Tube boost + overdrive running off a 9 volt battery

[tasos]

Good for the 9AU7, altough it won't give you huge differences...I used one in the past just for fun.

yeah the scratchy pot is intended in this ckt, it comes from varyng the value of the resistance at the cathode...

I think someone should try a parallel capacitor....
100pf probably?I boxed upped mine so too late!
could someone try that please?

[ayayay!]

I suppose I could do that.  See you later tonight...

(Y'know tasos, it would be super easy to just put a socket on the gain lugs...)

[iccaros]

I built this last night.  Normal original schem provided by Dano:  12AU7, 9vdc supply.   Started tweaking a little.  I think my PS was only supplying 150mA, which is probably why it couldn't quite reach unity gain.  I'll be addressing that shortly.  

My question is:  Is anyone getting a bit of static sounding "wool" when turning the gain knob?  Yes, this is a very simple circuit, but I was hoping I could clear that up.  Of course, it was late and I didn't measure voltage from pin 3 to ground.  Still, this crackle is not okay...

I picked up a 9AU7 to try tonight.   

A parallel Cap would not eliminate DC on the pot so that may not work, and depending on the value would change tone.

you can change the design to get ride of the DC noise on the pot, if you Bias the cathode with between 2 - 10K (inplace of VR1) , and place the pot between the tirode stages  after the cap, so replace R4 with a 500K audio pot

I would go to 12V..but that limits battery  use unless you go to 18V

[ayayay!]

Cool, thanks Steve!

[frequencycentral]

you can change the design to get ride of the DC noise on the pot, if you Bias the cathode with between 2 - 10K (inplace of VR1) , and place the pot between the tirode stages  after the cap, so replace R4 with a 500K audio pot

+1 on that, so I introduce you to the Valvemaster:



- interstage gain pot - no more crackle - like a 'real' tube circuit.  
- proper cathode biasing of both triodes - like a 'real' tube circuit.  

Would be worth playing around with the values of R2 and R3, as well as the (nominal) 1K and 10uF cathode biasing. Quite why it's taken 131 pages to get rid of the crackle and get some proper biasing......

[Bill Mountain]

Great post!

I've been doing this for a while on my starved plate fender/marshall style builds.  I haven't read the thread in about 30 pages so I assumed it had been covered by now.  I should check it more often.

[Renegadrian]

I'd put 500k or 1M as the interstage volume. Also it would be nice to swap the "regular" but pretty useless tone control with a princeton-like tone...I have to try!!!

also I'd swap the input cap with a 22k-33k res. (after the 1M to ground) as in a tube amp (no cap, yes resistor)

[juansolo]

Go back a few pages Rick, something very similar

Last one I built looked something like this

[zambo]

i think the input cap is required because without it you get grid conducting and it makes crackly dc  volume pot on your guitar. It also tames a little bass which is nice cause starved plate tubes tend to sound a bit farty sometimes. I have been building normal amps mostly now, but i still mess with starved plate tube preamps to see if I can make that "magic" low voltage one. Results are just ok so far in the high gain dept.

[Dongle]

Hi guys!

I just tried to put the valvecaster in Eagle. In the next days, I will try to make a PCB out of this. As I am new to this, it will take me days... So it would be great, if you could check the schematic first.
I wanted to save money and space - so I exchanged the first stage of the twincaster with an opamp. The second half of the opamp is used for an output buffer.
I also put the footswitch (3pdt) and the LED on the schematic - just for having it complete... Not sure, if I will put them also on the PCB.
In addition, I added the charge pump. So I can switch between 12 V and apprx. 40 V.

Some of the resistors I put as trimmers, so I can adjust the value for tuning later, without soldering too much.

Again: It would be great, if some of you can check it... It will be time consuming for me to design the PCB and expensive to print...

Thanks!!

[iccaros]

its looks ok.. I would put 2m resistors on my first op-amp input.. but that is me..

are those Zener's in your voltage multiplier?

