Assuming you're staying within the reasonable limits of an output tube (6v6, 6l6, el34, etc.), are there tonal qualities that would dictate designing an amp around a specific B+ voltage? Or is it ok, to just keep it wherever is convenient for available transformers and the rest of the amp design? I ask because I am designing an amp that will have a parallel EL84 and octal tube. EL84s can not take the plate voltage I would normally run a 6v6 at.
Why would you parallel an EL84 and an octal tube in a HIFI amp?
I can see it in an instrument amp, but...
I can see it in an instrument amp, but...
Yes, sorry. I should have mentioned that this was a guitar amp. the idea is to have the 2 tubes, each with a separate volume control allowing for a wide range of rich tones. Iv'e seen this done a few times, but only in amps designed for the lower voltage EL84
In the world of hifi, generally, the higher the supply voltage the lower the harmonic distortion, all else being equal.
In the world of guitar we don't care about simple harmonic distortion (it's too small to fuss over); we care about the distortion which happens during overdrive. For pentodes and tetrodes, the higher the screen voltage the lower the control-grid current and therefore the softer the clipping, and the less severe any blocking distortion will be, all else being equal. Most guitarists prefer this sound (which is one reason why triode and UL output stages are unpopular for guitar- their control-grid current is higher). But you certainly can get good tone at low voltages too; it's all about tweaking everything else to match. Beside, an EL84 will handle 300V or even 350V, which ain't exactly low.
In the world of guitar we don't care about simple harmonic distortion (it's too small to fuss over); we care about the distortion which happens during overdrive. For pentodes and tetrodes, the higher the screen voltage the lower the control-grid current and therefore the softer the clipping, and the less severe any blocking distortion will be, all else being equal. Most guitarists prefer this sound (which is one reason why triode and UL output stages are unpopular for guitar- their control-grid current is higher). But you certainly can get good tone at low voltages too; it's all about tweaking everything else to match. Beside, an EL84 will handle 300V or even 350V, which ain't exactly low.
I'm sorry. I figured since this was about the specific operation characteristics of a vacuum tube, the "Tubes" section was probably a reasonable choice.
Hi Guys
The "300V" rating for EL-84 is incorrect, so forget about it. They are used routinely in amps with 450Va and higher.
It is generally preferred to keep Va <450V so that one can use single caps rather than series-connected caps in the power supply. In this voltage range, paralleling any tube types that are commonly used in guitar amps is no problem whatsoever. You can do this for hifi as well.
In a SE amp, it is not a "requirement" that it be low voltage, rather it is the common way to do things, following the app notes from the tube manufacturer. Tubes will handle voltage stress better than current stress, and will do so their entire life. Forty years down the road the tube will still sustain full voltage even if its cathode emission has sunk to half its rating, at which point the tube maker considers the unit to have reached the end of its useful life.
Because SE is usually cathode biased and usually set up to run the tube hot, it is imperative that proper value screen-stops be used - 1k or greater per tube - unless you like to replace tubes a lot. You can also fix-bias an SE amp, and mix bias methods just as in a PP amp. This has great sonic benefits especially if you provide separate level controls for each output tube. TUT3 shows all these tricks in its 36 page chapter about the Champ.
Have fun
The "300V" rating for EL-84 is incorrect, so forget about it. They are used routinely in amps with 450Va and higher.
It is generally preferred to keep Va <450V so that one can use single caps rather than series-connected caps in the power supply. In this voltage range, paralleling any tube types that are commonly used in guitar amps is no problem whatsoever. You can do this for hifi as well.
In a SE amp, it is not a "requirement" that it be low voltage, rather it is the common way to do things, following the app notes from the tube manufacturer. Tubes will handle voltage stress better than current stress, and will do so their entire life. Forty years down the road the tube will still sustain full voltage even if its cathode emission has sunk to half its rating, at which point the tube maker considers the unit to have reached the end of its useful life.
Because SE is usually cathode biased and usually set up to run the tube hot, it is imperative that proper value screen-stops be used - 1k or greater per tube - unless you like to replace tubes a lot. You can also fix-bias an SE amp, and mix bias methods just as in a PP amp. This has great sonic benefits especially if you provide separate level controls for each output tube. TUT3 shows all these tricks in its 36 page chapter about the Champ.
Have fun
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