Re: Reasons for massive 6v6 current on DIY 15W?

"Jaret" <jbilewitch@xxxxxxxxx> wrote in message
news:64968ebd-49df-438f-8b9e-75c6e580817b@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
So I decided to get my feet wet building my own amp using a schematic
for a Bogen DB110 (3x 12AX7s, 2X 6V6GTs) I found on
schematicheaven.com:

http://www.schematicheaven.com/hifiamps/bogen_db-110_texas_blues_special.pdf

When I assembled it and fired it up, I quickly noticed the distortion
was horrible! Almost like the speaker sounded blown (it's not- I just
bought it). I checked the output tubes (cathode biased) and found, to
my horror, that they were running at almost twice the rated
dissipation for 6v6's (like 20W!). After doing alot of puzzling and
alot of internet searching on cathode biasing, I cranked the cathode
resistor up to a 600 ohm and now have values that are about 12W.

My question is, given that I adhered to the schematic (which I realize
could've been created by anyone), how can there be such a huge
difference in what was needed for a cathode resistor value? I've
checked some other similar schematics and 600 ohms seems high.

Also, even though it's biased below 12W in what (I think) is the
pentode setting (screens -> B+), when I switch it over (screens ->
plates/OT) it jumps up to 20W again.

As this is my first project and, as you can probably guess from this
message, my knowledge is pretty poor on amp design, I was hoping
someone could help explain what's going on! This problem aside, I've
got some ideas for improving the design, like de-paralleling the 1st
stage for more pre-amp gain and maybe adding a master volume control.
Any advice for other improvements would be appreciated, too!

I've taken a quick look to the linked schematic:

DUH !!, the B+ DC supply voltage is connected directly to the
plate of V5, look not further. The OT has no center tap, it's a
single ended transfomer. Then, both output valve's plates should
be connected in parallel to one end of the OT's primary (usually
the blue wire) and the other end (red wire) to the B+ DC supply.
Either, the schematic has a major error or it was drawn by an
imcompetent dude (lots of 'em around ...)


Chucktronic
http://chucktronic.ca/
http://www.myspace.com/chucktronic



.

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