Really simple low-volt valve amp

[markeebee]

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:



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:





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  I originally made a filter between the heater supply and the anode supply, but I found that it wasn't necessary - that explains the 3W resistor and the mini-tagboard.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.....

[Earthscum]

I can't say anything about the design, but the finished product is superb!

Whippet Huffer... lol. I love it. Good ol' NO₂. 

[petey twofinger]

well done , that is really nice looking .

[tiges_ tendres]

I'm going to pretend this amp is named after my favourite activity, which is sniffing miniature grey hounds.

[a soBer Newt]

Can you tell me what the current draw of this is?

[markeebee]

Current draw is about 650mA to 700mA.

[merlinb]

Is that textured paint? How did you manage to paint only the core and clamp of the transformer while leaving the laminations untouched?

[Taylor]

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. 

Really cool amp Mark. I want to buy/build/steal one. Do you live in a first floor flat?

[markeebee]

Is that textured paint? How did you manage to paint only the core and clamp of the transformer while leaving the laminations untouched?

It's this stuff, got it in B&Q:



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.

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.

[Earthscum]

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. 

I gotta disagree... I don't think puppies will make you fish on the floor. 

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.  I'd sniff one if I had it.

[zambo]

That amp is really really cool! I would love to hear it. it runs on 20 volts? So sweet!

[nosamiam]

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?

[allesz]

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
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

[markeebee]

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!

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.

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
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?    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.

[allesz]

Swapping tubes shoul be no problem.... but since I have just one 12au7.... I will have to buy one more just to try . 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.

[deadastronaut]

what a wonderfully bent amp....nice one mark, looks great!..and suits brighton  

[zambo]

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.

[markeebee]

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.

[bluebunny]

Nice.  (Another one for the build list, methinks!)  Girl in the poster seemed to like it too.

BTW, mess looks much like my "office"... 

[Scruffie]

Where would I find one of these transformers Mark? Can't seem to spot one unless i'm searching for the wrong thing.