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Greg Robinson's mini tube amp

PostPosted: Tue Apr 23, 2013 2:18 pm
by Greg Robinson
Hi everyone,
This will be the first challenge I have participated in, as I think I've got something well suited that I will have the time for.
My entry will be a mini tube amp, around 1 watt or less, using some "reclaimed" parts. Here's a picture:

In the picture you can see a power transformer from a Vox Lil' Night Train, which a client brought to me recently, suffering from a clicking sound. It took me a while to track down the problem, but it turned out that one of the windings on the centre-tapped high voltage secondary had gone intermittently open. I removed the end-bells to inspect the windings and see if there was any easy way to repair it, but wherever the fault is, it's beneath the faraday shield (which if I damaged would have made it useless in such a compact amp) and the nasty potting compound. End result was that my client decided to go with a Hammond replacement transformer, and left this one with me.
I'll be able to use it with just half the high voltage secondary by using a bridge rectifier, rather than the full wave it was originally intended for, so long as I don't pull too much current from it, and am not too bothered by it getting warm. It had been running a little warm in the Vox, without burning up, and the Vox did have 1 more tube than I'm intending to use, so I'm not too worried. Maybe I'll even try pulling it apart to fix it at some point.

Also pictured is an output transformer from a Fender Blues Junior, from a client who had me install an upgraded Heyboer replacement. As they had no use for it, they left it with me. I don't re-use parts like this in my business, so I'm going to call it "reclaimed". They go for around $15 fairly regularly from a number of vendors on blowout anyway.

Also there are a couple of tube sockets (and matching shields) out of a cheap Chinese tube stereo hifi, which were a little loose, and were replaced.

There are four mismatched 1MEG pots, which were either upgraded or replaced because they were scratchy.

One 12AX7/ECC83, and one 12AU7/ECC82 which were removed from high-gain amps due to microphony, but should be OK in this low powered amp.

And finally, a brand-new enclosure, which is 6-3/4" x 4-5/8" x 2-1/8", which I purchased from a local retailer some time ago for around $20.

I'll also need some electrolytic capacitors, I'm pretty sure I've got some used ones that are still good (a lot of people want to "upgrade" their sound by replacing filter caps that still have 10-20 years life in them with whatever "mojo" caps someone has convinced them of, and there's only so far that I can talk people out of it, sometimes it's best to just do what they want). A few coupling caps will be needed, I'll use some cheap metalized polyester ones and a couple silver mica, they're about $1 each, and some resistors, about $0.01 each.

I keep a lot of these used parts around for prototyping or the occasional absolute emergency (I can't keep every transformer in stock, but if someone has a gig the next night, and I have the right one second hand, it's better than nothing, and they can keep using their gear until I receive a new replacement).

I'm sure there's some things I've forgotten, but I'll add up a total towards the end.

Below I've posted a schematic of my basic plan:
Challenge amp.JPG

So, the idea is two 12AX7/ECC83 gain stages, fully bypassed for maximum gain, gain control between them. Then I've decided to use the "Roy Bean" tone stack as shared by Balijukka on the AX84 forum. He calls the two controls the "Judge" and the "Bear". Neat name.
It's basically the "Tilt" control of Big Muff Pi fame, with an added mid boost/cut control. I had been toying with a similar idea for a while, but the values are all very interactive and it was difficult to come up with something that works, so it was very nice to find that someone had done the hard work for me! This will be the first time I have used this tone stack, so it should be interesting.
After the tone stack is a "master volume" type thing, but the two previous gain stages should put out more than enough signal to overdrive the power section (hence the huge grid-stopper), so this will have some grit of it's own.
This will go into a self-split long-tailed pair 12AU7/ECC82 push-pull amp, which should produce around a watt at max output. The output transformer from the Blues Junior is 8k-8ohms, which is too heavy of a load for a 12AU7, so instead I will mis-match the speaker load to give me 16k-16ohms.

Anyway, I'll probably make some changes to the amp after I first get it up and running, this is just a preliminary design. Any input would be great.

Hope everyone's ok with my interpretation of "reclaimed" materials, if you think I'm not, let me know!

Good luck to all the other participants!

Re: Greg Robinson's mini tube amp

PostPosted: Tue Apr 23, 2013 2:32 pm
by Jim McConkey
This project is clearly as reclaimed as any! The rules say the project can be any sort of "instrument," but I don't see requirement that it be the playable kind. Go for it!

Re: Greg Robinson's mini tube amp

PostPosted: Wed Apr 24, 2013 11:04 am
by Nick Middleton
I'm looking forward to hearing this one when your done!

