6V6 amp on the cheap

[mesherm]

http://webpages.charter.net/mesherm/6V6 ... v6back.jpg
http://webpages.charter.net/mesherm/6V6 ... 6front.jpg

This amp started out using one 6BX7 per channel but after a month of fooling with it trying to get more than about 3 watts I just punched an extra hole and set it up for 6V6s instead.
I built this amp mainly because I had all the parts and because I wanted to explore using TI's TL783 high voltage 3 terminal regulators as constant current sinks for the cathodes. The maximum voltage differential is 125 volts with max current being 700 ma...plenty for tube use. I had also modified my GSG to use EL34s with the TL783s as constant current sinks but that's another story.
The circuit is simple. Instead of a cathode resistor at each 6V6, I used a TL783 in a constant current configuration with a 47 ohm fixed resistor in series with a 20ohm trimpot. There was a 10 ohm resistor outside the loop for measuring tube current. Each TL783 was bypassed with 200mf 50 volt caps.
I had originally planned to have the two halves of the 6BX7s in a differential configuration with a TL783 as a constant current tail but it proved difficult to balance the two halves for equal current draw. With one of the 6BX7s triodes grounded I would not need a PI section.
When I switched to a regular PP configuration I had to change the front end from a SRPP to a differential circuit so I used an LM234 constant current sink ala Shannon's ST70 PI circuit. Since I had designs on selling it eventually on ebay I added a volume pot and a headphone jack.
I have the idle current set at 25ma on all the 6V6s and I have swapped them around and no matter which tube is where all the idle currents stay at 25.0 ma. One benifit of the constant current adjustable cathode sinks.
I measured the power output as around 5.2 watts per channel RMS at 1k Hz, not much but adequate. I was in for a real surprise though when I hooked it into my sound system. The bass was fantastic. I put it back on the bench and stepped down in frequency to 30 hz while scoping the output...no appreciable distortion or voltage drop. I went to 20 and the waveform was a bit distorted but who cares. 30 hz is plenty for me.
The upper ranges seemed a bit harsh to my ears so I added a little negative feedback via a 68pf silver mica and a 3 k resistor. I had grounded one side of the LTP using a 100 ohm resistor just in case I need some NFB. I have been doing some extensive listening and am still amazed at the great bass response of this simple little amp. It could be the fact that there is only one coupling cap stage in the circuit or it could be the Hammond OPTs. I had those same OPTs in my S-12 amp though and never got bass response this good.
Now I'm not sure I want to sell it.....LOL

[Gingertube]

One benefit of exactly matched idle currents in the push pull sides is always improved bass response. I've done the same thing with both 6V6 and EL84 but using "ring of two" transistor current sources.

I would never go back to resistor based cathode biasing.

Cheers,
Ian

[TomMcNally]

Nice job Mike (from a fellow Hammond black chassis lover) !

Any chance you could sketch up a schematic showing how the TL783 is used in the circuit ? That might be the way to go in a PPP amp I'm thinking about. I have a new pair of Hammond 1615's I wanna do something with. I used the 272HX power transformer in the EL-34 SE amp ... but they'll make more !

... tom

[sorenj07]

awesome! my impulse is to go with the hand-tuned dinosaur of fixed bias but if this can take care of it hassle-free, it might be the ticket for my next amp. which is coming up as soon as i get rid of this one.

[mesherm]

I will get a schematic drawn up tonight and post it.
For my EL34 SE amp I reduced the 47 ohm resistor to 20 ohm in order to have more current. The three terminal regulators must be mounted on a heat sink. I used small clip on heat sinks for the 6V6s but larger ones for the EL34s where I was drawing 60ma through each. The TL783s use MOS for the final regulation and seem to be much more stable than the LM317 types in addition to being able to go to higher voltage.

[hasafraker]

wow Mike that's very sharp lookin, what was the ballpark total cost? After I wrap up my current projects I want to build a new from scratch amp. I'm lookin for ideas. Yellow_Light_Colorz_PDT_17

[mesherm]

I had most of the parts on hand so it wasn't too bad. The PT was from Musical Power Supplies on ebay $35+10 for shipping =$45.
Hammond chassis and plate are like $35.
Six tube sockets about $30.
Four 6V6 $35.
Two 7F7 $12
Hammond 1608/9s $80.
odds and ends about $25
Total= $262 so less than $300 for the whole deal.
You don't have to use 6V6s. EL84s or any of those Russian clones would work also. 12AT7s or any dual medium mu triode will work for the driver.
Edcors would work in place of Hammonds although the Hammonds have pretty good bandwidth.
Using EL84 type tubes would give more output power.