pentodes and power supplies

[Andy Le Blanc]

I spent the last couple days noodle-ing with pentodes...... 6AU6 and EF86
theres alway tubes in the bone yard that you wonder about so haveing the
time I recycled some of the crap laying around the shop.... the remains of a
marshal and the bin of scrounged tube..... I tried out a typical fender power
side with a pair of 6l6 and a longtailed inverter with a single pentode pre....

these sound good and the gain is about right for the amp to respond about as
a fender should...... but the hum is a real issue..... now its not the heaters
and its not grounding or a wiring issue..... its the power supply.....

with both the 6au6 and the ef86 the operating point can be tweaked to reduce
the hum but at the sacrifice of gain.... the whole point is to have a single gain
stage with enough gain to drive the power side as you would expect....
you dont get the distortion of multiple stages.... so you hear more of your
instrument and the coloration of your amp without the blurr....

the reused power supply was out of a marshall.... a bridge rectifier and
no center tap.... the hum was not apparent in the supply or the power side
but reared its head in the pre.... unchanged when the heaters were run on
battery..... Ive heard pentodes can be quirky with age and are prone to
supply issues because of the large gains.... any ideas?

[CaseyJones]

Cheez Whiz Andy, got too much time on yer hands or what?

Sell those 6AU6s on eBay and buy some real tubes!

Once you get rid of the hum you'll find that EF86s are painfully microphonic. If you want some real excitement use them in a small combo right next to the speaker. I don't know how Vox did it...

[Phil_S]

I tried to sell my 6AU6's on eBay, starting bid $1. You have a better chance if you offer to pay someone to take them away.

[CaseyJones]

I tried to sell my 6AU6's on eBay, starting bid $1. You have a better chance if you offer to pay someone to take them away.

Toss keywords, "Hammond", "Leslie" and "NOS" in the description.

The Hammond organ guys are consumers of 6AU6s. A Hammond plugs into a Leslie so you can be pretty sure anyone with a Leslie either owns a Hammond or wants a Hammond. And "NOS" because every jerk on the web is selling their nasty dirty used-up pulls as "NOS".

[Andy Le Blanc]

despite some of the the hidrances...... these things sound alright.....
they do have a tone... the purr a bit.... its like a paraphase inverter tone
never totally clean...... one of my favorite text refferances is a book from the
late fifties titled "The Electrical Production of Music" it bridges the gap between
how acoustical instruments create tone... what tone is as it is perceived by the
ear and how to "synthisize" tone electronically.... and it places an emphasis
on harmonic complexity.... an electric guitar is a synth... an electro-mechanical
tone source..... and any way to increase the harmonic complexity of the guitar/amp
system in a favorable way to the ear is a good thing....
you know just how bad some of those CBS fenders sounded..... the distortion
that gave fenders their tone was eliminated..... its always good to explore
this choice of tube and pre gives a tone like an old Watkins Dominator
but with the power and break up of a Bassman ....its really nice...

[Andy Le Blanc]

hum reduced....... funny thing ....had to go with zero bias operation.....
the cathode and suppressor grid are at ground potential and bias provided at
the control grid by either a battery cell or by a cap and high value resistor
grid leak... a .25 mfd cap and 15M resistor dressed directly to the socket pin
I found the recomendation on noise reduction in a publication from the mid
forties..... for pentodes in general..... its the application that counts
we'll see about microphonics...... thats tube selection..... those old fashioned
metel shields that you have to cram on might be ticket..... anti resonace...