Suncalc wrote:
There are a whole set of these dissimilar triodes that I like for small low power amps. The 6EM7/6EA7, 6CY7, 6EW7, 6FM7, etc. (and all their higher filament voltage equivalents). They are all dual triodes with one moderate to high µ signal section and one power section designed to drive magnetic deflection coils. The requirements to drive an output transformer are very similar.
EXACTLY!!! The power sections have a lovely low Rp and they drive a transformer very well. Plenty of gain from the high-µ section.
I always thought this would be exactly what I'd use if I ever wanted to build a 5.1 setup for home theater. One tube per channel, nice and simple.
Suncalc wrote:
This and the fact that there were millions of these tubes made and never used because of the rapid switch over to solid state televisions, means that they are cheap and plentiful. Just the situation required for building small inexpensive low power amps.
I find, in general, that the 12 pin compactron sockets for these types are more expensive than the tubes!
If you get adventurous enough to use the 13 or 15V versions the tubes cost practically nothing - no one wants to use these voltages even though they are just as easy to derive as 6V or 12V. The power transformer I used was running well under its current spec so it provided a fair amount more than 12.6 V which is exactly what I was looking for. The power transformers look identical but one is 120:12 and the other is flipped to give 12:240 into the voltage doubler. Apex Jr. had a boatload of these super-cheap which made it VERY easy to build these amps cheaply. A smaller 12:240 would be just the ticket for making a little preamp or whatever that could be supplied by a 12VAC wall wart, if you like that idea. The ones you see here are surely overkill.
Suncalc wrote:
Tubegeek, do you have a picture of this amp? I'd love to see it.
Why indeed I do!
The one in the photos has some botched metalwork which is why I still have it. The 2nd one I built has mesh around the top deck and was recently removed from service at a local restaurant - after 8 years of playing all day every day, it outlasted the restaurant. The owner brought it home and for all I know it's still rockin'.
Unit #2 got the better Hammond single ended output transformers as spec'ed in the design, the ones in the pictures are universal "experimenter" SE/PP if I'm not mistaken. Size is 8"D x 12" W x 3.5" H to the top deck, 5.5" H to the tops of the tubes.
OVERVIEW:Attachment:
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BACK PANEL:Attachment:
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TOPLESS:You can see the two back to back power transformers to the left, with the 6 filter electrolytic caps in front of them and to the right.
The little circuit board has the divider which keeps the filament supply biased up (I'm not sure why there are two sets of resistors?) and then in front of that is the L channel output transformer. On the right side is the R channel output transformer and both the loading chokes snuggled in close to each other.
When I built the 2nd one it was considerably neater than this one!
Attachment:
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UNDER THE TOP DECK:(one of the things I find cool about these choke-loaded DC-coupled amps is that there's almost nothing inside there but iron.)
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