I wanted an ultra-low output amp for late night noodles. So I've been mucking about with this amp design, which is essentially a valvecaster with an ECC86/6GM8 tacked on the end:
(http://i476.photobucket.com/albums/rr124/markeebee/20v2.jpg)
It's powered by a laptop power supply. Output transformer is a 100V line output transformer, less than £5.
Surprisingly, it sounds really ok. Lowest gain setting gives a nice crunchy sound, not skweeky clean but nice for strumming and picking, good note seperation. As you wick the gain up the volume doesn't increase much, but it goes quite AC/DC gnarly. If you stick an overdrive in front, the world's your lobster.
Background hum and hiss levels are really low, pretty non-existant actually.
Volume is surprisingly loud. I'd say it's about the same level as a decent acoustic.
Overall, I'm dead chuffed with it. Only drawback is that ECC86 valves are a little thin on the ground, but I've managed to pick up a few for around a fiver each on eBay. There are some silly prices, though.
Here's my build:
(http://i476.photobucket.com/albums/rr124/markeebee/IMG_0824.jpg)
(http://i476.photobucket.com/albums/rr124/markeebee/IMG_0830.jpg)
(http://i476.photobucket.com/albums/rr124/markeebee/IMG_0832.jpg)
I tried out a few tone controls - SWTC, Moonlight, 18 Watt - but preferred it without, so the tone control in the pic is disconnected. I uprated R8 to 10W wirewound cos it was getting pretty hot. I originally made a filter between the heater supply and the anode supply, but I found that it wasn't necessary - that explains the 3W resistor and the mini-tagboard. Can't be arsed to take them out.
There are probably a load of screw-ups in the design, but it works and sounds good. I'd be interested in any suggestions for improvements.....
I can't say anything about the design, but the finished product is superb!
Whippet Huffer... lol. I love it. Good ol' NO2. :icon_lol:
well done , that is really nice looking .
I'm going to pretend this amp is named after my favourite activity, which is sniffing miniature grey hounds.
Can you tell me what the current draw of this is?
Current draw is about 650mA to 700mA.
Is that textured paint? How did you manage to paint only the core and clamp of the transformer while leaving the laminations untouched? :icon_razz:
Quote from: tiges_ tendres on April 09, 2012, 06:14:29 PM
I'm going to pretend this amp is named after my favourite activity, which is sniffing miniature grey hounds.
Hmm, I think I'd call N
20 the more wholesome activity of the two. :icon_neutral:
Really cool amp Mark. I want to buy/build/steal one. Do you live in a first floor flat?
Quote from: merlinb on April 10, 2012, 04:44:04 AM
Is that textured paint? How did you manage to paint only the core and clamp of the transformer while leaving the laminations untouched? :icon_razz:
It's this stuff, got it in B&Q:
(http://www.rustoleumaerosols.co.uk/aerosol/images/products/AE0080003E8.jpg?)
It covers blemishes well, and dries to a kinda classy matte stone effect. It's not very tough, so it's not much use for stompboxes.
The clamp was just fixed by a couple of bent-over lugs on the bottom of the txf. I unbent them and lifted the clamp off carefully, but it turned out that the laminations were really solid, so I needn't have worried. So spraying the clamp was easy.
I sprayed the lams, core and bobbins with silver metallic (put some masking fluid on the exposed core ends) and then masked off everything but the core and sprayed the textured stuff.
Quote from: Taylor on April 10, 2012, 05:03:13 AM
Really cool amp Mark. I want to buy/build/steal one. Do you live in a first floor flat?
Thanks Taylor. You were the inspiration for the name.
Quote from: Taylor on April 10, 2012, 05:03:13 AM
Quote from: tiges_ tendres on April 09, 2012, 06:14:29 PM
I'm going to pretend this amp is named after my favourite activity, which is sniffing miniature grey hounds.
