Russian mini tubes

[Eb7+9]

You guys should also check out the Nuvistor!
These were used in some old Ampex preamps. They’re pretty cool.

... keep an eye on the last generation tube-based 200Mhz Tektronik scopes ... the front-end preamps where all nuvistor based and built on Silver plated chassis with space age materials used for damping ... fun to turn these into phono preamps ... the Nuvistors used there were hand picked so you can bet they had better than average microphonic specs ...

[@ndy]

Is it ok to increase the anode resistor to get the voltage at the anode down? Because with the 47K resistor (see above), the output at the anode measures around 13V (with 18V supply voltage), doesn't that seem a little bit too high? I replaced the resistor with a pot and found that the voltage drops to 9V with a resistance of 300k. Is this approach ok?

I think plate resistor has to be defined by the maximum allowed plate current for each particular tube. When the tube is fully "open" the current (defined by plate resistor according to Mr. Ohm law) shouldn't exceed allowed maximum. Why do you try to decrease the plate voltage? I think it's not a problem if you have 13V with 18V power supply.

When building an overdrive pedal with these tubes, I don't think two stages have enough gain for some serious distortion, maybe putting a JFET gain stage in front of it would be a good idea? Something like this:

Jfet stage -> Tube Stage -> Volume Pot (Gain) -> Tube Stage -> Volume Pot

IMHO, number of tube stages depends on the sound you want. Even two stages provide enough gain for some serious signal clipping, but personaly I don't like the sound in this case: it's too cracky. I prefer more stages when each stage just slightly overdrive the sound, adding mostly even harmonics. After each stage there should be tone forming circuit and resistor divider decreasing the amplitude of the signal to avoid next stage overclipping.

The preamp structure you offered seems to be OK, but the best way to find the result is an experiment, I think  

[spongebob]

Even two stages provide enough gain for some serious signal clipping, but personaly I don't like the sound in this case: it's too cracky. I prefer more stages when each stage just slightly overdrive the sound, adding mostly even harmonics. After each stage there should be tone forming circuit and resistor divider decreasing the amplitude of the signal to avoid next stage overclipping.

That's an interesting approach, how many stages would you suggest? I guess 4 should be enough?

I tested the two pentode stage idea this weekend, it consists of a JFET gain stage driving two pentodes, with a volume "gain" control between the two pentodes and a low pass filter + volume control at the output. The JFET produces a high enough output signal to overdrive the first tube slightly, the amount of distortion produced by the second tube can be controlled with the potentiometer between the two tubes. It doesn't sound too bad, but nothing spectacular either. I still have to try the pentodes in triode mode next...

Here's a picture of the board:

[Peter Snowberg]

That looks great Spongebob!

How is the clean tone?

Take care,
-Peter

[@ndy]

Nice picture Spongebob!

Personally I didn't try to overdrive pentodes in preamp, only power ones in output stages. All I said in previous topic concerns triodes (like 12AX7). But such small tubes like 1sh18b may be excellent to make tube preamp in small stompbox envelope  

Andrei

[@ndy]

2 Spongebob

how many stages would you suggest? I guess 4 should be enough?

Look at most tube preamps schematics (like MESA, Marshall etc). There are 3-5 stages in most cases. It depends on tubes type, the sound you want to get and other things.

As to me, I haven't made any solid state FX box yet (maybe later   ). Tubes are amazing!

[@ndy]

Yesterday night I have made 1 stage with 1sh29b (similar to Spongebob's but in triode mode) and tested it. A kind of usual tube sound, nothing special. One thing disappoints me: the tube is "microphoning" even when I put my combo to a low volume. It seems the only advantage of 1sh29b is low plate/heater voltages (and small size surely)  :cry:

[zjokka]

hi all,

this thread seemed to have dried out on a negative note. maybe not all minitubes are so unusuable? came across some 1sh18 1sh17 and 1sh24s. Is anybody still experimenting with this?

Should be possible to put them to some use. There's this hammond tech that has developped a preamp based on these tubes for transistorbased hammonds.
http://www.keyboardpartner.de/hammond/t-mod-skill4.htm

will try out spongebob designs anyway, but later this week.
zj

[rockgardenlove]

Wow, this is certainly an ancient thread...quite a resurrection you've managed.

Cool stuff though for sure.

[zachary vex]

submini tubes aren't good for anything! 

8^P

[Meanderthal]

 I once read an article about the possibility of using nano-tubes in an IC. Very interesting idea! They were talking about processor chips though, not audio... but imagine the possibility of an all tube opamp... in an 8 pin DIP(or SMT) package!