choke instead of resistor

[phsyconoodler]

What are the advantages of a choke instead of the resistor?Will it tighten up the bass response enough to make it worth while?I know it works on many other amps,but was wondering why it's not used on many D style amps?

[Luthierwnc]

I didn't notice much difference in the low end. The main change is that the resistor has a little more grain or rasp to it. The choke stays cleaner longer. I had a switch in the back to alternate. It is a fairly simple adaptation. You see resistors a lot on these amps. Do a search on the Robben Ford mod for more.

I've also used a HD DPDT on-off-on switch on the standby. For standby, run the rectifier to the pole. Tie the throws together and send that to the OT, choke, resistor and first cap. The center position is standby. On the other side of the switch, tie the output of the choke and resistor to either throw and send the pole to the next cap and screens. There are probably lots of other ways to do it.

This is one of the easier mods that can make a useful difference in tone. I used a 330R aluminum resistor bolted to the chassis. It saves the need for garolite and it has a built-in heatsink.

FWIW, Skip

[Structo]

I'm still re-learing tube amp theory.
Isn't the main purpose of a choke in a power supply to smooth the ripple even more after the filter caps?
So this choke resistor is just a voltage dropping resistor or since it is wire wound does it have inductance properties?

An inductor opposes rapid current change?
But I'm learning that power supply components can have an effect on tone as well.
Some guys say slap whatever in the PS while others are more discerning about the amount of capacitance and reactance.

[Trace]

What are the advantages of a choke instead of the resistor? Will it tighten up the bass response enough to make it worth while? I know it works on many other amps, but was wondering why it's not used on many D style amps?

A choke offers better power supply regulation (some what similar to a voltage regulator) and while it allows the voltage to pass one way it tends to restrict voltage from going the opposite way where as a resistor offers less regulation.

Regarding bass response; Comparatively a choke will give you a tighter bass response and the over all response or attack of the amp is more immediate. The general effect depends on the value and how the choke is made (all chokes are not created equal). Try a Mercury Magnetics MC10H choke as I think you will find it to your liking.

Sag; For those looking for more sag or "give" you can install a 100ohm resistor in the B+ line to simulate the effects of a tube rectifier (a chassis mount power resistor is recommended). You can vary the value depending on how much sag or give you want. If you have a favorite type of tube rectifier you can look up the internal resistance on the spec-sheet and select a value to match.

Tighter response; Going the opposite way, you can install a diode in series with the B+ line on either side of the phase inverters B+ line and this will also tighten things up noticeably. This tends to work well in amps that use 5Y3 rectifiers, amps that loosen up considerably when cranked, etc and is cost effective (make sure the diode is properly rated). So long as it is properly rated you can use a switch to short out the diode, thus making it a selectible option.

Installing a switch to A/B the difference between a choke and a resistor is not an apples to apples comparison. To be fair one should document the voltages prior to installing the choke. Having done so you will want o also mirror the voltages on the phase inverter as well as the rest of the preamp as they tend to change/vary after installing the choke, how much so depends on the value and manufacturer of the choke.


Hope it helps
Trace

[Trace]

I'm still re-learing tube amp theory. Isn't the main purpose of a choke in a power supply to smooth the ripple even more after the filter caps?

Essentially, yes.

So this choke resistor is just a voltage dropping resistor or since it is wire wound does it have inductance properties? An inductor opposes rapid current change?

A resistor is not an inductor therefore it will (edit) not (end edit) give you the same results as as an inductor.

But I'm learning that power supply components can have an effect on tone as well.

A more factual to look at this is that psu components not only impact the tone but they also have a direct impact on the feel of an amp.

Some guys say slap whatever in the PS while others are more discerning about the amount of capacitance and reactance.

Tone is subjective and what I can tell you is that those who say "slap whatever in the PS" are either inexperienced and/or they cannot hear or feel the difference.


