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

[frequencycentral]

I just started playing with these 5672 submini pentodes on the breadboard tonight, basically to check that they work. I bought four - they were manufactured in 1958! They are tiny, and are completely dwarfed by my 6111 submini dual triodes.

They require a 1.25 volt heater voltage (at only 50ma), I used a LM317 to provide that.

When I hooked up the heater voltage I expected to see a glowing tube..............but...........nothing.

So I rigged up a simple test circuit to check out if the tube was actually functioning - it was!

All of a sudden I was overdriving my VJ clone to saturation - basically what I usually use an LPB to do.

Here's the circuit, the clean boost is about 10db - and very tubey:

[mth5044]

Weird, I was just about too search for something along these lines. Can't wait to see other responces. Very nice 

[Renegadrian]

Rick, I got a space charge 12bl6 tube - got to take out something from that tube...cheap and easy to find on the bay...
still I love to see how pentodes can be used!!!Gotta learn from your circuit! 

[frequencycentral]

Rick, I got a space charge 12bl6 tube - got to take out something from that tube...cheap and easy to find on the bay...
still I love to see how pentodes can be used!!!Gotta learn from your circuit! 

Didn't you buy a submini pentode a while back? I remember you mentioned it but I can't remember it's name (began with a 5 though) - maybe if you check the pinout against the 5672 you could use it in this circuit.

[Renegadrian]

Yes Sir, I got 5678 and 6205.

[frequencycentral]

Yes Sir, I got 5678 and 6205.

I don't think the 5678 is a pin for pin replacement for the 5672 (the specs are similar though). The heater pins are the opposite way round. In both tubes Grid #3 is connected to the filament pins. You should be able to work something close out though........

[ambulancevoice]

have you tried other pentodes???
just to see what they sound like

[frequencycentral]

have you tried other pentodes???
just to see what they sound like

5672 are the only ones I have right now........

..........and for sure I will be cascading them - for a "PentaDrive" !!

[Steben]

AFAIK,

pentodes have similar response curves to MOSfets (harmonic contents), more logarithmic, where triodes are square root.
Yet I think they still clip softer than any semiconductor.

[~arph]

..the odds,

I just received 10 of these tuesday... I expected glowing too, none of that but they do work!
I'll try your heater supply, I was having troubles with it..

Regards,

a

EDIT:  btw, I allready figured out that these tubes do not have enough gain to do a PSO (for an all tube vibracaster)

[frequencycentral]

..the odds,

I just received 10 of these tuesday... I expected glowing too, none of that but they do work!
I'll try your heater supply, I was having troubles with it..

10 you say - maybe cascade the lot!!

[~arph]

(read again)   

[frequencycentral]

I realised that the 100k resistor from pin 3 of the pentode to earth is completely pointless - don't know how that got there! So i have removed it from the schematic.

Rick

[sweetwilly]

1st attempt at a layout using Bancika's layout creator.  Apart from the LM317t needing a heatsink and the extra space that that will require does this look ok?



  I'm working on a layout with both the 5672 and pot on board to cut wiring (thanks Rick!).

[Renegadrian]

Spent this saturday morning breadboarding, I tried the Pentaboost with the submini pentode I have, the 5678.
It works, and the 5678 is a good sub(mini)substitute.
I am powering the heaters with a AAA battery. It should be easy to wire the neg. to the input jack to cut it out when not connected, as all the other battery  powered pedals...
As for the valvy I am surprised to get a good sound out of a stupidly simple circuit...That's the beauty of DIY...
Rick, as I am looking for more gain, should I try the Pentadriver?

[frequencycentral]

Spent this saturday morning breadboarding, I tried the Pentaboost with the submini pentode I have, the 5678.
It works, and the 5678 is a good sub(mini)substitute.
I am powering the heaters with a AAA battery. It should be easy to wire the neg. to the input jack to cut it out when not connected, as all the other battery  powered pedals...
As for the valvy I am surprised to get a good sound out of a stupidly simple circuit...That's the beauty of DIY...
Rick, as I am looking for more gain, should I try the Pentadriver?

Hi Adriano - pleased you have success with that. I thought it might work with a 5678 too.

More gain you say? Metal head!!   

I think you would be pleased with a PentaDriver. Maybe I should do some A/B/C soundclips of the Valvecaster, Pepper Shredder and PentaDriver.

You should maybe try your PentaBoost in front of your Valvecaster - or you Valvecaster in front of the Pentaboost. Could be interesting!

Adriano - you're a Veroboard man (I don't use Vero) - could you verify sweeywilly's layout - or suggest your own layout?

EDIT: I think if should be possible to mount both the tube and the pot on the Veroboard.

[Renegadrian]

More gain you say? Metal head!!   

YEAH!!!

Adriano - you're a Veroboard man (I don't use Vero) - could you verify sweeywilly's layout - or suggest your own layout?

Well, I breadboarded right from the vero layout posted by Willy...It seems correct all the way, so yep, I'd say

VERIFIED

Maybe he wired the VOL backwards? Usually is OUTPUT-->pin 3 and pin 1-->ground. Just flip the two wires.

[sweetwilly]

Yup, more than likely.  Seem to have a habit of reversing pot wiring.  Pin outs - fine, but pots?!??!?!

Thanks for the verification on the layout Adriano!

Rick, I'll incorporate at least the tube on the board in next version!

[frequencycentral]

Just a bump to publish the final schematic. I've been deleting and re-uploading the schematic in Photobucket when I have made changes - but it occurs to me that 'cookies' might prevent the viewing unless you click 'refresh'.

[mth5044]

soundclips of this bad boy yet?