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

[frequencycentral]

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!

[tux320]

That its awesome! and I am very jealous

[liddokun]

Does this little design get loud enough for bedroom levels?

[frequencycentral]

Does this little design get loud enough for bedroom levels?

Yes - I'm driving it at the moment with my stock Valvecaster and its cool. Clean its a little quiet at the moment, but i think a few more voltage increments using extra diode/cap stages after the MAX1044 wil bring it up to a nice level. I'm running it into a 12" 8 ohm speaker.

[tux320]

I've been thinking about building a valvy for a while but this has pushed it over the edge. Perhaps you could incorporate the valvy you are driving the amp with into the amp itself and then have it relay switched, or maybe that's defeating the simplicity factor of it?

[frequencycentral]

I've been thinking about building a valvy for a while but this has pushed it over the edge. Perhaps you could incorporate the valvy you are driving the amp with into the amp itself and then have it relay switched, or maybe that's defeating the simplicity factor of it?

Yeah that has crossed my mind to use two dual triodes. I have a bunch of 6111 subminis - I'll have to try some ideas using a couple of them.

[kurtlives]

I would make the it push pull with a dual triode. Bit more output, kinda fun.

Then you would need another 12AX7 for the "pre-amp", got an extra triode now. Could do fun stuff with it.

[andrew_k]

I would make the it push pull with a dual triode. Bit more output, kinda fun.

Then you would need another 12AX7 for the "pre-amp", got an extra triode now. Could do fun stuff with it.

...and it edges ever closer to being DougH's Firefly 

For the size and expense of all those caps, not to mention running the MAX over spec, I'd be tempted to just run two 12v transformers back-to-back and get a B+ of 240V... hmmm.

[frequencycentral]

For the size and expense of all those caps, not to mention running the MAX over spec, I'd be tempted to just run two 12v transformers back-to-back and get a B+ of 240V... hmmm.

Well, its only £1.00's worth of caps and diodes. But you are right. I'm running this from the MAX just because its on my breadboard already for the Murder One. The Murder One has a voltage limit of 90 volts due to that being the maximum voltage for the 5672 pentodes. That issue doesnt exist with 12AU7/6111, so maybe a 555 switch mode or the 12v transformers you suggest are the way to go. Quite frankly, I bought the fender reverb transformer just to mess about with, I'm amazed how easy good results have come.

I would make the it push pull with a dual triode. Bit more output, kinda fun.

Then you would need another 12AX7 for the "pre-amp", got an extra triode now. Could do fun stuff with it.

I would need a centre tap transformer for push/pull right? I do have a small multi-tap 25v line transformer I could use for that.

$h!t!! Tubes are fun! Tube amps are double the fun.

[Darkness, Darkness]

$h!t!! Tubes are fun! Tube amps are double the fun.

+1 

[Caferacernoc]

Nice job! I think you have built the Blackheart Ant Killer for like 20% of the cost.
How does the output compare to a LM386 amp?

[frequencycentral]

Nice job! I think you have built the Blackheart Ant Killer for like 20% of the cost.
How does the output compare to a LM386 amp?

Thanks!

I've got a LM386 amp that I built but never boxed. I'll dig it out and do a comparision.

[frequencycentral]

Well I just tried this idea with two 6111 submini dual triodes (I only have one 12AU7 in stock). It worked really well. I just have to study some similar topology tube amp schematics, as I'm easily getting nice overdrive, but I need to be able to clean it up.

[Salvatore]

Cool work frequencycentral.

I was wondering how much mA the 12 volt supply must give.
Would 500mA be enough ?

[frequencycentral]

Cool work frequencycentral.

I was wondering how much mA the 12 volt supply must give.
Would 500mA be enough ?

Thanks Salvatore!

Yes 500ma is enough. I'm using a 500ma filtered/regulated puwer supply.
A 12AU7 requires 150ma for the heater.
Two 6111, run with the heaters in series require 300ma for the heaters.
The plate ma requirement is minimal - I'm using the puny ma output of the MAX to provide that. If I remember rightly the MAX ma output is 20ma.

[Renegadrian]

Rick I  have seen this one and also the (in)famous MURDER ONE
Basically this is a Valvy with a trafo at the end, so it has one triode for the gain and one for the power (am I right?!)
The MURDER ONE can use a 12AU7 as the pre tube and a pentode as a power tube (am I right?!)
So I guess you could just have more gain with a murder one with a dual triode gain stage and a pentode at the end than this one, using one triode as pre and the other haf as a power tube (am I right?!)

