"Valvecaster Head" - How to turn your Valvecaster into a Practice Amp

[biggy boy]

Hi guys
frequency central that's cool!
I'm going to give this a try once the twincaster is  finished.
I'll use the twincaster as the preamp.
I was wondering about the impedance rating of the matching transformer (fender reverb) it's 2800/1
The plate impedance of a 12au7 at 100 volts is 6250 ohms  at 250 volts its 7700 ohms. Indicating that the impedance goes up as the plate voltage is increased.
The circuit that this transformer originally was taken from uses a plate voltage of 12 volts, I assume at 12 volts the plate impedance would be quite low.
I'm wondering if the higher plate voltage you are using would cause this matching transformer to no longer be a good impedance match for the 12au7 with a 60-100v plus plate voltage?

Glen

[frequencycentral]

Hi guys
frequency central that's cool!
I'm going to give this a try once the twincaster is  finished.
I'll use the twincaster as the preamp.
I was wondering about the impedance rating of the matching transformer (fender reverb) it's 2800/1
The plate impedance of a 12au7 at 100 volts is 6250 ohms  at 250 volts its 7700 ohms. Indicating that the impedance goes up as the plate voltage is increased.
The circuit that this transformer originally was taken from uses a plate voltage of 12 volts, I assume at 12 volts the plate impedance would be quite low.
I'm wondering if the higher plate voltage you are using would cause this matching transformer to no longer be a good impedance match for the 12au7 with a 60-100v plus plate voltage?

Glen

Yes you are right - there is probably a better match than the transformer I used. I got a Fender reverb transformer some weeks ago and just tried a few different ideas - I eventually used it to build my Murder One, the output impedance of a 5672 tube being 20K, so well matched. I'm not a very technical guy and am just getting my head around impedance issues - it takes someone like yourself to improve on my idea.

I came across this today: http://www.ax84.com/static/corepoweramps/2W_SE/AX84_2W_SE_Poweramp_Schematic.pdf which would suggest that the 12AU7 would work well with a 4K primary. So maybe the Hammond 125ASE would be a better match than the Fender reverb transformer as it has 2K5/5K/10K primary taps.

[biggy boy]

Yes you are right - there is probably a better match than the transformer I used. I got a Fender reverb transformer some weeks ago and just tried a few different ideas - I eventually used it to build my Murder One, the output impedance of a 5672 tube being 20K, so well matched. I'm not a very technical guy and am just getting my head around impedance issues - it takes someone like yourself to improve on my idea.

I came across this today: http://www.ax84.com/static/corepoweramps/2W_SE/AX84_2W_SE_Poweramp_Schematic.pdf which would suggest that the 12AU7 would work well with a 4K primary. So maybe the Hammond 125ASE would be a better match than the Fender reverb transformer as it has 2K5/5K/10K primary taps.

Here's something  this site shows you how to select and test any transformer to determin it's impedance and how to matchg it to you tubes. very good stuff.     
http://www.radioremembered.org/outimp.htm

I'll have a look at that link when I get home from work! thanks. This tube stuff is alll new to me too.  I does help that I'm an electrician thou, makes it a bit easier to understand. LOL

Glen

[Renegadrian]

Glen, are you a lefty too?!?

[frequencycentral]

Glen, are you a lefty too?!?

Thread hijacked by southpaws alert!

[biggy boy]

Glen, are you a lefty too?!?

Sure am thats why I make my own guitars and basses!

[biggy boy]

You may have seen my "Murder One" amp thread: http://www.diystompboxes.com/smfforum/index.php?topic=73222.0

I thought I'd see how the same design philosophy works with a 12AU7. It does!

