"PentaBoost" - 12 volt Pentode Booster using Submini 5672 tube

[sweetwilly]

I tried it, but as Rick said, very subtle if anything.  Haven't tried it on the turbo version or on the Cold War Drive yet, but don't know what if any effect that'll have on it.

[frequencycentral]

Fresh from my "Murder One" amp, experiments I still have the MAX1044 circuit on my breadboard, so I thought I'd see what happens to the PentaBoost with 70 volts running through its veins.

This is how to do it:

- Apply 70 volts from the MAX cap/diode array to pin 1 of the 5672 via the 100K resistor.

- Leave pin 2 just as it is, connected to 12 volts (or 9 volts).

The boost from 70 volts is MASSIVE. That's using twelve diodes and caps. The caps get pretty big physically when rated for 70 volts. And that's a lot more components too.

However:

The boost from just three doubling stages is excellent too, just not quite as massive. And that's only six caps/diodes - and the caps can be very physically small when dealing with these voltages.

Just thought I'd contribute this idea - not sure if it's any use to anyone, but well worth breadboarding if you have the parts about anyway.

For details of the MAX circuit, see my "Murder One" thread, or go to 'Circuit Sweepings' at GEOFEX.

[svstee]

Worked up a vero layout for this based on my experiences with using resistors in place of the lm317 on the most excellent Red Star Drive. runs best at 9v, you might have to play with R1's value a bit to use higher voltage.
Not verified, please let me know if there are any problems, And a big thanks to frequencycentral for all his work with subminis.


Thanks!

[frequencycentral]

Worked up a vero layout for this based on my experiences with using resistors in place of the lm317 on the most excellent Red Star Drive. runs best at 9v, you might have to play with R1's value a bit to use higher voltage.
Not verified, please let me know if there are any problems, And a big thanks to frequencycentral for all his work with subminis.

Hi, looks like it should work - but C1 should be 0.047 (47n) not 0.0047. Here's a cap conversion chart: http://www.justradios.com/uFnFpF.html

You are aware that C1 and C2 are polarised electrolytic? You would normally use the polarised cap symbol for them.

You could route R1 from d2 to a2, save yourself a jumper and a track cut.

Probably not good practice to have C3 over R3 - but I've done worse in the past! I assume you have a cut at b7.

You should get a bit moore boost by changing the pot for a higher value - 250K or 500K.

Good luck!

Rick

[svstee]

Thanks much. Revisions are on the way!

[svstee]

Here it is:

Thanks for all the help, man. And to clear up a few things:
C2 and C3 are multilayer film caps, I use them on everything from 0.1 uf to 2.2 uf. beter tolerances than electrolytic, smaller, not polarity sensitive, and cheaper.
Why is it not a good idea to have C3 over R3? I do have a trace cut under them.
Just to let everyone know, R1 does get warm, so giving it a bit of room is a good idea.

Edit: Just built it with this layout, works great, sounds awesome! Adds a bit of high end and shimmer and a lot of volume. Can be run off a 9v battery, but it sucks a lot of power. Thanks for the schematic, this is a great circuit.

[frequencycentral]

Why is it not a good idea to have C3 over R3? I do have a trace cut under them.

Nothing actually wrong with doing that electronically, just not prefered practice to have one component on top of another on a board - I guess because it could make debugging or changing out a part more difficult. I would probably have run R3 from c3 to a3 to avoid having C3 on top of it. Like I said, I've done worse when struggling for space, and it works so don't lose sleep over it.

Just built it with this layout, works great, sounds awesome! Adds a bit of high end and shimmer and a lot of volume. Can be run off a 9v battery, but it sucks a lot of power. Thanks for the schematic, this is a great circuit.

Hey glad it worked out and that you like it! Enjoy! 

[svstee]

Hey, what is the best way to deal with microphonics? it wasn't an issue for me when the boost was just a circuit board with juice and jacks attached but when I boxed her up they became loud and nasty. Tried double sided tape, foam insulation, and a combination of the two with poor results.

[frequencycentral]

Hey, what is the best way to deal with microphonics? it wasn't an issue for me when the boost was just a circuit board with juice and jacks attached but when I boxed her up they became loud and nasty. Tried double sided tape, foam insulation, and a combination of the two with poor results.

Yes they can be microphonic little buggers at times - some are worse than others. The filament in these tubes is long and spring-like, which doesn't help. There is some discussion of how to combat microphonics earlier in this thread. What I do is make sure the tubes are not touching anything and pack a little foam around them - it cuts the problem down to size.

As noted earlier in this thread, the tubes can be quite microphonic, so take that into account when planning the enclosure - you want something to dampen the stomp.  Most of the "mechanical" noise seems to transfer through the leads of the tube on mine, so not a great deal can be done there, but foam padding around and under the tube (you can glue a small piece onto the board under where the tube will lie) will make a big difference.

