Why no choke in tube preamp pedals?

[Vorg]

Hello everybody, this is my first post here so I guess I should point out immediately that I'm a total noob in amp building, in fact that's the reason why I registered to this wonderful board: a huge learning curve I'm definitely willing to climb!

Coming to the question: I'm seeing more and more tube preamp pedals hitting the market. I love this solution, 'think this format is eager to find a lot of space in countless rigs; I also see some of these pedals are rather high-gain but I never see choke transformers implied.

As far as my knowledge goes, chokes are particularly relevant as to ripple filtering duties and perform much better (in terms of playing feel, if I'm allowed to say so) than an array of caps and resistors; especially high-gain circuits shall benefit from introducing such component.

I can see mounting a choke in a pedal would make them quite clumsy, in terms of size, but I wonder if there couldn't be smaller choke designs made purposely for pedal footprints and, in general, how do today's manufacturers deal with this: is a choke effectively that relevant to the extent of the preamp section in an amp? Would tone and feel change that much without it in a preamp pedal? Which viable alternatives are currently implied?

Thanks to those who'll chime in!

[chaccmgr]

Hi Vorg,

chokes are used to filter ripple in the power supply which only should occur in AC supplies. When talkin about pedals you usually have a DC supply which comes already filtered (best case, there are bad DC supplies...).

In tube pedals you usually have a normal low voltage DC supply which gets stepped up by a voltage pump inside the pedal to the DC voltages required for the tubes, say 180V ish HT.

There is also a heater voltage required. As most tube have a quite wide tolerance wrt voltage, a 12V DC supply is ok for heating (series connected for a dual tube).

Thats why you don't need a choke in pedals.

[Stevem]

Please answer these 3 questions about what you want more out of a tube stomp box by stating a percentage.

1) tone change?

2) gain increases?

3) feel as in you can just look at a string and it plays itself almost?

[sluckey]

A tube preamp is a very light current load on a B+ supply. And that light load is very constant. The main characteristic of a choke is to oppose changes in current. Since there is little if any change in current in a preamp, there's really not much for a choke to do. With light loads a simple CRC filter using big caps and big resistor will produce a very smooth DC output. No need to waste money and space for a useless choke in this case.

[Vorg]

Hi Vorg,

chokes are used to filter ripple in the power supply which only should occur in AC supplies. When talkin about pedals you usually have a DC supply which comes already filtered (best case, there are bad DC supplies...).

In tube pedals you usually have a normal low voltage DC supply which gets stepped up by a voltage pump inside the pedal to the DC voltages required for the tubes, say 180V ish HT.

There is also a heater voltage required. As most tube have a quite wide tolerance wrt voltage, a 12V DC supply is ok for heating (series connected for a dual tube).

Thats why you don't need a choke in pedals.

Thanks a ton, that's super clear and super helpful!

@Stevem... I'm not quite sure as to what your comment is aimed at, besides, I personally couldn't give any percentage to the 3 alternatives you suggested since I'd use a tube preamp pedal to go straight into a p.a., hence it'd "make" the tone rather than change an already existent one.

[cdemike]

Interesting idea. As others have pointed out, the preamp tubes won't likely draw enough current to let a choke do much in terms of power supply sag. Another way of getting there, though, is to draw a lot of current relative to a power transformer's size. One way to do that is to use voltage multipliers (though you'd need a separate 120v supply apart from the usual 9/18v pedal supplies. If, for example, you used something like the 7v Antek AN-0207, you'd have about 6.5VAC (loaded) for the heater supply, and you could put the secondaries in series for ~13VAC. If you used about 10 Cockcroft-Walton multipliers, that'd give ~180V assuming zero losses to ESR, so my guess is that you'd get something significantly lower than that for a B+ rail, but still something you could run tubes off of. Because the transformer is only rated for a certain output in terms of watts, i.e. P=IV, and V is much higher, your power supply will be much more sensitive to changes in current, meaning you'd be able to get power supply sag in a preamp pedal.

Just spitballing -- haven't tried this.