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I'm new to Ampage, and as such I thought I'd start off by presenting a bit about my latest amp project, the Miniphant. Miniphant was assembled in January and since then has somewhere in the range of 150 hours of use on it, used in practice sessions with a live drummer and onstage miked through a PA. There has been one major maintenance overhaul since then, in which a defective potentiometer was replaced and the speaker was swapped. But mostly, it's been a reliable companion and definitely worth building.
It was tempting to post this in the 'conversions' section but it's really not a conversion because it's a from-scratch circuit and design. It is built in the chassis of a scruffy 1970s solid-state amp that I bought in barely-working order as junk. The old thing might have been a Univox but was missing a logo, it was hard to sell. As a solid-state chassis, it had no punch holes, so I was free to punch them for the tubes wherever I wanted. The iron came from a 1960s console stereo chassis, which originally ran 2 EL84 tubes and one 12AX7 per channel. Some of the power resistors came from an old rear-projection TV that the neighbors discarded and the local children smashed into tiny pieces, making it easy for me to pick up whatever components looked handy. Not so much a conversion as a junkpile project, designed around parts that I had and wanted to put into something. Caps, though, are all brand new because I'm not that cheap or into watching smoke pour out of things!
I've drawn a schematic of the circuit here:
http://img686.imageshack.us/img686/9532/miniphant.png
The output stage might appear strange to some. As I mentioned, the power and output transformers are for a stereo amp with 2 EL84s per channel. So why am I using EL34 output tubes? Mostly because it's cheaper to buy 2 EL34s then it is to buy 4 EL84s, just slightly. Also because I've built EL84 amps before and had to deal with the darn tubes going microphonic or arcing out on me. EL34s are bigger and possibly tougher. Electrically, there's a huge safety factor on B+ voltage, 310 V is hot on an EL84 but it's nothing to an EL34! I went with the EL34BSTR because it's a big, heavy looking tube and is reputed to be ruggedly built, not at all microphonic, and so far it seems to live up to that. While 2 EL34s can make 50W, mine only make about 30W. They're limited by the power supply voltage, about 310V, and by the load impedance of the output transformers.
The output transformer wiring may also look a bit weird - I've parallelled the primaries of the stereo OPTs and put the secondaries in series. This splits the power evenly between both, and also changes the impedance ratio. Originally the transformers had a 50:1 ratio, likely indicating a 8k plate-to-plate load for 2 EL84s, matching a 3.2-4 ohm speaker. They now produce a 25:1 voltage ratio wired this way, giving a 5k plate-to-plate load for a 8 ohm speaker. This is a bit high for 2 EL34s, but then again, maxing them out for power isn't possible with this power supply anyway.
As the impedance load is just a bit high, a load line may cut below the pentode 'knee' and cause some heavy screen current during high-volume playing. To avoid screen current spikes, I nailed the suppressor grids to the bias supply instead of ground, just to keep things down a little there. So far, the tubes haven't complained.
The preamp is also a bit strange. It's pretty much a copy of the 'cascaded' Marshall JCM800 preamp, but has a couple tweaks to it. The first tube is wired with a footswitchable feedback loop which keeps the gain low and the frequency response flat - upon using the footswitch to ground the junction of the 33K resistor and the 0.44 uF capacitor, the gain is strongly boosted due to the loss of the negative feedback, and there is a further gain-boost with a brightness boost due to the 0.44 uF capacitor now working as a cathode bypass - so the footswitch provides both a gain and a voicing adjustment.
There are also two volume controls, a standard master volume control ("LEVEL") and a post-PI master volume ("VOLUME"). This is to allow the use of the second footswitch button - as most footswitches have 2 buttons, I wanted to use them both - the master volume pot may be lifted from ground by the footswitch, giving an adjustable loudness boost. This isn't a pure loudness boost, because PI clipping is a major factor if you turn the MV up, and it's a great way to get a bit of extra gain without adding preamp stages, PPMV is good like that, but it's a good bit of extra footswitchable adjustment.
The power supply is a voltage doubler - that seems weird - it's what this transformer was designed for, apparently, that's how the console stereo used it. I do wonder if it is actually a B+ transformer at all, or if it's simply a dual-primary 6.3 VAC filament transformer originally designed for 120/240 V operation! I'm sure that, if I could find such a transformer on the cheap, and made sure it had sufficient VA to handle the B+ load as well as the filament current, it would work pretty well in this circuit. The bias supply is a complete hack, lacking a separate bias winding it's the best I could do, problem with that big dropping resistor is that it takes a few seconds to bring up the bias voltage and as a result there is a bit of a B+ current surge when it turns on. Can't use a fast-blow fuse in the B+ in this amp. So far, the tubes haven't complained, at least.
I think this may actually be a good design! It's the sad sort of thing that you just can't buy anywhere. Not fancy enough to be made into a 'boutique' amp product. Too classic-rock sounding for any company to make. If I had magic parts access and could make this thing and sell it for $350 or so I bet it would sell, but I don't know where the Chinese get their parts from so I can't. There may only ever be one Miniphant. It isn't the awesomest amp in the world, but it doesn't suck. It's a Chevy sort of amplifier. Sounds like an old Marshall, sort of. Nice sound, cuts through a mix really well. Not a high-gain buzzsaw amp. But that's what Heavy Metal pedals are for! Just a plain, regular amp here.
Haven't done a photo shoot, and this poor amp spends most of it's time in the band trailer now, but if you look to the left of this photo you'll see me (wearing the hat) and Miniphant just to my right, behind me.
Here's me playing with my band, I'm using Miniphant no pedals or anything, just plain amp tone, the other guitarist is a bit hard to hear here because the cameraman was on my side, but that's fine, you can hear the Miniphant:
YouTube - NO MEANS YES
Perhaps someday I'll get some good photos up-close of the amp itself. Until then, well, curious what people think! I did post about Miniphant on alt.guitar.amps but wow that place is full of noise. I'm curious what Ampage is like.
