Greg Robinson's mini tube amp
Posted: Tue Apr 23, 2013 2:18 pm
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: 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!
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: 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!