Re: Greg Robinson's mini tube amp

PostPosted: Wed Apr 24, 2013 2:49 pm
by Bryan Bear

Re: Greg Robinson's mini tube amp

PostPosted: Wed Apr 24, 2013 6:28 pm
by Warren May
Welcome to the pool, Greg. Great "reclaimed" project. It would be nice to see others join the party and I think the rules allow a lot of different concepts of "reclaimed" that maybe your project will inspire others to come in to make it more interesting.

Re: Greg Robinson's mini tube amp

PostPosted: Thu Apr 25, 2013 1:29 am
by Greg Robinson
Thanks everyone!

I've done a bit of drilling of the enclosure:

I've drilled holes for the control pots, input and speaker jacks, tube sockets and screw mounts (which I've tapped), power switch, cut and filed a hole for an IEC 14 power socket with inbuilt fuse socket, drilled a hole for the chassis safety ground lug right next to the IEC, and holes for transformer leads.
I haven't drilled mounting holes for the transformers yet, as I will try to find a position with least coupling between the two once it's up and running (which isn't always exactly on different axes), before mounting them permanently.

I've got a DPDT power switch that was $4.10, and the IEC socket retails for $6.95. The input and speaker jacks were $2.00 each, from memory. I get a lot of wholesale prices for parts, so I'm just going with retail prices from the website of one of the local (and expensive) electronics hobby suppliers. Looks like I'll be well under budget regardless.
I've got a bunch of scraps of wire (a lot from trimming transformer leads) that I'll be using, and all the screws and nuts and bolts come from my old jar of bits saved from scrapping various bits of gear.

Re: Greg Robinson's mini tube amp

PostPosted: Thu Apr 25, 2013 8:37 am
by John Kingma
Cool idea for the challenge.

I've got a little one watt tube amp that a friend built. It's awesome.

I can't wait to see what you come up with here.

Re: Greg Robinson's mini tube amp

PostPosted: Thu Apr 25, 2013 3:05 pm
by Hans Bezemer
Cool project Greg!

Re: Greg Robinson's mini tube amp

PostPosted: Thu Apr 25, 2013 3:16 pm
by Greg Robinson
Thanks Jim, Nick, Bryan, Warran, John and Hans for looking in on my project, and the encouragement!
And John, I remember you posting that amp, it looked cool!

I've made some more progress:

I've mounted all the jacks, pots, tube sockets, a bridge rectifier, the switch, and wired the power socket, switch, heater wires, safety grounds, etc.

I decided to add a humdinger, which you can see mounted on the left, to balance the AC heaters.
I will also elevate them using a voltage divider/bleeder resistor.

You'll also notice a 1 ohm 3 watt metal oxide resistor mounted on the humdinger pot, which is in series with the 6.3V transformer secondary.
As the Vox the power transformer came from ran 3 tubes, it can supply 900mA, but as I am only using two drawing 600mA, and the transformer is small and poorly regulated, the heater voltage was sitting around 7.5V, too high for the tubes, which would have caused premature tube failure.
The 1 ohm resistor drops that to about 6.5V, which is within a 5% tolerence and good enough, but I expect that once the high voltage circuit is wired up and drawing current, it will drop a little further.

You can't see it in this picture, but as I am wiring this mostly point-to-point, I am making use of three dimensions. The heater wiring is quite low and close to the chassis, and the output transformer leads fly above them and approach the tube socket from directly above. Those two parts of the circuit aren't very susceptable to interfering with each other, but it is good practise to avoid coupling and interaction, so as to minimise noise.

Next step I think will be wiring the tone stack and adding some terminal strips for flying components.

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 8:06 am
by Greg Robinson
Bit more progress:

I've wired up the tone stack, and a few miscellaneous parts. It looks a mess, but should work fine. I didn't have any 500pF capacitors on hand, so I used two 250pF in parallel instead.

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 1:48 pm
by Greg Robinson
Ok, and here it is wired up!

I know it looks ugly as all hell, but I've fired it up, and once I'd adjusted the placement of the transformers and adjusted the humdinger (using an oscilloscope, because there really is that little noise), you wouldn't be able to tell it's on until you plug in and strum.
The point-to-point wiring may look hap-hazard, but each component is placed so as to minimise interaction and coupling between nearby parts, and I managed to get away without needing to use any terminal strips at all.
The large electrolytic capacitors are stuck down using silicon sealant, and the whole wiring array is actually quite rigid.