For what it's worth
Trace

[phsyconoodler]

Thanks guys,
I understand what a choke does,I was just kind of curious why most D'Lites and a lot of builds don't use them here.I guess I knew that it would tighten things up on the bottom,but this circuit does things differently than most other amps so I was curious.
By the way structo,a choke would RAISE the voltages because it replaces a resistor.So it would make the amp cleaner sounding on the clean channel.One might have to adjust things to the preamp a little.It helps with power supply ripple too.
I think I'll just have to put one in and see what it sounds like.

[Bob-I]

Thanks guys,
I understand what a choke does,I was just kind of curious why most D'Lites and a lot of builds don't use them here.

The D'Lite goal was to produce a Dumble inspired amp kit at a price many folks could afford. $10 here, $2 there brought the kit to a reasonable price plus the resistor is a common mod in real Dumbles so why not.

Personally I like the cleans better with the choke and OD better with a resistor. Also, the difference is fairly subtle and tends to be more obvious as volume goes up.

[heisthl]

It is overkill to use a chassis mounted aluminum body resistor here. Ok it does look good, but the reality is it drops so little voltage that a 1 watt will usually work and a 5 watt is well over the 20% tolerance factor. I like the 390 ohm value here and have never seen it drop more than 7 volts(it's usually dropping 4 volts). But even at 7 volts the power is 7 x 7/390 = .126 watts

[Trace]

It is overkill to use a chassis mounted aluminum body resistor here. Ok it does look good, but the reality is it drops so little voltage that a 1 watt will usually work and a 5 watt is well over the 20% tolerance factor. I like the 390 ohm value here and have never seen it drop more than 7 volts(it's usually dropping 4 volts). But even at 7 volts the power is 7 x 7/390 = .126 watts

Assuming you are referring to using a resistor in place of a choke; It's also worth considering the over all current draw when cranking an amp to higher levels. I have seen a number of 1 watters short/burn open and the common thread is that players were cranking the amps up to higher levels. Generally speaking; If you crank an amp up for 5 minutes and you will find that a 5 watt resistor will be very hot to the touch. In my humble opinion it's better to go over spec verses under-spec.


For what it's worth
Trace

[heisthl]

I agree completely on NOT using a 1 watt and I never have or would but I think the 5 watt has enough of a margin to be acceptable. Does a 5 watt get warm - yes, does it get detrimentaly hot - no. Hot is when it makes the white cement block types turn yellow and stink. Max current would be something like 22ma total for screens and 8ma(or is it less?) per 12AX7 so still not much. I always run a 1 amp safety fuse on the secondary of the P.T. and I've never had an amp blow one from running full out and that includes the current of the O.T./ output tubes. Don't misunderstand I think the aluminum one looks cool and is a good idea but is not a "must" item and is rarely implemented. Commercial amp companies use 5,7 or 10 watt unsinked resistors there depending on the manufacturer and even the 5 watt ones don't run hot enough to discolor the PCB underneath unless there's a problem down the line. Now cathode bias is a great place to use them and is commonly seen. I must be in a beligerent mood today to even be talking about this....sorry

[Structo]

A resistor is not an inductor therefore it will give you the same results as as an inductor.

I'm not sure I understand that statement.

[Luthierwnc]

As far as the aluminum housed resistors go, I just use the 10 or 15 watters. It is a little bit overkill but the fact that they can be put almost anywhere in a crowded chassis gives me some options. One drawback is that the mounting holes are too small for even a 4/40 bolt. Generally I tap them for a 4/40 bolt and tighten it from the outside.

I got into the habit by making switchable fixed or cathode bias amps.

sh

[Trace]

even the 5 watt ones don't run hot enough to discolor the PCB underneath unless there's a problem down the line.

When a power tube shorts it can put a strain on the screen grid resistors and generally speaking it's not a bad to go with 5 watt screen grids (a little extra insurance never hurts).


For what it's worth
Trace

[Trace]

I'm not sure I understand that statement.

Sorry it was a type-o



Trace