21-05-2021, 06:30 PM
Over the last few months I've been fettling away on a new valve amp build. I swore this time I'd build something more simpler but as one does I've got a bit carried away and ended up going a bit off piste'. As all valve amp designs start I started experimenting first with the OP stage trying to find a good fit a few odd valves I've had stashed away for a while and mating them with some clones of Dynaco ST70 OPT's that Ed Dinning wound for me.
OP stage - After much experimenting with various odd valves including QQV03-20's, 6BG6GA's & 6DQ6B's I settled on 6146's running in class AB2. The OP valves are really wasted here as I could get over 70w with them in AB2 however 30w is plenty enough and I'm more interested in the direct connection of the driver stage to the OP stage that AB2 gives me rather than raw power.
Driver stage - The OP stage is driven by 12BH7 cathode followers run off a CCS this gives plenty of oomph at low-ish distortion. I'm not after ultra low THD, I'm after "sculpturing" the sound, adding just the right amount of 2nd harmonic, if I wanted low THD/straight wire with gain I'd use trannys and loads of NFB.
Phase splitter - again I'm using transistors for a CCS to get good AC balance using an ECC82 LTP, the only slight set back being the LTP has a bit of 3rd harmonic distortion,not to worry about though.
The part of the circuit that has been vexing me the most is the tone control/IP gain stage. I tried various circuit modifications to an active Baxandall tone circuit by for instance trying to increase gain by subbing an ECF82 for the 12AU7/12AX7 OP "opamp" not realising that than gain has to be spot on else the circuit won't work, see att. I'd painted myself into a corner by not having a clear design right from the start and so only had 1 1/2 B9A valve bases to play with as I fettled the chassis before having a clear idea of what I would build.
Lastly - power supply... again, like all my amp builds I tend to base my design around what bits I have, rather than the best or correct design,so with this amp as with the last one, the 807 amp, I used a choke I had as well as four 700u 250v caps, two in series to give 350u before and after the 15H choke. I had to wind two power transformers- the original ST70 clone was very loud and badly wound so I re-wound it to give me 0v, 450v, 470v, 500v HT, the original had a 330v 0v 330v HT sec with a 55v bias tap. With the extra room on the bobbin I overwound the primary and secondary with heavier gauge wire. The original pri was 220v, I rewound for 245v and put some extra turns on to give a higher V/T ratio. I also rewound a Japanese tfmr to give me a 100v bias supply & 100mA, a 3A heater winding and a 250v supply for the g2 supply. With AB2, the OP valve screen grid's have to be fed by a regulated PSU with a +/- 5% spec, the seperate tfmr and board gives a well regulated 175v with bullet proof SC protection.
. I built a circuit board cutting the traces with a Dremmel using the circuit Dave Cox kindly knocked up for me, see - https://www.vintage-radio.net/forum/show...p?t=167901&page=3 for details. This caused me loads of grief, it took me weeks to de-bug the circuit as well a small pile of dead Q's and mosfets, a lot of this was down to the construction method, Dremmeling a PCB just isn't good enough, it is a quick way to make a board,but just isn't accurate enough. In future I intend to get some PCB's made up, spending weeks of frustrated de-bugging is a waste of time and is bad for the soul.
Lastly, lastly, the chassis also took quite a bit of time to make, this is another aspect that calls for software design & professional manufacture, I came across this firm - https://www.meface.co.uk/bespoke-anodise...nt-panels/ in future I'll get the top panel and perhaps the front and back done to get a decent aesthetic result and hopefully save a lot of time mucking about with an angle grinder and files as well as lots of prep & painting.
The amp isn't quite finished but the left channel is built, it'll need testing then hopefully the rest won't take long. Anyhoo,enough of my waffle, here's some pics.....Andy.
OP stage - After much experimenting with various odd valves including QQV03-20's, 6BG6GA's & 6DQ6B's I settled on 6146's running in class AB2. The OP valves are really wasted here as I could get over 70w with them in AB2 however 30w is plenty enough and I'm more interested in the direct connection of the driver stage to the OP stage that AB2 gives me rather than raw power.
Driver stage - The OP stage is driven by 12BH7 cathode followers run off a CCS this gives plenty of oomph at low-ish distortion. I'm not after ultra low THD, I'm after "sculpturing" the sound, adding just the right amount of 2nd harmonic, if I wanted low THD/straight wire with gain I'd use trannys and loads of NFB.
Phase splitter - again I'm using transistors for a CCS to get good AC balance using an ECC82 LTP, the only slight set back being the LTP has a bit of 3rd harmonic distortion,not to worry about though.
The part of the circuit that has been vexing me the most is the tone control/IP gain stage. I tried various circuit modifications to an active Baxandall tone circuit by for instance trying to increase gain by subbing an ECF82 for the 12AU7/12AX7 OP "opamp" not realising that than gain has to be spot on else the circuit won't work, see att. I'd painted myself into a corner by not having a clear design right from the start and so only had 1 1/2 B9A valve bases to play with as I fettled the chassis before having a clear idea of what I would build.
Lastly - power supply... again, like all my amp builds I tend to base my design around what bits I have, rather than the best or correct design,so with this amp as with the last one, the 807 amp, I used a choke I had as well as four 700u 250v caps, two in series to give 350u before and after the 15H choke. I had to wind two power transformers- the original ST70 clone was very loud and badly wound so I re-wound it to give me 0v, 450v, 470v, 500v HT, the original had a 330v 0v 330v HT sec with a 55v bias tap. With the extra room on the bobbin I overwound the primary and secondary with heavier gauge wire. The original pri was 220v, I rewound for 245v and put some extra turns on to give a higher V/T ratio. I also rewound a Japanese tfmr to give me a 100v bias supply & 100mA, a 3A heater winding and a 250v supply for the g2 supply. With AB2, the OP valve screen grid's have to be fed by a regulated PSU with a +/- 5% spec, the seperate tfmr and board gives a well regulated 175v with bullet proof SC protection.
. I built a circuit board cutting the traces with a Dremmel using the circuit Dave Cox kindly knocked up for me, see - https://www.vintage-radio.net/forum/show...p?t=167901&page=3 for details. This caused me loads of grief, it took me weeks to de-bug the circuit as well a small pile of dead Q's and mosfets, a lot of this was down to the construction method, Dremmeling a PCB just isn't good enough, it is a quick way to make a board,but just isn't accurate enough. In future I intend to get some PCB's made up, spending weeks of frustrated de-bugging is a waste of time and is bad for the soul.
Lastly, lastly, the chassis also took quite a bit of time to make, this is another aspect that calls for software design & professional manufacture, I came across this firm - https://www.meface.co.uk/bespoke-anodise...nt-panels/ in future I'll get the top panel and perhaps the front and back done to get a decent aesthetic result and hopefully save a lot of time mucking about with an angle grinder and files as well as lots of prep & painting.
The amp isn't quite finished but the left channel is built, it'll need testing then hopefully the rest won't take long. Anyhoo,enough of my waffle, here's some pics.....Andy.