It was tempting to post this in the 'conversions' section but it's really not a conversion because it's a from-scratch circuit and design. It is built in the chassis of a scruffy 1970s solid-state amp that I bought in barely-working order as junk. The old thing might have been a Univox but was missing a logo, it was hard to sell. As a solid-state chassis, it had no punch holes, so I was free to punch them for the tubes wherever I wanted. The iron came from a 1960s console stereo chassis, which originally ran 2 EL84 tubes and one 12AX7 per channel. Some of the power resistors came from an old rear-projection TV that the neighbors discarded and the local children smashed into tiny pieces, making it easy for me to pick up whatever components looked handy. Not so much a conversion as a junkpile project, designed around parts that I had and wanted to put into something. Caps, though, are all brand new because I'm not that cheap or into watching smoke pour out of things!
I've drawn a schematic of the circuit here:
http://img686.imageshack.us/img686/9532/miniphant.png
The output stage might appear strange to some. As I mentioned, the power and output transformers are for a stereo amp with 2 EL84s per channel. So why am I using EL34 output tubes? Mostly because it's cheaper to buy 2 EL34s then it is to buy 4 EL84s, just slightly. Also because I've built EL84 amps before and had to deal with the darn tubes going microphonic or arcing out on me. EL34s are bigger and possibly tougher. Electrically, there's a huge safety factor on B+ voltage, 310 V is hot on an EL84 but it's nothing to an EL34! I went with the EL34BSTR because it's a big, heavy looking tube and is reputed to be ruggedly built, not at all microphonic, and so far it seems to live up to that. While 2 EL34s can make 50W, mine only make about 30W. They're limited by the power supply voltage, about 310V, and by the load impedance of the output transformers.
The output transformer wiring may also look a bit weird - I've parallelled the primaries of the stereo OPTs and put the secondaries in series. This splits the power evenly between both, and also changes the impedance ratio. Originally the transformers had a 50:1 ratio, likely indicating a 8k plate-to-plate load for 2 EL84s, matching a 3.2-4 ohm speaker. They now produce a 25:1 voltage ratio wired this way, giving a 5k plate-to-plate load for a 8 ohm speaker. This is a bit high for 2 EL34s, but then again, maxing them out for power isn't possible with this power supply anyway.
As the impedance load is just a bit high, a load line may cut below the pentode 'knee' and cause some heavy screen current during high-volume playing. To avoid screen current spikes, I nailed the suppressor grids to the bias supply instead of ground, just to keep things down a little there. So far, the tubes haven't complained.
The preamp is also a bit strange. It's pretty much a copy of the 'cascaded' Marshall JCM800 preamp, but has a couple tweaks to it. The first tube is wired with a footswitchable feedback loop which keeps the gain low and the frequency response flat - upon using the footswitch to ground the junction of the 33K resistor and the 0.44 uF capacitor, the gain is strongly boosted due to the loss of the negative feedback, and there is a further gain-boost with a brightness boost due to the 0.44 uF capacitor now working as a cathode bypass - so the footswitch provides both a gain and a voicing adjustment.
There are also two volume controls, a standard master volume control ("LEVEL") and a post-PI master volume ("VOLUME"). This is to allow the use of the second footswitch button - as most footswitches have 2 buttons, I wanted to use them both - the master volume pot may be lifted from ground by the footswitch, giving an adjustable loudness boost. This isn't a pure loudness boost, because PI clipping is a major factor if you turn the MV up, and it's a great way to get a bit of extra gain without adding preamp stages, PPMV is good like that, but it's a good bit of extra footswitchable adjustment.
The power supply is a voltage doubler - that seems weird - it's what this transformer was designed for, apparently, that's how the console stereo used it. I do wonder if it is actually a B+ transformer at all, or if it's simply a dual-primary 6.3 VAC filament transformer originally designed for 120/240 V operation! I'm sure that, if I could find such a transformer on the cheap, and made sure it had sufficient VA to handle the B+ load as well as the filament current, it would work pretty well in this circuit. The bias supply is a complete hack, lacking a separate bias winding it's the best I could do, problem with that big dropping resistor is that it takes a few seconds to bring up the bias voltage and as a result there is a bit of a B+ current surge when it turns on. Can't use a fast-blow fuse in the B+ in this amp. So far, the tubes haven't complained, at least.
I think this may actually be a good design! It's the sad sort of thing that you just can't buy anywhere. Not fancy enough to be made into a 'boutique' amp product. Too classic-rock sounding for any company to make. If I had magic parts access and could make this thing and sell it for $350 or so I bet it would sell, but I don't know where the Chinese get their parts from so I can't. There may only ever be one Miniphant. It isn't the awesomest amp in the world, but it doesn't suck. It's a Chevy sort of amplifier. Sounds like an old Marshall, sort of. Nice sound, cuts through a mix really well. Not a high-gain buzzsaw amp. But that's what Heavy Metal pedals are for! Just a plain, regular amp here.
Haven't done a photo shoot, and this poor amp spends most of it's time in the band trailer now, but if you look to the left of this photo you'll see me (wearing the hat) and Miniphant just to my right, behind me.
Here's me playing with my band, I'm using Miniphant no pedals or anything, just plain amp tone, the other guitarist is a bit hard to hear here because the cameraman was on my side, but that's fine, you can hear the Miniphant:
YouTube - NO MEANS YES
Perhaps someday I'll get some good photos up-close of the amp itself. Until then, well, curious what people think! I did post about Miniphant on alt.guitar.amps but wow that place is full of noise. I'm curious what Ampage is like.
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