• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Otl

Status
Not open for further replies.
Jump to Latest
#104
Re: MEANING?

fdegrove said:
Any resistor on the grid when tied to ground is going to have an effect on the grid no?
Click to expand...

What do you mean by effect? Remember Frank, the "grid" is just a wire, hanging in the air. We either give it a potential using a battery, or we make the cathode more postive. There is no current flow in either case. The grid is an open circuit... EXCEPT in the case where we make the grid have a postive potential, and then it acts like a little mini plate. Then there is some (small) current flow between cathode and grid - because, just like the plate the grid attracts electrons when *positive*. But that is not Class A, or Class AB operation, and most tubes are not designed to operate that way.
 
#105
Any resistor on the grid when tied to ground is going to have an effect on the grid no?
Click to expand...
Like I said in my previous post, it's when . I contend that current does not need flow during operation.
Consider a small signal valve with a 10M leak R. Sometimes it is possible to remove the R during operation, and the circuit continues to work, perhaps until re-powered.
 
#108
dshortt9 said:
Until the leakage pulls the grid slowly less and less negative... then Whammo! Which is exactly why there is a maximum grid circuit resistance specified.
Click to expand...

Please explain to all of us how there can be a "whammo" when using self-adjusting bias?

Let's say the grid goes all the way down to zero, ok? The current in the plate circuit goes up, right? Fine, then that means the current across the cathode resistor goes up... which means the voltage drop across it goes up.... which means the grid swings more "negative" with respect to the cathode... which means the current goes down, etc. That's why it's called "self bias"!!!:bawling:
This is tubes 101, and I think we've spent enough time on it.
 
#109
dshortt9 said:
Remove the grid resistors from your tube amp and watch the plates glow red.
Click to expand...

Well, of course - because you are removing the grid's connection (referrence) to ground! It has nothing to do with the resistance. And in the case of fixed bias amps, you would be removing the bias supply's connection to the grid! Also not recommended.

Please don't take this the wrong way dshort, but I would go read up on bias some more before making any more of these silly points.:nod:
 
#110
Hello,

... and here is redrawed driver stage power supply, and bias
circuit diagram of the 6AS7GA OTL amplifier that is posted earlier.
 

Attachments

  • 6as7ga-pwr.gif
    6as7ga-pwr.gif
    25.5 KB · Views: 1,267
#118
OTL

Hi,

David and Scott,

Hey Frank, if you used the 8 or 10 tubes suggested by Scott
Click to expand...

At the risk of repeating myself:with OTL amps you always face the dilemma of low Zo and power output.

So,yes you could add a few tubes to that circuit,actually 4 of them in a series // configuration + NFB should bring the Zo down to usable levels.
You should discard the zobel network at the output as well since most modern speakers have one build in the x-over or don't need any.
I am not sure that circuit uses 4 // 6080s or 2.
The circuit does not show that clearly.
Keep in mind that the low mu triodes present a rather high input capacitance which is //ed by adding more tubes.
It will require a decent driver stage.

But using the 6336A is not cheap unless you're already sitting on a huge pile of them.

In the long run the 6AS7G/6080 is a better choice.
Other candidates are the EL509/6KG6 and the monster 6C33C.

Cheers,😉
 
#119
OTL

Hi,

I will build it as monoblocks.
Click to expand...

Fine.
I actually used 8 output tubes per channel on a friends' amp.

It is easy enough to do as long as you keep in mind that your powerxformers needs to be up to it.
Also I //ed the second 6SN7 on each block.

I set bias for 80 mA but it is not critical for sonic results IME.

The best way to avoid problems is to match the output tubes before you stick them in the amp.

Especially discard any tube with widely varying halves.

Cheers,😉
 
#120
Re: OTL

fdegrove said:
It is easy enough to do as long as you keep in mind that your powerxformers needs to be up to it.
Click to expand...

I looked through the Hammond catalog, and they have a line of toroidal x-formers that are small and could easily supply an OTL's 1-2 amps of B+. You can start to see why these didn't catch on!🙁
You take out the output tranny, and replace it with 20 lbs of power transformer!😉
 
Status
Not open for further replies.
Top Bottom