Todays jam amp

[Andy Le Blanc]

I make the time to set up or mod an amp to go to jam every few sundays.
heres a ruff scheme of todays amp. Ive found a limit with this form of diode bias.
Shot noise has become very apparent, still usable, the tones are gonna be
a hoot. They all act as response controls. Other than the bias in the pre, and
choice of tones, its a generic 50w 6l6, cathode bias, fender circuit.

[Andy Le Blanc]

This amp had a great tone, it quickly became apparent the the gain was way
too hot, I replaced one of the 12ax7 with a 12au7 to reduce some of the gain
at the jam. I'll replace the plate resistors in the pre with something in the range
of 39k to 47k and give it another try next week. It should reduce the noise
I encountered this morning after tacking it together. It nailed fender tones
pretty easy clean and had a nice natural OD even using the motley assortment
of tone tweakers as is, really seems flexible. Tone controls are subtractive,
its more fun to play with the response, so to speak.

[Phil_S]

Very cool that you can a) do this to an amp and b) have a forum for a live test.

[Andy Le Blanc]

Open mic's are good for some things. Many customers show just drop gear off.
Picked up work yesterday too, an order for a custom bottom with a 10' for a
harp player, and a opt replacement.

That diode hook up is ridiculously simple and it sounds good. It works marvelously
in a simple one gain stage with inverter amp. So with two, and hi mu tubes,
gain and gain related issues popped up, 12ay7 might be a better choice.
I found that diode configuration in an old high school electronics book.
I'm still very impressed with it. You can reduce your stage to socket dress,
one resistor and one cap. I'm sure there are issues with it, but so far the
practicality of the circuit more than makes up for it.

[Structo]

So are you talking about using a diode for the cathode bias of the preamp tube?
I didn't see one on your sketch so I was wondering.

[Andy Le Blanc]

One half of a 12ax7 is dressed to act as a diode, and then used to bias the
other side, fun trick. You can do it to any dual triode. One of the issues with
it is the mu of the tube, it drifts with voltage condition and age of the tube.
This makes the gain less predictable and, of coarse, its not an ideal diode.
But it sounds good, and really simplifies a hand fabricated circuit.

Here's the image I found in the text book, they've replaced a crystal with a tube.
It goes on from here to modify the circuit as an amplifier, and once you get
there, then you have with little more modification a regenerative receiver,
and radio oscillator.

The example is done with a 6av6. They're going through the history of radio
and trying to make a Flemming Valve, in 1969 on a $30 lab fee for the course.

[Colossal]

One half of a 12ax7 is dressed to act as a diode, and then used to bias the other side, fun trick. You can do it to any dual triode. One of the issues with it is the mu of the tube, it drifts with voltage condition and age of the tube. This makes the gain less predictable and, of coarse, its not an ideal diode. But it sounds good, and really simplifies a hand fabricated circuit.

Here's the image I found in the text book, they've replaced a crystal with a tube. It goes on from here to modify the circuit as an amplifier, and once you get there, then you have with little more modification a regenerative receiver, and radio oscillator.

The example is done with a 6av6. They're going through the history of radio and trying to make a Flemming Valve, in 1969 on a $30 lab fee for the course.

That's really cool Andy, thanks for posting that. I appreciate the lesson. It's always nice to see these sorts of things presented in practical ways and accessible language.

[Andy Le Blanc]

Your very welcome. I'm always looking to try stuff out and looking for odd
circuits to put into amps. I found this looking up tube constants, I've had the
book for a long while, just recently put 2 and 2 together. Ive tried biasing
tube stages with LED's, to get gobs of harmonic distortion, mesa used a
diode bias in some early models, theres a brief mention of diode bias on the
Valve Wizard site and I think on the Steve Bench site.

[Colossal]

Ive tried biasing tube stages with LED's, to get gobs of harmonic distortion, mesa used a diode bias in some early models, theres a brief mention of diode bias on the Valve Wizard site and I think on the Steve Bench site.

Andy,

I've read Valve Wizards treatise on LED biasing and it seems really cool. Besides you get a nice little blinking light you could mount on the front panel when you're clipping; how cool is that?? Seriously, how did you find the distortion using LED biasing? I've always wondered if it was worth checking out. I really want to build a breadboard style amp so I can have a platform to work with without building a "real" amp to answer my own questions.

[Andy Le Blanc]

In a back issue of audioXpress, 6/03, theres an article by Graham Dicker,
"A Tube Preamp for Computer Soundcards". And in it are reviews of this guys
preamp by studio techs from Australia, where one mentions using it to get
some of the best overdriven sounds he'd ever heard. Its another thing that
is stupid simple and fun to play with. You can bias any old stage like this,
the led lights a bit then goes bright when current is being drawn.
A couple of these stages put together really sound the balls. Again, you'll have to
fiddle with it to get it just so. I think last time I used the clear radio sack LED's.

[Merlinb]

What sort of bias voltage does that give you? The 12AX7 is pretty high impedance, so I would expect it gives you quite a cold bias, right?

[Structo]

Depending on the color of the led, isn't the forward voltage on them all over 2v?
Here is a chart of different colors and forward voltages.

http://www.oksolar.com/led/led_color_chart.htm

[Andy Le Blanc]

Yes and no, the LED's forward voltage makes the bias and can be cold in
relative terms, but that depends on the device chosen, that's the "color"
reference you'll find. You pick a forward bias or color that's close to the desired operation point.

The tube diode hookup seems to settle out at around .85v to .95v, which is
pretty hot in relative terms. There's always a trade off, this simple hook up
is much less predictable and is affected by heater voltage fluctuation.
The amp I have set up at the moment also uses the heaters of the first to
preamp tubes to bias a pair of 6l6 or el34, which helps to regulate the diode
heaters some.

[Andy Le Blanc]

I've replaced the tubes in the pre with 12au7, and 39k plate resistors. The
gain of the amp is much better, about where one would expect, maybe a tad
hotter than a typical fender, funny thing, the amplification factor of a 12au7
being around 17. Other twin triodes like 12bh7a, 6sn7 and 6cf6 should work.
You could probably reduce the pre to one 12ax7. The amp purr's nicely, and
sounds a bit like a paraphase, nice departure from the fender tone, but still
retains the qualities one would expect, it breaks up like fender.

I did find a reference in 6av6 data sheets that was confusing, a statement
that diode bias should be avoided, 12ax7 shares the same characteristics.
After a little more digging I found that the statement had been carried through
from the diodes incorporated in a 6av6. It appears to have been developed for
automotive radios as a detector and high gain amp, and has separate diodes
around the cathode. The statement referring to attempts to use these to
provide a diode bias for the triode section of the tube.

[Andy Le Blanc]

More ......... I ran the first gain stage thru a Rightmark 6.2.1 audio analyzer.
I was able to run the pre with a 195v, and around 140v supply voltage.
This is one half 12au7 with the other half wired to act as a diode and provide bias, with a 39k plate resistor.
So ..... at 140v this method made a bias of around 1.4v,
and at 195v made a bias of around 1.7v.
The analyzer revealed a THD figure of less than 1% at the higher plate voltage.
I was surprised at how good they were, its a guitar amp, with no efforts made in that direction.
At 140v it had a 1.12% THD and at 195v the THD dropped to .75% .