Here's an updated and more complete schematic:
Challenge amp 2.JPG

I made some changes to the earlier iteration based on what I had on hand, and what better fit the layout requirements.

Guess I'd best put together a parts and price list?

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 2:03 pm
by Charlie Schultz
Nice! Reminds me of the Heathkits I built back in the '60s.

Where does the digital whizbang get installed?? <g>

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 2:42 pm
by Greg Robinson
Thanks Charlie!

Haha, no digital here!

I guess that in my line of work I'm pretty spoilt, in that I get to try out soo much and such a variety of equipment, and while many of the digital toys are amazing, and allow you to create sounds that are not otherwise possible (not to mention being lighter, cheaper, lower powered, etc), I always find myself preferring a guitar and tube amp, connected with a cable. Call me a snob!

I am considering adding a boost, using an LND150 depletion mode FET (which many might consider sacrilege! Silicon? NO!), to add a bit more grind, but I'll wait until tomorrow so I can crank it before passing judgement (1 watt is pretty loud!).

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 4:27 pm
by Greg Robinson
Ok, I've been tallying up materials and prices, so here it goes:
Challenge pricelist.JPG

This isn't what I actually paid for most of the parts, I usually buy wholesale or from cheaper suppliers or in bulk, but I thought I should just go with fairly high-priced local retailers to base my costs on to make things a little more challenging. I do shop at the three retailers I've listed though, so it's not completely misleading.

Those prices are in Australian Dollars, so that works out to about $49.30USD.

Guess I'll have to add some knobs, but I think I'm also going to add a line-out so I can run it into a power amp, and I think I just have to add the LND150 boost stage idea. Maybe I'll do a bit of a paint job too?

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 4:31 pm
by Jason Rodgers
Jim McConkey wrote:This project is clearly as reclaimed as any! The rules say the project can be any sort of "instrument," but I don't see requirement that it be the playable kind. Go for it!

Screw the rules! If he can Macgyver a tube amp from a Radio Shack sneeze, then I'm all for it!

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 4:35 pm
by Greg Robinson
Haha, thanks Jason!

Re: Greg Robinson's mini tube amp

PostPosted: Fri Apr 26, 2013 4:43 pm
by Greg Robinson
Oops! Forgot a couple of 3 watt metal oxide resistors from Evatco at $0.18 each, so make it $48.30.

Re: Greg Robinson's mini tube amp

PostPosted: Sat Apr 27, 2013 9:21 am
by Hans Bezemer

You've made some progress!!!
Looking forward to sound clips!

What colour will it be painted?


Re: Greg Robinson's mini tube amp

PostPosted: Sat Apr 27, 2013 12:55 pm
by Greg Robinson
Hi Hans,
Yeah, I've been tearing through this while I have the time. I actually think I've spent more time documenting it here than I have on design and assembly though!

Not sure how I'll paint it, painting the entire chassis would require disassembly, which would be a right pain, but I've got a couple of ideas, I'll keep that a surprise!

I've installed a line out jack, with a 10k level control, which is in parallel with the speaker load, which should prevent any arcing/flyback voltages should an open or unterminated speaker cable be used. I've also added two 8 ohm resistors in series for a 16 ohm load that switches in when there is no speaker plugged in, so that the line out can be used by itself to run into a power amp or as a distortion/boost effect before another amp.

I've also added an LND150 depletion mode FET boost stage at the front end, which can be switched in or out with a switch on the top, with a level/gain control too. This can provide ludicrous amounts of gain, which does get quite noisy at maximum, and it does oscillate with all the gains maxed, but I actually kind of like it, it gives it this weird gating, swirling sound that is just over-the-top and ridiculous. Think Zvex Fuzz Factory with the "Stab" control turned all the way up.

Anyway, here's the final schematic, I think I'm done playing with the circuit.
Challenge Amp ver 1.3.JPG

And a photo of the guts:

A shot of the front:

And the back:

And here's an updated bill of materials:
Materials list.JPG

$66.35 Australian Dollars is about $68.22USD today.

Not sure I've got any knobs that would suit on hand, I'll have to track something down.
Onto some sort of paint job next.
When that's done, I'll record some clips. I don't have a great recording setup right now, I'll be running an SM57 into a little Behringer mixer then into the inbuilt soundcard on my laptop, which I haven't used before, so I'm not sure how it will turn out.

Re: Greg Robinson's mini tube amp

PostPosted: Sun Apr 28, 2013 5:12 pm
by Steve Senseney
I always get into trouble when I try to play with any electronics.

You project has made me more overwhelmed than I usually am.

Nice work.

Don't go over the cost limits with the paint job..