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

[frequencycentral]

Well, what can I say, that really makes a lot of difference! actually usable now at 9v.
Thanks, I had seen the same kind of arrangement too with a j201 somewhere (I believe on tim escobedo's site)

Yeah, nice isnt it? I'm just not technical enough to have worked it out myself - so all thanks to Jasper Oosthoek for his great work. Jasper has done the math on this, and has come to the conclusion that the the transistor addition gives the circuit an output equivalent if the circuit had been run at 22 volts, here's a qoute from one of his posts on the AX84 thread:

"If you're wondering how this system works:

In the old schematic the anode voltage I measured was about 3.5 volts and the (AC) load resistance was 50k. (The 100k load resistor parallel with the volume pot). The input signal modulates the current in the tube by a certain amount: Irms. This get's converted to a voltage by the AC load resistance to a voltage equal to Irms * Rac. So if the Rac increases, the output will increase too.

The transistor is a trick to raise the anode voltage and load resistance without changing the supply voltage. Raising the anode voltage will create more gain too because it raises the "Transconductance" which is the ratio between the input voltage and Irms.
The (AC) load resistance in your new schematic will be 100k (only the volume pot). The load resistance of the transistor is much higher and can therefore be neglected. Also the anode voltage can be raised to 9 volts (with 130uA). This limits the maximum voltage swing to about 2 volts before it becomes dangerous. Still that's MUCH higher than the old circuit.

This means the output is identical to the original circuit working on 22 volts (9 + 130uA*100k). And that's without a volume pot that steals 6dB (because the AC load resistance is 100k not 50k)."

[mth5044]

hey stereovoid thats pretty cool, but I got two questions

1) Did you just glue of those rocks down?
2) how does your chicken head knob rotate paste the top of the lighthouse? not the pointer, but the other end looks like it would get stuck. I guess if you have it up high enough..

[sweetwilly]

Yeah, sorry Sterevoid, forgot to say that's a sweet looking pedal!

[~arph]

The PNP power does seem to  have a slight drawback..

I tested it wtih a 1spot.. worked fine. Then I used a reasonably fresh battery

Here are the results:

1spot   9.4V @ 61mA
batt     8.8V  @ 72mA

Different voltages (or should that rather be currents) into the PNP's base means rebiasing the base current
So, do we need a regulated powersupply for this or is there a way around.

What struck me most though is that with the battery the gain was MUCH higher than with the 1spot..   

One other thing I noticed (this was not involved in the test above) is the 100k and 1uF on the grid plate.. (pin 2) If you remove both and just stick a 100R resistor from pin 2 to 9V you get a lot more headroom. (it breaks up a lot later)

[frequencycentral]

I just tried all the different PNP transistors I have and I can report that they ALL work really well.

Here's what I tried:

Si: BC212, BC557C

Ge: 2n2000, 2n404, SFT353, GT403

A couple of the Ge I tried didn't work so well - leakage? But at least one of each type did work fine

The silicons both required the trimpot to be about 68/32, so a voltage divider ratio of 15:1 approx, so 68k / 1M should be close enough.

The germaniums all required the trimpot to be 30/70, a voltage divider ratio of 35:1 approx, so 27K / 1M should be close enough.

In terms of 'mojo' sonic difference between the Si and Ge, there was none. I guess because the transistor is not in the audio path.

In terms of 'mojo' visual difference I obviously be using the big fat black metal can 2n404!

[sweetwilly]

and you'll be installing a teeny weeny plexi window and a backlighting LED and all I guess?   

[frequencycentral]

The PNP power does seem to  have a slight drawback..

I tested it wtih a 1spot.. worked fine. Then I used a reasonably fresh battery

Here are the results:

1spot   9.4V @ 61mA
batt     8.8V  @ 72mA

Different voltages (or should that rather be currents) into the PNP's base means rebiasing the base current
So, do we need a regulated powersupply for this or is there a way around.

I guess it's a case of finding the 'sweet spot' for whatever power supply you are using, so it might be best to leave the trimmer in rather than replace it with fixed resistors.

From what you say, the downside of battery powering this circuits is that as the battery dies the sweet spot wil be lost. Hmm.

What struck me most though is that with the battery the gain was MUCH higher than with the 1spot..   

Weird - I have no idea why.....

One other thing I noticed (this was not involved in the test above) is the 100k and 1uF on the grid plate.. (pin 2) If you remove both and just stick a 100R resistor from pin 2 to 9V you get a lot more headroom. (it breaks up a lot later)

I just tried this but only got lower gain. 