Hmm, I think I'd call N20 the more wholesome activity of the two. :icon_neutral:
I gotta disagree... I don't think puppies will make you fish on the floor. :icon_lol:
btw, I love Whippets... I got jealous when my cousin got one, but he lives in Florida... first hand experience is they don't do good in mountain snow. ;D I'd sniff one if I had it.
That amp is really really cool! I would love to hear it. it runs on 20 volts? So sweet!
That badge with the labeling looks really pro. Looks like old equipment in the lab where I work. How did you do it? Where'd you get the stock?
Hallo, and great work markeebee. I followed your first experiments (in another tread if I remember right) because they are very interesting.
And very nice build.
Also I have a couple of tube sockets, a small otput transformer and some unused 19V notebook power supplies, and I confess I would like to give the circuit a try ;D
Since it is difficult scoring a ecc86, and since your head is already built... how does it sound with just two 12au7 ? Is it worth a try or is it better to wait and find the proper tube?
thank you.
Alessandro
Quote from: zambo on April 11, 2012, 01:17:21 AM
That amp is really really cool! I would love to hear it. it runs on 20 volts? So sweet!
Thanks Zambo. That means a lot, coming from you. I'm a big fan of your amp adventures.
I'll post some clips as soon as I can, but I'm busy with other things for a couple of days. It sounds alright, though, honest!
Quote from: nosamiam on April 11, 2012, 07:48:29 AM
That badge with the labeling looks really pro. Looks like old equipment in the lab where I work. How did you do it? Where'd you get the stock?
Ha, thanks. I was hoping it would look like an old rating plate or equipment label. I used a scrap of polished aluminium (bought a 150mm square offcut on eBay for £2), and made a waterslide decal.
Quote from: allesz on April 11, 2012, 11:40:34 AM
Hallo, and great work markeebee. I followed your first experiments (in another tread if I remember right) because they are very interesting.
And very nice build.
Also I have a couple of tube sockets, a small otput transformer and some unused 19V notebook power supplies, and I confess I would like to give the circuit a try ;D
Since it is difficult scoring a ecc86, and since your head is already built... how does it sound with just two 12au7 ? Is it worth a try or is it better to wait and find the proper tube?
thank you.
Alessandro
Aw, thanks Alessandro. I haven't tried it with a 'normal' triode as the output but it's definitely worth a try. What's the worst that can happen? :icon_mrgreen: I'll give it a go in a couple of days. If you try it as well, please remember that the heater in a 12AU7 is slightly different to a ECC86. You would need to connect both pins 4 and 5 of the 12AU7 V2 to the 6V point at pin 5 on V1, and also connect pin 9 of V2 to ground.
Swapping tubes shoul be no problem.... but since I have just one 12au7.... I will have to buy one more just to try ;D. And you know, 12au7 have not great use in everiday tubes amps.
That's why I asked if you can give it a go for me (or for the others interested by the way).
About the heaters.... I would like to try connecting them in series (it should be 24V this way, but I hope that 19V could be enough).
So to try a 12au7 in the power section will you have to mod your work to the different heater wiring?.. well if this is the case leave it as it is. I suspect it is not worth the work.
what a wonderfully bent amp....nice one mark, looks great!..and suits brighton ;) :icon_cool:
a 12au7 will work in the power section it looks like, but i dont know if you will hear it at all. 20 volts isnt very much to run it on and the impedance mismatch on the ot wont help much i think. You may hear it but i bet its so quiet its not much good. I buitlt one running on around 60v with au7 and its audible for late night but man..... i dont know. wont hurt to try it. curious to know.
Tried it with 12AX7 in the output position. Results were not good. Very very quiet, and sounds like a misbiased fuzz.
In other news, I'm making one for a friend and I filmed him noodling around on the original one through a 1x12" combo cab. We used my son's bedroom so we could get to his recording gear, so excuse the teenage-ness of the room. And DO NOT ask what the thing under the bed near Jimmy's left leg is.
Nice. (Another one for the build list, methinks!) Girl in the poster seemed to like it too.
