....damn Thrintun!

• http://diyparadise.com/simplepreamp.html

It is my opinion that if you're going to the bother of cutting holes in a chassis, it should be better.

A 300-0-300 plate winding and two 12AX4 damper diodes is a good place to start. Feed a choke, like Hammond's 30 Hy/40 mA and 100 uF of motor run cap and if you're past critical current( 20 or so mA ), you'll have less than a volt of ripple peak-to-peak.

Also, what about gain? be sure of the requirement, so as to avoid padding the volume control as a lot of folks have had to do when building with 12AU7's.

A single 12B4 is one of my fav's. or get nuts, and pick the 26, or 45, or 10...

And then, there is the question of plate loads. Resistive, CCS, or inductive? There are advantages to each to be sure. A single DN2540N5, a gate stoper resistor and a 1k/10 turn pot is one of the simpler ways of doing a CCS. Cost ~$5 depending on sourcing and shipping costs. Cascoding the 2540 is better but a bit more complex as well.
regards,
Douglas

....damned Thrintun!

This particular preamp was chosen by another member because there is already a wealth of literature on this particluar design. Yes, you are correct, new audio ground is not being broken here. What we are trying achieve is a basis for future projects. The logistics of an internet group build is daunting in itself, muchless without struggling over modifications from the outset. Please, do not mistake me, I am all for "playing" with designs; that is what this hobby is all about. I will build the preamp stock; others will not.

Douglas, your knowlege of this science is apparent. To be honest, what I know about tubes could be written on the back of a postage stamp. It is people like you and others that I want to learn from. I truely look forward to future discussions with you....Regards, Colin

Take a slightly smaller simpler means of doing it.

180vac( or thereabouts, there is a 190-0190 Hammond, 2 or 369EX ), double for a CT winding or a full bridge on a single...hybrid bridge, SF4007 diodes to ground and a 6CA4 for the forward diodes. The same 30 Hy Hammond, and an eaBay-ed motor run of 240vac rating and ~100 uF.

This will get a reasonable B+. The single DN2540N5 with a reasonable heatsink will also allow the use of another output point, namely the source of the MOSFET. You'll have what is effectively a source follower output buffer. This also simplifies the cathode circuit because the triode still sees a constant curent, and no bypass is needed. Use a pot instead of a fixed resistor to set the current. try 10-15 mA.

The output coupling cap has already been mentioned.

the EX Hammond has 2.5A of 6.3, plenty for a 6CA4 and single 5687. DC can be done with a RadioShack TX and an LM317 should it be required at ~$20 additional.

Biasing of ~3 volts ought to give a good operating point, 2-4v is a good place to play. V=iR.

so, 2 nine pin sockets, 1 5687, 1 6CA4, 1 369EX( note 370AX is on sale for $36 ), 1 157G, one 80-100 uF/240vac motor run oiler, 2 5 uF motor run caps for the output coupling, 2 DN2540N5 mosfets, 510R 1/4 watt carbon comp gate stoppers, 2 1kOhm pots for current set, 4 RCA jacks, 1 stereo volume control ( R-S 100k Alps is ~$3 ), 4 red/20 mA LED's for bias, wire and chassis material.

so where are we?
regards,
Douglas

....damned Thrintun!

Regulated, depends on what youcall regulated. recitfier to an LC filter, runniing at past critical current is pretty well regulated.

The recitfier thing. V-0-V allows two forward facing diodes( like tubes ). Full bridge it is called. V-V, or 0-V requires a full bridge of four diodes. One can substitute a twin diode like a 6CA4 or 5U4, or 6X4 in place of the two forward facing diodes and use SS for the 'back' side of the bridge.

It depends on how you use the CCS. A Bottlehead C4S won't work as a mu-follower style circuit. A mosfet can. if the load ( the amp ) is connected to the plate, and we're using a CCS as a plate load, the AC current taken by the load leaves a varying current through the valve, and therefore the cathode. Take the output at the source of a mosfet, above the current setting resistor, and the device will still provide the valve with CC, and supply the AC to the load. If the current is constant through the valve, we don't *REQUIRE* a bypasesed cathode. It may be useful to avoid hum coupling from the heater to bypass but that's another topic.

What is a 360-360?
regards,
Douglas

....damned Thrintun!

Own design is an interesting concept. I did not see this exact thing elsewhere in the exact form I will provide you( or anybody who asks ) with. I have build a few linestages, and will build a few more before I am done. This is about as simple as I think it can be done and still deliver 'the goods'.

If you are going to go DC on the filaments, the Hammond 261C6, E6, and G6 are all capable of running the rectifier, and are a bit cheaper.
regards,
Douglas