[~arph]

I will continue my experiments.

But so far battery powering doesn't seem like a good idea, perhaps if you use a 7V zener as a regulator, but then you are lowering the voltage again..

[stereovoid]

"hey stereovoid thats pretty cool, but I got two questions

1) Did you just glue of those rocks down?
2) how does your chicken head knob rotate paste the top of the lighthouse? not the pointer, but the other end looks like it would get stuck. I guess if you have it up high enough.."

cheers, pretty chuffed with it! yep just glued them down with gel super glue and they all hold fine! might put a couple of coats of clear lacker over to give them shine! the knob just clears the stones, i left a 3mm gap and the highest stone is 2mm, so all cool! going to make a little battery box to go along the top as i screwed up measurements with the jacks and can't get one in!

[frequencycentral]

I've just doing been doing a bit of tweaking - change the volume control to a 470K for a tad more boost.

[forsakenrider]

I have built two of these now using 6088's. One is stand alone running off two 9v in series (but im going to try the pnp mod to see how it sounds off a single 9v) and the other is in my valvecaster wired infront of it with a "boost" stomp switch.

for me the 6088's are CRAZY microphonic. like, yikes. if you tap it you get a DINGGGGGGGG for atleast a minute... and playing loud with a little floor rumbling it doesnt stop. do the tubes you use have the same issue? or have you found a way to minimize it?

also, for my valvecaster-pentaboost i have an extra hole to fill with a switch or a pot. ive tried the "squish", and noticed noting really, even with a few different caps. ive tried using it as a volume for the boost, but it seems pointless. right now its wired for cliping like the Red Star Drive, but im noticing minimal differences. The only thing i can think of putting in there would be another tone, but that seems boring. Any ideas?

[frequencycentral]

The 5672 are a little microphonic - they certainly don't DING for a minute though. I've put foam around mine which mostly cures it.

[stereovoid]

same for me with the 5672's. slight chime if you flick them but nothing like when your amp goes microphonic! i raised mine off the vero board with some round cork stickers (you can get them from focus) and held the valve in place with a plastic tie clip. did the job for me!

[forsakenrider]

Ugh, its really not working out at 9volts!!! i wish it would, as that the only easy way i can lend it out! but with the PNP mod it nneeds re-biasing as the voltage drops. hmmm

Im curious as to how the valvecaster works at 9v (so people have said) yet this little guy doesnt seem to handle it. or maybe its just me.

Has anyone had success with a 9v bat??

[frequencycentral]

Ugh, its really not working out at 9volts!!! i wish it would, as that the only easy way i can lend it out! but with the PNP mod it nneeds re-biasing as the voltage drops. hmmm

Im curious as to how the valvecaster works at 9v (so people have said) yet this little guy doesnt seem to handle it. or maybe its just me.

Has anyone had success with a 9v bat??

I built my first Valvecaster as a 9 volt battery only device. I soon replaced it with a 12 volt wall wart only device. Much happier.

With tubes the simple answer is more volts = more gain/boost.

As you point out, the Pentaboost plus PNP turbo isnt going to work well with batteries due to the baising issue.

Other options to explore (without the PNP boost) would be deriving 18 volts from two 9 volt batteries in series, or using a 9 volt battery together with a voltage doubler circuit such as MAX1044/LT1054/LMC7660 - I would guess good things could result from that, you could get the plate voltage up quite high!

[forsakenrider]

Yeah, Ive already done the 18v.... But after 18 with the PNP mod.... I dont think i can go back to the 100ohm resistor
I guess Its back to regulated 12v.


A voltage doubler or trippler is a good idea! too bad ill have to order a special IC for that. Maybe i will try that though.

[kurtlives]

Thought it was in this thread....

Rick you posted a schem from an idea at AX84...something about this simple compression control. Don't remember much about it but the pot connected to the cathode haha.

Know what I am talking about?

[frequencycentral]

I think that might be the 'squish' idea which was discussed in the Red Star Drive thread. I haven't had the opportunity to experiment with that yet.

[kurtlives]

edit

Guess that control would only work with pentodes and not triodes...

[frequencycentral]

edit

Guess that control would only work with pentodes and not triodes...

Yup - correct.

I seem to remember someone tried it and reported it to b 'very subtle'. Maybe it works better at high voltage.