Still trying to put my head around tubes!
I notice there is no resistor on the Cathode terminal 3 and ground.
I guess this will give the circuit the maximum  gain possible right?
So then the gain control is just in the preamp part?? in your case the Valvecaster preamp?
From what I have read so far I thought a resistor was needed on the cathode to create a situation were
the voltage to the cathode would be slightly higher then the grid voltage.
Sorry for so many questions. I'm not trying to question whether this circuit is right or not, I'm just trying to learn and understand how all this stuff works
I'm a very curious person by nature. Man so much to learn. At least its Fun!!!

Thanks!
Glen

[frequencycentral]

Hi Glen. This circuit was more a 'design for discussion' than a finished project. As you have already correctly suggested, it would work better (louder?) with a different transformer.

I notice there is no resistor on the Cathode terminal 3 and ground.
I guess this will give the circuit the maximum  gain possible right?

Correct, if you have a Valvecatser and turn up the Gain to maximum you are shorting the cathode to ground, so that triode is set up for maximum gain. With such a relatively low voltage amp clean sound are just not an option - too quiet.

From what I have read so far I thought a resistor was needed on the cathode to create a situation were
the voltage to the cathode would be slightly higher then the grid voltage.

Many tubes designs use a low value resistor (say 1K) and a cap (say 1uf) between the cathode and ground. Thats probably better engineering than the Valvecaster's approach - but it works. "By altering the values of the cathode resistor and cathode bypass cap, it is possible to roll off various degrees of bass with this triode stage." http://www.diystompboxes.com/pedals/tubedummy.html

Sorry for so many questions. I'm not trying to question whether this circuit is right or not, I'm just trying to learn and understand how all this stuff works
I'm a very curious person by nature. Man so much to learn. At least its Fun!!!

Keep asking the question - I ask a few too! And yes it's real good fun. 

[petemoore]

  That cathode gnd. is buried, i checked yesterday while adding a 220k to my 220k plate resistor to reduce gain on stage 1...
  12aT7 though, and at 38vdc on the plate resistor.
  I am about to dig that cathode ground wire up and put a little resistor in it, >470 I think. Then maybe fix gain starting 'low' 15k or something on stage 1 cathode, then through the 25k pot allowing cathode R adjustment between 15k and ~40k.
  The way it gets played a lot now, it is nice 'meat'n potatoes' tone, I want to keep that, maybe make it to where it's even adjustable to less gainy !
  Can anyone guide me to schematic of output stages of single ended tube amps that are paralleled or seriesed tube sharing the output load ?
  I'm looking for examples...
12ax7:  Anode current: 0.5 mA
12au7:  Plate current: 10.5 mA
  I have some reading to do...
  I haven't been able to find the cathode current max rating on the 12ax7, perhaps a cathode follower could be designed to supply current and be of useful impedance to drive a speaker ?
  Trying to get past the idea that a 12ax7 couldn't be made into an amp...thinking too hard about sketchy ideas of reducing plate resistance by making a parallel plate output...or something..well, that and having plenty of other things I know I should be thinking about.
 
 

[frequencycentral]

I am about to dig that cathode ground wire up and put a little resistor in it, >470 I think. Then maybe fix gain starting 'low' 15k or something on stage 1 cathode, then through the 25k pot allowing cathode R adjustment between 15k and ~40k.

That's a good idea Pete - I find the 'thunk' when you turn the Valvecaster gain pot to max slighly annoying. Some 12AU7  can 'whistle' when bypassed too if the cathode is dirctly connected to ground.

[biggy boy]

You may have seen my "Murder One" amp thread: http://www.diystompboxes.com/smfforum/index.php?topic=73222.0

I thought I'd see how the same design philosophy works with a 12AU7. It does!

Hi
Rick just checking to see if you have added any more stages to your power supply?
If so how is the sound at the higher voltage.

I take it higher supply voltage equates to more headroom?


Glen

[frequencycentral]

Hi
Rick just checking to see if you have added any more stages to your power supply?
If so how is the sound at the higher voltage.

I take it higher supply voltage equates to more headroom?