Which of those solution is better in term of clean and distorted sound!?

[petemoore]

  Cool thread FC !
  A tube amp that relies on a 'wimpy' enough chip that I think it qualifies as a non-lethal approach?
  Less high tension...you know what high tension lines do in amps and how hard it can be to deal with that.
  Pretty cool indeed !
  Nice to have a milder amp. My smallest usable tube amp is my best buddy these days, the very obvious perfect choice until it's not loud enough to compete.
  I feel like scaling down another few notches...might just be the next project !
  My best new amp [built this week] is a friendly tube head, 12ax7 / 6v6 @204 on the plates. Small-watt has so many advantages over bigger [especially for producing a wide frequency response in 'idle air'], any more than that, IMO, is simply an obnoxious nuisance, with a stronger tendancy to be too powerful to control itself at low volumes, keeps wanting to jump up, doesn't respond right, tends to need comp. etc.
  Once the PS and OT are figured out, getting a tube to do the trick isn't that difficult, about the same as wiring a couple transistors [well, of course the heater].

 

[frequencycentral]

Basically this is a Valvy with a trafo at the end, so it has one triode for the gain and one for the power (am I right?!)

You are right, one triode is the preamp, one is the power amp.

The MURDER ONE can use a 12AU7 as the pre tube and a pentode as a power tube (am I right?!)

That's correct.

So I guess you could just have more gain with a murder one with a dual triode gain stage and a pentode at the end than this one, using one triode as pre and the other haf as a power tube (am I right?!)

Yes - I did quite a few different experiments as well as 'Valvecaster Head' and 'Murder One'. I also tried something similar to a Pepper Shredder with the fender reverb transformer at the end. I tried parallel triodes as the output stage too - not push/pull, but putting two triodes in parallel. Of all the ideas I tried, the final version of 'Murder One' seemed to me to be the loudest, and also the brightest.

Which of those solution is better in term of clean and distorted sound!?

At such relatively low voltage there is not much clean headroom. For a clean amp design more volts/headroom are needed than in 'Valvecaster Head' and 'Murder One'. My next amp experiment will hopefully get there, I'll be using two 7327 dual triodes for a push/pull amp with a Hammond 125A O/T, and a 555 based SMPS getting up to 100s of volts.

I think you should experiment a bit - breadboard a few ideas. If you want a dirty amp at low volume 'Murder One' would do the job, but it's clean sound is too quiet.

  Cool thread FC !
  A tube amp that relies on a 'wimpy' enough chip that I think it qualifies as a non-lethal approach?

Thanks Pete! That's the idea - a little tube amp that pedal builders can build safely without having to worry about lethal voltages.

[Renegadrian]

Can't find both my breadboard and some free time... 

Can you suggest me a normal size pentode to sub the 5672?!

[frequencycentral]

Can't find both my breadboard and some free time... 

Isn't that illegal???

Can you suggest me a normal size pentode to sub the 5672?!

5672 is the only pentode I've ever used.  I know you have some pentodes in stock. So it really is a case of breadboarding/experimenting. The fact that the 5672 is directly heated (grid internally connected to the heater +ve) means any sub you used would have to account for this - you could have a look at other amp schematics for how to wire up grid 3.

You would also have to consider the sub tubes' output impedance - the 5672 output impedance is 20k so it's a close match to the transformer I used - I'm just getting started in how to work out impedance matching. Hammond do output transformers with many impedance options, the 125A for push/pull, the 125SE for single-ended for example.

Also, using a normal size pentode would mean you could work with higher voltages, as the 5672 is limited to ~67v in 'Murder One':

It would appear that although the maximum voltage for a 5672 is 90 volts it's not possible to apply a 90 volt plate voltage via a transformer (though ok via a resistor). Here's what Jasper Oosthoek over at the AX84 forum has to say about that: "If 90 volts is the maximum for the tubes, you cannot use it with a transformer (resistor is not a problem). As soon as you get a signal in the transformer its primary AC voltage will swing around B+. This means that if the AC voltage is 30 volts peak to peak, the anode voltage of the tube will swing between 75 and 105 volts. 60 volts on the anode might be a lot safer and will enable you to have 60 volts AC P2P (30 to 90 volts)".

There are lots of possibilities, but I think all require some expermenting!