Liriodendron
Tele-Holic
I've done two of them so far, one a combo, one a head. They've been a bit of work in progress. I think the combo I'm going to rework into a head at some point, I don't like the sound of the cab. Aspect ratio isn't right. But the amps have been great.
The first one I built with with EL84s, but I actually started the design out with EL844s to get a bit lower power/volume. Turned out I didn't like those tubes, so I stuck with EL84s. As it sits, it probably only puts out 7-8W (I could boost the voltage a tad bit and still keep it in Class A) but it can go full tilt without fizzling out, so I'm pretty happy. I've since adjusted the design so I could push it to the max 10W, and perhaps if I build another, I will do that.
I don't know if I'll post the schematic just yet, but it's only 3 knobs, Volume, Treble and Bass. It's somewhat similar to an AC30 but with no cathode follower. And I've biased the PI to be a bit warmer. To me it sounds kind of in between a Marshall and a Vox, which I like! Lots of chime, but not too bright and not too much mids. And not too fizzy like an AC30 PI can sound.
I used the OT off a Gibson GA-5 crest, which is a 10W PP, 10K primary. It's a MM and it's pretty small, but holds together nicely. It's also quite inexpensive (I paid $60 for it). The PT is custom, but it's a tad on the low side. Again, I may modify or rework. I think there's an Edcor that might be right for this amp.
As you can see, I used a choke. That forms a Pi filter between the plate and screens, and ideally it's to keep the screen voltage up high, almost at the plate voltage, but highly regulated. The bonus is it gives the amp some nice dynamics.
The other one I built with 6V6s and used more of a Plexi type preamp. I've been fine tuning it and it's pretty good. It doesn't have NFB now and I might add that in and switch the "cut" control to a "presence". As of now the preamp can overpower the power amp a bit, so cutting a bit of gain and/or bass will be in order.
As you might recognize, the cab and chassis are an old Judybox. I sold the transformers and used a rather unique combo here. The PT is an Edcor, and the OT is a MM for a Supro meant to run 6L6s in Class A at 30W, 5K primary, 4ohm secondary. I run 6V6s at 10K with 8ohm on the secondary. It was also a cheap wind, only around $60 as well although significantly bigger than the Gibson. I can't say which sounds better or different as both the amps are so different, but both work well for this type of power amp. I'm tempted to snub my nose at the Gibson because it's so small, but so far I can't detect any real weakness. I think they're both very simple, probably primary/secondary/primary winds and hence the low cost.
The class A power amps do sound a bit different though. They sound "cleaner" than a typical AB power amp. I'd assume that's because there's no crossover distortion, but perhaps also it might be due to their harmonic content. I purposely inbalanced the phase inverters on both to give them more 2nd harmonic distortion. But still some may perfer the actual tone of an AB power amp over this. I actually prefer it. It doesn't sound anything like a SE class A amp. It has none of the rattiness, thinness or harshness that sometimes can happen with SE amps. And of course PP, even without NFB is much more damped than SE, so they are quite tight given the right bass response. They overdrive very, very easily and the volume is only a touch more than a typical 5W SE amp with 6V6 or EL84.
Cost is fairly comparable as the OTs are cheaper - you could also use a cheap 5E3 OT at 8K, it would still likely be "almost" pure class A if the bias and voltages are right. The choke and extra tube adds a bit more cost and of course the PT has to be able to supply a lot more current. These run around 120mA (they are NOT efficient) and of course, you need low voltages. One could easily add a VVR mosfet and associated circuitry and have it adjustable between true Class A and a hot AB. I don't notice a huge difference in tone even when I bias them a bit cold, but both these I have them running a lot of current!
Overall I've been quite pleased with them. I'm not sure they're something that many people would buy, given cost and lack of headroom, but for someone looking for great overdriven tone at near SE volume, these are the ticket!
The first one I built with with EL84s, but I actually started the design out with EL844s to get a bit lower power/volume. Turned out I didn't like those tubes, so I stuck with EL84s. As it sits, it probably only puts out 7-8W (I could boost the voltage a tad bit and still keep it in Class A) but it can go full tilt without fizzling out, so I'm pretty happy. I've since adjusted the design so I could push it to the max 10W, and perhaps if I build another, I will do that.
I don't know if I'll post the schematic just yet, but it's only 3 knobs, Volume, Treble and Bass. It's somewhat similar to an AC30 but with no cathode follower. And I've biased the PI to be a bit warmer. To me it sounds kind of in between a Marshall and a Vox, which I like! Lots of chime, but not too bright and not too much mids. And not too fizzy like an AC30 PI can sound.
I used the OT off a Gibson GA-5 crest, which is a 10W PP, 10K primary. It's a MM and it's pretty small, but holds together nicely. It's also quite inexpensive (I paid $60 for it). The PT is custom, but it's a tad on the low side. Again, I may modify or rework. I think there's an Edcor that might be right for this amp.
As you can see, I used a choke. That forms a Pi filter between the plate and screens, and ideally it's to keep the screen voltage up high, almost at the plate voltage, but highly regulated. The bonus is it gives the amp some nice dynamics.
The other one I built with 6V6s and used more of a Plexi type preamp. I've been fine tuning it and it's pretty good. It doesn't have NFB now and I might add that in and switch the "cut" control to a "presence". As of now the preamp can overpower the power amp a bit, so cutting a bit of gain and/or bass will be in order.
As you might recognize, the cab and chassis are an old Judybox. I sold the transformers and used a rather unique combo here. The PT is an Edcor, and the OT is a MM for a Supro meant to run 6L6s in Class A at 30W, 5K primary, 4ohm secondary. I run 6V6s at 10K with 8ohm on the secondary. It was also a cheap wind, only around $60 as well although significantly bigger than the Gibson. I can't say which sounds better or different as both the amps are so different, but both work well for this type of power amp. I'm tempted to snub my nose at the Gibson because it's so small, but so far I can't detect any real weakness. I think they're both very simple, probably primary/secondary/primary winds and hence the low cost.
The class A power amps do sound a bit different though. They sound "cleaner" than a typical AB power amp. I'd assume that's because there's no crossover distortion, but perhaps also it might be due to their harmonic content. I purposely inbalanced the phase inverters on both to give them more 2nd harmonic distortion. But still some may perfer the actual tone of an AB power amp over this. I actually prefer it. It doesn't sound anything like a SE class A amp. It has none of the rattiness, thinness or harshness that sometimes can happen with SE amps. And of course PP, even without NFB is much more damped than SE, so they are quite tight given the right bass response. They overdrive very, very easily and the volume is only a touch more than a typical 5W SE amp with 6V6 or EL84.
Cost is fairly comparable as the OTs are cheaper - you could also use a cheap 5E3 OT at 8K, it would still likely be "almost" pure class A if the bias and voltages are right. The choke and extra tube adds a bit more cost and of course the PT has to be able to supply a lot more current. These run around 120mA (they are NOT efficient) and of course, you need low voltages. One could easily add a VVR mosfet and associated circuitry and have it adjustable between true Class A and a hot AB. I don't notice a huge difference in tone even when I bias them a bit cold, but both these I have them running a lot of current!
Overall I've been quite pleased with them. I'm not sure they're something that many people would buy, given cost and lack of headroom, but for someone looking for great overdriven tone at near SE volume, these are the ticket!
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