2 x 12au7 amp running on 9 volt adaptor and smps

[zambo]

  I just put this amp back together in a blues jr. cab. I will post video asap. Sounds ok and is loud enough to kill an open mic. great for at home playing. I am guessing 2 or 3 watts but not real sure. I bet the smart guys on here could figure it out. smps is putting out 200 volts before the r/c filter. Feel free to redraw the schematic and post/build sell whatever with my blessing. Just do the right thing and mention my name in the inspiration section as I rightfully stole these ideas from Doug H. and Rick Holt aka Frequency Central. Cheers and be safe, Greg

[zambo]

ps . output trans is hammond 125c using the 11k taps on the primary side.

[tasos]

can't wait for the soundclips!
any chance you try this with bass?

[zambo]

uploading to youtube now havent tried with bass yet but not sure how good it would sound as its in a guitar combo... I can try though. if it sounds good i will post a vid clip of it!

[zambo]

<iframe width="425" height="349" src="http://www.youtube.com/embed/rkhOqfyzdN4" frameborder="0" allowfullscreen></iframe>

I hope this works...demo vid.....

[MetalUpYerEye]

That sounds pretty decent... 

Just thinking out loud... It looks like you just paralleled the two 12au7's in the power stage... Would the same idea be possible in one of FC's submini amps? For example the use of two 6111's in the Superfly instead of one, for a higher wattage output. How would that affect the way the transformer is wired up?

[zambo]

Thanks! I think that would work as the principal is the same. You would need a transformer with half the primary resistance at that point. I suppose if you hooked up a 4 ohm speaker instead of 8 it would do the same thing in theory. I think thats right....check first.

[amptramp]

Where is the grid return resistor for the upper output triodes?  Also, your coupling capacitor to the output stage comes off the wrong side of the plate resistor and so does the coupling cap from the first stage to the second stage.  If it works, you have something different in there from what you drew.  BTW, is that a tone stack at the input to the second stage?  I don't see any pots and there is nothing in a position to act as a simple volume control.

Sounds like the 11K taps are a bit high for a push-pull parallel triode output.  I would expect the optimum to be about half of that.  Some people drive reverb tanks from a single-ended 12AU7 and the impedance is on the order of 2500 ohms.  PPP 12AU7's may be around 2500 ohms plate-to-plate, but I don't have actual figures.  Triode output impedances should definitely be less that pentode or beam power tube impedances.

I would also advise a lot more capacitance at the input to the SMPS.  A power converter is a negative resistance at the input since the internal feedback (if regulated) causes the current to increase when the voltage decreases.

[PRR]

You surely don't have preamp plates wired as shown, and no grid-leak will work only for a short while. More like this:



> Sounds like the 11K taps are a bit high for a push-pull parallel triode output

It's not way-wrong. Matchbook analysis: each 12AU7 section is 6K, a good load is twice or 12K, 6K per side, in class A (which this must work in) the P-P impedance rating is twice, so nominal 12K p-p. 11K is spot-on.

For cleanliness you might go higher: THD falls faster than power.

For small hard guitar you might go 6K-5K for less speaker damping.

[zambo]

yeah i pretty much hate drawing schematics for this reason, I stink at it. There is a volume after the tone stack and a master right before the pa that is the grid ground reference. They are 1 meg . Also yeah, the caps are in the right spot in reality....just not on the schematic  I will correct and repost tomorow if i can.

[iccaros]

This is mine, I have added a valve caster to the beginning since this drawing, and some things have changed, but it may give you ideals. This is using 12V (30max) rated tubes


you can hear it here. I am not playing anything, just messing with how it sounds, so I can go back and tweak. this is through a Carvin 2x12 cab with Carvin speakers.

Skip the first min as I wait for tubes to warm up. Between my wife opening the garage door and the bread board it is not as clean as it could be
This is recorded using a cheep $60 aiptek camcorder, using its mike.

As for yours, you need grid leak as stated, the tubes will die a painful death with out them as the grid will need to get rid of the little current it collects from the cathode.  do not exceed what the datasheet has for the tube though. I believe in class A it list 1m Ohm max in the datasheet.

[iccaros]

Watched the video, sounds good.. most of it is the player, I was not sure how much that jalapeño like EQ would get you, I was thinking of adding it to mine.. cleans up nice..
do you have  a link to that power transformer (voltage booster)?
I am assuming that you are taking heater voltage strait from the one spot. and is that a ~9 dc one spot?

