I have two preamps in my collection earlier (well there was a third). Think my SRPP line stage copied from Elektor Electronics is not functioning all rigt. The Octal 6 (from Mapletree Audio Design), which actually is an Octal 4 as I only use the version with a 6J5 + 6R7, is working OK. The third was a mysterious thing from Russia (AML+).
The reason for this project was the two inductive potentiometers I had gotten from a friend in Malaysia, a great amount of tubes a chassis and some transformers suitable for preamplifiers.
I picked the Blue Velvet of no particular reason, maybe becasue it was so un-kinky. The schematic is copied from Dick Olsher and Kara Chaffee), though the graphic is rather hard to read, so I redraw the schematic. The ECL85/6F5P is an add-on that will be installed (maybe) later on. It's a simple regulator, but until then I will feed the amp from a 240V-transformer, rectified through a diodbridge (4 x 1N4007) and filtered by a CRC-filter (100uF + 1,1kohm + 100uF).
I will use a pair of 6N8S (Russian equivalent to 6SN7) and else very standardized components. Resistors are metalfilm (1% 0,6W) and capacitors are polypropylene (RIFA) and polystyrene (no-name) , as I had those at home. Else it isn't shown anywhere that special kinds of capacitors are "better" than not special kinds. It is even so that many manufacturers randomly pick standard caps, marks them with some HighEnd-label and sets the price tag 10 times higher.
Cables are also "what I found in my drawers". Switches are cheap ones from Conrad's Swedish web shop. Transformer is a standard one from Danbury in UK, now sadle closed down. RCA connectors are great stuff from VT4C in HongKong. Chassis is a vintage from a tube amp kit (Swedish?), and so are knobs, main switch and neon lamp holder.
Shortly this is an easy and financially reasonable project.
I wouldn't call this my first serious preamplifier project. But it is serious, it is a premplifier project but my third.
Maybe slightlu more serious as I am using inductive volume controls and will (later on) add some gizmos that will enhance the operation. More about that later.
The idea is to use an old chassis I have cut and drilled before but never gotten around to do anything serious. Well nothing at all.
The original idea was to use a motorpotentiometer with remote control but I dropped that. I will use a pair of 6SN7 (Russian 6N8S). So in this photo - the motorpot, the relay board and remote control will go. The transformer will be replaced as this one is intended for another project.
About tha chassis - it was intended for a simple 3-tube amplifier kit once. The power switch and neon lamp holer are original as well as the two knobs for input selector and volume.
I have made a small subchassis for the tubes. There weren't many components but I managed to make the small alu-angle really crowded.
I am using whatever components I have handy and that can withstand the voltages, wattages or currents in the
circuits. Here can the RIFA polypropylene capacitors (output) be seen as well as some (yellow) noname polystyrene caps.
Religious fanatics may have a heavenly belief that certain caps "sound better" better that others, but I do not even involve in that kind of discussions, and avoid that kind of charlatans.
This looks more accurate. Still the wrong transformer, but two 100uF caps are there, the tube assy, the assy with the rotary s witches (selector and volume) and the inductive "potentiometers".
The two inductors are mounted on top each other. IF the chassis had ben laid out different I would have mounted them in another fashion to make it easier to solder wires between them and the switch.
I use a set of RCA connectors bought from VT4C. These were nice to mount with the mounting screw on the outside and VT4C sells them at a super price.
Spaghetti. I've just soldered the input selector and have soldered small pieces of cable to the inductors. Now the fun begins!
Mains transformer is a standard thing from Danbury Electronics. I am just slightly worried that the total 1,2A the tubes will eat is a too big burden on the transformer. I always go for a 50% headroom. However the 125mA for HV is too much.
Tubes!? Got some 12SN7 - 6SN7 running on 12V heater voltage. Even have a bunch of Russian 6N8S and a pair of RCA 6SN7.
There simply have to be a blue LED somewhere!!!
Backpanel using the xcellent RCA jacks from VT4C.
So, where are we right now? Well the nuce chassis has been occupied by a kit built 6P1P (large 6AQ5 in 9-pin socket). The original idea of the Blue Velvet isn't dead at all. Got the sub assy still intact and even have a idea where to mount it (the SRPP preamp chassis describes elsewhere. Got some ideas ...
Dick Olsher and Kara Chaffee website