Yes, I ended up with six stages, that's 12 caps and 12 diodes - that's what I used in Murder One. I have the same PS on my breadboard for some more experiments I'm doing. Each stage increases the volume, volts=volume with tubes. I stopped at ~70 volts as that's the limit for the 5672 tube in Murder One. 12AU7 can go higher though (check the data sheet) - so it would be worth exploring a switched mode power supply like this: http://www.ledsales.com.au/kits/nixie_supply.pdf

[biggy boy]

Hi
Rick just checking to see if you have added any more stages to your power supply?
If so how is the sound at the higher voltage.

I take it higher supply voltage equates to more headroom?

Yes, I ended up with six stages, that's 12 caps and 12 diodes - that's what I used in Murder One. I have the same PS on my breadboard for some more experiments I'm doing. Each stage increases the volume, volts=volume with tubes. I stopped at ~70 volts as that's the limit for the 5672 tube in Murder One. 12AU7 can go higher though (check the data sheet) - so it would be worth exploring a switched mode power supply like this: http://www.ledsales.com.au/kits/nixie_supply.pdf

Ya I was looking at that power supply design looks good and fairly easy to make!

I'm thinking 100 volts plus would be cool to try out?

Glen

[Jered]

  I just built a Nixie last week and so far, so good. Mine adjust's from 73 volts to 241 volts. I used a 10 turn 1K Helipot instead of the trimmer, so I could dial in precise voltages if need be.
  I haven't tried it yet with a tube circuit. That will be the true test.

[frequencycentral]

  I just built a Nixie last week and so far, so good. Mine adjust's from 73 volts to 241 volts. I used a 10 turn 1K Helipot instead of the trimmer, so I could dial in precise voltages if need be.
  I haven't tried it yet with a tube circuit. That will be the true test.

Excellent - did you build the one I linked to above? If not can you post a schematic of what you built? How did you get the adjustment so wide? Will be interesting to see what you do with it.

[biggy boy]

Any idea how much milli amp load the nixie can handle?
Was wondering if it can handle two or three tubes a 12ax7 and one or two 6L6

Glen

[petemoore]

   Google found this:
  http://www.desmith.net/NMdS/Electronics/NixiePSU.html
   Great Page...
  Mentions current draw of larger tubes is more than of smaller tubes...

[frequencycentral]

   Google found this:
  http://www.desmith.net/NMdS/Electronics/NixiePSU.html
   Great Page...
  Mentions current draw of larger tubes is more than of smaller tubes...

There was a thread about that one recently: http://www.diystompboxes.com/smfforum/index.php?topic=74020.msg601045#msg601045

[petemoore]

  The noise generated and picked up by the audio circuit is one of the problems that concerened me as the inductors do emit a lot of RFI, you can use a shielded inductor but the efficiency will drop dramatically.
  ...so don't start with the PS inductors permanently fixed near signal paths?
  Put the SP portion in a separate shielded containter?
  Apparently the PS inductor works quite differently when shielding, perhaps a ground plane between Indy and the Signal path would keep inductor effeciency drop from being so dramatic and yet harvest fewer RFI problems in SP.
  Put the PS inductors...'over there', then run a DC wire to the SP area ?
  How much separation is necessary if...?
  Experimental layouts will tell much more than the theories.
  Perhaps some feedback from a sensor of RFI can test positions around the placed inductor [on a ground plane?] to sense what positions around the device have the highest and lowest outputs, or to find out it has omnidirectional characteristics to it's output.
  I have not done much in the way of psu design before but its strange how you can get quite hooked into psu design. (always considered it a boring, secondary neccessity). I've always avoided circuits with inductors as they have been a mystery to me in the past, so this is also all about learning. 
  The above was typed before discovering how much of a problem the RFI into SP is, or what simple steps make satisfactory PS noise rejection to say pre-amp circuits. 

[petemoore]

  This recent thread has surfaced some 'useful tips':
   http://www.diystompboxes.com/smfforum/index.php?topic=74265.0