[Dongle]

Thanks for checking!
I agree, it should be 1 or 2 M resistors at the input..
And yes, I once heard that Zeners are more effective than normal ones in the charge pump... Not sure, if this is correct.
I started yesterday with the PCB - is there anything, I should look at? I think, its the most important thing, to keep the length of the signal path short?!?
Anything else?

Thanks again!

[frequencycentral]

And yes, I once heard that Zeners are more effective than normal ones in the charge pump... Not sure, if this is correct.

Geofex: "1N400x diodes work and are cheap, but the losses are higher than they really need to be. For higher performance and lower losses, it's better to use something like the 1N5817 schottky diodes for low losses. But both will work." http://www.geofex.com/circuits/+9_to_33.htm

[rutabaga bob]

Gentlemen...sorry to jump in late, but we have indeed had the 'interstage pot like a tube amp' discussion before, in the postings regarding Adriano's 'Tube Star'.  In fact, he made two or three different layouts with the gain pot implemented different ways.    Larry S.

[Renegadrian]

Yeah Larry, thx for reminding that and for all the support! 

[iccaros]

And yes, I once heard that Zeners are more effective than normal ones in the charge pump... Not sure, if this is correct.

Geofex: "1N400x diodes work and are cheap, but the losses are higher than they really need to be. For higher performance and lower losses, it's better to use something like the 1N5817 schottky diodes for low losses. But both will work." http://www.geofex.com/circuits/+9_to_33.htm

I was looking for that quote. Since Zeners are made to allow voltage to flow backward (know.. bad wording I am tired) I think that they could be a problem once you hit a stage that exceeds the reverse turn on voltage, not sure if that would happen here but I would think about it.  Schottky diodes drop less voltage so when you do the math on the stages you have 2x voltage - diodes, so you end up with a higher voltage with the Schottky

The rest is fine, Tubes make simple designs, its why they are popular, and we sell the some tone dripping dream to kids learning guitar   

[Dongle]

Right... I got mixed up here. So it should not be Zener diodes but Schottky...

Anyway, before I destroy something or make it wrong, I will just place the same there as in my last charge pump... I think, they were probably also Schottky...

Yes, just checked! It was the 1N5817!

Btw.: What do you think about the use of the opamp as gain control? Its maybe not so "pure" anymore - but it should sound? And work?

[Bill Mountain]

Right... I got mixed up here. So it should not be Zener diodes but Schottky...

Anyway, before I destroy something or make it wrong, I will just place the same there as in my last charge pump... I think, they were probably also Schottky...

Yes, just checked! It was the 1N5817!

Btw.: What do you think about the use of the opamp as gain control? Its maybe not so "pure" anymore - but it should sound? And work?

Go ahead.  A lot of tube pedals do this.  Check out the Shakatube.

[iccaros]

Right... I got mixed up here. So it should not be Zener diodes but Schottky...

Anyway, before I destroy something or make it wrong, I will just place the same there as in my last charge pump... I think, they were probably also Schottky...

Yes, just checked! It was the 1N5817!

Btw.: What do you think about the use of the opamp as gain control? Its maybe not so "pure" anymore - but it should sound? And work?

Go ahead.  A lot of tube pedals do this.  Check out the Shakatube.

just to add, I think the opamp as a gain control is better than a pot on the cathode.
, and not re-bias the tube as the original is doing, better is the pot between stages as that is more like a "Tube" amp.

If you plan this on going to a tubeamp or even another pedal you may want some adjustment on the output, I have found that my Valvecaster overloads a unity gain opamp on the output, but then I do have  a 6au6 added to mine 

[Dongle]

just to add, I think the opamp as a gain control is better than a pot on the cathode.
, and not re-bias the tube as the original is doing, better is the pot between stages as that is more like a "Tube" amp.

If you plan this on going to a tubeamp or even another pedal you may want some adjustment on the output, I have found that my Valvecaster overloads a unity gain opamp on the output, but then I do have  a 6au6 added to mine 

Thanks  -I dont completely get this...
I am going to a tube amp - but thats why I piut the buffer at the end. Does this not help?

And what does "re-bias" mean? What should I change here?