BTW, mess looks much like my "office"... ;)
Where would I find one of these transformers Mark? Can't seem to spot one unless i'm searching for the wrong thing.
Quote from: Scruffie on April 22, 2012, 12:04:18 PM
Where would I find one of these transformers Mark? Can't seem to spot one unless i'm searching for the wrong thing.
It's one of these:
http://uk.farnell.com/jsp/displayProduct.jsp?sku=1778009&CMP=e-2072-00001000&gross_price=true (http://uk.farnell.com/jsp/displayProduct.jsp?sku=1778009&CMP=e-2072-00001000&gross_price=true)
You can get them at Maplin as well. Mine was from eBay.
Use the 2W tap.
Cheers Mark! This looks like it should suffice... http://www.ebay.co.uk/itm/100V-LINE-TRANSFORMER-0-25-0-5-1-2-4W-PA-SYSTEM-VOLUME-CONTROL-/310330501667?pt=UK_Consumer_Professional_RL&hash=item484123aa23
Why the 2W tap? Can different ratios be achieved with the others?
Yeah, that'll crack it.
TBH, the maths that dictates the choice of transformer tap is beyond me, especially as data on the ECC86 as an output valve is fairly scant. I found this document http://cool386.tripod.com/6bm8/6BM8.html (http://cool386.tripod.com/6bm8/6BM8.html) which gives info about the relative impedance of the taps on similar 100V line transformers. The 2W 5kohm tap just felt right, so I tried it and it worked.
If anybody can enlighten with regard to the theory I'd be very grateful.
If you google "line transformer as output transformer" you should get a link to ozvalveamps which has loads of info and a link to another page with the maths for calculating the impedances for different transformers. I would post the link, but I still haven't figured how to do that on my phone.
I built something very similar a while back and just tried different taps until I found the one that sounded best. I've also used a line transformer as the output tranny on a real valve amp, works great.
Thanks Slacker, that's good stuff. TBH, though, the txf wasn't the real problem; I was more stymied trying to ascertain the load impedance of the valve
treat the load like a resistor, which it is, ohms law will do the rest. you have a datasheet with max dissapation and you know your voltage, so you have current and voltage..
to give you ideals
http://www.diystompboxes.com/smfforum/index.php?topic=41776.0
and my 24v tube amp
http://www.diystompboxes.com/smfforum/index.php?topic=91606.0
Hi
not sure why the comments about low output from a 12AU7. They are similar to the 12U7, which was the output tube for valve radios in cars (@12V). About half a watt??
If you want more, you could always run it through a little transistor amp (5 clean watts are easy to get from the 20V supply into 8 ohms).
cheers
12U7 were preamp tubes in car radio's. 12K5 and such like it, were the output tubes. Most ran to transistors of the day.
While the 12u7 and 12AU7 were simular, they are not the same. I have read the internet rumors that 12u7 were 12AU7's that did not pass all of the test, but they react differently, if not as much to NOS 12AU7 they do with newer tubes.
You could put a 12AU7 or 12U7 as an output tube.. but you would not be happy I believe..
Quote from: iccaros on April 23, 2012, 03:15:09 AM
You could put a 12AU7 or 12U7 as an output tube.. but you would not be happy I believe..
Really? :o
http://www.ax84.com/index.php/oldprojects.html?project_id=firefly (http://www.ax84.com/index.php/oldprojects.html?project_id=firefly)
Low voltage (12 - 24).. Not 230v like my firefly... ??? There is a huge difference.. the 12au7 @ 12v and 3 -5 ma is not very loud. A 12K5 at 50ma is not very loud but much louder than 12au7 and at more reasonable loads. sitting at 1K and not 22,000
But I will put one together, simple enough and see how much power and what it sounds like.. I will post
If you search the forum for rascal you'll find a little amp I built basically the same as this using 12au7s or 12at7s in the power section. That worked fine at about 20 volts although it was much better and louder at 30ish.