I have a compactron (6af11) with two triodes and a pentode in the same tube, but I need to keep it at around 200v DC.. and I do not want to spend $60 on  a transformer if I do not need to..
Great thread,, I will have to add this to my set of projects.

thanks

[tasos]

Skip the first min as I wait for tubes to warm up.

so how do we warm up the tubes?

[iccaros]

heaters... heaters have to heat the cathodes to make them discharge electrons.
On bread board it took 30- 45 Seconds to heat up. with the two 12K5's in parallel they could take a min before they produce sound as they both need to conduct

[Steben]

It's a pitty you didn't try a class AB push-pull attempt, since you are using 2 12K5's anyway...
If you use push-pull, you get dynamic current draw ... chance of getting sag ... compression ...

and ... lethal voltages? at 12V-24V? I thought non-lethality was specific feature of these tubes?

[tasos]

heaters... heaters have to heat the cathodes to make them discharge electrons.
On bread board it took 30- 45 Seconds to heat up. with the two 12K5's in parallel they could take a min before they produce sound as they both need to conduct

so we just turn power on right?

[iccaros]

It's a pitty you didn't try a class AB push-pull attempt, since you are using 2 12K5's anyway...
If you use push-pull, you get dynamic current draw ... chance of getting sag ... compression ...

and ... lethal voltages? at 12V-24V? I thought non-lethality was specific feature of these tubes?

Zmbo is using 200V from a voltage step system. Mine is only 12- 24, but its best to be safe

also this is my first build, I have some 26A7GT which are push pull @ 27 volts .5 watt, so I will do a tube rectifier to get SAG. and I have some 12AL8's which in  AB push pull are nice as they have a triode and pentode built in, so from a the phase inverter to the triode to push the power tube a bit more..

[amptramp]

Iccaros, the capacitors on the output tube cathodes will give you a high-frequency boost starting at 3290 Hz and above.  You may want to increase the cap to reduce this frequency or remove the caps altogether if you have enough gain.  Your coupling capacitance of 0.0022 µF gives you a low-frequency turnover of 72.4 Hz.  You could afford to go up in capacitance a bit to bring this down.  Is R5 really 220K?  This would give you mere 24 microamps in that stage.

[iccaros]

Iccaros, the capacitors on the output tube cathodes will give you a high-frequency boost starting at 3290 Hz and above.  You may want to increase the cap to reduce this frequency or remove the caps altogether if you have enough gain.  Your coupling capacitance of 0.0022 µF gives you a low-frequency turnover of 72.4 Hz.  You could afford to go up in capacitance a bit to bring this down.  Is R5 really 220K?  This would give you mere 24 microamps in that stage.

@amptramp
Thanks for the input, I am lost on these calculations, I was pulling my hair out using load lines with these tubes, so I put a pot on each cathode and twisted until I liked, then measured and put in the closed resistor for the value I read. I really believe my issue are the bread boards, so I am putting this on turret to test, but this is my starting point.

as for the 220K cathode, it is, and it boost according to the oscope from 10's of millivolts to hundreds of millivolts and the next stage boost from that to 6 volts. These tubes are meant for boosting radio signals, so this is the values that gave me the cleanest signal, any lower I started to get blocking distortion with most other settings,  when I put a pedal before to get more overdrive it just died... It may have been the breadboards causing this.
Caps, I am really lost on calculating tone values and what they should be.  2.2 is the largest I have right now with out going to 220uf so this is what I have, what would be a good value 6.8? with out caps the power section farts out into oscillation and blocking distortion, so the thought is they would stabilize current,  these tubes at 30V are a little different than normal, they are very sensitive.
As for coupling I have a .01 in C3 right now, did not update the drawing.

I started out with 220K on the plates and 2.2k on the cathodes, this was really clean, but when I added another stage just got blocking distortion, with these settings I was able to add the valve caster in front if this to get dirty.


@all Sorry I am not intending to take over this thread, Just wanted to compare Zambo's 12V amp to mine to get ideals going. I really like his version of the firefly using the nexitube power supply.  next payday I will have to get 2 of those boosters.

[amptramp]

No problem with the calculations, R*C is a time constant.  1/(RC) is a frequency in radians/second where there are 2 pi radians in a cycle.  Therefore, frequency is 1/(2 * pi * R * C).  You can calculate this by hand or use the following link for a calculator:

http://www.muzique.com/schem/filter.htm

But you should confirm your first calculations by hand, just to make sure you don't become dependent on having someone else's algorithm available.