linear Low voltage tube project(pedal)

[iccaros]

are you still running 9 volts? I would not expect 12 to be diffrnet, I will try my other tubes.. I am using Sovtek i pulled from my Carvin.

I have some JJ's I can try.

[Johan]

clearly there is a misstake in this schematic.. R5 surely must be intended to go to the Anode (pin 5 of the 5840) and not to the cap, otherwise there is no voltage to the tube.
J

[ThunderShowers]

Ok, 9-volts, different tubes run down:
Ruby 12ax7a; high output, lots of bass.
Some old Canadian Tube that I can just read 12ax7a on; Smoooooth, medium high output.
Sovtek 5751; low to Medium output in this circuit, adds it's own little bit of breakup.
GT 12ay7; Low output, barely adds any boost.
Mesa 12ax7a; lower output: barely breaks even.

Out of these, the only old tube is the Canadian one.

Plate lengths, from small to large are:
Mesa
GT
Ruby
Canadian
Sovtek

Gonna buy a Sovtek 12AX7LPS and maybe a Sungtol, see what happens.

Edit: It should be noted this is a pretty common thing, as plate lengths and Gain are indeed Related: it's just more pronounced then I expected.

[iccaros]

clearly there is a misstake in this schematic.. R5 surely must be intended to go to the Anode (pin 5 of the 5840) and not to the cap, otherwise there is no voltage to the tube.
J

Thanks will fix.. The cap goes to the output...

[iccaros]

Ok, 9-volts, different tubes run down:
Ruby 12ax7a; high output, lots of bass.
Some old Canadian Tube that I can just read 12ax7a on; Smoooooth, medium high output.
Sovtek 5751; low to Medium output in this circuit, adds it's own little bit of breakup.
GT 12ay7; Low output, barely adds any boost.
Mesa 12ax7a; lower output: barely breaks even.

Out of these, the only old tube is the Canadian one.

Plate lengths, from small to large are:
Mesa
GT
Ruby
Canadian
Sovtek

Gonna buy a Sovtek 12AX7LPS and maybe a Sungtol, see what happens.

Edit: It should be noted this is a pretty common thing, as plate lengths and Gain are indeed Related: it's just more pronounced then I expected.

In other low starved plate designs, keeping linear is not the goal.. So I expect some things to be weird..  I am just happy the 12AX7 worked at all @ 9 volts for you. I could not get any of mine to work in a valve caster @ 9volts.. It took at least 20..

[panterafanatic]

You're going to want a buffer at the output, pentodes have a much larger output impedance than triodes, you gotta pay for that extra gain somehow. A direct coupled MosFET source follower would do the job well. http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm - First image, just hook the 1uF cap to the source (along with a little series resistance, say 1k-4k7), and perhaps change the 1M pot to 10k, another advantage of using a buffered output. Another thought it to AC couple the buffer, keep the high value pot control, and instead of connecting the lug of the pot to ground, connect it to 9V or so. Here's another Geofex link with a little info on source followers and MosFET boosts. http://www.geofex.com/Article_Folders/mosboost/mosboost.htm

Just a thought, you could turn the buffer that's pre-pentode into a MosFET, and use a joint anode line paralleled triode arrangement. This would allow a high-low gain input or even allow two guitarists to play into it.

[iccaros]

You're going to want a buffer at the output, pentodes have a much larger output impedance than triodes, you gotta pay for that extra gain somehow. A direct coupled MosFET source follower would do the job well. http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm - First image, just hook the 1uF cap to the source (along with a little series resistance, say 1k-4k7), and perhaps change the 1M pot to 10k, another advantage of using a buffered output. Another thought it to AC couple the buffer, keep the high value pot control, and instead of connecting the lug of the pot to ground, connect it to 9V or so. Here's another Geofex link with a little info on source followers and MosFET boosts. http://www.geofex.com/Article_Folders/mosboost/mosboost.htm

Just a thought, you could turn the buffer that's pre-pentode into a MosFET, and use a joint anode line paralleled triode arrangement. This would allow a high-low gain input or even allow two guitarists to play into it.

Thanks for links, if you read the first link I give, the author talks about using buffers, but I am attempting to do this all tube as I would have to buy silicon as a I have few opamps and a couple 2n222, I am stubborn like that  , but if someone wants to test adding silicon and report back, that would be cool add sound samples Please!!! , the first stage could be replaced with MosFet it would lose the fattening in "Clean mode" as I am putting a bypass switch to switch in and out the pentode. I could be wrong though.

So far this works great for me, but may not for others (I need to test with my delay and my Wah pedal) .. I am going into an all tube amp, so this just acts like another stage, with some nice "power tube" distortion. Which allow me to keep the amp clean, use the clean boost to give a little bit of 2nd order distortion (fatter tone) and then kick in a nice smooth distortion that is very pick sensitive.

I am guessing I just want this set for unity gain for the output..?

[iccaros]

I have added a opamp to the input set for unity,  biasing the input with two 2meg resistors on the input, one going to ground and one going to + voltage. This does a good job eliminating issues form other pedals or changing guitars.

my issue now is that I get 2.1 volts out. This is too hot I believe, I place a 120k resistor on the output and that have voltage swinging between 1.5 and 2 volts.
so the question, what is a good way to reduce the output, I thought of putting knee compressor at the output, but I do not know a good design for one.

I appreciated any help..
thanks


PS I tried a opamp on the output, but it just farts out.. and since the gain control is really adjusting plate voltage, turning it down just changes the sound, not real volume. 

[vendettav]

how about a master volume pot at the very end??? or will it suck tone or something 

[iccaros]

how about a master volume pot at the very end??? or will it suck tone or something 

Not sure, I can try a small 1meg trimmer which I could set output voltage I guess? would that add noise?

[Renegadrian]

Dead links...

[merlinb]

Dead links...

Which links? The one in the first post is here:
http://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

[Renegadrian]

Dead links...

Which links? The one in the first post is here:
http://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

yeah I meant dead images links, poster said he had a schem or something. I got valvewizard pdf safely stored in my hard disk and it's so useful, along with all the other info at the site!

[amptramp]

Here is a site:

http://www.sophtamps.ca/www.sophtamps.ca/mambo/index862b.html?option=com_content&task=view&id=25&Itemid=37

you might want to try for information on low-voltage tube amps.  These use 12 volt car radio tubes but the 12K5 space-charge grid tetrode is interesting.  The first grid is pulled to the +12 volt supply and the second grid is used as the input.  I wonder what would happen if you used this with a 12AU6 with the control grid at +12, the signal going into the screen grid and the suppressor could be +12 or connected to the screen.  The space-charge grid is one way to accelerate electrons before the pass the second grid which is used as the control grid.

[Renegadrian]

Ron I do know the sopht site well, thx anyway for posting!

[PRR]

> with a 12AU6 with the control grid at +12

Technically, this is tube abuse. G1 is not made to go positive. G.E. data specifically says Max Positive 0 Volts. Counting my thumbs, I'd guess it will flow 12mA, at 12V, is 144 milliWatts in a grid with NO dissipation rating.

Given the current value of 6AU6 ($8-$13, $6 for the 12AU6), I really can't care what you do to your poor _AU6es. Maybe the worst can happen is a red-hot grid comes loose and shorts-out your power supply, be ready.