In search of the perfect linestage...
Over the past few months I've started constructing linestages. Unlike amplifiers, linestages are fairly inexpensive to put together, especially with a well stocked cellar. Many of these designs were optimized to use parts I had on hand - most power transformers were scavenged as were many sockets, caps and even knobs (Yup, most of the knobs came from an old Tektronics scope).
My first attempt was derived from the SRPP design that Russ Sadd had on his website. I made use of minature types (6CG7, 0A2) as well as solid state rectification. It's a wonderful sounding, but unfortunately has way too much gain. I ended up padding down the input to around -30 to -50db.
My second attempt made use of a simple design based on the 5687 that I found thru the Joelist. I don't know the original designer but would love to give appropriate credit if anyone does! I used a 6X5 rectifier along with a regulated B+ similar to the above design. It is a very nice sounding linestage for such a simple design. It's currently living across town with James Melhuish - take a peek at his full range speaker page when you get a second!
Attempt #3 was based on the same design, but I abandoned the regulation. I like the unregulated version better sonically as well as aesthetically.
My latest linestage started out as Jack Elliano's Ultrapath design. Unfortunately I underestimated the distance needed between the power transformer and the output transformers. The hum was quite excessive. I'll rebuild it later using a separate chassis for the power transformer.
I changed the circuit to a simple voltage gain stage. The schematics are here for the linestage and power supply. Two more views are here and here. A few words about the power supply. I made use of a interesting part by Keystone Thermometrics - the CL110 inrush current limiter. This gives a nice slow start. The filter cap on the filament is clearly excessive, but I had it lying around.
Ok, there's a reward for reading this far down the page - I'll let you in on a tip about one of my favorite tubes! Did you notice the 2C22 (or 7193) tubes? These unusual tubes are unique in that they have both a grid and plate cap in a GT style bottle. Take a look at the spec's here. They are surprisingly similar to a 6J5 or 1/2 of a 6SN7. These blow away any 6J5 or 6SN7 I've yet tried and are inexpensive. Just watch the plate caps around your kids.
Baby Blue - A Western Electric Based Linestage!
My latest linestage was prompted by a hamfest purchase of a moderate quantity of Western Electric 407A tubes. Specs for the 407A are available on my Data page The 407A is essentially a 20/40 volt version of the WE 396A / 2C51 twin triode. Don't be turned off by the odd filament voltage - it is easily provided by a 25vac transformer and a LM7820 voltage regulator. Each tube draws 100ma @ 20v and the regulator barely gets warm powering both tubes.
While Western Electric 396A's or 2C51's have become very pricey, the Western Electric 407A is still an orphan and quite inexpensive. I've seen them for < $10 on various websites. Even less expensive, Ned Carlson at Triode Electronics had some very nice looking Sylvania Gold Brand 407a's on special for $2.95
A very inexpensive PCB and bare kit for 78xx based voltage regulator is the kit #60 by KitsRus and distributed in the USA by Amazon Electronics.
Other versions of this tube include the 5670 - a rather complete datasheet for the 5670 is on Pete Millett's site.
The schematic is based on a design I downloaded from a site in Taiwan. I couldn't read a single thing on the site, but thankfully electronics are universal. I made up a small PCB using ExpressPCB's rapid service. They provide a simple PCB layout program free, and are set up to provide small batches of PC boards to the hobbiest at attractive prices.
The output caps are a pair of 5uf motor run caps that have a wonderful tone. The case is tightly packed as you can see here. Power supply uses a surplus transformer from a vtvm that was beyond repair and supplies 25vac to the filament regulator and 185vac for B+. The high voltage supply uses a bridge rectifier and CLCLC filtering to provide 215vdc. The linestage is dead quiet, and even drives some of my more difficult amplifiers with ease.
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