a standby is
less important.....
Ive seen one Idea that looked really good.... use a rectifier with a indirectly
heated cathode as the "switch" on the ground side of the PT or in series with
the B+..... its always a soft start..... as the fil. heats..... put the switch on the
tubes heater...... no pop....

[Andy Le Blanc]

I think its from a M. Kornacker (GA 3/00, p.70) based on a P. Becker also from
"Glass Audio" (GA 4/94, p.20)...... looks good and very simple....

[dehughes]

Before I go and buy a resistor for my amp and wire up a choke/R switch, I have a question:

The amps that most people are doing this 330 ohm/choke RF mod in are ODS amps, yes? These are running quartets of 6L6s, or something like that... So, if the 330 ohm resistor is switchable with the choke and doesn't necessitate a bias adjustment when switched in place of the choke, then would this 330 value be the "correct" value to use in a Blonde Basssman amp running 5881s? That is, should I raise or lower the 330 value to account for a pair of power tubes as opposed to the quad as in the ODS amps? I don't want to go source and purchase nice, aluminum housed 25w resistors only to find the value is incorrect for my uses. And yes, I know this requires some measure of guessing, but I'd prefer as educated a guess as I can manage.

[Andy Le Blanc]

your changing the B supply for everything but the plates of the power tubes
I'd guess anything from twice the DC resistance of the choke to maybe 3x or 4x
whats the DC resistances of the chokes used as examples so far in the thread?......

[dehughes]

Thanks Andy. I was going to actually put the resistor in parallel with the choke (switchable), so it would change voltages for the plates of the power tubes as well as the rest of the amp. What do you recommend in that regard? Think 6G6-B Bassman circuit... My instincts say 330 ohm would be fine, but I'd like to be sure...

[Andy Le Blanc]

switchable..... as either/or ......... or both parallel?
I think you mean putting B+ tap after the whole assembly so it functions as a
standby....... that would make sence as a sag option ... if your not relying on
the choke for its smoothing action of ripple in the supply then paralleling the
resistor and choke makes sence too......

measure the DC resistance of the choke and choose....... so that you know
what resistances your inserting before the powersupply filter.....

if the option is going to be used inpart for its smoothing action or as part of
the supply filter I think a parallel choke/resistor would increase the ripple present
to a degree...... and the value of resistance might need to be larger than you
think if ripple is a major worry... anyway you choose.... mind the bias point
so you can compensate if the bias jumps past maximum dissipation...