^ Ah, yeah, of course. I remember the Rascal now. I have probably inadvertantly ripped you off, and many apologies for that. Please send the invoice to the usual address.
I recall that was going to build a Rascal, and you kind of told me to go for one of Frequency Central's amps instead. Oooops - he'll probably send me an invoice now, as well :icon_wink:.
Quote from: slacker on April 23, 2012, 01:42:48 PM
If you search the forum for rascal you'll find a little amp I built basically the same as this using 12au7s or 12at7s in the power section. That worked fine at about 20 volts although it was much better and louder at 30ish.
Good sound clips, Transistor source follower is a good add, It sounds quite, which is not bad, I am always ready to admit when I am wrong.. Will have to build one. I wonder if a ECC99 at 50V would be better though..
Hi
RE: 12U7 and 12AU7. Just sub one for the other - in the circuits I've built (9 to 32V supply) they sound and scope more-or-less the same.
If you want to avoid loading the 12AU7 (it is quite a low load at present) an option would be to stick a class A MOSFET output stage on the end. About 6 components and is perfect for a 20V supply. The most expensive part - an MTP3055 or similar - only costs about $2, which is cheaper than the line transformer. There have been several threads about simple class A MOSFET amps.
cheers
Oh boys I really hate you now: first comes markeebee and his amp, then comes the rascal.... Ok I will buy another 12au7 and start experimenting, it seems I have no alternatives right now.
Quote from: allesz on April 25, 2012, 09:09:13 AM
Oh boys I really hate you now: first comes markeebee and his amp, then comes the rascal.... Ok I will buy another 12au7 and start experimenting, it seems I have no alternatives right now.
I just bought 20 off ebay for $20.. some may be bad or weak, but at that price.. look around you can find really good deals. Preamp tubes almost never fail and in Hammond organs they would change them every year no matter what, so there are a lot of them sitting in a box in someones house and they sell them on ebay. like here
http://www.ebay.com/itm/RCA-12AU7-Clear-Top-Tubes-/120898639830?pt=Vintage_Electronics_R2&hash=item1c261ed7d6#ht_1801wt_1396
Most likely some of these are weak, and one may not work, but a "Weak" tube will be OK for this design as we are not pushing high voltage.
> enlighten with regard to the theory
Life is too short.
Use Rules-Of-Thumb.
Triode Power Amplifer: find the plate resistance. This varies with plate voltage and grid bias, and on a "tuner" tube will vary a LOT. You want the zero grid voltage line from zero to your nominal plate voltage.
ECC86 data:
http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/ECC86.pdf
Page 9: Vg=0V hits Va (plate voltage) of 20V at 8mA current.
20V/8mA= 2.5K ohms.
You have two sections in parallel, 1.2K.
Rule-Of-Thumb:
For good power and distortion, the load should be double that.
For less power and lower distortion, or for high supply voltage, go 3X to 5X.
To force maximum power from insufficient supply voltage, rough-match the plate resistance. With a "good audio triode" the distortion will be a bit obvious. With this tuner triode the distortion will be gross, but not sour (up to a point).
Since your voltage is awful low, you would compare 2X (2.5K) and 1X (1.2K) loading, to see how the power and distortion work out.
In each case you "must" re-optimize standing bias to center the tube on the available current/voltage limits for that load. Put DC voltmeter on cathode bias resistor. Note reading at idle and at FULL WAIL. If it idles 0.7V and wails 2V or 0.3V, that's way too much bias shift (unless it sounds kewl). Change bias resistor until the idle/wail shift is under 50%, under 20% for "clean". Or until you decide you like it not-centered (a valid musical choice).
Thanks PRR. That makes sense and is very helpful.
I found the plate resistance (like iccaros suggested above, thanks iccaros) but I wasn't aware of the rule of thumb about doubling it. Seems I fumbled my way there anyway.
I have to say that I quite like the sound of this little thing, but on my next build I'll try a lower impedance tap and monkey around with the bias as you suggest. A bit more output and a bit less distortion might be good things. Thanks again.