Hello all you smart helpful people. 
I have an EL34 P-P tube amp that currently runs in UL mode.
I'd like to convert it to Triode mode to see if I like the sound.
I have done some research and come up with the plan on the diagram.
Is it correct?
Is there anything else I need to be aware of?
Also, where I have marked the red question marks, is this a feedback loop? Should I leave it alone?
I have an EL34 P-P tube amp that currently runs in UL mode.
I'd like to convert it to Triode mode to see if I like the sound.
I have done some research and come up with the plan on the diagram.
Is it correct?
Is there anything else I need to be aware of?
Also, where I have marked the red question marks, is this a feedback loop? Should I leave it alone?
An externally hosted image should be here but it was not working when we last tested it.
That's not too bad, really.
I just stopped by my local electronics shop and picked up 4x 100R 2w. Can't wait to get home and do some tinkering.
I don't really know how to adjust cathode bias (or any bias, for that matter).
WRT bypass caps, I wouldn't know where to put them...
You definitely want to *measure* the screen current as you reach clipping.
The role of that resistor (which is essential) is to limit the screen current in order to prevent premature tube failure.
Ignore the power loss, it ought to be marginally audible, if at all.
There is no need to change the feedback values, except if there is HF compensation, that ought to be adjusted (anyhow, regardless if a change is made) for best square wave response - 1kHz as a midband point is a typical freq to check with. The feedback resistor sets the overall amplifier gain, and while in triode mode you have less overall gain, the difference is small. You could consider reducing the feedback, increasing the gain, but that result will need to be evaluated in light of the harmonic distortion levels and spectrum, and of course how it sounds to you.
_-_-
The role of that resistor (which is essential) is to limit the screen current in order to prevent premature tube failure.
Ignore the power loss, it ought to be marginally audible, if at all.
There is no need to change the feedback values, except if there is HF compensation, that ought to be adjusted (anyhow, regardless if a change is made) for best square wave response - 1kHz as a midband point is a typical freq to check with. The feedback resistor sets the overall amplifier gain, and while in triode mode you have less overall gain, the difference is small. You could consider reducing the feedback, increasing the gain, but that result will need to be evaluated in light of the harmonic distortion levels and spectrum, and of course how it sounds to you.
_-_-
I hate to say it, but I'm one of those guys who knows just enough electronics to be dangerous. I need things spelled out for me or I end up breaking things.
For example, how exactly would I go about measuring the screen current? I assume I should do this before making any changes?
I have both a standard DMM and a very nice old Sangamo Weston Schlumberger analog meter (which works like it is brand new, AFAIK).
For example, how exactly would I go about measuring the screen current? I assume I should do this before making any changes?
I have both a standard DMM and a very nice old Sangamo Weston Schlumberger analog meter (which works like it is brand new, AFAIK).
When UL is changed to triode the gain is reduced essentially.
As the GNFB components are not adjusted the feedback is smaller and THD higher than with UL.
Screen grid resistors are mainly for preventing oscillation. Put some 470...820 ohms to screen grids and you need to measure or worry about nothing.
The tube failure due to excessive G2 current is not potential or even possible with triode connection as it is in pentode connection.
As the GNFB components are not adjusted the feedback is smaller and THD higher than with UL.
Screen grid resistors are mainly for preventing oscillation. Put some 470...820 ohms to screen grids and you need to measure or worry about nothing.
The tube failure due to excessive G2 current is not potential or even possible with triode connection as it is in pentode connection.
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I suppose it would help to see the whole schematic?
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Typically 1/2 of what it is running in Pentode. M EL34 PP is 40 W pentode/20 W triode.
If you bias it into Class A triode about 8w.
25w UL is less than the typical 35 w of so.
dave
Thanks. I like the sound of that. Could you be more specific about exactly what I need to do here?
Assume I know nothing.
If that's the case, then perhaps some folks have other suggestions on how to get the best out of this amp.
I've already replaced C4 and C5 with Russian K40Y PIO, and C3 with Sprague orange drops.
Been running it like this for a long time and love how it sounds, but want to hear triode.
Looks awfully familiar, doesn't it.
I went all K40's for coupling in my 302b.
I really liked my amp in triode mode, running Russian 6P3S-E's. Sadly, it's collecting dust at the moment. I built a Pete Millett DCPP amp, which has become my "daily driver".
jeff
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