Low wattage tube (1/8 watt) Amp

[iccaros]

So I was intreaged by a post my DRBOB on another board. I have found designs for Low wattage tube amps using 12V tubes This is my first take so I would like some opinions or people willing to test. I am awaiting tubes and transformer. And I have no issues changing types and may have to but here it is:
B+ is ~21vdc (not to exceed 30 no less than 12V as mentioned in the thread) with heater (pin 4 and 5 being ~12.6VDC With 4 to pos and 5 to ground)


here is the Bill of materials to match
http://dl.dropbox.com/u/14312589/B.O.M.V2.csv

Group Name Value Part # Quantity
Capacitors C1 0.022 1
Capacitors C3, C2 0.001 2
Capacitors C4 470pf 1
Capacitors C5 100n 1
Capacitors C6, C7 .01uf 2
Jacks J1 1
Jacks J2 1
Resistors R1 69k 1
Resistors R12 56k 1
Resistors R13 500K Log 1
Resistors R14 250k Lin 1
Resistors R15 50k Lin 1

Resistors R18 100K A 1
Resistors R19 2M 1
Resistors R2, R7, R10 1M 3
Resistors R20 150 1 (thanks Redelephant for catching this should not be K)

Resistors R4, R6 100k 2
Resistors R5, R3 30k 2

Resistors R8, R9, R11, R16 22k 4
Tubes V1 , V2 12u7 2

Tubes V3 12AL8 1


and here are the datasheets
http://dl.dropbox.com/u/14312589/12AL8%20%281%29.pdf
http://dl.dropbox.com/u/14312589/12u7.pdf

The output transformer is a 70v line transformer.
I have found topics on this but they are all a few years old, unless I missed one.

[redelephant]

Very cool. Ditch R20!
Not sure I'd go for grid bias like this tho. Very uncertain about the quiescent conditions and u have very little dynamic headroom. Besides if u go that route u must AC couple the potentiometer wipers going into the grids, or u get scratching noise from current thru the pot's wiper. I think u need cathode resistors. To get any appreciable headroom, b+ being 30V, u need the Vp at around 20V. That means Ip at 0.5mA, and a cathode resistor of 2kohm. That is actually good b/c u can then tune the response with the bypass cap, small value for more high end, large value for more low end, etc.
U can also ditch C1 when using cathode resistors.
What I've read about these low voltage tubes, it is important not to exceed 30V on the plates. The output tube will clearly swing above 30V if that is b+.

[iccaros]

All good points.
I put 30VDC as a thought but the loads are setup for 12.6 in the math (sorry should have stated that ) I think at say 25 volts (good high number) 220K for example

I put R20 in because the line transformer gives about 650ohms and the datasheet calls for 800ohms. So I was thinking it would give more clean headroom, and the sopha amp Sopht amps - all tube 12v amps Version 2 made that comment.

But you don't think I need It? I do not think it it will hurt at such small voltages to not match.

the no resister on the cathode is out of the data sheet, but that is for a 12.6 voltage so I think your right I will need it for higher. Then your right I could tune with bypass.

This is basically a Ax84 Lead II and Soph amp mix. I will hopefully have some tubes to test with next week and will play and make changes if work and school don't stop me.


Thanks for the input.

I am trying to get my hands on tubes, tube depot is out of 12k5 and 12U7 (mine are back ordered) and they said they are having issues finding 12u7 (then why let me order... ) but such is life with NOS tubes. I have some bids in for 12AE7's which may replace them at lower gain. But I have to play the ebay game...

[redelephant]

Well, on your bill of materials R20 is 150kohms! If having a resistor in series here helps the tone, then I guess that's only good, but 100-200ohms maybe?
I've never played with these tubes, so dont know, but as u remarked with 30V b+ I would use cathode resistores on the 12U7 at least.

[iccaros]

good catch... note do not create BOM at 2am... it should be 150 ohm not K ohm

also thinking about what you said, B+ should be no more than 21 volts (70% of max 30)

the datasheet shows a 1ma plate center operation so the .5 you showed would work, and give room to tune both sides for best performance/sound

thanks for the advice.

[redelephant]

Nah...keep the b+ at 30V, or 29 to be safe. The operating point should be with the plate around 20V, (2/3 of b+), which means .5mA thru the 22kohm plate resistor. Looking at hte graph, with 20volt on plates, 0.5mA current, u should have 1volt negative on grid, so a 2kohm cathode resistor should fit the bill.