[svstee]

If the foam touched the walls of the enclosure and the tube I had the same problems. What about using a foaming glue to hold the leads in place, cuuting of the microphonics from that end?

[svstee]

The who enclosure is hypersensitive, just touching the top makes things get microphonic, and stomping the switch is out of the question. I think my problem is the direct mounting of the pot onto the stripboard, the whole thing is acting like a spring.

[frequencycentral]

The who enclosure is hypersensitive, just touching the top makes things get microphonic, and stomping the switch is out of the question. I think my problem is the direct mounting of the pot onto the stripboard, the whole thing is acting like a spring.

Hmm, dont know what to suggest. My Red Star Drive isn't microphonic at all, and the pots and tube are mounted on the circuit board. My Pentadriver was a little microphonic - just the first tube, not the second - sorted with foam. My Murder One isn't at all either. Maybe the ultimate protection would be to shock mount the circuit board within foam (with wires between the board and pots) so that there is not direct contact between board and enclosure. seems a little extreme though. Out of interest, what brand are your 5672's? I have Valvo (German), Raytheon (USA) and CEI (French) - I prefer the Raytheon.

Did you get your Pentadriver working yet?

[svstee]

Maybe a rubber washer on the knob might work. would glue on the underside of the circuit board cause problems?

And yeah, the pentadriver is mostly working, gain pot is backwards though, and doesn't give much volume boost. I think I had a bad battery.

[frequencycentral]

Maybe a rubber washer on the knob might work. would glue on the underside of the circuit board cause problems?

And yeah, the pentadriver is mostly working, gain pot is backwards though, and doesn't give much volume boost. I think I had a bad battery.

Yeah the washer should help. Glue would be fine as long as its not conductive. I'm amazed that you even run the Pentadriver from battery. A low battery would cause the heater to be starved - all sorts of unpredictable behaiour could ensue! The circuit draws 100ma +, so usable battery life isnt going to be long either.

[petemoore]

  Tubes are known for having various microphonic tendancies, one tube more than the other, so to swap.

[svstee]

 

  Tubes are known for having various microphonic tendancies, one tube more than the other, so to swap.

I would, but this tube was fine before I boxed it up, so I don't think that's it. I built the circuit with just in/out jacks and it wasn't microphonic at al.
On another note, would longer leads make the problem better or worse?

[~arph]

The filament in these tubes is long and spring-like, which doesn't help.

Anyone tried running the guitarsignal through the filament? 

[petemoore]

The who enclosure is hypersensitive, just touching the top makes things get microphonic, and stomping the switch is out of the question. I think my problem is the direct mounting of the pot onto the stripboard, the whole thing is acting like a spring.
  Tubes always make a little noise, sometimes more.
  "For car use" tends to mean "may be subjected to intense or prolonged vibration"..to what degree shock endurance was designed into them of course is different than suggesting the design took into accout that vibration might occur...I guess these are intended to be used where vibrations may be present.
  Tankin' on the tube lightly with a chopstick, bumping it a touch, well...experienced tube bumpers know what to listen for...a sound, but with very little resonance or sustain, once you've heard an old rattley tube...remember that sound...it'll probably be different with the next tube you hear rattling.
  The tube noise of the
  How 'used' is the tube?
  Between shock damping the tube and shocking it with a little vibration [not that I haven't had some funky wire make funny noises when tube-tapping...and think it was the tube...] sorting out what type of noises the tube makes as a 'source', decide whether that tube is the one.
  Increasing and decreasing the vibrations a 'hot tube is subjected to...it isn't as easy as flipping a switch or turning a knob, but ~ has about the same range of effect sometimes.
  On a similar note my 'AT@38' ~valvecaster makes 'nosey' sounds and the olifactory of the tube absolutely can be heard if my foot even gets near the box it seems. Oh yea, microphonic...
  I have the tubes waistline in a 3x3'' ring/clamp dampener, and doesn't jump into whining until the guitar gets close...and that's when the gains up...and I keep the gain down for a less 'thunky' tone.
  And it sounds just great and I don't mess with much it other than exacting more deliberate, accurate stomponit moves than my Chip boxes, mostly it just stays on, and I play with that through a tube amp that is the smallest I've yet to complete [2w+]...very nice to have super sweet tones @ sweet volume...Otherwise the thing is quite stable and low noise floor-O.

[esdiezy28]

Hi All! I just got in some 6088 sub mini's yesterday, very similar to 5672 but lower power output, and I'd like to share that the Penta Boost is VERIFIED with a 6088 tube, run at 9V. Although I'm quite confident that it will work at 12V as well, but my test board's power supply is variable only up to 9V.

[svstee]

Petemoore, I think you are onto something, because it got WAY worse with the amp distorted. I was running it through an Ampeg J-12T reissue, which breaks up pretty quick. I'll run it through the clean channel on my Randall and see if it is still there. Problem is, it sounds really good pushing existing amp distortion to the next level.