When using a choke or transformer as a load, the plate voltage equals the b+ (minus the small drop over the OT) and the voltage will swing up and down around that voltage. So with b+ at 30volts on the output tube, u may see swings up to 60volts on the plate (if the idle current and load permits). So the voltage supply for output tube should be 15-20volts or so. But keep the preamp b+ as high as possible.

[iccaros]

Got ya, my concern is that the last triode is apart of the final tentode so reducing voltage on one limits the other. I need to play with these... I was out bid on a EBay buy so I am still searching, I have some lower mu (13 instaed of 20's) tubes comming in.

I need to break out the math and stop guessing on some of this .. which means I need to figure out the power supply.

Do you see any issues with the EQ where it is?

[redelephant]

even tho the triode and tetrode is in the same glass tube, they are 100% separated and will not interact. They share the same heater, but that is not part of the circuit. One can even be not connected to anything and the other will do what u make out of it. No probs using separate b+ for them.

To be honest I assumed the EQ to be some clone or such and haven't bothered looking at it. If it's from a known good design I'm sure it's good to go. No probs modifying that later when all is up and running and u decide if u need to tune one way or the other.

Now just build the thing and let us know how it sounds. I may have to get some of those tubes myself

[iccaros]

Tubes are on their way.
Tube depot turned out to have one 12u7 and they had the 12AL8's
Antique Electronic parts (aka Antique Electronic Supply) has a few 12u7s
So they are shipping. So I hope to have them by this weekend. This will build easily on breadboard so we shall soon see
Thanks for the input.

This is my first take a strange tube and build so, I am thankful for the steerage. For some reason I was thinking of the heaters in the 12AL8 as a same as pos and neg on a dual op amp..

[iccaros]

found this on a radio forum, according the author its very audible through an 8 ohm speaker. But while its a radio, look at the parallel out dual 12k5 power section
http://oldradiobuilder.com/12vdiagram.pdf


looks interesting...

[iccaros]

first attempt, Had issues with blowing out lm317's and I could not get my 12.6 -0 - 12.6to put out less than 40volts once I added filter caps to the supply...
This things pulls almost a amp of power.. but its ok, I ended up going to 13 volts from a wall wart (rated at 1 amp.. pushing it)
I did not have the eq or the 3rd stage or the recovery stage after the EQ yet, need more wire. Also I need a few pots,,,

Used a 12K5 for output for this one.. (warning the 12K5 get hot, probably the .5 amp heater, had a fan on it...)

[iccaros]

Test 2, now at 30 volts, need to clean it up as I want to add an overdrive boots at the beginning so I can tune the overdrive. I also would like more "power amp" overdrive. I am getting a little too much preamp overdrive
this is just a Les Paul strait in.

forgive the mess, I finished at 1:30 am and have not cleaned all my junk up.

[riz]

Sounds like rock to me--thanks for the video links! I want to do something similar one of these days. Once upon a time I found a little tube intercom on the street, and simply wired an input jack right to the grid of the first tube stage. Sounded freakin' awesome till it blew up! This was back before I had much of a clue about tubes, and now that I know a bit more, I also have more tinnitus, and I really need to make the time to build a little ripper like this to dime out around the house--even my Champ rings my bell some days anymore.

[iccaros]

I am with you riz.. I need something I can practice with and use in small venues where my 5 watt, with volume on 3 is still too loud. I plan on using a pair of 26A7GT, which is a push pull in a tube (easy way to say it)

[iccaros]

Transformer question
I was using a 4 watt 70 volt line transformer, This in my calculation should put 70*70/4 = 1225OHM with a 8 ohm speaker connected, Correct? so I have a multi-tap from 5 watts to 15 watts. So 70*70/5 = 980 ohms? yes which is closer to the 800 ohms the tube wants for output load. Am I doing my math correctly? my voltage out went from 14 volts to 28 volts from the tube read with a DMM. I need to connect the scope but that will require me to clean my bench. I do not have a low voltage ac generator to test the transformer so any help would be great.

on a side note would my 7.5 watt be my center tap so to do push pull I would connect to 5watt with center being 7.5watt and the common? or would I use the 10 watt as center? or vice versa..